51工具盒子通过管理一套图书的完整代码示例，来探索轻量级的 RESTful 服务。
Web 服务，以这样或那样的形式，已经存在了近二十年。比如，XML-RPC 服务出现在 90 年代后期，紧接着是用 SOAP 分支编写的服务。在 XML-RPC 和 SOAP 这两个开拓者之后出现后不久，REST 架构风格的服务在大约 20 年前也出现了。REST 风格（以下简称 Restful）服务现在主导了流行的网站，比如 eBay、Facebook 和 Twitter。尽管分布式计算的 Web 服务有很多替代品（如 Web 套接字、微服务和远程过程调用的新框架），但基于 Restful 的 Web 服务依然具有吸引力，原因如下：
- Restful 服务建立在现有的基础设施和协议上，特别是 Web 服务器和 HTTP/HTTPS 协议。一个拥有基于 HTML 的网站的组织可以很容易地为客户添加 Web 服务，这些客户对数据和底层功能更感兴趣，而不是对 HTML 的表现形式感兴趣。比如，亚马逊就率先通过网站和 Web 服务（基于 SOAP 或 Restful）提供相同的信息和功能。
- Restful 服务将 HTTP 当作 API，因此避免了复杂的软件分层，这种分层是基于 SOAP 的 Web 服务的明显特征。比如，Restful API 支持通过 HTTP 命令（POST-GET-PUT-DELETE）进行标准的 CRUD（增加-读取-更新-删除）操作；通过 HTTP 状态码可以知道请求是否成功或者为什么失败。
- Restful Web 服务可以根据需要变得简单或复杂。Restful 是一种风格，实际上是一种非常灵活的风格，而不是一套关于如何设计和构造服务的规定。（伴随而来的缺点是，可能很难确定哪些服务不能算作 Restful 服务。）
- 作为使用者或者客户端，Restful Web 服务与语言和平台无关。客户端发送 HTTP(S) 请求，并以适合现代数据交换的格式（如 JSON）接收文本响应。
- 几乎每一种通用编程语言都至少对 HTTP/HTTPS 有足够的（通常是强大的）支持，这意味着 Web 服务的客户端可以用这些语言来编写。
这篇文章将通过一段完整的 Java 代码示例来探讨轻量级的 Restful 服务。
基于 Restful 的"小说" Web 服务 {#%E5%9F%BA%E4%BA%8E-restful-%E7%9A%84%E5%B0%8F%E8%AF%B4-web-%E6%9C%8D%E5%8A%A1}
基于 Restful 的"小说" web 服务包含三个程序员定义的类：
Novel类代表一个小说，只有三个属性：机器生成的 ID、作者和标题。属性可以根据实际情况进行扩展，但我还是想让这个例子看上去更简单一些。Novels类包含了用于各种任务的工具类：将一个Novel或者它们的列表的纯文本编码转换成 XML 或者 JSON；支持在小说集合上进行 CRUD 操作；以及从存储在文件中的数据初始化集合。Novels类在Novel实例和 servlet 之间起中介作用。NovelsServlet类是从HttpServlet中继承的，HttpServlet是一段健壮且灵活的代码，自 90 年代末的早期企业级 Java 就已经存在了。对于客户端的 CRUD 请求，servlet 可以当作 HTTP 的端点。 servlet 代码主要用于处理客户端的请求和生成相应的响应，而将复杂的细节留给Novels类中的工具类进行处理。
一些 Java 框架，比如 Jersey（JAX-RS）和 Restlet，就是为 Restful 服务设计的。尽管如此，HttpServlet 本身为完成这些服务提供了轻量、灵活、强大且充分测试过的 API。我会通过下面的"小说"例子来说明。
部署"小说" Web 服务 {#%E9%83%A8%E7%BD%B2%E5%B0%8F%E8%AF%B4-web-%E6%9C%8D%E5%8A%A1}
当然，部署"小说" Web 服务需要一个 Web 服务器。我的选择是 Tomcat，但是如果该服务托管在 Jetty 或者甚至是 Java 应用服务器上，那么这个服务应该至少可以工作（著名的最后一句话！）。在我的网站上有总结了如何安装 Tomcat 的 README 文件和代码。还有一个附带文档的 Apache Ant 脚本，可以用来构建"小说"服务（或者任何其他服务或网站），并且将它部署在 Tomcat 或相同的服务。
Tomcat 可以从它的官网上下载。当你在本地安装后，将 TOMCAT_HOME 设置为安装目录。有两个子目录值得关注：
TOMCAT_HOME/bin目录包含了类 Unix 系统（startup.sh和shutdown.sh）和 Windows（startup.bat和shutdown.bat） 的启动和停止脚本。Tomcat 作为 Java 应用程序运行。Web 服务器的 servlet 容器叫做 Catalina。（在 Jetty 中，Web 服务器和容器的名字一样。）当 Tomcat 启动后，在浏览器中输入http://localhost:8080/可以查看详细文档，包括示例。TOMCAT_HOME/webapps目录是已部署的 Web 网站和服务的默认目录。部署网站或 Web 服务的直接方法是复制以.war结尾的 JAR 文件（也就是 WAR 文件）到TOMCAT_HOME/webapps或它的子目录下。然后 Tomcat 会将 WAR 文件解压到它自己的目录下。比如，Tomcat 会将novels.war文件解压到一个叫做novels的子目录下，并且保留novels.war文件。一个网站或 Web 服务可以通过删除 WAR 文件进行移除，也可以用一个新版 WAR 文件来覆盖已有文件进行更新。顺便说一下，调试网站或服务的第一步就是检查 Tomcat 已经正确解压 WAR 文件；如果没有的话，网站或服务就无法发布，因为代码或配置中有致命错误。- 因为 Tomcat 默认会监听 8080 端口上的 HTTP 请求，所以本机上的 URL 请求以
http://localhost:8080/开始。
通过添加不带 .war 后缀的 WAR 文件名来访问由程序员部署的 WAR 文件：
http://locahost:8080/novels/
如果服务部署在 TOMCAT_HOME 下的一个子目录中（比如，myapps），这会在 URL 中反映出来：
http://locahost:8080/myapps/novels/
我会在靠近文章结尾处的测试部分提供这部分的更多细节。
如前所述，我的主页上有一个包含 Ant 脚本的 ZIP 文件，这个文件可以编译并且部署网站或者服务。（这个 ZIP 文件中也包含一个 novels.war 的副本。）对于"小说"这个例子，命令的示例（% 是命令行提示符）如下：
% ant -Dwar.name=novels deploy
这个命令首先会编译 Java 源代码，并且创建一个可部署的 novels.war 文件，然后将这个文件保存在当前目录中，再复制到 TOMCAT_HOME/webapps 目录中。如果一切顺利，GET 请求（使用浏览器或者命令行工具，比如 curl）可以用来做一个测试：
% curl http://localhost:8080/novels/
默认情况下，Tomcat 设置为 热部署hot deploys：Web 服务器不需要关闭就可以进行部署、更新或者移除一个 web 应用。
"小说"服务的代码 {#%E5%B0%8F%E8%AF%B4%E6%9C%8D%E5%8A%A1%E7%9A%84%E4%BB%A3%E7%A0%81}
让我们回到"小说"这个例子，不过是在代码层面。考虑下面的 Novel 类：
例 1：Novel 类 {#%E4%BE%8B-1novel-%E7%B1%BB}
package novels;
import java.io.Serializable;
public class Novel implements Serializable, Comparable<Novel> {
static final long serialVersionUID = 1L;
private String author;
private String title;
private int id;
`<span class="token keyword">public</span> <span class="token class-name">Novel</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setAuthor</span><span class="token punctuation">(</span><span class="token keyword">final</span> <span class="token class-name">String</span> author<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">this</span><span class="token punctuation">.</span>author <span class="token operator">=</span> author<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getAuthor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>author<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setTitle</span><span class="token punctuation">(</span><span class="token keyword">final</span> <span class="token class-name">String</span> title<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">this</span><span class="token punctuation">.</span>title <span class="token operator">=</span> title<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getTitle</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>title<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setId</span><span class="token punctuation">(</span><span class="token keyword">final</span> <span class="token keyword">int</span> id<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">this</span><span class="token punctuation">.</span>id <span class="token operator">=</span> id<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">getId</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>id<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">compareTo</span><span class="token punctuation">(</span><span class="token keyword">final</span> <span class="token class-name">Novel</span> other<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>id <span class="token operator">-</span> other<span class="token punctuation">.</span>id<span class="token punctuation">;</span> <span class="token punctuation">}</span>
`
}
这个类实现了 Comparable 接口中的 compareTo 方法，因为 Novel 实例是存储在一个线程安全的无序 ConcurrentHashMap 中。在响应查看集合的请求时，"小说"服务会对从映射中提取的集合（一个 ArrayList）进行排序；compareTo 的实现通过 Novel 的 ID 将它按升序排序。
Novels 类中包含多个实用工具函数：
例 2：Novels 实用工具类 {#%E4%BE%8B-2novels-%E5%AE%9E%E7%94%A8%E5%B7%A5%E5%85%B7%E7%B1%BB}
package novels;
import java.io.IOException;
import java.io.File;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.nio.file.Files;
import java.util.stream.Stream;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.Collections;
import java.beans.XMLEncoder;
import javax.servlet.ServletContext; // not in JavaSE
import org.json.JSONObject;
import org.json.XML;
public class Novels {
private final String fileName = "/WEB-INF/data/novels.db";
private ConcurrentMap<Integer, Novel> novels;
private ServletContext sctx;
private AtomicInteger mapKey;
`<span class="token keyword">public</span> <span class="token class-name">Novels</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
novels <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ConcurrentHashMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">,</span> <span class="token class-name">Novel</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
mapKey <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">AtomicInteger</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setServletContext</span><span class="token punctuation">(</span><span class="token class-name">ServletContext</span> sctx<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">this</span><span class="token punctuation">.</span>sctx <span class="token operator">=</span> sctx<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">ServletContext</span> <span class="token function">getServletContext</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>sctx<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">ConcurrentMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">,</span> <span class="token class-name">Novel</span><span class="token punctuation">></span></span> <span class="token function">getConcurrentMap</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">getServletContext</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span> <span class="token comment">// not initialized</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>novels<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator"><</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token function">populate</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>novels<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">toXml</span><span class="token punctuation">(</span><span class="token class-name">Object</span> obj<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// default encoding</span>
<span class="token class-name">String</span> xml <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">ByteArrayOutputStream</span> out <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ByteArrayOutputStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">XMLEncoder</span> encoder <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">XMLEncoder</span><span class="token punctuation">(</span>out<span class="token punctuation">)</span><span class="token punctuation">;</span>
encoder<span class="token punctuation">.</span><span class="token function">writeObject</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
encoder<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
xml <span class="token operator">=</span> out<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token keyword">return</span> xml<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">toJson</span><span class="token punctuation">(</span><span class="token class-name">String</span> xml<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// option for requester</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">JSONObject</span> jobt <span class="token operator">=</span> XML<span class="token punctuation">.</span><span class="token function">toJSONObject</span><span class="token punctuation">(</span>xml<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> jobt<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3 is indentation level</span>
<span class="token punctuation">}</span>
<span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">addNovel</span><span class="token punctuation">(</span><span class="token class-name">Novel</span> novel<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> id <span class="token operator">=</span> mapKey<span class="token punctuation">.</span><span class="token function">incrementAndGet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
novel<span class="token punctuation">.</span><span class="token function">setId</span><span class="token punctuation">(</span>id<span class="token punctuation">)</span><span class="token punctuation">;</span>
novels<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span>id<span class="token punctuation">,</span> novel<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> id<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">populate</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">InputStream</span> in <span class="token operator">=</span> sctx<span class="token punctuation">.</span><span class="token function">getResourceAsStream</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>fileName<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Convert novel.db string data into novels.</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>in <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">InputStreamReader</span> isr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">InputStreamReader</span><span class="token punctuation">(</span>in<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">BufferedReader</span> reader <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedReader</span><span class="token punctuation">(</span>isr<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> record <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>record <span class="token operator">=</span> reader<span class="token punctuation">.</span><span class="token function">readLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> parts <span class="token operator">=</span> record<span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">"!"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>parts<span class="token punctuation">.</span>length <span class="token operator">==</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Novel</span> novel <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Novel</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
novel<span class="token punctuation">.</span><span class="token function">setAuthor</span><span class="token punctuation">(</span>parts<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
novel<span class="token punctuation">.</span><span class="token function">setTitle</span><span class="token punctuation">(</span>parts<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">addNovel</span><span class="token punctuation">(</span>novel<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// sets the Id, adds to map</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
in<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">IOException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
`
}
最复杂的方法是 populate，这个方法从一个包含在 WAR 文件中的文本文件读取。这个文本文件包括了"小说"的初始集合。要打开此文件，populate 方法需要 ServletContext，这是一个 Java 映射类型，包含了关于嵌入在 servlet 容器中的 servlet 的所有关键信息。这个文本文件有包含了像下面这样的记录：
Jane Austen!Persuasion
这一行被解析为两部分（作者和标题），由感叹号（!）分隔。然后这个方法创建一个 Novel 实例，设置作者和标题属性，并且将"小说"加到容器中，保存在内存中。
Novels 类也有一些实用工具函数，可以将"小说"容器编码为 XML 或 JSON，取决于发出请求的人所要求的格式。默认是 XML 格式，但是也可以请求 JSON 格式。一个轻量级的 XML 转 JSON 包提供了 JSON。下面是关于编码的更多细节。
例 3：NovelsServlet 类 {#%E4%BE%8B-3novelsservlet-%E7%B1%BB}
package novels;
import java.util.concurrent.ConcurrentMap;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.util.Arrays;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.OutputStream;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.beans.XMLEncoder;
import org.json.JSONObject;
import org.json.XML;
public class NovelsServlet extends HttpServlet {
static final long serialVersionUID = 1L;
private Novels novels; // back-end bean
`<span class="token comment">// Executed when servlet is first loaded into container.</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>novels <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Novels</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
novels<span class="token punctuation">.</span><span class="token function">setServletContext</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">getServletContext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// GET /novels</span>
<span class="token comment">// GET /novels?id=1</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">doGet</span><span class="token punctuation">(</span><span class="token class-name">HttpServletRequest</span> request<span class="token punctuation">,</span> <span class="token class-name">HttpServletResponse</span> response<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">String</span> param <span class="token operator">=</span> request<span class="token punctuation">.</span><span class="token function">getParameter</span><span class="token punctuation">(</span><span class="token string">"id"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Integer</span> key <span class="token operator">=</span> <span class="token punctuation">(</span>param <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token keyword">null</span> <span class="token operator">:</span> <span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">(</span>param<span class="token punctuation">.</span><span class="token function">trim</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Check user preference for XML or JSON by inspecting</span>
<span class="token comment">// the HTTP headers for the Accept key.</span>
<span class="token keyword">boolean</span> json <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> accept <span class="token operator">=</span> request<span class="token punctuation">.</span><span class="token function">getHeader</span><span class="token punctuation">(</span><span class="token string">"accept"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>accept <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> accept<span class="token punctuation">.</span><span class="token function">contains</span><span class="token punctuation">(</span><span class="token string">"json"</span><span class="token punctuation">)</span><span class="token punctuation">)</span> json <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token comment">// If no query string, assume client wants the full list.</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>key <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">ConcurrentMap</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">,</span> <span class="token class-name">Novel</span><span class="token punctuation">&gt;</span></span> map <span class="token operator">=</span> novels<span class="token punctuation">.</span><span class="token function">getConcurrentMap</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Object</span> list <span class="token operator">=</span> map<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span>list<span class="token punctuation">)</span><span class="token punctuation">;</span>
&lt;span class=&quot;token class-name&quot;&gt;String&lt;/span&gt; payload &lt;span class=&quot;token operator&quot;&gt;=&lt;/span&gt; novels&lt;span class=&quot;token punctuation&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;token function&quot;&gt;toXml&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;list&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt; &lt;span class=&quot;token comment&quot;&gt;// defaults to Xml&lt;/span&gt;
&lt;span class=&quot;token keyword&quot;&gt;if&lt;/span&gt; &lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;json&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt; payload &lt;span class=&quot;token operator&quot;&gt;=&lt;/span&gt; novels&lt;span class=&quot;token punctuation&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;token function&quot;&gt;toJson&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;payload&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt; &lt;span class=&quot;token comment&quot;&gt;// Json preferred?&lt;/span&gt;
&lt;span class=&quot;token function&quot;&gt;sendResponse&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;response&lt;span class=&quot;token punctuation&quot;&gt;,&lt;/span&gt; payload&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt;
<span class="token punctuation">}</span>
<span class="token comment">// Otherwise, return the specified Novel.</span>
<span class="token keyword">else</span> <span class="token punctuation">{</span>
<span class="token class-name">Novel</span> novel <span class="token operator">=</span> novels<span class="token punctuation">.</span><span class="token function">getConcurrentMap</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>novel <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// no such Novel</span>
<span class="token class-name">String</span> msg <span class="token operator">=</span> key <span class="token operator">+</span> <span class="token string">" does not map to a novel.\n"</span><span class="token punctuation">;</span>
<span class="token function">sendResponse</span><span class="token punctuation">(</span>response<span class="token punctuation">,</span> novels<span class="token punctuation">.</span><span class="token function">toXml</span><span class="token punctuation">(</span>msg<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">else</span> <span class="token punctuation">{</span> <span class="token comment">// requested Novel found</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>json<span class="token punctuation">)</span> <span class="token function">sendResponse</span><span class="token punctuation">(</span>response<span class="token punctuation">,</span> novels<span class="token punctuation">.</span><span class="token function">toJson</span><span class="token punctuation">(</span>novels<span class="token punctuation">.</span><span class="token function">toXml</span><span class="token punctuation">(</span>novel<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">else</span> <span class="token function">sendResponse</span><span class="token punctuation">(</span>response<span class="token punctuation">,</span> novels<span class="token punctuation">.</span><span class="token function">toXml</span><span class="token punctuation">(</span>novel<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">// POST /novels</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">doPost</span><span class="token punctuation">(</span><span class="token class-name">HttpServletRequest</span> request<span class="token punctuation">,</span> <span class="token class-name">HttpServletResponse</span> response<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">String</span> author <span class="token operator">=</span> request<span class="token punctuation">.</span><span class="token function">getParameter</span><span class="token punctuation">(</span><span class="token string">"author"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> title <span class="token operator">=</span> request<span class="token punctuation">.</span><span class="token function">getParameter</span><span class="token punctuation">(</span><span class="token string">"title"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Are the data to create a new novel present?</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>author <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span> title <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_BAD_REQUEST<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Create a novel.</span>
<span class="token class-name">Novel</span> n <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Novel</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
n<span class="token punctuation">.</span><span class="token function">setAuthor</span><span class="token punctuation">(</span>author<span class="token punctuation">)</span><span class="token punctuation">;</span>
n<span class="token punctuation">.</span><span class="token function">setTitle</span><span class="token punctuation">(</span>title<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Save the ID of the newly created Novel.</span>
<span class="token keyword">int</span> id <span class="token operator">=</span> novels<span class="token punctuation">.</span><span class="token function">addNovel</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Generate the confirmation message.</span>
<span class="token class-name">String</span> msg <span class="token operator">=</span> <span class="token string">"Novel "</span> <span class="token operator">+</span> id <span class="token operator">+</span> <span class="token string">" created.\n"</span><span class="token punctuation">;</span>
<span class="token function">sendResponse</span><span class="token punctuation">(</span>response<span class="token punctuation">,</span> novels<span class="token punctuation">.</span><span class="token function">toXml</span><span class="token punctuation">(</span>msg<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// PUT /novels</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">doPut</span><span class="token punctuation">(</span><span class="token class-name">HttpServletRequest</span> request<span class="token punctuation">,</span> <span class="token class-name">HttpServletResponse</span> response<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">/* A workaround is necessary for a PUT request because Tomcat does not
generate a workable parameter map for the PUT verb. */</span>
<span class="token class-name">String</span> key <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> rest <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token keyword">boolean</span> author <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
<span class="token comment">/* Let the hack begin. */</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">BufferedReader</span> br <span class="token operator">=</span>
<span class="token keyword">new</span> <span class="token class-name">BufferedReader</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">InputStreamReader</span><span class="token punctuation">(</span>request<span class="token punctuation">.</span><span class="token function">getInputStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> data <span class="token operator">=</span> br<span class="token punctuation">.</span><span class="token function">readLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">/* To simplify the hack, assume that the PUT request has exactly
two parameters: the id and either author or title. Assume, further,
that the id comes first. From the client side, a hash character
# separates the id and the author/title, e.g.,
id=33#title=War and Peace
*/&lt;/span&gt;
&lt;span class=&quot;token class-name&quot;&gt;String&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;[&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;]&lt;/span&gt; args &lt;span class=&quot;token operator&quot;&gt;=&lt;/span&gt; data&lt;span class=&quot;token punctuation&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;token function&quot;&gt;split&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;token string&quot;&gt;&quot;#&quot;&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt; &lt;span class=&quot;token comment&quot;&gt;// id in args[0], rest in args[1]&lt;/span&gt;
&lt;span class=&quot;token class-name&quot;&gt;String&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;[&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;]&lt;/span&gt; parts1 &lt;span class=&quot;token operator&quot;&gt;=&lt;/span&gt; args&lt;span class=&quot;token punctuation&quot;&gt;[&lt;/span&gt;&lt;span class=&quot;token number&quot;&gt;0&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;]&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;token function&quot;&gt;split&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;token string&quot;&gt;&quot;=&quot;&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt; &lt;span class=&quot;token comment&quot;&gt;// id = parts1[1]&lt;/span&gt;
key &lt;span class=&quot;token operator&quot;&gt;=&lt;/span&gt; parts1&lt;span class=&quot;token punctuation&quot;&gt;[&lt;/span&gt;&lt;span class=&quot;token number&quot;&gt;1&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;]&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt;
&lt;span class=&quot;token class-name&quot;&gt;String&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;[&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;]&lt;/span&gt; parts2 &lt;span class=&quot;token operator&quot;&gt;=&lt;/span&gt; args&lt;span class=&quot;token punctuation&quot;&gt;[&lt;/span&gt;&lt;span class=&quot;token number&quot;&gt;1&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;]&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;token function&quot;&gt;split&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;token string&quot;&gt;&quot;=&quot;&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt; &lt;span class=&quot;token comment&quot;&gt;// parts2[0] is key&lt;/span&gt;
&lt;span class=&quot;token keyword&quot;&gt;if&lt;/span&gt; &lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;parts2&lt;span class=&quot;token punctuation&quot;&gt;[&lt;/span&gt;&lt;span class=&quot;token number&quot;&gt;0&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;]&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;token function&quot;&gt;contains&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;token string&quot;&gt;&quot;author&quot;&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt; author &lt;span class=&quot;token operator&quot;&gt;=&lt;/span&gt; &lt;span class=&quot;token boolean&quot;&gt;true&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt;
rest &lt;span class=&quot;token operator&quot;&gt;=&lt;/span&gt; parts2&lt;span class=&quot;token punctuation&quot;&gt;[&lt;/span&gt;&lt;span class=&quot;token number&quot;&gt;1&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;]&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt;
<span class="token punctuation">}</span>
<span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_INTERNAL_SERVER_ERROR<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// If no key, then the request is ill formed.</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>key <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_BAD_REQUEST<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Look up the specified novel.</span>
<span class="token class-name">Novel</span> p <span class="token operator">=</span> novels<span class="token punctuation">.</span><span class="token function">getConcurrentMap</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">(</span>key<span class="token punctuation">.</span><span class="token function">trim</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>p <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// not found</span>
<span class="token class-name">String</span> msg <span class="token operator">=</span> key <span class="token operator">+</span> <span class="token string">" does not map to a novel.\n"</span><span class="token punctuation">;</span>
<span class="token function">sendResponse</span><span class="token punctuation">(</span>response<span class="token punctuation">,</span> novels<span class="token punctuation">.</span><span class="token function">toXml</span><span class="token punctuation">(</span>msg<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">else</span> <span class="token punctuation">{</span> <span class="token comment">// found</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>rest <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_BAD_REQUEST<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// Do the editing.</span>
<span class="token keyword">else</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>author<span class="token punctuation">)</span> p<span class="token punctuation">.</span><span class="token function">setAuthor</span><span class="token punctuation">(</span>rest<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">else</span> p<span class="token punctuation">.</span><span class="token function">setTitle</span><span class="token punctuation">(</span>rest<span class="token punctuation">)</span><span class="token punctuation">;</span>
&lt;span class=&quot;token class-name&quot;&gt;String&lt;/span&gt; msg &lt;span class=&quot;token operator&quot;&gt;=&lt;/span&gt; &lt;span class=&quot;token string&quot;&gt;&quot;Novel &quot;&lt;/span&gt; &lt;span class=&quot;token operator&quot;&gt;+&lt;/span&gt; key &lt;span class=&quot;token operator&quot;&gt;+&lt;/span&gt; &lt;span class=&quot;token string&quot;&gt;&quot; has been edited.\n&quot;&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt;
&lt;span class=&quot;token function&quot;&gt;sendResponse&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;response&lt;span class=&quot;token punctuation&quot;&gt;,&lt;/span&gt; novels&lt;span class=&quot;token punctuation&quot;&gt;.&lt;/span&gt;&lt;span class=&quot;token function&quot;&gt;toXml&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;(&lt;/span&gt;msg&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;)&lt;/span&gt;&lt;span class=&quot;token punctuation&quot;&gt;;&lt;/span&gt;
&lt;span class=&quot;token punctuation&quot;&gt;}&lt;/span&gt;
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">// DELETE /novels?id=1</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">doDelete</span><span class="token punctuation">(</span><span class="token class-name">HttpServletRequest</span> request<span class="token punctuation">,</span> <span class="token class-name">HttpServletResponse</span> response<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">String</span> param <span class="token operator">=</span> request<span class="token punctuation">.</span><span class="token function">getParameter</span><span class="token punctuation">(</span><span class="token string">"id"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Integer</span> key <span class="token operator">=</span> <span class="token punctuation">(</span>param <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token keyword">null</span> <span class="token operator">:</span> <span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">(</span>param<span class="token punctuation">.</span><span class="token function">trim</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Only one Novel can be deleted at a time.</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>key <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_BAD_REQUEST<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
novels<span class="token punctuation">.</span><span class="token function">getConcurrentMap</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> msg <span class="token operator">=</span> <span class="token string">"Novel "</span> <span class="token operator">+</span> key <span class="token operator">+</span> <span class="token string">" removed.\n"</span><span class="token punctuation">;</span>
<span class="token function">sendResponse</span><span class="token punctuation">(</span>response<span class="token punctuation">,</span> novels<span class="token punctuation">.</span><span class="token function">toXml</span><span class="token punctuation">(</span>msg<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_INTERNAL_SERVER_ERROR<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">// Methods Not Allowed</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">doTrace</span><span class="token punctuation">(</span><span class="token class-name">HttpServletRequest</span> request<span class="token punctuation">,</span> <span class="token class-name">HttpServletResponse</span> response<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_METHOD_NOT_ALLOWED<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">doHead</span><span class="token punctuation">(</span><span class="token class-name">HttpServletRequest</span> request<span class="token punctuation">,</span> <span class="token class-name">HttpServletResponse</span> response<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_METHOD_NOT_ALLOWED<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">doOptions</span><span class="token punctuation">(</span><span class="token class-name">HttpServletRequest</span> request<span class="token punctuation">,</span> <span class="token class-name">HttpServletResponse</span> response<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_METHOD_NOT_ALLOWED<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// Send the response payload (Xml or Json) to the client.</span>
<span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">sendResponse</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span> response<span class="token punctuation">,</span> <span class="token class-name">String</span> payload<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">OutputStream</span> out <span class="token operator">=</span> response<span class="token punctuation">.</span><span class="token function">getOutputStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
out<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>payload<span class="token punctuation">.</span><span class="token function">getBytes</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
out<span class="token punctuation">.</span><span class="token function">flush</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RuntimeException</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token class-name">HttpServletResponse</span><span class="token punctuation">.</span>SC_INTERNAL_SERVER_ERROR<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
`
}
上面的 NovelsServlet 类继承了 HttpServlet 类，HttpServlet 类继承了 GenericServlet 类，后者实现了 Servlet 接口：
NovelsServlet extends HttpServlet extends GenericServlet implements Servlet
从名字可以很清楚的看出来，HttpServlet 是为实现 HTTP(S) 上的 servlet 设计的。这个类提供了以标准 HTTP 请求动词（官方说法，方法methods）命名的空方法：
doPost（Post = 创建）doGet（Get = 读取）doPut（Put = 更新）doDelete（Delete = 删除）
其他一些 HTTP 动词也会涉及到。HttpServlet 的子类，比如 NovelsServlet，会重载相关的 do 方法，并且保留其他方法为空no-ops。NovelsServlet 重载了七个 do 方法。
每个 HttpServlet 的 CRUD 方法都有两个相同的参数。下面以 doPost 为例：
public void doPost(HttpServletRequest request, HttpServletResponse response) {
request 参数是一个 HTTP 请求信息的映射，而 response 提供了一个返回给请求者的输出流。像 doPost 的方法，结构如下：
- 读取
request信息，采取任何适当的措施生成一个响应。如果该信息丢失或者损坏了，就会生成一个错误。 - 使用提取的请求信息来执行适当的 CRUD 操作（在本例中，创建一个
Novel），然后使用response输出流为请求者编码一个适当的响应。在doPost例子中，响应就是已经成功生成一个新"小说"并且添加到容器中的一个确认。当响应被发送后，输出流就关闭了，同时也将连接关闭了。
关于方法重载的更多内容 {#%E5%85%B3%E4%BA%8E%E6%96%B9%E6%B3%95%E9%87%8D%E8%BD%BD%E7%9A%84%E6%9B%B4%E5%A4%9A%E5%86%85%E5%AE%B9}
HTTP 请求的格式相对比较简单。下面是一个非常熟悉的 HTTP 1.1 的格式，注释由双井号分隔：
GET /novels ## start line
Host: localhost:8080 ## header element
Accept-type: text/plain ## ditto
...
[body] ## POST and PUT only
第一行由 HTTP 动词（在本例中是 GET）和以名词（在本例中是 novels）命名目标资源的 URI 开始。报头中包含键-值对，用冒号分隔左面的键和右面的值。报头中的键 Host（大小写敏感）是必须的；主机名 localhost 是当前机器上的本地符号地址，8080 端口是 Tomcat web 服务器上等待 HTTP 请求的默认端口。（默认情况下，Tomcat 在 8443 端口上监听 HTTP 请求。）报头元素可以以任意顺序出现。在这个例子中，Accept-type 报头的值是 MIME 类型 text/plain。
一些请求（特别是 POST 和 PUT）会有报文，而其他请求（特别是 GET 和 DELETE）没有。如果有报文（可能为空），以两个换行符将报头和报文分隔开；HTTP 报文包含一系列键-值对。对于无报文的请求，比如说查询字符串，报头元素就可以用来发送信息。下面是一个用 ID 2 对 /novels 资源的 GET 请求：
GET /novels?id=2
通常来说，查询字符串以问号开始，并且包含一个键-值对，尽管这个键-值可能值为空。
带有 getParameter 和 getParameterMap 等方法的 HttpServlet 很好地回避了有报文和没有报文的 HTTP 请求之前的差异。在"小说"例子中，getParameter 方法用来从 GET、POST 和 DELETE 方法中提取所需的信息。（处理 PUT请求需要更底层的代码，因为 Tomcat 没有提供可以解析 PUT 请求的参数映射。）下面展示了一段在 NovelsServlet中被重载的 doPost 方法：
@Override
public void doPost(HttpServletRequest request, HttpServletResponse response) {
String author = request.getParameter("author");
String title = request.getParameter("title");
...
对于没有报文的 DELETE 请求，过程基本是一样的：
@Override
public void doDelete(HttpServletRequest request, HttpServletResponse response) {
String param = request.getParameter("id"); // id of novel to be removed
...
doGet 方法需要区分 GET 请求的两种方式：一种是"获得所有"，而另一种是"获得某一个"。如果 GET 请求 URL 中包含一个键是一个 ID 的查询字符串，那么这个请求就被解析为"获得某一个"：
http://localhost:8080/novels?id=2 ## GET specified
如果没有查询字符串，GET 请求就会被解析为"获得所有"：
http://localhost:8080/novels ## GET all
一些值得注意的细节 {#%E4%B8%80%E4%BA%9B%E5%80%BC%E5%BE%97%E6%B3%A8%E6%84%8F%E7%9A%84%E7%BB%86%E8%8A%82}
"小说"服务的设计反映了像 Tomcat 这样基于 Java 的 web 服务器是如何工作的。在启动时，Tomcat 构建一个线程池，从中提取请求处理程序，这种方法称为 "每个请求一个线程one thread per request" 模型。现在版本的 Tomcat 使用非阻塞 I/O 来提高个性能。
"小说"服务是作为 NovelsServlet 类的单个实例来执行的，该实例也就维护了一个"小说"集合。相应的，也就会出现竞态条件，比如出现两个请求同时被处理：
- 一个请求向集合中添加一个新"小说"。
- 另一个请求获得集合中的所有"小说"。
这样的结果是不确定的，取决与 读 和 写 的操作是以怎样的顺序进行操作的。为了避免这个问题，"小说"服务使用了线程安全的 ConcurrentMap。这个映射的关键是生成了一个线程安全的 AtomicInteger。下面是相关的代码片段：
public class Novels {
private ConcurrentMap<Integer, Novel> novels;
private AtomicInteger mapKey;
...
默认情况下，对客户端请求的响应被编码为 XML。为了简单，"小说"程序使用了以前的 XMLEncoder 类；另一个包含更丰富功能的方式是使用 JAX-B 库。代码很简单：
public String toXml(Object obj) { // default encoding
String xml = null;
try {
ByteArrayOutputStream out = new ByteArrayOutputStream();
XMLEncoder encoder = new XMLEncoder(out);
encoder.writeObject(obj);
encoder.close();
xml = out.toString();
}
catch(Exception e) { }
return xml;
}
Object 参数要么是一个有序的"小说" ArraList（用以响应"获得所有get all"请求），要么是一个 Novel 实例（用以响应"获得一个get one"请求），又或者是一个 String（确认消息）。
如果 HTTP 请求报头指定 JSON 作为所需要的类型，那么 XML 就被转化成 JSON。下面是 NovelsServlet 中的 doGet 方法中的检查：
String accept = request.getHeader("accept"); // "accept" is case insensitive
if (accept != null && accept.contains("json")) json = true;
Novels类中包含了 toJson 方法，可以将 XML 转换成 JSON：
public String toJson(String xml) { // option for requester
try {
JSONObject jobt = XML.toJSONObject(xml);
return jobt.toString(3); // 3 is indentation level
}
catch(Exception e) { }
return null;
}
NovelsServlet会对各种类型进行错误检查。比如，POST 请求应该包含新"小说"的作者和标题。如果有一个丢了，doPost 方法会抛出一个异常：
if (author == null || title == null)
throw new RuntimeException(Integer.toString(HttpServletResponse.SC_BAD_REQUEST));
SC_BAD_REQUEST 中的 SC 代表的是 状态码status code，BAD_REQUEST 的标准 HTTP 数值是 400。如果请求中的 HTTP 动词是 TRACE，会返回一个不同的状态码：
public void doTrace(HttpServletRequest request, HttpServletResponse response) {
throw new RuntimeException(Integer.toString(HttpServletResponse.SC_METHOD_NOT_ALLOWED));
}
测试"小说"服务 {#%E6%B5%8B%E8%AF%95%E5%B0%8F%E8%AF%B4%E6%9C%8D%E5%8A%A1}
用浏览器测试 web 服务会很不顺手。在 CRUD 动词中，现代浏览器只能生成 POST（创建）和 GET（读取）请求。甚至从浏览器发送一个 POST 请求都有点不好办，因为报文需要包含键-值对；这样的测试通常通过 HTML 表单完成。命令行工具，比如说 curl，是一个更好的选择，这个部分展示的一些 curl 命令，已经包含在我网站的 ZIP 文件中了。
下面是一些测试样例，没有展示相应的输出结果：
% curl localhost:8080/novels/
% curl localhost:8080/novels?id=1
% curl --header "Accept: application/json" localhost:8080/novels/
第一条命令请求所有"小说"，默认是 XML 编码。第二条命令请求 ID 为 1 的"小说"，XML 编码。最后一条命令通过 application/json 添加了 Accept 报头元素，作为所需要的 MIME 类型。"获得一个get one"命令也可以用这个报头。这些请求用了 JSON 而不是 XML 编码作为响应。
下面两条命令在集合中创建了一个新"小说"，并且确认添加了进去：
% curl --request POST --data "author=Tolstoy&title=War and Peace" localhost:8080/novels/
% curl localhost:8080/novels?id=4
curl 中的 PUT 命令与 POST 命令相似，不同的地方是 PUT 的报文没有使用标准的语法。在 NovelsServlet 中关于 doPut 方法的文档中有详细的介绍，但是简单来说，Tomcat 不会对 PUT 请求生成合适的映射。下面是一个 PUT 命令和确认命令的的例子：
% curl --request PUT --data "id=3#title=This is an UPDATE" localhost:8080/novels/
% curl localhost:8080/novels?id=3
第二个命令确认了集合已经更新。
最后，DELETE 命令会正常运行：
% curl --request DELETE localhost:8080/novels?id=2
% curl localhost:8080/novels/
这个请求是删除 ID 为 2 的"小说"。第二个命令会显示剩余的"小说"。
web.xml 配置文件 {#webxml-%E9%85%8D%E7%BD%AE%E6%96%87%E4%BB%B6}
尽管官方规定它是可选的，web.xml 配置文件是一个生产级别网站或服务的重要组成部分。这个配置文件可以配置独立于代码的路由、安全性，或者网站或服务的其他功能。"小说"服务的配置通过为该服务的请求分配一个 URL 模式来配置路由：
<xml version = "1.0" encoding = "UTF-8">
<web-app>
<servlet>
<servlet-name>novels</servlet-name>
<servlet-class>novels.NovelsServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>novels</servlet-name>
<url-pattern>/*</url-pattern>
</servlet-mapping>
</web-app>
servlet-name 元素为 servlet 全名（novels.NovelsServlet）提供了一个缩写（novels），然后这个名字在下面的 servlet-mapping 元素中使用。
回想一下，一个已部署服务的 URL 会在端口号后面有 WAR 文件的文件名：
http://localhost:8080/novels/
端口号后斜杠后的 URI，是所请求资源的"路径"，在这个例子中，就是"小说"服务。因此，novels 出现在了第一个单斜杠后。
在 web.xml 文件中，url-patter 被指定为 /*，代表 "以 /novels 为起始的任意路径"。假设 Tomcat 遇到了一个不存在的 URL，像这样：
http://localhost:8080/novels/foobar/
web.xml 配置也会指定这个请求被分配到"小说" servlet 中，因为 /* 模式也包含 /foobar。因此，这个不存在的 URL 也会得到像上面合法路径的相同结果。
生产级别的配置文件可能会包含安全相关的信息，包括连接级别wire-level和用户角色users-roles。即使在这种情况下，配置文件的大小也只会是这个例子中的两到三倍大。
总结 {#%E6%80%BB%E7%BB%93}
HttpServlet 是 Java web 技术的核心。像"小说"这样的网站或 web 服务继承了这个类，并且根据需求重载了相应的 do 动词方法。像 Jersay（JAX-RS）或 Restlet 这样的 Restful 框架通过提供定制的 servlet 完成了基本相同的功能，针对框架中的 web 应用程序的请求，这个 servlet 扮演着 HTTP(S) 终端endpoint的角色。
当然，基于 servlet 的应用程序可以访问 web 应用程序中所需要的任何 Java 库。如果应用程序遵循关注点分离separation-of-concerns原则，那么 servlet 代码仍然相当简单：代码会检查请求，如果存在缺陷，就会发出适当的错误；否则，代码会调用所需要的功能（比如，查询数据库，以特定格式为响应编码），然后向请求这发送响应。HttpServletRequest 和 HttpServletReponse 类型使得读取请求和编写响应变得简单。
Java 的 API 可以从非常简单变得相当复杂。如果你需要用 Java 交付一些 Restful 服务的话，我的建议是在做其他事情之前先尝试一下简单的 HttpServlet。
via: https://opensource.com/article/20/7/restful-services-java
作者：Marty Kalin 选题：lujun9972 译者：Yufei-Yan 校对：wxy
