Spring Boot 为什么可以使用 Jar 包启动？ --20210625-工具盒子

可能很多初学者会比较困惑，Spring Boot 是如何做到将应用代码和所有的依赖打包成一个独立的 Jar 包，因为传统的 Java 项目打包成 Jar 包之后，需要通过 -classpath 属性来指定依赖，才能够运行。我们今天就来分析讲解一下 Spring Boot 的启动原理。

1. Spring Boot 打包插件 {#1-spring-boot-打包插件}

Spring Boot 提供了一个名叫 spring-boot-maven-plugin 的 maven 项目打包插件，可以方便的将 Spring Boot 项目打成 jar 包。这样我们就不再需要部署 Tomcat 、Jetty等之类的 Web 服务器容器啦。

我们先看一下 Spring Boot 打包后的结构是什么样的，打开 target 目录我们发现有两个jar包：

hello-0.0.1-SNAPSHOT.jar：17.3MB

hello-0.0.1-SNAPSHOT.jar.original：3KB

其中，hello-0.0.1-SNAPSHOT.jar 是通过 Spring Boot 提供的打包插件采用新的格式打成 Fat Jar，包含了所有的依赖；而 hello-0.0.1-SNAPSHOT.jar.original 则是Java原生的打包方式生成的，仅仅只包含了项目本身的内容。

2. SpringBoot FatJar 的组织结构 {#2-springboot-fatjar-的组织结构}

我们将 Spring Boot 打的可执行 Jar 展开后的结构如下所示：

|---------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 | . ├── BOOT-INF │ ├── classes │ │ ├── application.properties │ │ └── com │ │ └── javanorth │ │ └── hello │ │ └── HelloApplication.class │ └── lib │ ├── spring-boot-2.5.0.RELEASE.jar │ ├── spring-boot-autoconfigure-2.5.0.RELEASE.jar │ ├── spring-boot-configuration-processor-2.5.0.RELEASE.jar │ ├── spring-boot-starter-2.5.0.RELEASE.jar │ ├── ... ├── META-INF │ ├── MANIFEST.MF │ └── maven │ └── com.javanorth │ └── hello │ ├── pom.properties │ └── pom.xml │ ├── org │ └── springframework │ └── boot │ └── loader │ ├── ExecutableArchiveLauncher.class │ ├── JarLauncher.class │ ├── Launcher.class │ ├── MainMethodRunner.class │ ├── ... |

BOOT-INF目录：包含了我们的项目代码（classes目录），以及所需要的依赖（lib 目录）
META-INF目录：通过 MANIFEST.MF 文件提供 Jar包的元数据，声明了 jar 的启动类
org.springframework.boot.loader ：Spring Boot 的加载器代码，实现的 Jar in Jar 加载的魔法源

我们看到，如果去掉BOOT-INF目录，这将是一个非常普通且标准的Jar包，包括元信息以及可执行的代码部分，其/META-INF/MAINFEST.MF指定了Jar包的启动元信息，org.springframework.boot.loader 执行对应的逻辑操作。

3. MAINFEST.MF 元信息分析 {#3-mainfestmf-元信息分析}

元信息内容如下所示：

|------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 | Manifest-Version: 1.0 Created-By: Maven Jar Plugin 3.2.0 Build-Jdk-Spec: 11 Implementation-Title: hello Implementation-Version: 0.0.1-SNAPSHOT Main-Class: org.springframework.boot.loader.JarLauncher Start-Class: com.javanorth.hello.HelloApplication Spring-Boot-Version: 2.5.0 Spring-Boot-Classes: BOOT-INF/classes/ Spring-Boot-Lib: BOOT-INF/lib/ Spring-Boot-Classpath-Index: BOOT-INF/classpath.idx Spring-Boot-Layers-Index: BOOT-INF/layers.idx |

它相当于一个 Properties 配置文件，每一行都是一个配置项目。重点来看看两个配置项：

Main-Class 配置项：Java 规定的 jar 包的启动类，这里设置为 spring-boot-loader 项目的 JarLauncher 类，进行 Spring Boot 应用的启动。
Start-Class 配置项：Spring Boot 规定的主启动类，这里设置为我们定义的 Application 类。
Spring-Boot-Classes 配置项：指定加载应用类的入口
Spring-Boot-Lib 配置项: 指定加载应用依赖的库

4. 启动原理 {#4-启动原理}

Spring Boot 的启动原理如下图所示：

Spring Boot Start jar 1

5. 源码分析 {#5-源码分析}

org.springframework.boot.loader.JarLauncher {#orgspringframeworkbootloaderjarlauncher}

JarLauncher 类是针对 Spring Boot jar 包的启动类，完整的类图如下所示：

Spring Boot Start jar 2

其中的 WarLauncher 类，是针对 Spring Boot war 包的启动类。启动类 org.springframework.boot.loader.JarLauncher 并非为项目中引入类，而是 spring-boot-maven-plugin 插件 repackage 追加进去的。接下来我们先来看一下 JarLauncher 的源码，比较简单，如下图所示：

|------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 | public class JarLauncher extends ExecutableArchiveLauncher { private static final String DEFAULT_CLASSPATH_INDEX_LOCATION = "BOOT-INF/classpath.idx"; static final EntryFilter NESTED_ARCHIVE_ENTRY_FILTER = (entry) -> { if (entry.isDirectory()) { return entry.getName().equals("BOOT-INF/classes/"); } return entry.getName().startsWith("BOOT-INF/lib/"); }; public JarLauncher() { } protected JarLauncher(Archive archive) { super(archive); } @Override protected ClassPathIndexFile getClassPathIndex(Archive archive) throws IOException { // Only needed for exploded archives, regular ones already have a defined order if (archive instanceof ExplodedArchive) { String location = getClassPathIndexFileLocation(archive); return ClassPathIndexFile.loadIfPossible(archive.getUrl(), location); } return super.getClassPathIndex(archive); } private String getClassPathIndexFileLocation(Archive archive) throws IOException { Manifest manifest = archive.getManifest(); Attributes attributes = (manifest != null) ? manifest.getMainAttributes() : null; String location = (attributes != null) ? attributes.getValue(BOOT_CLASSPATH_INDEX_ATTRIBUTE) : null; return (location != null) ? location : DEFAULT_CLASSPATH_INDEX_LOCATION; } @Override protected boolean isPostProcessingClassPathArchives() { return false; } @Override protected boolean isSearchCandidate(Archive.Entry entry) { return entry.getName().startsWith("BOOT-INF/"); } @Override protected boolean isNestedArchive(Archive.Entry entry) { return NESTED_ARCHIVE_ENTRY_FILTER.matches(entry); } public static void main(String[] args) throws Exception { new JarLauncher().launch(args); } } |

当执行 java -jar 命令或执行解压后的 org.springframework.boot.loader.JarLauncher 类时，JarLauncher 会将 BOOT-INF/classes 下的类文件和 BOOT-INF/lib 下依赖的jar加入到classpath下，后调用 META-INF/MANIFEST.MF 文件 Start-Class 属性 [指向项目中的 com.javanorth.hello.HelloApplicatioin 启动类] 完成应用程序的启动。

JarLauncher 假定依赖项jar包含在 /BOOT-INF/lib 目录中，并且应用程序类包含在 /BOOT-INF/classes 目录中。它的 main 方法调用的则是基类 Launcher 定义的 launch 方法，而 Launcher 是ExecutableArchiveLauncher 的父类。

org.springframework.boot.loader.ExecutableArchiveLauncher {#orgspringframeworkbootloaderexecutablearchivelauncher}

ExecutableArchiveLauncher 是 JarLauncher 的直接父类，继承了 Launcher 基类，并实现部分抽象方法

|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 | public abstract class ExecutableArchiveLauncher extends Launcher { private static final String START_CLASS_ATTRIBUTE = "Start-Class"; protected static final String BOOT_CLASSPATH_INDEX_ATTRIBUTE = "Spring-Boot-Classpath-Index"; private final Archive archive; private final ClassPathIndexFile classPathIndex; public ExecutableArchiveLauncher() { try { this.archive = createArchive(); this.classPathIndex = getClassPathIndex(this.archive); } catch (Exception ex) { throw new IllegalStateException(ex); } } protected ExecutableArchiveLauncher(Archive archive) { try { this.archive = archive; this.classPathIndex = getClassPathIndex(this.archive); } catch (Exception ex) { throw new IllegalStateException(ex); } } protected ClassPathIndexFile getClassPathIndex(Archive archive) throws IOException { return null; } @Override protected String getMainClass() throws Exception { Manifest manifest = this.archive.getManifest(); String mainClass = null; if (manifest != null) { mainClass = manifest.getMainAttributes().getValue(START_CLASS_ATTRIBUTE); } if (mainClass == null) { throw new IllegalStateException("No 'Start-Class' manifest entry specified in " + this); } return mainClass; } @Override protected ClassLoader createClassLoader(Iterator archives) throws Exception { List urls = new ArrayList<>(guessClassPathSize()); while (archives.hasNext()) { urls.add(archives.next().getUrl()); } if (this.classPathIndex != null) { urls.addAll(this.classPathIndex.getUrls()); } return createClassLoader(urls.toArray(new URL[0])); } private int guessClassPathSize() { if (this.classPathIndex != null) { return this.classPathIndex.size() + 10; } return 50; } @Override protected Iterator getClassPathArchivesIterator() throws Exception { Archive.EntryFilter searchFilter = this::isSearchCandidate; Iterator archives = this.archive.getNestedArchives(searchFilter, (entry) -> isNestedArchive(entry) && !isEntryIndexed(entry)); if (isPostProcessingClassPathArchives()) { archives = applyClassPathArchivePostProcessing(archives); } return archives; } private boolean isEntryIndexed(Archive.Entry entry) { if (this.classPathIndex != null) { return this.classPathIndex.containsEntry(entry.getName()); } return false; } private Iterator applyClassPathArchivePostProcessing(Iterator archives) throws Exception { List list = new ArrayList<>(); while (archives.hasNext()) { list.add(archives.next()); } postProcessClassPathArchives(list); return list.iterator(); } protected boolean isSearchCandidate(Archive.Entry entry) { return true; } protected abstract boolean isNestedArchive(Archive.Entry entry); protected boolean isPostProcessingClassPathArchives() { return true; } protected void postProcessClassPathArchives(List archives) throws Exception { } @Override protected boolean isExploded() { return this.archive.isExploded(); } @Override protected final Archive getArchive() { return this.archive; } } |

org.springframework.boot.loader.Launcher {#orgspringframeworkbootloaderlauncher}

如下则是 Launcher 的源码

launch 方法会首先创建类加载器，而后判断是否 jar 是否在 MANIFEST.MF 文件中设置了 jarmode 属性。
如果没有设置，launchClass 的值就来自 getMainClass() 返回，该方法由子类实现，返回 MANIFEST.MF 中配置的 START_CLASS_ATTRIBUTE 属性值
调用 createMainMethodRunner 方法，构建一个 MainMethodRunner 对象并调用其 run 方法

jarmode 是创建 docker 镜像时用到的参数，使用该参数是为了生成带有多个 layer 信息的镜像，这里暂不注意

|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 | public abstract class Launcher { private static final String JAR_MODE_LAUNCHER = "org.springframework.boot.loader.jarmode.JarModeLauncher"; protected void launch(String[] args) throws Exception { if (!isExploded()) { JarFile.registerUrlProtocolHandler(); } ClassLoader classLoader = createClassLoader(getClassPathArchivesIterator()); String jarMode = System.getProperty("jarmode"); String launchClass = (jarMode != null && !jarMode.isEmpty()) ? JAR_MODE_LAUNCHER : getMainClass(); launch(args, launchClass, classLoader); } @Deprecated protected ClassLoader createClassLoader(List archives) throws Exception { return createClassLoader(archives.iterator()); } protected ClassLoader createClassLoader(Iterator archives) throws Exception { List urls = new ArrayList<>(50); while (archives.hasNext()) { urls.add(archives.next().getUrl()); } return createClassLoader(urls.toArray(new URL[0])); } protected ClassLoader createClassLoader(URL[] urls) throws Exception { return new LaunchedURLClassLoader(isExploded(), getArchive(), urls, getClass().getClassLoader()); } protected void launch(String[] args, String launchClass, ClassLoader classLoader) throws Exception { Thread.currentThread().setContextClassLoader(classLoader); createMainMethodRunner(launchClass, args, classLoader).run(); } protected MainMethodRunner createMainMethodRunner(String mainClass, String[] args, ClassLoader classLoader) { return new MainMethodRunner(mainClass, args); } protected abstract String getMainClass() throws Exception; protected Iterator getClassPathArchivesIterator() throws Exception { return getClassPathArchives().iterator(); } @Deprecated protected List getClassPathArchives() throws Exception { throw new IllegalStateException("Unexpected call to getClassPathArchives()"); } protected final Archive createArchive() throws Exception { ProtectionDomain protectionDomain = getClass().getProtectionDomain(); CodeSource codeSource = protectionDomain.getCodeSource(); URI location = (codeSource != null) ? codeSource.getLocation().toURI() : null; String path = (location != null) ? location.getSchemeSpecificPart() : null; if (path == null) { throw new IllegalStateException("Unable to determine code source archive"); } File root = new File(path); if (!root.exists()) { throw new IllegalStateException("Unable to determine code source archive from " + root); } return (root.isDirectory() ? new ExplodedArchive(root) : new JarFileArchive(root)); } protected boolean isExploded() { return false; } protected Archive getArchive() { return null; } } |

org.springframework.boot.loader.MainMethodRunner {#orgspringframeworkbootloadermainmethodrunner}

从名字可以判断这是一个目标类main方法的执行器，此时的 mainClassName 被赋值为 MANIFEST.MF 中配置的 START_CLASS_ATTRIBUTE 属性值，也就是 com.javanorth.hello.HelloApplication，之后便是通过反射执行 HelloApplication 的 main 方法，从而达到启动 Spring Boot 的效果。

|------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 | public class MainMethodRunner { private final String mainClassName; private final String[] args; public MainMethodRunner(String mainClass, String[] args) { this.mainClassName = mainClass; this.args = (args != null) ? args.clone() : null; } public void run() throws Exception { Class<?> mainClass = Class.forName(this.mainClassName, false, Thread.currentThread().getContextClassLoader()); Method mainMethod = mainClass.getDeclaredMethod("main", String[].class); mainMethod.setAccessible(true); mainMethod.invoke(null, new Object[] { this.args }); } } |

总结 {#总结}

jar 包类似于 zip 压缩文件，只不过相比 zip 文件多了一个 META-INF/MANIFEST.MF 文件，该文件在构建 jar 包时自动创建
想要制作可执行 JAR 包，在 MANIFEST.MF 中指定 Main-Class 是关键。使用 java 执行 jar 包的时候，实际上等同于使用 java 命令执行指定的 Main-Class 程序。
Spring Boot 提供了一个插件 spring-boot-maven-plugin ，用于把程序打包成一个可执行的jar包
使用 java -jar 启动 Spring Boot 的 jar 包，首先调用的入口类是 JarLauncher，内部调用 Launcher 的 launch 后构建 MainMethodRunner 对象，最终通过反射调用 HelloApplication 的 main 方法实现启动效果。

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