spring boot jar的启动原理解析

2019-11-26 10:18:59

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1.前言

近来有空对公司的open api平台进行了些优化，然后在打出jar包的时候，突然想到以前都是对spring boot使用很熟练，但是从来都不知道spring boot打出的jar的启动原理，然后这回将jar解开了看了下，与想象中确实大不一样，以下就是对解压出来的jar的完整分析。

2.jar的结构

spring boot的应用程序就不贴出来了，一个较简单的demo打出的结构都是类似，另外我采用的spring boot的版本为1.4.1.RELEASE网上有另外一篇文章对spring boot jar启动的分析，那个应该是1.4以下的，启动方式与当前版本也有着许多的不同。

在mvn clean install后，我们在查看target目录中时，会发现两个jar包，如下：

xxxx.jarxxx.jar.original

这个则是归功于spring boot插件的机制，将一个普通的jar打成了一个可以执行的jar包，而xxx.jar.original则是maven打出的jar包，这些可以参考spring官网的文章来了解，如下：

http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#executable-jar

以下是spring boot应用打出的jar的部分目录结构，大部分省略了,仅仅展示出其中重要的部分。

.├── BOOT-INF│ ├── classes│ │ ├── application-dev.properties│ │ ├── application-prod.properties│ │ ├── application.properties│ │ ├── com│ │ │ └── weibangong│ │ │  └── open│ │ │   └── openapi│ │ │    ├── SpringBootWebApplication.class│ │ │    ├── config│ │ │    │ ├── ProxyServletConfiguration.class│ │ │    │ └── SwaggerConfig.class│ │ │    ├── oauth2│ │ │    │ ├── controller│ │ │    │ │ ├── AccessTokenController.class│ │ ├── logback-spring.xml│ │ └── static│ │  ├── css│ │  │ └── guru.css│ │  ├── images│ │  │ ├── FBcover1200x628.png│ │  │ └── NewBannerBOOTS_2.png│ └── lib│  ├── accessors-smart-1.1.jar├── META-INF│ ├── MANIFEST.MF│ └── maven│  └── com.weibangong.open│   └── open-server-openapi│    ├── pom.properties│    └── pom.xml└── org └── springframework  └── boot   └── loader    ├── ExecutableArchiveLauncher$1.class    ├── ExecutableArchiveLauncher.class    ├── JarLauncher.class    ├── LaunchedURLClassLoader$1.class    ├── LaunchedURLClassLoader.class    ├── Launcher.class    ├── archive    │ ├── Archive$Entry.class    │ ├── Archive$EntryFilter.class    │ ├── Archive.class    │ ├── ExplodedArchive$1.class    │ ├── ExplodedArchive$FileEntry.class    │ ├── ExplodedArchive$FileEntryIterator$EntryComparator.class     ├── ExplodedArchive$FileEntryIterator.class

这个jar除了我们写的应用程序打出的class以外还有一个单独的org包，应该是spring boot应用在打包的使用spring boot插件将这个package打进来，也就是增强了mvn生命周期中的package阶段，而正是这个包在启动过程中起到了关键的作用，另外中jar中将应用所需的各种依赖都打进来，并且打入了spring boot额外的package，这种可以all-in-one的jar也被称之为fat.jar,下文我们将一直以fat.jar来代替打出的jar的名字。

3.MANIFEST.MF文件

这个时候我们再继续看META-INF中的MANIFEST.MF文件，如下：

Manifest-Version: 1.0Implementation-Title: open :: server :: openapiImplementation-Version: 1.0-SNAPSHOTArchiver-Version: Plexus ArchiverBuilt-By: xiaxuanImplementation-Vendor-Id: com.weibangong.openSpring-Boot-Version: 1.4.1.RELEASEImplementation-Vendor: Pivotal Software, Inc.Main-Class: org.springframework.boot.loader.PropertiesLauncherStart-Class: com.weibangong.open.openapi.SpringBootWebApplicationSpring-Boot-Classes: BOOT-INF/classes/Spring-Boot-Lib: BOOT-INF/lib/Created-By: Apache Maven 3.3.9Build-Jdk: 1.8.0_20Implementation-URL: http://maven.apache.org/open-server-openapi

这里指定的main-class是单独打入的包中的一个类文件而不是我们的启动程序，然后MANIFEST.MF文件有一个单独的start-class指定的是我们的应用的启动程序。

4.启动分析

首先我们找到类org.springframework.boot.loader.PropertiesLauncher，其中main方法为：

public static void main(String[] args) throws Exception {  PropertiesLauncher launcher = new PropertiesLauncher();  args = launcher.getArgs(args);  launcher.launch(args);}

查看launch方法，这个方法在父类Launcher中，找到父类方法launch方法，如下：

 protected void launch(String[] args, String mainClass, ClassLoader classLoader) throws Exception {  Thread.currentThread().setContextClassLoader(classLoader);  this.createMainMethodRunner(mainClass, args, classLoader).run(); } protected MainMethodRunner createMainMethodRunner(String mainClass, String[] args, ClassLoader classLoader) {  return new MainMethodRunner(mainClass, args); }

launch方法最终调用了createMainMethodRunner方法，后者实例化了MainMethodRunner对象并运行了run方法，我们转到MainMethodRunner源码中，如下：

package org.springframework.boot.loader;import java.lang.reflect.Method;public class MainMethodRunner { private final String mainClassName; private final String[] args; public MainMethodRunner(String mainClass, String[] args) {  this.mainClassName = mainClass;  this.args = args == null?null:(String[])args.clone(); } public void run() throws Exception {  Class mainClass = Thread.currentThread().getContextClassLoader().loadClass(this.mainClassName);  Method mainMethod = mainClass.getDeclaredMethod("main", new Class[]{String[].class});  mainMethod.invoke((Object)null, new Object[]{this.args}); }}

查看run方法，就很怎么将spring boot的jar怎么运行起来的了，由此分析基本也就结束了。

5、main程序的启动流程

讲完了jar的启动流程，现在来讲下spring boot应用中，main程序的启动与加载流程，首先我们看一个spring boot应用的main方法。

package cn.com.devh;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.cloud.client.discovery.EnableDiscoveryClient;import org.springframework.cloud.netflix.eureka.EnableEurekaClient;import org.springframework.cloud.netflix.feign.EnableFeignClients;/** * Created by xiaxuan on 17/8/25. */@SpringBootApplication@EnableFeignClients@EnableEurekaClientpublic class A1ServiceApplication { public static void main(String[] args) {  SpringApplication.run(A1ServiceApplication.class, args); }}

转到SpringApplication中的run方法，如下：

 /**  * Static helper that can be used to run a {@link SpringApplication} from the  * specified source using default settings.  * @param source the source to load  * @param args the application arguments (usually passed from a Java main method)  * @return the running {@link ApplicationContext}  */ public static ConfigurableApplicationContext run(Object source, String... args) {  return run(new Object[] { source }, args); } /**  * Static helper that can be used to run a {@link SpringApplication} from the  * specified sources using default settings and user supplied arguments.  * @param sources the sources to load  * @param args the application arguments (usually passed from a Java main method)  * @return the running {@link ApplicationContext}  */ public static ConfigurableApplicationContext run(Object[] sources, String[] args) {  return new SpringApplication(sources).run(args); }

这里的SpringApplication的实例化是关键，我们转到SpringApplication的构造函数。

 /**  * Create a new {@link SpringApplication} instance. The application context will load  * beans from the specified sources (see {@link SpringApplication class-level}  * documentation for details. The instance can be customized before calling  * {@link #run(String...)}.  * @param sources the bean sources  * @see #run(Object, String[])  * @see #SpringApplication(ResourceLoader, Object...)  */ public SpringApplication(Object... sources) {  initialize(sources); } private void initialize(Object[] sources) {  if (sources != null && sources.length > 0) {   this.sources.addAll(Arrays.asList(sources));  }  this.webEnvironment = deduceWebEnvironment();  setInitializers((Collection) getSpringFactoriesInstances(    ApplicationContextInitializer.class));  setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));  this.mainApplicationClass = deduceMainApplicationClass(); }

这里的initialize方法中的deduceWebEnvironment()确定了当前是以web应用启动还是以普通的jar启动，如下：

 private boolean deduceWebEnvironment() {  for (String className : WEB_ENVIRONMENT_CLASSES) {   if (!ClassUtils.isPresent(className, null)) {    return false;   }  }  return true; }

其中的WEB_ENVIRONMENT_CLASSES为:

 private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",   "org.springframework.web.context.ConfigurableWebApplicationContext" };

只要其中任何一个不存在，即当前应用以普通jar的形式启动。

然后setInitializers方法初始化了所有的ApplicationContextInitializer，

/**  * Sets the {@link ApplicationContextInitializer} that will be applied to the Spring  * {@link ApplicationContext}.  * @param initializers the initializers to set  */ public void setInitializers(   Collection<? extends ApplicationContextInitializer<?>> initializers) {  this.initializers = new ArrayList<ApplicationContextInitializer<?>>();  this.initializers.addAll(initializers); }setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class))**

这一步初始化所有Listener。

我们再回到之前的SpringApplication(sources).run(args);处，进入run方法，代码如下：

/**  * Run the Spring application, creating and refreshing a new  * {@link ApplicationContext}.  * @param args the application arguments (usually passed from a Java main method)  * @return a running {@link ApplicationContext}  */ public ConfigurableApplicationContext run(String... args) {  StopWatch stopWatch = new StopWatch();  stopWatch.start();  ConfigurableApplicationContext context = null;  configureHeadlessProperty();  SpringApplicationRunListeners listeners = getRunListeners(args);  listeners.started();  try {   ApplicationArguments applicationArguments = new DefaultApplicationArguments(     args);   context = createAndRefreshContext(listeners, applicationArguments);   afterRefresh(context, applicationArguments);   listeners.finished(context, null);   stopWatch.stop();   if (this.logStartupInfo) {    new StartupInfoLogger(this.mainApplicationClass)      .logStarted(getApplicationLog(), stopWatch);   }   return context;  }  catch (Throwable ex) {   handleRunFailure(context, listeners, ex);   throw new IllegalStateException(ex);  } }

这一步进行上下文的创建createAndRefreshContext(listeners, applicationArguments),

private ConfigurableApplicationContext createAndRefreshContext(   SpringApplicationRunListeners listeners,   ApplicationArguments applicationArguments) {  ConfigurableApplicationContext context;  // Create and configure the environment  ConfigurableEnvironment environment = getOrCreateEnvironment();  configureEnvironment(environment, applicationArguments.getSourceArgs());  listeners.environmentPrepared(environment);  if (isWebEnvironment(environment) && !this.webEnvironment) {   environment = convertToStandardEnvironment(environment);  }  if (this.bannerMode != Banner.Mode.OFF) {   printBanner(environment);  }  // Create, load, refresh and run the ApplicationContext  context = createApplicationContext();  context.setEnvironment(environment);  postProcessApplicationContext(context);  applyInitializers(context);  listeners.contextPrepared(context);  if (this.logStartupInfo) {   logStartupInfo(context.getParent() == null);   logStartupProfileInfo(context);  }  // Add boot specific singleton beans  context.getBeanFactory().registerSingleton("springApplicationArguments",    applicationArguments);  // Load the sources  Set<Object> sources = getSources();  Assert.notEmpty(sources, "Sources must not be empty");  load(context, sources.toArray(new Object[sources.size()]));  listeners.contextLoaded(context);  // Refresh the context  refresh(context);  if (this.registerShutdownHook) {   try {    context.registerShutdownHook();   }   catch (AccessControlException ex) {    // Not allowed in some environments.   }  }  return context; }// Create and configure the environment  ConfigurableEnvironment environment = getOrCreateEnvironment();  configureEnvironment(environment, applicationArguments.getSourceArgs());

这一步进行了环境的配置与加载。

  if (this.bannerMode != Banner.Mode.OFF) {   printBanner(environment);  }

这一步进行了打印spring boot logo，需要更改的话，在资源文件中加入banner.txt,banner.txt改为自己需要的图案即可。

// Create, load, refresh and run the ApplicationContext  context = createApplicationContext();return (ConfigurableApplicationContext) BeanUtils.instantiate(contextClass)

创建上下文，这一步中真正包含了是创建什么容器，并进行了响应class的实例化，其中包括了EmbeddedServletContainerFactory的创建，是选择jetty还是tomcat，内容繁多，留待下一次再讲。

if (this.registerShutdownHook) {   try {    context.registerShutdownHook();   }   catch (AccessControlException ex) {    // Not allowed in some environments.   }  }

这一步就是当前上下文进行注册，当收到kill指令的时候进行容器的销毁等工作了。

基本到此，启动的分析就结束了，但是还有一些细节讲述起来十分耗时，这个留待后续的博文中再来讲述，今天就到这里。