spring boot jar的启动原理解析

时间:2021-05-19

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。

6.总结

综上spring boot jar的启动流程基本就是下面几个步骤:

1、我们正常进行maven打包时,spring boot插件扩展maven生命周期,将spring boot相关package打入到jar中,这个jar中包含了应用所打出的jar以外还有spring boot启动程序相关的类文件。

2、我以前看过稍微低一些版本的spring boot的jar的启动流程,当时我记得是当前线程又起了一个新线程来运行main程序,而现在的已经改成了直接使用反射来启动main程序。

总结

以上所述是小编给大家介绍的spring boot jar的启动原理解析,希望对大家有所帮助,如果大家有任何疑问请给我留言,小编会及时回复大家的。在此也非常感谢大家对网站的支持!

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