spring boot jar的启动原理解析

 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.jar
xxx.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: 1.0
Implementation-Title: open :: server :: openapi
Implementation-Version: 1.0-SNAPSHOT
Archiver-Version: Plexus Archiver
Built-By: xiaxuan
Implementation-Vendor-Id: com.weibangong.open
Spring-Boot-Version: 1.4.1.RELEASE
Implementation-Vendor: Pivotal Software, Inc.
Main-Class: org.springframework.boot.loader.PropertiesLauncher
Start-Class: com.weibangong.open.openapi.SpringBootWebApplication
Spring-Boot-Classes: BOOT-INF/classes/
Spring-Boot-Lib: BOOT-INF/lib/
Created-By: Apache Maven 3.3.9
Build-Jdk: 1.8.0_20
Implementation-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
@EnableEurekaClient
public 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呐喊教程网站的支持!

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