spring boot jar的啟動原理解析

更新時間：2018年02月05日 16:45:54 作者：葉長風(fēng)

這篇文章主要介紹了 spring boot jar的啟動原理分析,非常不錯，具有參考借鑒價值,需要的朋友可以參考下

1.前言

近來有空對公司的open api平臺進行了些優(yōu)化，然后在打出jar包的時候，突然想到以前都是對spring boot使用很熟練，但是從來都不知道spring boot打出的jar的啟動原理，然后這回將jar解開了看了下，與想象中確實大不一樣，以下就是對解壓出來的jar的完整分析。

2.jar的結(jié)構(gòu)

spring boot的應(yīng)用程序就不貼出來了，一個較簡單的demo打出的結(jié)構(gòu)都是類似，另外我采用的spring boot的版本為1.4.1.RELEASE網(wǎng)上有另外一篇文章對spring boot jar啟動的分析，那個應(yīng)該是1.4以下的，啟動方式與當(dāng)前版本也有著許多的不同。

在mvn clean install后，我們在查看target目錄中時，會發(fā)現(xiàn)兩個jar包，如下：

xxxx.jar
xxx.jar.original

這個則是歸功于spring boot插件的機制，將一個普通的jar打成了一個可以執(zhí)行的jar包，而xxx.jar.original則是maven打出的jar包，這些可以參考spring官網(wǎng)的文章來了解，如下：

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

以下是spring boot應(yīng)用打出的jar的部分目錄結(jié)構(gòu)，大部分省略了,僅僅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

這個jar除了我們寫的應(yīng)用程序打出的class以外還有一個單獨的org包，應(yīng)該是spring boot應(yīng)用在打包的使用spring boot插件將這個package打進來，也就是增強了mvn生命周期中的package階段，而正是這個包在啟動過程中起到了關(guān)鍵的作用，另外中jar中將應(yīng)用所需的各種依賴都打進來，并且打入了spring boot額外的package，這種可以all-in-one的jar也被稱之為fat.jar,下文我們將一直以fat.jar來代替打出的jar的名字。

3.MANIFEST.MF文件

這個時候我們再繼續(xù)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

這里指定的main-class是單獨打入的包中的一個類文件而不是我們的啟動程序，然后MANIFEST.MF文件有一個單獨的start-class指定的是我們的應(yīng)用的啟動程序。

4.啟動分析

首先我們找到類org.springframework.boot.loader.PropertiesLauncher，其中main方法為：

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

查看launch方法，這個方法在父類Launcher中，找到父類方法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方法最終調(diào)用了createMainMethodRunner方法，后者實例化了MainMethodRunner對象并運行了run方法，我們轉(zhuǎn)到MainMethodRunner源碼中，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，就很怎么將spring boot的jar怎么運行起來的了，由此分析基本也就結(jié)束了。

5、main程序的啟動流程

講完了jar的啟動流程，現(xiàn)在來講下spring boot應(yīng)用中，main程序的啟動與加載流程，首先我們看一個spring boot應(yīng)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);
 }
}

轉(zhuǎn)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);
 }

這里的SpringApplication的實例化是關(guān)鍵，我們轉(zhuǎn)到SpringApplication的構(gòu)造函數(shù)。

 /**
  * 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();
 }

這里的initialize方法中的deduceWebEnvironment()確定了當(dāng)前是以web應(yīng)用啟動還是以普通的jar啟動，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為:

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

只要其中任何一個不存在，即當(dāng)前應(yīng)用以普通jar的形式啟動。

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))**

這一步初始化所有Listener。

我們再回到之前的SpringApplication(sources).run(args);處，進入run方法，代碼如下：

/**
  * 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);
  }
 }

這一步進行上下文的創(chuàng)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());

這一步進行了環(huán)境的配置與加載。

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

這一步進行了打印spring boot logo，需要更改的話，在資源文件中加入banner.txt,banner.txt改為自己需要的圖案即可。

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

創(chuàng)建上下文，這一步中真正包含了是創(chuàng)建什么容器，并進行了響應(yīng)class的實例化，其中包括了EmbeddedServletContainerFactory的創(chuàng)建，是選擇jetty還是tomcat，內(nèi)容繁多，留待下一次再講。

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

這一步就是當(dāng)前上下文進行注冊，當(dāng)收到kill指令的時候進行容器的銷毀等工作了。

基本到此，啟動的分析就結(jié)束了，但是還有一些細(xì)節(jié)講述起來十分耗時，這個留待后續(xù)的博文中再來講述，今天就到這里。