前言

我们生成一个spring boot 项目时,会自带一个启动类. 代码如下:

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

就是这么简单的代码,构成了spring boot的世界. 那么代码中只有一个@SpringBootApplication 注解 和 调用了SpringApplication#run方法.那么我们先来解析SpringApplication的run方法.

解析

1. 首先调用了org.springframework.boot.SpringApplication#run(Object, String…) 方法.代码如下:

   public static ConfigurableApplicationContext run(Object source, String... args) {
       return run(new Object[] { source }, args);
   }

   接着调用如下代码:

   public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
       return new SpringApplication(sources).run(args);
   }

   可以发现 首先初始化了SpringApplication,然后调用其实例方法:run.

2. 在 SpringApplication 的构造器中,调用了 initialize 方法.

   public SpringApplication(Object... sources) {
       initialize(sources);
   }

3. SpringApplication#initialize方法代码如下:

   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();
   }

   可以看到做了如下5件事:

   1. 如果sources长度大于0的话,加入到SpringApplication的sources中,该sources是一个LinkedHashSet.
   2. 调用deduceWebEnvironment方法判断是否是web环境
   3. 设置initializers.
   4. 设置Listeners.
   5. 设置mainApplicationClass.

4. deduceWebEnvironment代码如下:

   private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",
           "org.springframework.web.context.ConfigurableWebApplicationContext" };
   
   private boolean deduceWebEnvironment() {
       for (String className : WEB_ENVIRONMENT_CLASSES) {
           if (!ClassUtils.isPresent(className, null)) {
               return false;
           }
       }
       return true;
   }

   可以发现会调用ClassUtils类的isPresent方法，检查classpath中是否存在javax.servlet.Servlet类和org.springframework.web.context.ConfigurableWebApplicationContext类,如果存在的话,返回true.否则返回false.

5. 在设置Initializers时首先调用getSpringFactoriesInstances方法加载ApplicationContextInitializer.然后直接赋值给initializers.代码如下:

   private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type) { return getSpringFactoriesInstances(type, new Class<?>[] {}); }

   转而调用如下代码:

   private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
       ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
       // Use names and ensure unique to protect against duplicates
       // 使用Set保存names来避免重复元素
       Set<String> names = new LinkedHashSet<String>(
               SpringFactoriesLoader.loadFactoryNames(type, classLoader));
       // 根据names来进行实例化
       List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
               classLoader, args, names);
       // 对实例进行排序
       AnnotationAwareOrderComparator.sort(instances);
       return instances;
   }

   该方法逻辑如下:

   1. 首先获得ClassLoader.
   2. 调用SpringFactoriesLoader#loadFactoryNames进行加载,然后放入到LinkedHashSet进行去重.
   3. 调用createSpringFactoriesInstances进行初始化
   4. 排序

   其中SpringFactoriesLoader#loadFactoryNames代码如下:

   public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
       String factoryClassName = factoryClass.getName();
       try {
           Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                   ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
           List<String> result = new ArrayList<String>();
           while (urls.hasMoreElements()) {
               URL url = urls.nextElement();
               Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
               String factoryClassNames = properties.getProperty(factoryClassName);
               result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
           }
           return result;
       }
       catch (IOException ex) {
           throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                   "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
       }
   }

   逻辑如下:

   1. 获得factoryClassName,对于当前来说factoryClassName =org.springframework.context.ApplicationContextInitializer.
   2. 通过传入的classLoader加载META-INF/spring.factories文件.
   3. 通过调用PropertiesLoaderUtils#loadProperties将其转为Properties.
   4. 获得factoryClassName对应的值进行返回.

   对于当前来说,在spring-boot/META-INF/spring.factories中.org.springframework.context.ApplicationContextInitializer值如下:

   org.springframework.context.ApplicationContextInitializer=\
   org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
   org.springframework.boot.context.ContextIdApplicationContextInitializer,\
   org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
   org.springframework.boot.context.embedded.ServerPortInfoApplicationContextInitializer

   SpringApplication#createSpringFactoriesInstances方法如下:

   private <T> List<T> createSpringFactoriesInstances(Class<T> type,
           Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args, Set<String> names) { List<T> instances = new ArrayList<T>(names.size()); for (String name : names) { try { Class<?> instanceClass = ClassUtils.forName(name, classLoader);
               Assert.isAssignable(type, instanceClass);
               Constructor<?> constructor = instanceClass .getDeclaredConstructor(parameterTypes); T instance = (T) BeanUtils.instantiateClass(constructor, args); instances.add(instance); } catch (Throwable ex) { throw new IllegalArgumentException( "Cannot instantiate " + type + " : " + name, ex); } } return instances; }

   逻辑如下:遍历传入的names,也就是之前通过SpringFactoriesLoader加载的类名.通过遍历,依次调用其构造器进行初始化.加入到instances.然后进行返回.

   对于当前场景来说:
   ConfigurationWarningsApplicationContextInitializer,DelegatingApplicationContextInitializer,ServerPortInfoApplicationContextInitializer初始化没有做任何事.

   ContextIdApplicationContextInitializer在初始化时.会获得spring boot的应用名.搜索路径如下:

   1. spring.application.name
   2. vcap.application.name
   3. spring.config.name
   4. 如果都没有配置的话,返回application.

   代码如下:

   private static final String NAME_PATTERN = "${spring.application.name:${vcap.application.name:${spring.config.name:application}}}";
   
   public ContextIdApplicationContextInitializer() {
       this(NAME_PATTERN);
   }
   
   public ContextIdApplicationContextInitializer(String name) {
       this.name = name;
   }

6. 设置SpringApplication#setListeners时,还是同样的套路.调用getSpringFactoriesInstances加载META-INF/spring.factories中配置的org.springframework.context.ApplicationListener.
   对于当前来说.加载的类如下:

   org.springframework.context.ApplicationListener=\
   org.springframework.boot.ClearCachesApplicationListener,\
   org.springframework.boot.builder.ParentContextCloserApplicationListener,\
   org.springframework.boot.context.FileEncodingApplicationListener,\
   org.springframework.boot.context.config.AnsiOutputApplicationListener,\
   org.springframework.boot.context.config.ConfigFileApplicationListener,\
   org.springframework.boot.context.config.DelegatingApplicationListener,\
   org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener,\
   org.springframework.boot.logging.ClasspathLoggingApplicationListener,\
   org.springframework.boot.logging.LoggingApplicationListener

   这些类在构造器中都没有做任何事.

7. 调用SpringApplication#deduceMainApplicationClass方法.获得应用的启动类.该方法通过获取当前方法调用栈，找到main函数的类.代码如下:

   private Class<?> deduceMainApplicationClass() {
       try {
           StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
           for (StackTraceElement stackTraceElement : stackTrace) {
               if ("main".equals(stackTraceElement.getMethodName())) {
                   return Class.forName(stackTraceElement.getClassName());
               }
           }
       }
       catch (ClassNotFoundException ex) {
           // Swallow and continue
       }
       return null;
   }

流程图如下: