附3 springboot源码解析 - 构建SpringApplication

 package com.microservice.framework;

 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;

 @SpringBootApplication
 public class MySpringAplication {

     public void run(String[] args) {
         SpringApplication sa = new SpringApplication(MySpringAplication.class);
         sa.run(args);
     }

 }

SpringBoot启动过程：

1、构建SpringApplication对象

2、执行run()

一、构建SpringApplication对象

     /**
      * The application context will load beans from the specified sources
      */
     public SpringApplication(Object... sources) {
         initialize(sources);
     }

说明：

• 实例化该类的时候会加载bean到applicationContext中去
• 这里的入参是MySpringApplication.class这样一个Class<com.microservice.framework.MySpringApplication>对象

    private final Set<Object> sources = new LinkedHashSet<Object>();
    private boolean webEnvironment;
    private Class<?> mainApplicationClass;

    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();
    }

步骤：

• 将传入的MySpringApplication.class对象放入Set集合
• 判断是否是web环境
• 创建ApplicationInitializer列表
• 初始化ApplicationListener列表
• 初始化主类mainApplicationClass

1.1、将传入的MySpringApplication.class对象放入Set集合

1.2、判断是否是web环境：

    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;
    }

说明：通过在classpath中查看是否存在WEB_ENVIRONMENT_CLASSES这个数组中所包含的所有类（实际上就是2个类），如果存在那么当前程序即是一个Web应用程序，反之则不然。

1.3、创建ApplicationContextInitializer列表

     private List<ApplicationContextInitializer<?>> initializers;

     public void setInitializers(
             Collection<? extends ApplicationContextInitializer<?>> initializers) {
         this.initializers = new ArrayList<ApplicationContextInitializer<?>>();
         this.initializers.addAll(initializers);
     }

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

     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
         Set<String> names = new LinkedHashSet<String>(
                 SpringFactoriesLoader.loadFactoryNames(type, classLoader));
         List<T> instances = new ArrayList<T>(names.size());

         // Create instances from the names
         for (String name : names) {
             try {
                 Class<?> instanceClass = ClassUtils.forName(name, classLoader);
                 Assert.isAssignable(type, instanceClass);
                 Constructor<?> constructor = instanceClass.getConstructor(parameterTypes);
                 T instance = (T) constructor.newInstance(args);
                 instances.add(instance);
             }
             catch (Throwable ex) {
                 throw new IllegalArgumentException(
                         "Cannot instantiate " + type + " : " + name, ex);
             }
         }

         AnnotationAwareOrderComparator.sort(instances);
         return instances;
     }

步骤：

• 调用SpringFactoriesLoader.loadFactoryNames(type, classLoader)来获取所有Spring Factories的名字，（这里是获取了四个ApplicationContextInitializer实现类的全类名，见下边）
• 为每一个Spring Factories根据读取到的名字创建其对象。（这里创建了4个对象）
• 将创建好的对象列表排序并返回。

其中，SpringFactoriesLoader.loadFactoryNames(type, classLoader)如下：

     /**
      * The location to look for factories.
      * <p>Can be present in multiple JAR files.
      */
     public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

     /**
      * Load the fully qualified class names of factory implementations of the
      * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
      * class loader.
      */
     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);
         }
     }

META-INF/spring-factories

 # Application Context Initializers
 org.springframework.context.ApplicationContextInitializer=\
 org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
 org.springframework.boot.context.ContextIdApplicationContextInitializer,\
 org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
 org.springframework.boot.context.web.ServerPortInfoApplicationContextInitializer

说明：

• 从所有jar获取所有的META-INF/spring-factories文件。（这里只有spring-boot-1.3.0.RELEASE.jar下有一个）
• 遍历每一个spring-factories文件，并获取其下key为factoryClass.getName()（这里是入参

  org.springframework.context.ApplicationContextInitializer）的value（这里有以上四个ApplicationContextInitializer实现类）

以上四个类的作用：

至此，设置ApplicationContextInitialize就完成了。

总结：整个setInitializers实际上就是初始化了SpringApplication的属性List<ApplicationContextInitializer<?>> initializers为一个ArrayList列表，该列表中有四个实例：

• ConfigurationWarningsApplicationContextInitializer的实例
• ContextIdApplicationContextInitializer的实例
• DelegatingApplicationContextInitializer实例
• ServerPortInfoApplicationContextInitializer实例

1.4、初始化ApplicationListener列表

     private List<ApplicationListener<?>> listeners;    

         /**
      * Sets the {@link ApplicationListener}s that will be applied to the SpringApplication
      * and registered with the {@link ApplicationContext}.
      * @param listeners the listeners to set
      */
     public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {
         this.listeners = new ArrayList<ApplicationListener<?>>();
         this.listeners.addAll(listeners);
     }    

META-INF/spring-factories

 # Application Listeners
 org.springframework.context.ApplicationListener=\
 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

以上八个listener的作用如下：

至此，整个setListeners方法结束，初始化了一个包含以上8个ApplicationListener实例的List集合。

1.5、初始化主类mainApplicationClass

     private Class<?> mainApplicationClass;

     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;
     }

说明：获取main()方法所在的主类Class对象，并赋值给SpringApplication的mainApplicationClass属性。

至此，SpringApplication对象初始化完成了。

总结：整个SpringApplication初始化的过程，就是初始化了

• 一个包含入参MySpringApplication.class的sources的Set<Object>
• 一个当前环境是否是web环境的boolean webEnvironment
• 一个包含4个ApplicationContextInitializer实例的List
• 一个包含8个ApplicationListener实例的List
• 一个main方法所在的主类的Class对象。

注意：

本文基本参照http://zhaox.github.io/java/2016/03/22/spring-boot-start-flow完成，该文的作者已经解析的很好了，我这里再抄一遍，只是为了加深记忆！！！