springboot源码解析 - 构建SpringApplication-白红宇

springboot源码解析 - 构建SpringApplication

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发布时间：2019-06-18

本文共 8919 字，大约阅读时间需要 29 分钟。

1 package com.microservice.framework; 2  3 import org.springframework.boot.SpringApplication; 4 import org.springframework.boot.autoconfigure.SpringBootApplication; 5  6 @SpringBootApplication 7 public class MySpringAplication { 8  9     public void run(String[] args) {10         SpringApplication sa = new SpringApplication(MySpringAplication.class);11         sa.run(args);12     }13 14 }

SpringBoot启动过程：

1、构建SpringApplication对象

2、执行run()

一、构建SpringApplication对象

1     /**2      * The application context will load beans from the specified sources 3      */4     public SpringApplication(Object... sources) {5         initialize(sources);6     }

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

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

1     private List
     
      > initializers; 2  3     public void setInitializers( 4             Collection
      > initializers) { 5         this.initializers = new ArrayList
      
       >(); 6         this.initializers.addAll(initializers); 7     } 8  9     private 
       
         Collection
         getSpringFactoriesInstances(Class
        
          type) {10         return getSpringFactoriesInstances(type, new Class
         [] {});11     }12 13     private 
         
           Collection
           getSpringFactoriesInstances(Class
          
            type,14 Class
           [] parameterTypes, Object... args) {15 ClassLoader classLoader = Thread.currentThread().getContextClassLoader();16 17 // Use names and ensure unique to protect against duplicates18 Set
           
             names = new LinkedHashSet
            
             (19 SpringFactoriesLoader.loadFactoryNames(type, classLoader));20 List
             
               instances = new ArrayList
              
               (names.size());21 22 // Create instances from the names23 for (String name : names) {24 try {25 Class
                instanceClass = ClassUtils.forName(name, classLoader);26 Assert.isAssignable(type, instanceClass);27 Constructor
                constructor = instanceClass.getConstructor(parameterTypes);28 T instance = (T) constructor.newInstance(args);29 instances.add(instance);30 }31 catch (Throwable ex) {32 throw new IllegalArgumentException(33 "Cannot instantiate " + type + " : " + name, ex);34 }35 }36 37 AnnotationAwareOrderComparator.sort(instances);38 return instances;39 }

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：

1     /** 2      * The location to look for factories. 3      * Can be present in multiple JAR files. 4      */ 5     public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"; 6  7     /** 8      * Load the fully qualified class names of factory implementations of the 9      * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given10      * class loader.11      */12     public static List
      
        loadFactoryNames(Class
        factoryClass, ClassLoader classLoader) {13         String factoryClassName = factoryClass.getName();14         try {15             Enumeration
       
         urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :16                     ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));17             List
        
          result = new ArrayList
         
          ();18             while (urls.hasMoreElements()) {19                 URL url = urls.nextElement();20                 Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));21                 String factoryClassNames = properties.getProperty(factoryClassName);22                 result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));23             }24             return result;25         }26         catch (IOException ex) {27             throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +28                     "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);29         }30     }

META-INF/spring-factories

1 # Application Context Initializers2 org.springframework.context.ApplicationContextInitializer=\3 org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\4 org.springframework.boot.context.ContextIdApplicationContextInitializer,\5 org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\6 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

1     private List
     
      > listeners;     2  3         /** 4      * Sets the {@link ApplicationListener}s that will be applied to the SpringApplication 5      * and registered with the {@link ApplicationContext}. 6      * @param listeners the listeners to set 7      */ 8     public void setListeners(Collection
      > listeners) { 9         this.listeners = new ArrayList
      
       >();10         this.listeners.addAll(listeners);11     }

META-INF/spring-factories

1 # Application Listeners 2 org.springframework.context.ApplicationListener=\ 3 org.springframework.boot.builder.ParentContextCloserApplicationListener,\ 4 org.springframework.boot.context.FileEncodingApplicationListener,\ 5 org.springframework.boot.context.config.AnsiOutputApplicationListener,\ 6 org.springframework.boot.context.config.ConfigFileApplicationListener,\ 7 org.springframework.boot.context.config.DelegatingApplicationListener,\ 8 org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener,\ 9 org.springframework.boot.logging.ClasspathLoggingApplicationListener,\10 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

1     private Class
      mainApplicationClass; 2  3     private Class
      deduceMainApplicationClass() { 4         try { 5             StackTraceElement[] stackTrace = new RuntimeException().getStackTrace(); 6             for (StackTraceElement stackTraceElement : stackTrace) { 7                 if ("main".equals(stackTraceElement.getMethodName())) { 8                     return Class.forName(stackTraceElement.getClassName()); 9                 }10             }11         }12         catch (ClassNotFoundException ex) {13             // Swallow and continue14         }15         return null;16     }

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