从servlet到springboot（1） springboot启动 初始化SpringApplication

直接从启动类开始说起，springboot的入口就是 在main启动类的

SpringApplication.run方法，这是Application的一个静态方法，那么要执行这个静态方法，肯定首先要对SpringApplication进行初始化，该类初始化的方法如下，前面几步都是给变量赋值，没什么好说的，主要看最后一步：initialize()；

public static void main(String[] args) {
    SpringApplication.run(Demo22Application.class, args);
}

   ---->

public SpringApplication(ResourceLoader resourceLoader, Object... sources) {
    this.bannerMode = Mode.CONSOLE;
    this.logStartupInfo = true;
    this.addCommandLineProperties = true;
    this.headless = true;
    this.registerShutdownHook = true;
    this.additionalProfiles = new HashSet();
    this.resourceLoader = resourceLoader;
//上面几步都是对SpringApplication中的的几个变量进行赋值
主要看下下面这个方法
    this.initialize(sources);
}
---------->
private void initialize(Object[] sources) {
    if (sources != null && sources.length > 0) {
        this.sources.addAll(Arrays.asList(sources));
    }
    // 判断是否是web程序(javax.servlet.Servlet和org.springframework.web.context.ConfigurableWebApplicationContext都必须在类加载器中存在)，并设置到webEnvironment属性中    
    this.webEnvironment = this.deduceWebEnvironment();
------>
private boolean deduceWebEnvironment() {
    String[] var1 = WEB_ENVIRONMENT_CLASSES;//这个变量就是javax.servlet.Servlet和//org.springframework.web.context.ConfigurableWebApplicationContext
    int var2 = var1.length;

    for(int var3 = 0; var3 < var2; ++var3) {
        String className = var1[var3];
        if (!ClassUtils.isPresent(className, (ClassLoader)null)) {
            return false;
        }
    }

   return true;
}

<---------

// 从spring.factories文件中找出key为ApplicationContextInitializer的类并实例化后设置到SpringApplication的initializers属性中。这个过程也就是找出所有的应用程序初始化器

this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));

进入这个方法看一下

这个方法首先调用了getSpringFactoriesInstances方法，这个方法调用了重载方法

ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
//看这里
Set names = new LinkedHashSet(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
---->
loadFactoryNames
z这个方法会找到类路径下jar包中的META-INF/spring.factories下所有的文件中以org.springframework.context.ApplicationContextInitializer为key的value
比如我们看spring-boot-1.4.7.relase下的

----省略--
# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

# Application Context Initializers
#这个文件下以org.springframework.context.ApplicationContextInitializer为key的value又四个
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

# Application Listeners
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
---省略----

由于这个方法传入的clazz是ApplicationContextInitializer.class，最终找到的result=6

String factoryClassName = factoryClass.getName();

    try {
        Enumeration urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
        ArrayList result = new ArrayList();

        while(urls.hasMoreElements()) {
            URL 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 var8) {
        throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + "META-INF/spring.factories" + "]", var8);
    }
}
<---------
初始化这几个类
List instances = this.createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
排序
AnnotationAwareOrderComparator.sort(instances);
return instances;

----->

然后进入setInitializers方法，会发现是吧上一步初始化的集合赋值给SpringApplication中的initializers

 

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

接着initialize方法

<-------

这个方法和上面的this.setInitializers方法类似，这个方法最终把SpringApplication上的listeners设置上值，

 

  setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));

// 找出main类，这里是启动类
this.mainApplicationClass = this.deduceMainApplicationClass()

 以上吧initialize 的全过程