springboot源码解析-SpringApplication

水深无声 2022-02-18 08:13 199阅读 0赞

# 说明 #

用过springboot的人应该都知道SpringApplication做boot的一个启动点，但是有多少人知道这个类做了多少事情？  
我们来看下源码里面的解释：  
可以通过java主程序（main）来启动spring应用，执行时需要按照以下步骤执行：

1、创建一个实例应用也就是程序；

2、注册一个资源文件来执行spring属性也就是ResourceLoader需要做的事情；

3、刷新应用程序的上下文，加载所有的单例bean；

4、触发一些需要执行的bean，指的是监听和runner；

# 几种启动方式 #

## 实现方式 ##

第一种  
`SpringApplication.run(Application.class, args);`

第二种  
`SpringApplication.main(args);`

第三种

SpringApplication springApplication = new SpringApplication(Application.class);
    springApplication.run(args);

## 说明 ##

前面两种SpringApplication的一个静态方法run，通过run构造一个SpringApplication对象，一般直接启动会直接选择第一种；第二种我觉得作用不大，因为需要构造primarySource资源，第三种一般需要改造启动类，做一些定制化代码。

/**
    	 * Static helper that can be used to run a {@link SpringApplication} from the
    	 * specified source using default settings.
    	 * @param primarySource the primary source to load
    	 * @param args the application arguments (usually passed from a Java main method)
    	 * @return the running {@link ApplicationContext}
    	 */
    	public static ConfigurableApplicationContext run(Class<?> primarySource,
    			String... args) {
    		return run(new Class<?>[] { primarySource }, args);
    	}
    
    	/**
    	 * Static helper that can be used to run a {@link SpringApplication} from the
    	 * specified sources using default settings and user supplied arguments.
    	 * @param primarySources the primary sources to load
    	 * @param args the application arguments (usually passed from a Java main method)
    	 * @return the running {@link ApplicationContext}
    	 */
    	public static ConfigurableApplicationContext run(Class<?>[] primarySources,
    			String[] args) {
    		return new SpringApplication(primarySources).run(args);
    	}
    		/**
    	 * A basic main that can be used to launch an application. This method is useful when
    	 * application sources are defined via a {@literal --spring.main.sources} command line
    	 * argument.
    	 * <p>
    	 * Most developers will want to define their own main method and call the
    	 * {@link #run(Class, String...) run} method instead.
    	 * @param args command line arguments
    	 * @throws Exception if the application cannot be started
    	 * @see SpringApplication#run(Class[], String[])
    	 * @see SpringApplication#run(Class, String...)
    	 */
    	public static void main(String[] args) throws Exception {
    		SpringApplication.run(new Class<?>[0], args);
    	}

## 参数说明 ##

运行run方法会用两个参数primarySource、args

primarySource：主要是用于创建一个实例bean，通过这个主类加载应用的上下文，相当于所有bean的源头；

args：主要用于参数传递

## SpringAppication构造方法 ##

1、初始化资源，启动传的值为null；

public SpringApplication(Class<?>... primarySources) {
    		this(null, primarySources);
    	}

2、设置bean的来源，这个是必须要有的；

this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));

3、推断SpringBoot的应用类型，主要分为SERVLET：基于servlet启动的应用，默认是这个；REACTIVE：基于Reactive Web启动的用于；

this.webApplicationType = deduceWebApplicationType();

4、初始化上下文初始化器；

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

5、加载应用事件监听器

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

6、运行SpringApplication

/**
    	 * Create a new {@link SpringApplication} instance. The application context will load
    	 * beans from the specified primary sources (see {@link SpringApplication class-level}
    	 * documentation for details. The instance can be customized before calling
    	 * {@link #run(String...)}.
    	 * @param resourceLoader the resource loader to use
    	 * @param primarySources the primary bean sources
    	 * @see #run(Class, String[])
    	 * @see #setSources(Set)
    	 */
    	@SuppressWarnings({ "unchecked", "rawtypes" })
    	public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    		this.resourceLoader = resourceLoader;
    		Assert.notNull(primarySources, "PrimarySources must not be null");
    		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    		this.webApplicationType = deduceWebApplicationType();
    		setInitializers((Collection) getSpringFactoriesInstances(
    				ApplicationContextInitializer.class));
    		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    		this.mainApplicationClass = deduceMainApplicationClass();
    	}