springboot源码解析-SpringApplication

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();
	}