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Spring中的@Async

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WHAT

@Async由Spring定义，在方法上添加此注解，方法的执行将会是异步的(在另一个线程中执行)，调用者不需要等待方法执行完成

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@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Async {
    // 用于指定异步执行的线程池名称
	String value() default "";
}

WHY

被@Async标记的方法都可以异步执行，注解的方式非常灵活，异步的线程资源可以集中管理

异步执行 BEFORE

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public void sayWorld() {
    new Thread(() -> {
        try {
            TimeUnit.SECONDS.sleep(30);
            System.out.println("world");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }).start(); 
}

public void sayHello() {
   sayWorld();
   System.out.println("hello");
}

异步执行 AFTER

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@Async
public void sayWorld() throws InterruptedException {
    TimeUnit.SECONDS.sleep(30);
    System.out.println("world");
}

public void sayHello() {
   sayWorld();
   System.out.println("hello");
}

HOW

@Async如何使用？又是怎么运作的？

事例

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@Configuration
@EnableAsync
public class AsyncConfigDemo implements AsyncConfigurer {	// 优先级低，相对于 @Async指定异步线程池

    @Override
    public Executor getAsyncExecutor() {
        ThreadPoolTaskExecutor threadPoolTaskExecutor = new ThreadPoolTaskExecutor();
        threadPoolTaskExecutor.setCorePoolSize(10);
        threadPoolTaskExecutor.setThreadNamePrefix("async-");
        threadPoolTaskExecutor.initialize();
        return threadPoolTaskExecutor;
    }

    @Override
    public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
        return new SimpleAsyncUncaughtExceptionHandler();
    }
}

@Service
public class AsyncDemo {
    @Async(value = "taskScheduler") // 优先级高
    public void sayWorld() throws InterruptedException {
        TimeUnit.SECONDS.sleep(3);
        System.out.println("world");
    }
}

// 使用

    @Resource
    private AsyncDemo asyncDemo;

    @Test
    public void testSync() throws IOException {
        try {
            asyncDemo.sayWorld();
            System.out.println("hello");
            System.in.read();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

原理

  1. AsyncConfigDemo由ConfigurationClassParser进行解析，包括注解@EnableAsync上的AsyncConfigurationSelector类

  2. AsyncConfigurationSelector根据@EnableAsync mode()选择代理方式实现，默认为ProxyAsyncConfiguration

  3. Configuration类ProxyAsyncConfiguration被创建并注入AsyncConfigurer扩展的executor、exceptionHandler，创建AsyncAnnotationBeanPostProcessor并通过setBeanFactory方法创建AsyncAnnotationAdvisor

  4. AsyncAnnotationAdvisor设置advice(AnnotationAsyncExecutionInterceptor，设置缺省获取线程池方法getDefaultExecutor，当未扩展AsyncConfigurer的实现则在执行异步的时候getDefaultExecutor会从Spring的上下文中查找TaskExecutor的实现类实例)。设置pointcut(@Async的注解类或方法或是自定义的异步注解)

  5. AnnotationAsyncExecutionInterceptor通过determineAsyncExecutor方法确认执行的Executor。getExecutorQualifier方法查找名为@Async指定的value值的Executor，若找到则执行，若无法确认Executor则调用getDefaultExecutor方法查询类型为TaskExecutor的类，若有多个实现则会查找名字为”taskExecutor”的实现Bean，最终都没有找到则创建SimpleAsyncTaskExecutor(每一个异步的方法任务都会创建一个新的线程)

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