原文地址：http://www.baeldung.com/spring-async

1. Overview

In this article, we’ll explore the?asynchronous execution support in Spring?– and the?@Async?annotation.

Simply put – annotating a method of a bean with?@Async?will make it?execute in a separate thread?i.e. the caller will not wait for the completion of the called method.

One interesting aspect in Spring is that the event support in the framework?also has support for async processing?if you want to go that route.

2. Enable Async Support

Let’s start by?enabling asynchronous processing?with?Java configuration?– by simply adding the?@EnableAsync?to a configuration class:

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@Configuration @EnableAsync public class SpringAsyncConfig { ... }

The enable annotation is enough, but as you’d expect, there are also a few simple options for configuration as well:

• annotation?– by default,?@EnableAsync?detects Spring’s?@Async?annotation and the EJB 3.1?javax.ejb.Asynchronous; this option can be used to detect other, user-defined annotation types as well
• mode?– indicates the type of?advice?that should be used – JDK proxy-based or AspectJ weaving
• proxyTargetClass?– indicates the type of?proxy?that should be used – CGLIB or JDK; this attribute has effect only if the?mode?is set to?AdviceMode.PROXY
• order?– sets the order in which?AsyncAnnotationBeanPostProcessor?should be applied; by default, it runs last, just so that it can take into account all existing proxies

Asynchronous processing can also be enabled using?XML configuration?– by using the?task?namespace:

1 2	<task:executor id="myexecutor" pool-size="5"? /> <task:annotation-driven executor="myexecutor"/>

3. The?@Async?Annotation

First – let’s go over the rules –?@Async?has two limitations:

• it must be applied to?public?methods only
• self-invocation – calling the async method from within the same class – won’t work

The reasons are simple –?the method needs to be?public?so that it can be proxied. And?self-invocation doesn’t work?because it bypasses the proxy and calls the underlying method directly.

3.1. Methods with void Return Type

Following is the simple way to configure a method with void return type to run asynchronously:

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@Async public void asyncMethodWithVoidReturnType() { ????System.out.println("Execute method asynchronously. " ??????+ Thread.currentThread().getName()); }

3.2. Methods With Return Type

@Async?can also be applied to a method with return type – by wrapping the actual return in the Future:

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@Async public Future<String> asyncMethodWithReturnType() { ????System.out.println("Execute method asynchronously - " ??????+ Thread.currentThread().getName()); ????try { ????????Thread.sleep(5000); ????????return new AsyncResult<String>("hello world !!!!"); ????} catch (InterruptedException e) { ????????// ????} ????return null; }

Spring also provides an?AsyncResult?class which implements?Future. This can be used to track the result of asynchronous method execution.

Now, let’s invoke the above method and retrieve the result of the asynchronous process using the?Future?object.

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public void testAsyncAnnotationForMethodsWithReturnType() ??throws InterruptedException, ExecutionException { ????System.out.println("Invoking an asynchronous method. " ??????+ Thread.currentThread().getName()); ????Future<String> future = asyncAnnotationExample.asyncMethodWithReturnType(); ????while (true) { ????????if (future.isDone()) { ????????????System.out.println("Result from asynchronous process - " + future.get()); ????????????break; ????????} ????????System.out.println("Continue doing something else. "); ????????Thread.sleep(1000); ????} }

4. The Executor

By default, Spring uses a?SimpleAsyncTaskExecutor?to actually run these methods asynchronously. The defaults can be overridden at two levels – at the application level or at the individual method level.

4.1. Override the Executor at the Method Level

The required executor needs to be declared in a configuration class:

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@Configuration @EnableAsync public class SpringAsyncConfig { ????? ????@Bean(name = "threadPoolTaskExecutor") ????public Executor threadPoolTaskExecutor() { ????????return new ThreadPoolTaskExecutor(); ????} }

Then the executor name should be provided as an attribute in?@Async:

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@Async("threadPoolTaskExecutor") public void asyncMethodWithConfiguredExecutor() { ????System.out.println("Execute method with configured executor - " ??????+ Thread.currentThread().getName()); }

4.2. Override the Executor at the Application Level

The configuration class should implement the?AsyncConfigurer?interface – which will mean that it has the implement the?getAsyncExecutor()?method. It’s here that we will return the executor for the entire application – this now becomes the default executor to run methods annotated with?@Async:

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@Configuration @EnableAsync public class SpringAsyncConfig implements AsyncConfigurer { ????? ????@Override ????public Executor getAsyncExecutor() { ????????return new ThreadPoolTaskExecutor(); ????} ????? }

5. Exception Handling

When a method return type is a?Future, exception handling is easy –?Future.get()?method will throw the exception.

But, if the return type is?void,?exceptions will not be propagated to the calling thread. Hence we need to add extra configurations to handle exceptions.

We’ll create a custom async exception handler by implementing?AsyncUncaughtExceptionHandler?interface. The?handleUncaughtException()?method is invoked when there are any uncaught asynchronous exceptions:

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public class CustomAsyncExceptionHandler ??implements AsyncUncaughtExceptionHandler { ????@Override ????public void handleUncaughtException( ??????Throwable throwable, Method method, Object... obj) { ?? ????????System.out.println("Exception message - " + throwable.getMessage()); ????????System.out.println("Method name - " + method.getName()); ????????for (Object param : obj) { ????????????System.out.println("Parameter value - " + param); ????????} ????} ????? }

In the previous section, we looked at the?AsyncConfigurer?interface implemented by the configuration class. As part of that, we also need to override the?getAsyncUncaughtExceptionHandler()?method to return our custom asynchronous exception handler:

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

6. Conclusion

In this tutorial, we looked at?running asynchronous code with Spring. We started with the very basic configuration and annotation to make it work but also looked at more advanced configs such as providing our own executor, or exception handling strategies.

轉(zhuǎn)載于:https://www.cnblogs.com/davidwang456/p/7160586.html

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