深入理解Spring的异步机制

一、Spring中实现异步执行

　　在这里我先以事件的机制举例，注意默认情况下事件的发布与监听都是同步执行的。那么我们来看一看基于异步事件的例子该怎么写

　　首先还是定义事件：

package com.bdqn.lyrk.ssm.study.app.entity.event;

import org.springframework.context.ApplicationEvent;

/**
 * 定义一个饥饿的事件
 *
 * @author chen.nie
 * @date 2018/6/26
 **/
public class HungryEvent extends ApplicationEvent {
    /**
     * Create a new ApplicationEvent.
     *
     * @param source the object on which the event initially occurred (never {@code null})
     */
    public HungryEvent(Object source) {
        super(source);
    }
}

　　定义一个Person类，该类主要发布相关事件　

package com.bdqn.lyrk.ssm.study.app.entity.event;

import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.ApplicationEventPublisherAware;
import org.springframework.stereotype.Component;
import static com.bdqn.lyrk.ssm.study.app.utils.Printer.print;
@Component
public class Person implements ApplicationEventPublisherAware {

    private int hungry;

    private String name = "admin";

    public String getName() {
        return name;
    }

    private ApplicationEventPublisher applicationEventPublisher;

    public void setHungry(int hungry) {
        this.hungry = hungry;

    }

    public void work(){
        if (hungry < 10) {
            print("饿了，谁来帮我做做饭..");
            applicationEventPublisher.publishEvent(new HungryEvent(this));
        }
       print("继续工作....");
    }

    @Override
    public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher) {
        this.applicationEventPublisher = applicationEventPublisher;
    }
}

　　定义事件的监听者

package com.bdqn.lyrk.ssm.study.app.entity;

import com.bdqn.lyrk.ssm.study.app.entity.event.HungryEvent;
import com.bdqn.lyrk.ssm.study.app.entity.event.Person;
import org.springframework.context.event.EventListener;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;

import static com.bdqn.lyrk.ssm.study.app.utils.Printer.print;

@Component
public class TeacherEntity {

    @Async
    @EventListener(HungryEvent.class)
    public void cook(HungryEvent hungryEvent) throws InterruptedException {
        Person person = (Person) hungryEvent.getSource();

        print(person.getName() + "饿了开始做饭");
        Thread.sleep(5000);
        print("饭做好了...");
    }
}

　　在这里@Async表明调用该方法时，会开启一个线程进行异步执行。@EventListener表明该方法会监听对应的事件

　　AppConfig的配置

@Configuration
@ComponentScan
@EnableAsync
public class AppConfig{

     /**
     * 定义异步线程任务
     *
     * @author chen.nie
     * @date 2018/6/27
     **/
    @Bean
    public Executor taskExecutor() {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        return executorService;
    }
}

　　在这里顺便用Printer方法打印出线程名称

public class Printer {

    public static void print(String content) {
        System.out.printf("[%s]--%s%n", Thread.currentThread().getName(), content);
    }
}

　　main方法：

 public static void main(String[] args) {

        AnnotationConfigApplicationContext appApplicationContext = new AnnotationConfigApplicationContext(AppConfig.class);
        Person person = appApplicationContext.getBean(Person.class);
        person.setHungry(1);
        person.work();
        appApplicationContext.close();

    }

　　执行结果：

[main]--饿了，谁来帮我做做饭..
[main]--继续工作....
[pool-1-thread-1]--admin饿了开始做饭
[pool-1-thread-1]--饭做好了...

二、Async的原理分析

2.1 @EnableAsync

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AsyncConfigurationSelector.class)
public @interface EnableAsync {

    /**
     * Indicate the 'async' annotation type to be detected at either class
     * or method level.
     * <p>By default, both Spring's @{@link Async} annotation and the EJB 3.1
     * {@code @javax.ejb.Asynchronous} annotation will be detected.
     * <p>This attribute exists so that developers can provide their own
     * custom annotation type to indicate that a method (or all methods of
     * a given class) should be invoked asynchronously.
     */
    Class<? extends Annotation> annotation() default Annotation.class;

    /**
     * Indicate whether subclass-based (CGLIB) proxies are to be created as opposed
     * to standard Java interface-based proxies.
     * <p><strong>Applicable only if the {@link #mode} is set to {@link AdviceMode#PROXY}</strong>.
     * <p>The default is {@code false}.
     * <p>Note that setting this attribute to {@code true} will affect <em>all</em>
     * Spring-managed beans requiring proxying, not just those marked with {@code @Async}.
     * For example, other beans marked with Spring's {@code @Transactional} annotation
     * will be upgraded to subclass proxying at the same time. This approach has no
     * negative impact in practice unless one is explicitly expecting one type of proxy
     * vs. another &mdash; for example, in tests.
     */
    boolean proxyTargetClass() default false;

    /**
     * Indicate how async advice should be applied.
     * <p>The default is {@link AdviceMode#PROXY}.
     * @see AdviceMode
     */
    AdviceMode mode() default AdviceMode.PROXY;

    /**
     * Indicate the order in which the {@link AsyncAnnotationBeanPostProcessor}
     * should be applied.
     * <p>The default is {@link Ordered#LOWEST_PRECEDENCE} in order to run
     * after all other post-processors, so that it can add an advisor to
     * existing proxies rather than double-proxy.
     */
    int order() default Ordered.LOWEST_PRECEDENCE;

}

　　绝大多数的特性注解@EnableXXX都有个特性使用@Import进行导入操作，那么我们不妨在看一下AsyncConfigurationSelector这个类

public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> {

    private static final String ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME =
            "org.springframework.scheduling.aspectj.AspectJAsyncConfiguration";

    /**
     * {@inheritDoc}
     * @return {@link ProxyAsyncConfiguration} or {@code AspectJAsyncConfiguration} for
     * {@code PROXY} and {@code ASPECTJ} values of {@link EnableAsync#mode()}, respectively
     */
    @Override
    public String[] selectImports(AdviceMode adviceMode) {
        switch (adviceMode) {
            case PROXY:
                return new String[] { ProxyAsyncConfiguration.class.getName() };
            case ASPECTJ:
                return new String[] { ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME };
            default:
                return null;
        }
    }

}

　　看到这里我们已经能猜到Spring中的Async是基于AOP实现的，毕竟我们看到了AspectJ与Proxy了嘛

2.2、ProxyAsyncConfiguration

/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.scheduling.annotation;

import java.lang.annotation.Annotation;

import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Role;
import org.springframework.core.annotation.AnnotationUtils;
import org.springframework.scheduling.config.TaskManagementConfigUtils;
import org.springframework.util.Assert;

/**
 * {@code @Configuration} class that registers the Spring infrastructure beans necessary
 * to enable proxy-based asynchronous method execution.
 *
 * @author Chris Beams
 * @author Stephane Nicoll
 * @since 3.1
 * @see EnableAsync
 * @see AsyncConfigurationSelector
 */
@Configuration
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public class ProxyAsyncConfiguration extends AbstractAsyncConfiguration {

    @Bean(name = TaskManagementConfigUtils.ASYNC_ANNOTATION_PROCESSOR_BEAN_NAME)
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public AsyncAnnotationBeanPostProcessor asyncAdvisor() {
        Assert.notNull(this.enableAsync, "@EnableAsync annotation metadata was not injected");
        AsyncAnnotationBeanPostProcessor bpp = new AsyncAnnotationBeanPostProcessor();
        Class<? extends Annotation> customAsyncAnnotation = this.enableAsync.getClass("annotation");
        if (customAsyncAnnotation != AnnotationUtils.getDefaultValue(EnableAsync.class, "annotation")) {
            bpp.setAsyncAnnotationType(customAsyncAnnotation);
        }
        if (this.executor != null) {
            bpp.setExecutor(this.executor);
        }
        if (this.exceptionHandler != null) {
            bpp.setExceptionHandler(this.exceptionHandler);
        }
        bpp.setProxyTargetClass(this.enableAsync.getBoolean("proxyTargetClass"));
        bpp.setOrder(this.enableAsync.<Integer>getNumber("order"));
        return bpp;
    }

}

　　在这里我们可以看到此类是一个Spring的配置类，主要用于创建AsyncAnnotationBeanPostProcessor的Bean

2.3、AsyncAnnotationBeanPostProcessor

　　首先我先贴一下这个类图：

我们先看一下它的父类AbstractAdvisingBeanPostProcessor里重写的方法，在该方法里动态改变Bean的相关属性

@Override
    public Object postProcessAfterInitialization(Object bean, String beanName) {
        if (bean instanceof AopInfrastructureBean) {
            // Ignore AOP infrastructure such as scoped proxies.
            return bean;
        }
        // 如果为Advised对象且当前的bean适配当前的Advice，则将advisor添加到Advised里
        if (bean instanceof Advised) {
            Advised advised = (Advised) bean;
            if (!advised.isFrozen() && isEligible(AopUtils.getTargetClass(bean))) {
                // Add our local Advisor to the existing proxy's Advisor chain...
                if (this.beforeExistingAdvisors) {
                    advised.addAdvisor(0, this.advisor);
                }
                else {
                    advised.addAdvisor(this.advisor);
                }
                return bean;
            }
        }
        // 如果当前的bean复合当前的通知
        if (isEligible(bean, beanName)) {
            ProxyFactory proxyFactory = prepareProxyFactory(bean, beanName);
            if (!proxyFactory.isProxyTargetClass()) {
                evaluateProxyInterfaces(bean.getClass(), proxyFactory);
            }
            proxyFactory.addAdvisor(this.advisor);
            customizeProxyFactory(proxyFactory);
            return proxyFactory.getProxy(getProxyClassLoader());
        }

        // No async proxy needed.
        return bean;
    }

而AsyncAnnotationBeanPostProcessor 重写了BeanFactoryAware接口方法，请大家关注一下属性Executor和setBeanFactory方法：

public class AsyncAnnotationBeanPostProcessor extends AbstractBeanFactoryAwareAdvisingPostProcessor {

    /**
     * The default name of the {@link TaskExecutor} bean to pick up: "taskExecutor".
     * <p>Note that the initial lookup happens by type; this is just the fallback
     * in case of multiple executor beans found in the context.
     * @since 4.2
     * @see AnnotationAsyncExecutionInterceptor#DEFAULT_TASK_EXECUTOR_BEAN_NAME
     */
    public static final String DEFAULT_TASK_EXECUTOR_BEAN_NAME =
            AnnotationAsyncExecutionInterceptor.DEFAULT_TASK_EXECUTOR_BEAN_NAME;

    protected final Log logger = LogFactory.getLog(getClass());

    private Class<? extends Annotation> asyncAnnotationType;

    private Executor executor;

    private AsyncUncaughtExceptionHandler exceptionHandler;

    public AsyncAnnotationBeanPostProcessor() {
        setBeforeExistingAdvisors(true);
    }

    /**
     * Set the 'async' annotation type to be detected at either class or method
     * level. By default, both the {@link Async} annotation and the EJB 3.1
     * {@code javax.ejb.Asynchronous} annotation will be detected.
     * <p>This setter property exists so that developers can provide their own
     * (non-Spring-specific) annotation type to indicate that a method (or all
     * methods of a given class) should be invoked asynchronously.
     * @param asyncAnnotationType the desired annotation type
     */
    public void setAsyncAnnotationType(Class<? extends Annotation> asyncAnnotationType) {
        Assert.notNull(asyncAnnotationType, "'asyncAnnotationType' must not be null");
        this.asyncAnnotationType = asyncAnnotationType;
    }

    /**
     * Set the {@link Executor} to use when invoking methods asynchronously.
     * <p>If not specified, default executor resolution will apply: searching for a
     * unique {@link TaskExecutor} bean in the context, or for an {@link Executor}
     * bean named "taskExecutor" otherwise. If neither of the two is resolvable,
     * a local default executor will be created within the interceptor.
     * @see AsyncAnnotationAdvisor#AsyncAnnotationAdvisor(Executor, AsyncUncaughtExceptionHandler)
     * @see AnnotationAsyncExecutionInterceptor#getDefaultExecutor(BeanFactory)
     * @see #DEFAULT_TASK_EXECUTOR_BEAN_NAME
     */
    public void setExecutor(Executor executor) {
        this.executor = executor;
    }

    /**
     * Set the {@link AsyncUncaughtExceptionHandler} to use to handle uncaught
     * exceptions thrown by asynchronous method executions.
     * @since 4.1
     */
    public void setExceptionHandler(AsyncUncaughtExceptionHandler exceptionHandler) {
        this.exceptionHandler = exceptionHandler;
    }

    @Override
    public void setBeanFactory(BeanFactory beanFactory) {
        super.setBeanFactory(beanFactory);

        AsyncAnnotationAdvisor advisor = new AsyncAnnotationAdvisor(this.executor, this.exceptionHandler);
        if (this.asyncAnnotationType != null) {
            advisor.setAsyncAnnotationType(this.asyncAnnotationType);
        }
        advisor.setBeanFactory(beanFactory);
        this.advisor = advisor;
    }

}

　　在这里指定对应的this.advisor为AsyncAnnotationAdvisor。

2.4、AsyncAnnotationAdvisor

在这里我们先看看它的构造函数：

public AsyncAnnotationAdvisor(Executor executor, AsyncUncaughtExceptionHandler exceptionHandler) {
        Set<Class<? extends Annotation>> asyncAnnotationTypes = new LinkedHashSet<Class<? extends Annotation>>(2);
        asyncAnnotationTypes.add(Async.class);
        try {
            asyncAnnotationTypes.add((Class<? extends Annotation>)
                    ClassUtils.forName("javax.ejb.Asynchronous", AsyncAnnotationAdvisor.class.getClassLoader()));
        }
        catch (ClassNotFoundException ex) {
            // If EJB 3.1 API not present, simply ignore.
        }
        if (exceptionHandler != null) {
            this.exceptionHandler = exceptionHandler;
        }
        else {
            this.exceptionHandler = new SimpleAsyncUncaughtExceptionHandler();
        }
        this.advice = buildAdvice(executor, this.exceptionHandler);
        this.pointcut = buildPointcut(asyncAnnotationTypes);
    }

在这里主要是构建通知和切点，下面我们分别来看看怎么实现的：

buildAdvice:

protected Advice buildAdvice(Executor executor, AsyncUncaughtExceptionHandler exceptionHandler) {
        return new AnnotationAsyncExecutionInterceptor(executor, exceptionHandler);
    }

AnnotationAsyncExecutionInterceptor的父类实现了MethodInterceptor接口，我们来看看它重写的方法：

/**
     * Intercept the given method invocation, submit the actual calling of the method to
     * the correct task executor and return immediately to the caller.
     * @param invocation the method to intercept and make asynchronous
     * @return {@link Future} if the original method returns {@code Future}; {@code null}
     * otherwise.
     */
    @Override
    public Object invoke(final MethodInvocation invocation) throws Throwable {
        Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
        Method specificMethod = ClassUtils.getMostSpecificMethod(invocation.getMethod(), targetClass);
        final Method userDeclaredMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);
        //取@Async里的value属性对应Bean，如果没有值取名字为taskExecutor的Bean
        AsyncTaskExecutor executor = determineAsyncExecutor(userDeclaredMethod);
        if (executor == null) {
            throw new IllegalStateException(
                    "No executor specified and no default executor set on AsyncExecutionInterceptor either");
        }

        Callable<Object> task = new Callable<Object>() {
            @Override
            public Object call() throws Exception {
                try {
                    //异步执行本体方法
                    Object result = invocation.proceed();
                    if (result instanceof Future) {
                        return ((Future<?>) result).get();
                    }
                }
                catch (ExecutionException ex) {
                    handleError(ex.getCause(), userDeclaredMethod, invocation.getArguments());
                }
                catch (Throwable ex) {
                    handleError(ex, userDeclaredMethod, invocation.getArguments());
                }
                return null;
            }
        };

        return doSubmit(task, executor, invocation.getMethod().getReturnType());
    }

buildPointCut:

/**
     * Calculate a pointcut for the given async annotation types, if any.
     * @param asyncAnnotationTypes the async annotation types to introspect
     * @return the applicable Pointcut object, or {@code null} if none
     */
    protected Pointcut buildPointcut(Set<Class<? extends Annotation>> asyncAnnotationTypes) {
        ComposablePointcut result = null;
        for (Class<? extends Annotation> asyncAnnotationType : asyncAnnotationTypes) {
            Pointcut cpc = new AnnotationMatchingPointcut(asyncAnnotationType, true);
            Pointcut mpc = AnnotationMatchingPointcut.forMethodAnnotation(asyncAnnotationType);
            if (result == null) {
                result = new ComposablePointcut(cpc);
            }
            else {
                result.union(cpc);
            }
            result = result.union(mpc);
        }
        return result;
    }

在这里使用AnnotationMatchingPointcut，该切点会在有对应的annotation方法上切入相关的Advice，此处asyncAnnotationTypes对应的就是@Async

三、总结

　　Spring异步机制用到如下几种方式实现：

　　1）重写BeanPostProcessor来改变Bean的属性，在这里针对Advised或者ProxyFactory动态添加定义好的通知AsyncAnnotationAdvisor

　　2) 使用AOP的机制来，异步执行@Async标记的方法