SpringBoot @Async异步注解上下文透传-轻识

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上一篇文章说到，之前使用了@Async注解，子线程无法获取到上下文信息，导致流量无法打到灰度，然后改成线程池的方式，每次调用异步调用的时候都手动透传上下文（硬编码）解决了问题。

后面查阅了资料，找到了方案不用每次硬编码，来上下文透传数据了。

方案一：

继承线程池，重写相应的方法，透传上下文。

方案二：（推荐）

线程池ThreadPoolTaskExecutor，有一个TaskDecorator装饰器，实现这个接口，透传上下文。

方案一：继承线程池，重写相应的方法，透传上下文。

1、ThreadPoolTaskExecutor spring封装的线程池

@Bean(ExecutorConstant.simpleExecutor_3)
    public Executor asyncExecutor3() {
        MyThreadPoolTaskExecutor taskExecutor = new MyThreadPoolTaskExecutor();
        taskExecutor.setCorePoolSize(corePoolSize);
        taskExecutor.setMaxPoolSize(maxPoolSize);
        taskExecutor.setQueueCapacity(queueCapacity);
        taskExecutor.setThreadNamePrefix(threadNamePrefix_3);
        taskExecutor.initialize();
        return taskExecutor;
    }

    //------- 继承父类 重写对应的方法 start
    class MyCallable<T> implements Callable<T> {
        private Callable<T> task;
        private RequestAttributes context;

        public MyCallable(Callable<T> task, RequestAttributes context) {
            this.task = task;
            this.context = context;
        }

        @Override
        public T call() throws Exception {
            if (context != null) {
                RequestContextHolder.setRequestAttributes(context);
            }

            try {
                return task.call();
            } finally {
                RequestContextHolder.resetRequestAttributes();
            }
        }
    }
    class MyThreadPoolTaskExecutor extends ThreadPoolTaskExecutor{

        @Override
        public <T> Future<T> submit(Callable<T> task) {
            return super.submit(new MyCallable(task, RequestContextHolder.currentRequestAttributes()));
        }

        @Override
        public <T> ListenableFuture<T> submitListenable(Callable<T> task) {
            return super.submitListenable(new MyCallable(task, RequestContextHolder.currentRequestAttributes()));
        }
    }
    //------- 继承父类 重写对应的方法 end

1、MyCallable是继承Callable，创建MyCallable对象的时候已经把Attributes对象赋值给属性context了（创建MyCallable对象的时候因为实在当前主线程创建的，所以是能获取到请求的Attributes），在执行call方法前，先执行了RequestContextHolder.setRequestAttributes(context); 【把这个MyCallable对象的属性context 设置到setRequestAttributes中】所以在执行具体业务时，当前线程（子线程）就能取得主线程的Attributes

2、MyThreadPoolTaskExecutor类是继承了ThreadPoolTaskExecutor 重写了submit和submitListenable方法

为什么是重写submit和submitListenable这两个方法？

@Async AOP源码的方法位置是在：AsyncExecutionInterceptor.invoke

doSubmit方法能看出来

无返回值调用的是线程池方法：submit()

有返回值，根据不同的返回类型也知道：

返回值类型是：Future.class 调用的是方法：submit()
返回值类型是：ListenableFuture.class 调用的方法是：submitListenable(task)
返回值类型是：CompletableFuture.class调用的是CompletableFuture.supplyAsync这个在异步注解中暂时用不上的，就不考虑重写了。

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);

    AsyncTaskExecutor executor = determineAsyncExecutor(userDeclaredMethod);
    if (executor == null) {
      throw new IllegalStateException(
          "No executor specified and no default executor set on AsyncExecutionInterceptor either");
    }

    Callable<Object> task = () -> {
      try {
        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());
  }

  @Nullable
  protected Object doSubmit(Callable<Object> task, AsyncTaskExecutor executor, Class<?> returnType) {
    if (CompletableFuture.class.isAssignableFrom(returnType)) {
      return CompletableFuture.supplyAsync(() -> {
        try {
          return task.call();
        }
        catch (Throwable ex) {
          throw new CompletionException(ex);
        }
      }, executor);
    }
    else if (ListenableFuture.class.isAssignableFrom(returnType)) {
      return ((AsyncListenableTaskExecutor) executor).submitListenable(task);
    }
    else if (Future.class.isAssignableFrom(returnType)) {
      return executor.submit(task);
    }
    else {
      executor.submit(task);
      return null;
    }
  }

2、ThreadPoolExecutor 原生线程池

ThreadPoolExecutor线程池代码如下：

//------- ThreadPoolExecutor 继承父类 重写对应的方法 start
    class MyRunnable implements Runnable {
        private Runnable runnable;
        private RequestAttributes context;

        public MyRunnable(Runnable runnable, RequestAttributes context) {
            this.runnable = runnable;
            this.context = context;
        }

        @Override
        public void run() {
            if (context != null) {
                RequestContextHolder.setRequestAttributes(context);
            }
            try {
                runnable.run();
            } finally {
                RequestContextHolder.resetRequestAttributes();
            }
        }
    }

    class MyThreadPoolExecutor extends ThreadPoolExecutor{
        @Override
        public void execute(Runnable command) {
            if(!(command instanceof MyRunnable)){
                command = new MyRunnable(command,RequestContextHolder.currentRequestAttributes())
            }
            super.execute(command);
        }

        public MyThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
            super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
        }

        public MyThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) {
            super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory);
        }

        public MyThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler) {
            super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, handler);
        }

        public MyThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) {
            super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, handler);
        }
    }
    //------- ThreadPoolExecutor 继承父类 重写对应的方法 end

像ThreadPoolExecutor主要重写execute方法，在启动新线程的时候先把Attributes取到放到MyRunnable对象的一个属性中，MyRunnable在具体执行run方法的时候，把属性Attributes赋值到子线程中，当run方法执行完了在把Attributes清空掉。

为什么只要重写了execute方法就可以了？

ThreadPoolExecutor大家都知道主要是由submit和execute方法来执行的。

看ThreadPoolExecutor类的submit具体执行方法是由父类AbstractExecutorService#submit来实现。

具体代码在下面贴出来了，可以看到submit实际上最后调用的还是execute方法，所以我们重写execute方法就好了。

submit方法路径及源码：

public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

方案二：（推荐）ThreadPoolTaskExecutor线程池

实现TaskDecorator接口,把实现类设置到taskExecutor.setTaskDecorator(new MyTaskDecorator());

//------- 实现TaskDecorator 接口 start

    @Bean(ExecutorConstant.simpleExecutor_4)
    public Executor asyncExecutor4() {
        MyThreadPoolTaskExecutor taskExecutor = new MyThreadPoolTaskExecutor();
        taskExecutor.setCorePoolSize(corePoolSize);
        taskExecutor.setMaxPoolSize(maxPoolSize);
        taskExecutor.setQueueCapacity(queueCapacity);
        taskExecutor.setThreadNamePrefix(threadNamePrefix_4);
        taskExecutor.setTaskDecorator(new MyTaskDecorator());
        taskExecutor.initialize();
        return taskExecutor;
    }

    class MyTaskDecorator implements TaskDecorator{

        @Override
        public Runnable decorate(Runnable runnable) {
            try {
                RequestAttributes attributes = RequestContextHolder.getRequestAttributes();
                return () -> {
                    try {
                        RequestContextHolder.setRequestAttributes(attributes);
                        runnable.run();
                    } finally {
                        RequestContextHolder.resetRequestAttributes();
                    }
                };
            } catch (IllegalStateException e) {
                return runnable;
            }
        }
    }
    //------- 实现TaskDecorator 接口 end

为什么设置了setTaskDecorator就能实现透传数据了？

主要还是看taskExecutor.initialize()方法，主要是重写了ThreadPoolExecutor的execute方法，用装饰器模式增强了Runnable接口，源代码如下：

@Nullable
  private ThreadPoolExecutor threadPoolExecutor;

  //初始化方法
  public void initialize() {
    if (logger.isDebugEnabled()) {
      logger.debug("Initializing ExecutorService" + (this.beanName != null ? " '" + this.beanName + "'" : ""));
    }
    if (!this.threadNamePrefixSet && this.beanName != null) {
      setThreadNamePrefix(this.beanName + "-");
    }
    this.executor = initializeExecutor(this.threadFactory, this.rejectedExecutionHandler);
  }

  @Override
  protected ExecutorService initializeExecutor(
      ThreadFactory threadFactory, RejectedExecutionHandler rejectedExecutionHandler) {

    BlockingQueue<Runnable> queue = createQueue(this.queueCapacity);

    ThreadPoolExecutor executor;

    //判断是否设置了，taskDecorator装饰器
    if (this.taskDecorator != null) {
      executor = new ThreadPoolExecutor(
          this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
          queue, threadFactory, rejectedExecutionHandler) {
        @Override
        public void execute(Runnable command) {
          //执行装饰器方法包装Runnable接口
          Runnable decorated = taskDecorator.decorate(command);
          if (decorated != command) {
            decoratedTaskMap.put(decorated, command);
          }
          super.execute(decorated);
        }
      };
    }
    else {
      executor = new ThreadPoolExecutor(
          this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
          queue, threadFactory, rejectedExecutionHandler);

    }

    if (this.allowCoreThreadTimeOut) {
      executor.allowCoreThreadTimeOut(true);
    }
    //把初始化好的ThreadPoolExecutor线程池赋值给 当前类属性threadPoolExecutor
    this.threadPoolExecutor = executor;
    return executor;
  }

总结

无论是方案1还是方案2，原理都是先在当前线程获取到Attributes，然后把Attributes赋值到Runnable的一个属性中，在起一个子线程后，具体执行run方法的时候，把Attributes设置给当子线程，当run方法执行完了，在清空Attributes。

方案2实现比较优雅，所以推荐使用它。

出处：cnblogs.com/x-kq/p/14911497.html

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