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SpringBoot2.X异步编程@Async之请求上下文信息的传递

前两天研究了一下Spring中@Async这个注解，简单的说就是异步调用的一个注解。项目中也基本没用过，主要是没有响应的业务场景。比如：发短信、发邮件、发送队列消息等场景我觉得都可以使用异步编程。使用这个注解也比较简单，其中还是有一个坑的，就是请求的上下文信息的传递。

正文

异步调用对应的是 同步调用 ， 同步调用 指程序按照 定义顺序 依次执行，每一行程序都必须等待上一行程序执行完成之后才能执行； 异步调用 指程序在顺序执行时， 不等待 异步调用的语句 返回结果 就执行后面的程序。

准备完整代码: Controller层:

@ApiOperation(value = "t-1.5-异步执行测试")
@GetMapping("/task")
public String taskExecute() {
    try {
        Future<String> r1 = testTableService.doTaskOne();
        Future<String> r2 = testTableService.doTaskTwo();
        Future<String> r3 = testTableService.doTaskThree();

        ServletRequestAttributes requestAttributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = requestAttributes.getRequest();
        log.info("当前线程为 {}，请求方法为 {}，请求路径为：{}", Thread.currentThread().getName(), request.getMethod(), request.getRequestURL().toString());
        while (true) {
            if (r1.isDone() && r2.isDone() && r3.isDone()) {
                log.info("execute all tasks");
                break;
            }
            Thread.sleep(200);
        }
        log.info("/n" + r1.get() + "/n" + r2.get() + "/n" + r3.get());
    } catch (Exception e) {
        log.error("error executing task for {}", e.getMessage());
    }

    return "ok";
}


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Service层:

@Async("asyncExecutor") //一定要指明使用的哪个线程池
    @Override
    public Future<String> doTaskOne() throws InterruptedException {
        log.info("开始做任务一");
        long start = System.currentTimeMillis();
        Thread.sleep(1000);
        ServletRequestAttributes requestAttributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = requestAttributes.getRequest();
        log.info("当前线程为 {}，请求方法为 {}，请求路径为：{}", Thread.currentThread().getName(), request.getMethod(), request.getRequestURL().toString());
        long end = System.currentTimeMillis();
        log.info("完成任务一，耗时：" + (end - start) + "毫秒");
        return new AsyncResult<>("任务一完成，耗时" + (end - start) + "毫秒");
    }

    @Async("asyncExecutor")
    @Override
    public Future<String> doTaskTwo() throws InterruptedException {

        log.info("开始做任务二");
        long start = System.currentTimeMillis();
        Thread.sleep(1000);
        long end = System.currentTimeMillis();
        ServletRequestAttributes requestAttributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = requestAttributes.getRequest();
        log.info("当前线程为 {}，请求方法为 {}，请求路径为：{}", Thread.currentThread().getName(), request.getMethod(), request.getRequestURL().toString());
        log.info("完成任务二，耗时：" + (end - start) + "毫秒");
        return new AsyncResult<>("任务二完成，耗时" + (end - start) + "毫秒");
    }

    @Async("asyncExecutor")
    @Override
    public Future<String> doTaskThree() throws InterruptedException {
        log.info("开始做任务三");
        ServletRequestAttributes requestAttributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        HttpServletRequest request = requestAttributes.getRequest();
        log.info("当前线程为 {}，请求方法为 {}，请求路径为：{}", Thread.currentThread().getName(), request.getMethod(), request.getRequestURL().toString());
        long start = System.currentTimeMillis();
        Thread.sleep(1000);
        long end = System.currentTimeMillis();
        log.info("完成任务三，耗时：" + (end - start) + "毫秒");
        return new AsyncResult<>("任务三完成，耗时" + (end - start) + "毫秒");
    }
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配置类: executor.setTaskDecorator(new ContextCopyingDecorator()); 这行代码是重点，设置请求上下文的传递!

/**
 * @Description:
 * @author: ListenerSun(男, 未婚) 微信:810548252
 * @Date: Created in 2019-12-16 19:06
 */
@Slf4j
@Configuration
public class ConfigBean {

    public static final String ASYNC_EXECUTOR_NAME = "asyncExecutor";

    @Bean(name = ASYNC_EXECUTOR_NAME)
    public Executor asyncExecutor() {
        log.info("==========>开始注入线程池 Bean");
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        // for passing in request scope context
        // 线程上下文拷贝实现类
        executor.setTaskDecorator(new ContextCopyingDecorator());
        executor.setCorePoolSize(3);
        executor.setMaxPoolSize(5);
        executor.setQueueCapacity(100);
        executor.setWaitForTasksToCompleteOnShutdown(true);
        executor.setThreadNamePrefix("AsyncThread-");
        executor.initialize();
        return executor;
    }
}

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线程上下文信息拷贝类:

/**
 * @Description: 异步调用复制 请求 上下文
 * @author: ListenerSun(男, 未婚) 微信:810548252
 * @Date: Created in 2019-12-31 18:53
 */
public class ContextCopyingDecorator implements TaskDecorator {
    @Override
    public Runnable decorate(Runnable runnable) {
        RequestAttributes context = RequestContextHolder.currentRequestAttributes();
        return () -> {
            try {
                RequestContextHolder.setRequestAttributes(context);
                runnable.run();
            } finally {
                RequestContextHolder.resetRequestAttributes();
            }
        };
    }
}
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入口类: 添加@EnableAsync注解，代表允许开启异步编程！

@EnableAsync
@Slf4j
@SpringBootApplication
@EnableFeignClients("com.sqt.edu")
@EnableEurekaClient
@ComponentScan(basePackages = "com.sqt.edu")
public class Account_APP {
    public static void main(String[] args) {
        SpringApplication.run(Account_APP.class);
        log.info("==========>Account service start successful !");
    }
}

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首先如果我们将 executor.setTaskDecorator(new ContextCopyingDecorator()); 这行代码注释掉

/**
 * @Description:
 * @author: ListenerSun(男, 未婚) 微信:810548252
 * @Date: Created in 2019-12-16 19:06
 */
@Slf4j
@Configuration
public class ConfigBean {

    public static final String ASYNC_EXECUTOR_NAME = "asyncExecutor";

    @Bean(name = ASYNC_EXECUTOR_NAME)
    public Executor asyncExecutor() {
        log.info("==========>开始注入线程池 Bean");
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        // for passing in request scope context
        // 线程上下文拷贝实现类
        //executor.setTaskDecorator(new ContextCopyingDecorator());
        executor.setCorePoolSize(3);
        executor.setMaxPoolSize(5);
        executor.setQueueCapacity(100);
        executor.setWaitForTasksToCompleteOnShutdown(true);
        executor.setThreadNamePrefix("AsyncThread-");
        executor.initialize();
        return executor;
    }
}
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打印输出的结果:

可以看到日志报错了，也就是在Service层中的Task任务中的有一行代码报了空指针异常! 就是以下代码:

HttpServletRequest request = requestAttributes.getRequest();
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可以看到获取到的 requestAttributes 为 null !

在 TaskService 中，每个异步线程的方法获取 RequestContextHolder 中的请求信息时，报了空指针异常。这说明了请求的上下文信息未传递到异步方法的线程中。我们可以看一下 RequestContextHolder 的实现，里面有两个 ThreadLocal 保存当前线程下的 request。关于ThreadLocal这个类，个人的理解就是根据字面意思 "本地线程"，意思就是保存在ThreadLocal中的是你自己的，就跟JVM内存模型中的本地内存一样，从主内存中拷贝到线程本地内存一个道理!所以如何将上下文信息传递到异步线程呢？Spring 中的 ThreadPoolTaskExecutor 有一个配置属性 TaskDecorator ， TaskDecorator 是一个回调接口，采用装饰器模式。装饰模式是动态的给一个对象添加一些额外的功能，就增加功能来说，装饰模式比生成子类更为灵活。因此 TaskDecorator 主要用于任务的调用时设置一些执行上下文，或者为任务执行提供一些监视/统计。

此时我们把之前的 executor.setTaskDecorator(new ContextCopyingDecorator()); 这行代码注释去掉

@Slf4j
@Configuration
public class ConfigBean {

    public static final String ASYNC_EXECUTOR_NAME = "asyncExecutor";

    @Bean(name = ASYNC_EXECUTOR_NAME)
    public Executor asyncExecutor() {
        log.info("==========>开始注入线程池 Bean");
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        // for passing in request scope context
        // 线程上下文拷贝实现类
        executor.setTaskDecorator(new ContextCopyingDecorator());
        executor.setCorePoolSize(3);
        executor.setMaxPoolSize(5);
        executor.setQueueCapacity(100);
        executor.setWaitForTasksToCompleteOnShutdown(true);
        executor.setThreadNamePrefix("AsyncThread-");
        executor.initialize();
        return executor;
    }
}

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此时控制台打印结果: