springmvc中异步转同步

在有些场景中，需要用到异步转同步，什么意思呢，我举个例子，假如你和第三方有一个交互，第三方的处理方式是一个异步的，需要你提供通知地址（可能连第三方提供的查询的机制也是只有这种通知的形式），第三方会把相关处理后的信息通知到这个地址中，你接收到信息后处理。接下来这个功能呢需要提供其他第三方使用，而这个第三方呢觉得这对他而言是一个完整的事物，需要你在这一次调用中直接告诉他业务处理成功还是失败。这个时候就需要使用异步转同步了。以下提供几种处理方案：

1.  AsyncContext 
2. Callable
3. WebAsyncTask
4. DeferredResult

下面分别针对这四种方式逐一介绍

AsyncContext 

在 Servlet3.0+之后就支持了异步请求，第一种方式比较原始，相当于直接借助 Servlet 的规范来实现，当然下面的 case 并不是直接创建一个 servlet，而是借助AsyncContext来实现

@RestController
@RequestMapping(path = "servlet")
public class ServletRest {
 Map asyncMap = new HashMap();
    @GetMapping(path = "get/{requestId}")
    public void get(HttpServletRequest request,@PathVariable("requestId") String requestId) {
        AsyncContext asyncContext = request.startAsync();
        asyncContext.addListener(new AsyncListener() {
            @Override
            public void onComplete(AsyncEvent asyncEvent) throws IOException {
                System.out.println("操作完成:" + Thread.currentThread().getName());
            }
 
            @Override
            public void onTimeout(AsyncEvent asyncEvent) throws IOException {
                System.out.println("超时返回!!!");
                asyncContext.getResponse().setCharacterEncoding("utf-8");
                asyncContext.getResponse().setContentType("text/html;charset=UTF-8");
                asyncContext.getResponse().getWriter().println("超时了！！！!");
            }
 
            @Override
            public void onError(AsyncEvent asyncEvent) throws IOException {
                System.out.println("出现了m某些异常");
                asyncEvent.getThrowable().printStackTrace();
 
                asyncContext.getResponse().setCharacterEncoding("utf-8");
                asyncContext.getResponse().setContentType("text/html;charset=UTF-8");
                asyncContext.getResponse().getWriter().println("出现了某些异常哦！！！!");
            }
 
            @Override
            public void onStartAsync(AsyncEvent asyncEvent) throws IOException {
                System.out.println("开始执行");
            }
        });
        asyncContext.setTimeout(3000L);
        asyncMap.put(requestId,asyncContext);
        System.out.println("主线程over!!! " + Thread.currentThread().getName());
    }
    @PostMapping(path = "send/{requestId}")
    public String send(@PathVariable("requestId") String requestId) {
        AsyncContext asyncContext = asyncMap.get(requestId);
        asyncContext.start(new Runnable() {
            @Override
            public void run() {
                try {
                    System.out.println("内部线程：" + Thread.currentThread().getName());
                    asyncContext.getResponse().setCharacterEncoding("utf-8");
                    asyncContext.getResponse().setContentType("text/html;charset=UTF-8");
                    asyncContext.getResponse().getWriter().println("异步返回!");
                    asyncContext.getResponse().getWriter().flush();
                    // 异步完成，释放
                    asyncContext.complete();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
return "发送完成";
    }
}

完整的实现如上，简单的来看一下一般步骤

• javax.servlet.ServletRequest#startAsync()获取AsyncContext
• 添加监听器 asyncContext.addListener(AsyncListener)（这个是可选的）
  • 用户请求开始、超时、异常、完成时回调
• 设置超时时间 asyncContext.setTimeout(3000L) （可选）
• 异步任务asyncContext.start(Runnable)

Callable

相比较于上面的复杂的示例，SpringMVC 可以非常简单的实现，直接返回一个Callable即可

@RestController
@RequestMapping(path = "call")
public class CallableRest {
 
    @GetMapping(path = "get")
    public Callable get() {
        Callable callable = new Callable() {
            @Override
            public String call() throws Exception {
                System.out.println("do some thing");
                Thread.sleep(1000);
                System.out.println("执行完毕，返回!!!");
                return "over!";
            }
        };
 
        return callable;
    }
 
 
    @GetMapping(path = "exception")
    public Callable exception() {
        Callable callable = new Callable() {
            @Override
            public String call() throws Exception {
                System.out.println("do some thing");
                Thread.sleep(1000);
                System.out.println("出现异常，返回!!!");
                throw new RuntimeException("some error!");
            }
        };
 
        return callable;
    }
}

这个就不多说了，其实就和runable差不多只是这个可以抛出异常，而且有返回值

WebAsyncTask

其实WebAsyncTask只是包装了一下Callable

@RestController
@RequestMapping(path = "task")
public class WebAysncTaskRest {
 
    @GetMapping(path = "get")
    public WebAsyncTask get(long sleep, boolean error) {
        Callable callable = () -> {
            System.out.println("do some thing");
            Thread.sleep(sleep);
 
            if (error) {
                System.out.println("出现异常，返回!!!");
                throw new RuntimeException("异常了!!!");
            }
 
            return "hello world";
        };
 
        // 指定3s的超时
        WebAsyncTask webTask = new WebAsyncTask<>(3000, callable);
        webTask.onCompletion(() -> System.out.println("over!!!"));
 
        webTask.onTimeout(() -> {
            System.out.println("超时了");
            return "超时返回!!!";
        });
 
        webTask.onError(() -> {
            System.out.println("出现异常了!!!");
            return "异常返回";
        });
 
        return webTask;
    }
}

我们现在主要要讲的就是下面这个了：

DeferredResult

DeferredResult与WebAsyncTask最大的区别就是DeferredResult不确定什么时候会返回结果

@RestController
@RequestMapping(path = "defer")
public class DeferredResultRest {
 
    private Map cache = new ConcurrentHashMap<>();
 
    @GetMapping(path = "get")
    public DeferredResult get(String id) {
        DeferredResult res = new DeferredResult<>();
        cache.put(id, res);
 
        res.onCompletion(new Runnable() {
            @Override
            public void run() {
                System.out.println("over!");
            }
        });
        return res;
    }
 
    @GetMapping(path = "send")
    public String publish(String id, String content) {
        DeferredResult res = cache.get(id);
        if (res == null) {
            return "no consumer!";
        }
 
        res.setResult(content);
        return "over!";
    }
}

上面实例中，请求第一个接口/defer/get不会马上获得结果，直到请求了第二个接口/defer/send之后第一个接口/defer/get才返回了结果，注意：记得设置超时时间

可以通过以下两种方式设置

1. 构造new DeferredResult<>(30000L)时
2. 设置全局超时时间，则是通过代码

   @Configuration
   @EnableWebMvc
   public class WebConf implements WebMvcConfigurer {
    
       @Override
       public void configureAsyncSupport(AsyncSupportConfigurer configurer) {
           // 超时时间设置为60s
           configurer.setDefaultTimeout(TimeUnit.SECONDS.toMillis(30));
       }
   }