9.4 dubbo异步调用原理

9.1 客户端发起请求源码、9.2 服务端接收请求消息并发送响应消息源码、9.3 客户端接收响应信息(异步转同步的实现) 分析了dubbo同步调用的源码，现在来看一下dubbo异步调用。

一、使用方式

服务提供方不变，调用方代码如下：

     <dubbo:reference id="demoService" check="false" interface="com.alibaba.dubbo.demo.DemoService">
         <dubbo:method name="sayHello" async="true" timeout="60000"/>
         <dubbo:method name="sayBye" async="true" timeout="60000"/>
     </dubbo:reference>

配置里添加<dubbo:method name="xxx" async="true"/>，表示单个方法xxx使用异步方式；如果demoService下的所有方法都使用异步，直接配置为<dubbo:reference async="true"/>。

     public static void main(String[] args) throws Exception {
         //Prevent to get IPV6 address,this way only work in debug mode
         //But you can pass use -Djava.net.preferIPv4Stack=true,then it work well whether in debug mode or not
         System.setProperty("java.net.preferIPv4Stack", "true");

         asyncFuture2();
     }

     public static void asyncFuture1() throws ExecutionException, InterruptedException {
         ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-consumer.xml"});
         context.start();
         DemoService demoService = (DemoService) context.getBean("demoService"); // get remote service proxy

         long start = System.currentTimeMillis();

         demoService.sayHello("zhangsan");
         Future<String> helloFuture = RpcContext.getContext().getFuture();

         demoService.sayBye("lisi");
         Future<String> byeFuture = RpcContext.getContext().getFuture();

         final String helloStr = helloFuture.get();//消耗5s
         final String byeStr = byeFuture.get();//消耗8s

         System.out.println(helloStr + " -- " + byeStr + " ,cost:" + (System.currentTimeMillis()-start));//总消耗8s
     }

     public static void asyncFuture2() throws ExecutionException, InterruptedException {
         ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-consumer.xml"});
         context.start();
         DemoService demoService = (DemoService) context.getBean("demoService"); // get remote service proxy

         long start = System.currentTimeMillis();

         Future<String> helloFuture = RpcContext.getContext().asyncCall(()-> demoService.sayHello("zhangsan"));
         Future<String> byeFuture = RpcContext.getContext().asyncCall(()->demoService.sayBye("lisi"));

         final String helloStr = helloFuture.get();//消耗5s
         final String byeStr = byeFuture.get();//消耗8s

         System.out.println(helloStr + " -- " + byeStr + " ,cost:" + (System.currentTimeMillis()-start));//总消耗8s
     }

Consumer启动主类。其中asyncFuture2()方法是推荐用法，注意Callable（asyncCall方法的入参）只是一个任务task，不会新建线程；所以asyncFuture2()和asyncFuture1()相似，资源占用相同，都是用一根线程进行异步操作的。

二、asyncFuture1()源码解析

先来看asyncFuture1()，总体步骤：

• demoService.sayHello("zhangsan"); 创建一个Future对象，存入当前线程的上下文中
• Future<String> helloFuture = RpcContext.getContext().getFuture(); 从当前线程的上下文中获取第一步存入的Future对象
• final String helloStr = helloFuture.get(); 阻塞等待，从Future中获取结果

代码主要执行流（代码详细执行流看文章开头的三篇博客）：

1、demoService.sayHello("zhangsan"); 

-->FutureFilter.invoke(final Invoker<?> invoker, final Invocation invocation)
   -->DubboInvoker.doInvoke(final Invocation invocation)

FutureFilter:

     public Result invoke(final Invoker<?> invoker, final Invocation invocation) throws RpcException {
         final boolean isAsync = RpcUtils.isAsync(invoker.getUrl(), invocation);

         fireInvokeCallback(invoker, invocation);
         // need to configure if there's return value before the invocation in order to help invoker to judge if it's
         // necessary to return future.
         Result result = invoker.invoke(invocation);
         if (isAsync) {
             asyncCallback(invoker, invocation);
         } else {
             syncCallback(invoker, invocation, result);
         }
         return result;
     }

对于如上异步操作（asyncFuture1()和asyncFuture2()），FutureFilter没起任何作用，该Filter主要会用在事件通知中，后续再说。

DubboInvoker.doInvoke(final Invocation invocation)：

     protected Result doInvoke(final Invocation invocation) throws Throwable {
         RpcInvocation inv = (RpcInvocation) invocation;
         final String methodName = RpcUtils.getMethodName(invocation);
         inv.setAttachment(Constants.PATH_KEY, getUrl().getPath());
         inv.setAttachment(Constants.VERSION_KEY, version);

         ExchangeClient currentClient;
         if (clients.length == 1) {
             currentClient = clients[0];
         } else {
             currentClient = clients[index.getAndIncrement() % clients.length];
         }
         try {
             boolean isAsync = RpcUtils.isAsync(getUrl(), invocation);
             boolean isOneway = RpcUtils.isOneway(getUrl(), invocation);
             int timeout = getUrl().getMethodParameter(methodName, Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);
             if (isOneway) { //无返回值
                 boolean isSent = getUrl().getMethodParameter(methodName, Constants.SENT_KEY, false);
                 currentClient.send(inv, isSent);
                 RpcContext.getContext().setFuture(null);
                 return new RpcResult();
             } else if (isAsync) { //异步有返回值
                 ResponseFuture future = currentClient.request(inv, timeout);
                 RpcContext.getContext().setFuture(new FutureAdapter<Object>(future));
                 return new RpcResult();
             } else { //同步有返回值
                 RpcContext.getContext().setFuture(null);
                 return (Result) currentClient.request(inv, timeout).get();
             }
         } catch (TimeoutException e) {
             throw new RpcException(RpcException.TIMEOUT_EXCEPTION, "Invoke remote method timeout. method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
         } catch (RemotingException e) {
             throw new RpcException(RpcException.NETWORK_EXCEPTION, "Failed to invoke remote method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
         }
     }

模式：

• 如果是isOneway（不需要返回值），不管同步还是异步，请求直接发出，不会创建Future，直接返回RpcResult空对象。
• 如果是isAsync（异步），则
  • 先创建ResponseFuture对象，之后使用FutureAdapter包装该ResponseFuture对象；（创建ResponseFuture对象与同步的代码相同，最后得到的是一个DefaultFuture对象）
  • 然后将该FutureAdapter对象设入当前线程的上下文中RpcContext.getContext()；
  • 最后返回空的RpcResult
• 如果是同步，则先创建ResponseFuture对象，之后直接调用其get()方法进行阻塞调用（见文章开头的三篇文章）

简单来看一下FutureAdapter：

 public class FutureAdapter<V> implements Future<V> {

     private final ResponseFuture future;

     public FutureAdapter(ResponseFuture future) {
         this.future = future;
     }

     public ResponseFuture getFuture() {
         return future;
     }

     public boolean cancel(boolean mayInterruptIfRunning) {
         return false;
     }

     public boolean isCancelled() {
         return false;
     }

     public boolean isDone() {
22         return future.isDone();
23     }

     @SuppressWarnings("unchecked")
     public V get() throws InterruptedException, ExecutionException {
         try {
             return (V) (((Result) future.get()).recreate());
         } catch (RemotingException e) {
             throw new ExecutionException(e.getMessage(), e);
         } catch (Throwable e) {
             throw new RpcException(e);
         }
     }

     @SuppressWarnings("unchecked")
     public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
         int timeoutInMillis = (int) unit.convert(timeout, TimeUnit.MILLISECONDS);
         try {
             return (V) (((Result) future.get(timeoutInMillis)).recreate());
         } catch (com.alibaba.dubbo.remoting.TimeoutException e) {
             throw new TimeoutException(StringUtils.toString(e));
         } catch (RemotingException e) {
             throw new ExecutionException(e.getMessage(), e);
         } catch (Throwable e) {
             throw new RpcException(e);
         }
     }
 }

最后，回头看一下FutureFilter：

     public Result invoke(final Invoker<?> invoker, final Invocation invocation) throws RpcException {
         final boolean isAsync = RpcUtils.isAsync(invoker.getUrl(), invocation);

         fireInvokeCallback(invoker, invocation);
         // need to configure if there's return value before the invocation in order to help invoker to judge if it's
         // necessary to return future.
         Result result = invoker.invoke(invocation);
         if (isAsync) {
             asyncCallback(invoker, invocation);
         } else {
             syncCallback(invoker, invocation, result);
         }
         return result;
     }

     private void asyncCallback(final Invoker<?> invoker, final Invocation invocation) {
         Future<?> f = RpcContext.getContext().getFuture();
         if (f instanceof FutureAdapter) {
             ResponseFuture future = ((FutureAdapter<?>) f).getFuture();
             future.setCallback(new ResponseCallback() {
                 public void done(Object rpcResult) {
                     if (rpcResult == null) {
                         logger.error(new IllegalStateException("invalid result value : null, expected " + Result.class.getName()));
                         return;
                     }
                     ///must be rpcResult
                     if (!(rpcResult instanceof Result)) {
                         logger.error(new IllegalStateException("invalid result type :" + rpcResult.getClass() + ", expected " + Result.class.getName()));
                         return;
                     }
                     Result result = (Result) rpcResult;
                     if (result.hasException()) {
                         fireThrowCallback(invoker, invocation, result.getException());
                     } else {
                         fireReturnCallback(invoker, invocation, result.getValue());
                     }
                 }

                 public void caught(Throwable exception) {
                     fireThrowCallback(invoker, invocation, exception);
                 }
             });
         }
     }

这里的future对象时之前创建好的DefaultFuture对象。

     private volatile Response response;
     private volatile ResponseCallback callback;

     public boolean isDone() {
         return response != null;
     }

     public void setCallback(ResponseCallback callback) {
         if (isDone()) {
             invokeCallback(callback);
         } else {
             boolean isdone = false;
             lock.lock();
             try {
                 if (!isDone()) {
                     this.callback = callback;
                 } else {
                     isdone = true;
                 }
             } finally {
                 lock.unlock();
             }
             if (isdone) {
                 invokeCallback(callback);
             }
         }
     }

这里判断响应是否已经返回了，如果返回了，直接执行invokeCallback(callback)，否则将传入的ResponseCallback对象赋值给callback对象。

2、Future<String> helloFuture = RpcContext.getContext().getFuture(); 

RpcContext：

     private static final ThreadLocal<RpcContext> LOCAL = new ThreadLocal<RpcContext>() {
         @Override
         protected RpcContext initialValue() {
             return new RpcContext();
         }
     };

     private Future<?> future;

     public static RpcContext getContext() {
         return LOCAL.get();
     }

     public <T> Future<T> getFuture() {
         return (Future<T>) future;
     }

从当前线程上下文中获取之前存进去的FutureAdapter对象。

3、final String helloStr = helloFuture.get(); 

helloFuture是上述的FutureAdapter对象，其get()调用的是内部的DefaultFuture的get()，该方法与同步调用时相同，源码分析见文章开头的三篇文章。

     public V get() throws InterruptedException, ExecutionException {
         try {
             return (V) (((Result) future.get()).recreate());
         } catch (RemotingException e) {
             throw new ExecutionException(e.getMessage(), e);
         } catch (Throwable e) {
             throw new RpcException(e);
         }
     }

get方法的超时设置除了直接在xml中配置之外，还可以在代码中手动执行（优先级高）

 final String helloStr2 = helloFuture.get(7000, TimeUnit.MILLISECONDS);

三、asyncFuture2()源码解析

下面来看一下asyncFuture2()源码：

1、Future<String> helloFuture = RpcContext.getContext().asyncCall(()-> demoService.sayHello("zhangsan"));

     public <T> Future<T> asyncCall(Callable<T> callable) {
         try {
             try {
                 setAttachment(Constants.ASYNC_KEY, Boolean.TRUE.toString());
                 // 1 执行传入的任务（此处创建FutureAdapter对象，并且设置到当前线程的RpcContext的future对象中）
                 final T o = callable.call();
                 //local invoke will return directly
                 if (o != null) {
                     FutureTask<T> f = new FutureTask<T>(new Callable<T>() {
                         public T call() throws Exception {
                             return o;
                         }
                     });
                     f.run();
                     return f;
                 } else {

                 }
             } catch (Exception e) {
                 throw new RpcException(e);
             } finally {
                 removeAttachment(Constants.ASYNC_KEY);
             }
         } catch (final RpcException e) {
             return new Future<T>() {
                 public boolean cancel(boolean mayInterruptIfRunning) {
                     return false;
                 }

                 public boolean isCancelled() {
                     return false;
                 }

                 public boolean isDone() {
                     return true;
                 }

                 public T get() throws InterruptedException, ExecutionException {
                     throw new ExecutionException(e.getCause());
                 }

                 public T get(long timeout, TimeUnit unit)
                         throws InterruptedException, ExecutionException,
                         TimeoutException {
                     return get();
                 }
             };
         }
         // 2 从当前线程的RpcContext中获取future对象
         return ((Future<T>) getContext().getFuture());
     }

这里外层的catch的作用是什么？没搞清楚 https://github.com/alibaba/dubbo/issues/1346

2、final String helloStr = helloFuture.get();

与同步相同。

总结：dubbo异步与同步的差别：

• 同步：创建DefaultFuture之后，直接get阻塞等待；
• 异步：创建DefaultFuture之后，使用FutureAdapter进行包装，之后设置到当前线程的RpcContext中；后续用户在合适的时候自己从RpcContext获取future，之后get。