dubbo异步调用原理

一、使用方式

服务提供方不变，调用方代码如下：哪里要用在那里配置（局部配置，不会影像全局）

1     <dubbo:reference id="demoService" check="false" interface="com.alibaba.dubbo.demo.DemoService">
2         <dubbo:method name="sayHello" async="true" timeout="60000"/>
3         <dubbo:method name="sayBye" async="true" timeout="60000"/>
4     </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"/>。

1     public static void main(String[] args) throws Exception {
 2         //Prevent to get IPV6 address,this way only work in debug mode
 3         //But you can pass use -Djava.net.preferIPv4Stack=true,then it work well whether in debug mode or not
 4         System.setProperty("java.net.preferIPv4Stack", "true");
 5 
 6         asyncFuture2();
 7     }
 8 
 9     public static void asyncFuture1() throws ExecutionException, InterruptedException {
10         ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-consumer.xml"});
11         context.start();
12         DemoService demoService = (DemoService) context.getBean("demoService"); // get remote service proxy
13 
14         long start = System.currentTimeMillis();
15 
16         demoService.sayHello("zhangsan");
17         Future<String> helloFuture = RpcContext.getContext().getFuture();
18 
19         demoService.sayBye("lisi");
20         Future<String> byeFuture = RpcContext.getContext().getFuture();
21 
22         final String helloStr = helloFuture.get();//消耗5s
23         final String byeStr = byeFuture.get();//消耗8s
24 
25         System.out.println(helloStr + " -- " + byeStr + " ,cost:" + (System.currentTimeMillis()-start));//总消耗8s
26     }
27 
28     public static void asyncFuture2() throws ExecutionException, InterruptedException {
29         ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-consumer.xml"});
30         context.start();
31         DemoService demoService = (DemoService) context.getBean("demoService"); // get remote service proxy
32 
33         long start = System.currentTimeMillis();
34 
35         Future<String> helloFuture = RpcContext.getContext().asyncCall(()-> demoService.sayHello("zhangsan"));
36         Future<String> byeFuture = RpcContext.getContext().asyncCall(()->demoService.sayBye("lisi"));
37 
38         final String helloStr = helloFuture.get();//消耗5s
39         final String byeStr = byeFuture.get();//消耗8s
40 
41         System.out.println(helloStr + " -- " + byeStr + " ,cost:" + (System.currentTimeMillis()-start));//总消耗8s
42     }

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（发出一个线程，建立future对象用于之后的返回结果的存储，并不是立即返回的调用）此时将结果包装成Future放在rpc上下文中---已经开始执行代理方法
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---执行完目标方法可以更新此代理对象

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:

1     public Result invoke(final Invoker<?> invoker, final Invocation invocation) throws RpcException {
 2         final boolean isAsync = RpcUtils.isAsync(invoker.getUrl(), invocation);
 3 
 4         fireInvokeCallback(invoker, invocation);
 5         // need to configure if there's return value before the invocation in order to help invoker to judge if it's
 6         // necessary to return future.
 7         Result result = invoker.invoke(invocation);
 8         if (isAsync) {
 9             asyncCallback(invoker, invocation);
10         } else {
11             syncCallback(invoker, invocation, result);
12         }
13         return result;
14     }

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)：

1     protected Result doInvoke(final Invocation invocation) throws Throwable {
 2         RpcInvocation inv = (RpcInvocation) invocation;
 3         final String methodName = RpcUtils.getMethodName(invocation);
 4         inv.setAttachment(Constants.PATH_KEY, getUrl().getPath());
 5         inv.setAttachment(Constants.VERSION_KEY, version);
 6 
 7         ExchangeClient currentClient;
 8         if (clients.length == 1) {
 9             currentClient = clients[0];
10         } else {
11             currentClient = clients[index.getAndIncrement() % clients.length];
12         }
13         try {
14             boolean isAsync = RpcUtils.isAsync(getUrl(), invocation);
15             boolean isOneway = RpcUtils.isOneway(getUrl(), invocation);
16             int timeout = getUrl().getMethodParameter(methodName, Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);
17             if (isOneway) { //无返回值
18                 boolean isSent = getUrl().getMethodParameter(methodName, Constants.SENT_KEY, false);
19                 currentClient.send(inv, isSent);
20                 RpcContext.getContext().setFuture(null);
21                 return new RpcResult();
22             } else if (isAsync) { //异步有返回值
23                 ResponseFuture future = currentClient.request(inv, timeout);
24                 RpcContext.getContext().setFuture(new FutureAdapter<Object>(future));
25                 return new RpcResult();
26             } else { //同步有返回值
27                 RpcContext.getContext().setFuture(null);
28                 return (Result) currentClient.request(inv, timeout).get();
29             }
30         } catch (TimeoutException e) {
31             throw new RpcException(RpcException.TIMEOUT_EXCEPTION, "Invoke remote method timeout. method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
32         } catch (RemotingException e) {
33             throw new RpcException(RpcException.NETWORK_EXCEPTION, "Failed to invoke remote method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
34         }
35     }

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：

1 public class FutureAdapter<V> implements Future<V> {
 2 
 3     private final ResponseFuture future;
 4 
 5     public FutureAdapter(ResponseFuture future) {
 6         this.future = future;
 7     }
 8 
 9     public ResponseFuture getFuture() {
10         return future;
11     }
12 
13     public boolean cancel(boolean mayInterruptIfRunning) {
14         return false;
15     }
16 
17     public boolean isCancelled() {
18         return false;
19     }
20 
21     public boolean isDone() {
22         return future.isDone();
23     }
24 
25     @SuppressWarnings("unchecked")
26     public V get() throws InterruptedException, ExecutionException {
27         try {
28             return (V) (((Result) future.get()).recreate());
29         } catch (RemotingException e) {
30             throw new ExecutionException(e.getMessage(), e);
31         } catch (Throwable e) {
32             throw new RpcException(e);
33         }
34     }
35 
36     @SuppressWarnings("unchecked")
37     public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
38         int timeoutInMillis = (int) unit.convert(timeout, TimeUnit.MILLISECONDS);
39         try {
40             return (V) (((Result) future.get(timeoutInMillis)).recreate());
41         } catch (com.alibaba.dubbo.remoting.TimeoutException e) {
42             throw new TimeoutException(StringUtils.toString(e));
43         } catch (RemotingException e) {
44             throw new ExecutionException(e.getMessage(), e);
45         } catch (Throwable e) {
46             throw new RpcException(e);
47         }
48     }
49 }

最后，回头看一下FutureFilter：

1     public Result invoke(final Invoker<?> invoker, final Invocation invocation) throws RpcException {
 2         final boolean isAsync = RpcUtils.isAsync(invoker.getUrl(), invocation);
 3 
 4         fireInvokeCallback(invoker, invocation);
 5         // need to configure if there's return value before the invocation in order to help invoker to judge if it's
 6         // necessary to return future.
 7         Result result = invoker.invoke(invocation);
 8         if (isAsync) {
 9             asyncCallback(invoker, invocation);
10         } else {
11             syncCallback(invoker, invocation, result);
12         }
13         return result;
14     }

1     private void asyncCallback(final Invoker<?> invoker, final Invocation invocation) {
 2         Future<?> f = RpcContext.getContext().getFuture();
 3         if (f instanceof FutureAdapter) {
 4             ResponseFuture future = ((FutureAdapter<?>) f).getFuture();
 5             future.setCallback(new ResponseCallback() {
 6                 public void done(Object rpcResult) {
 7                     if (rpcResult == null) {
 8                         logger.error(new IllegalStateException("invalid result value : null, expected " + Result.class.getName()));
 9                         return;
10                     }
11                     ///must be rpcResult
12                     if (!(rpcResult instanceof Result)) {
13                         logger.error(new IllegalStateException("invalid result type :" + rpcResult.getClass() + ", expected " + Result.class.getName()));
14                         return;
15                     }
16                     Result result = (Result) rpcResult;
17                     if (result.hasException()) {
18                         fireThrowCallback(invoker, invocation, result.getException());
19                     } else {
20                         fireReturnCallback(invoker, invocation, result.getValue());
21                     }
22                 }
23 
24                 public void caught(Throwable exception) {
25                     fireThrowCallback(invoker, invocation, exception);
26                 }
27             });
28         }
29     }

这里的future对象时之前创建好的DefaultFuture对象。

1     private volatile Response response;
 2     private volatile ResponseCallback callback;
 3 
 4     public boolean isDone() {
 5         return response != null;
 6     }
 7 
 8     public void setCallback(ResponseCallback callback) {
 9         if (isDone()) {
10             invokeCallback(callback);
11         } else {
12             boolean isdone = false;
13             lock.lock();
14             try {
15                 if (!isDone()) {
16                     this.callback = callback;
17                 } else {
18                     isdone = true;
19                 }
20             } finally {
21                 lock.unlock();
22             }
23             if (isdone) {
24                 invokeCallback(callback);
25             }
26         }
27     }

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：

1     private static final ThreadLocal<RpcContext> LOCAL = new ThreadLocal<RpcContext>() {
 2         @Override
 3         protected RpcContext initialValue() {
 4             return new RpcContext();
 5         }
 6     };
 7 
 8     private Future<?> future;
 9 
10     public static RpcContext getContext() {
11         return LOCAL.get();
12     }
13 
14     public <T> Future<T> getFuture() {
15         return (Future<T>) future;
16     }

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。

1     public V get() throws InterruptedException, ExecutionException {
2         try {
3             return (V) (((Result) future.get()).recreate());
4         } catch (RemotingException e) {
5             throw new ExecutionException(e.getMessage(), e);
6         } catch (Throwable e) {
7             throw new RpcException(e);
8         }
9     }

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）

1 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"));

1     public <T> Future<T> asyncCall(Callable<T> callable) {
 2         try {
 3             try {
 4                 setAttachment(Constants.ASYNC_KEY, Boolean.TRUE.toString());
 5                 // 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）
 6                 final T o = callable.call();
 7                 //local invoke will return directly
 8                 if (o != null) {
 9                     FutureTask<T> f = new FutureTask<T>(new Callable<T>() {
10                         public T call() throws Exception {
11                             return o;
12                         }
13                     });
14                     f.run();
15                     return f;
16                 } else {
17 
18                 }
19             } catch (Exception e) {
20                 throw new RpcException(e);
21             } finally {
22                 removeAttachment(Constants.ASYNC_KEY);
23             }
24         } catch (final RpcException e) {
25             return new Future<T>() {
26                 public boolean cancel(boolean mayInterruptIfRunning) {
27                     return false;
28                 }
29 
30                 public boolean isCancelled() {
31                     return false;
32                 }
33 
34                 public boolean isDone() {
35                     return true;
36                 }
37 
38                 public T get() throws InterruptedException, ExecutionException {
39                     throw new ExecutionException(e.getCause());
40                 }
41 
42                 public T get(long timeout, TimeUnit unit)
43                         throws InterruptedException, ExecutionException,
44                         TimeoutException {
45                     return get();
46                 }
47             };
48         }
49         // 2 从当前线程的RpcContext中获取future对象
50         return ((Future<T>) getContext().getFuture());
51     }

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。（把阻塞放在别的地方，最后获取的时候阻塞）

dubbo异步调用原理

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