13.1 dubbo服务降级源码解析

从 9.1 客户端发起请求源码 的客户端请求总体流程图中，截取部分如下：

//代理发出请求
proxy0.sayHello(String paramString)
-->InvokerInvocationHandler.invoke(Object proxy, Method method, Object[] args)
  -->new RpcInvocation(method, args)
  -->MockClusterInvoker.invoke(Invocation invocation)//服务降级的地方

dubbo就是通过MockClusterInvoker来实现服务降级的。

一、示例

 public interface DemoService {
 //    String sayHello(String name);
     Car sayHello(String name);
 }

将dubbo-demo中的服务接口定义一个返回模型Car。提供者实现如下：

 public class DemoServiceImpl implements DemoService {
     public Car sayHello(String name) {
         Car car = new Car();
         car.setCarNum("浙A10000");
         car.setGoMile(100);
         return car;
     }
 }

消费者使用如下：

 public class Consumer {
     public static void main(String[] args) {
         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

         while (true) {
             try {
                 Thread.sleep(1000);
                 Car hello = demoService.sayHello("world"); // call remote method
                 System.out.println(hello.getCarNum() + "-" + hello.getGoMile()); // get result
             } catch (Throwable throwable) {
                 throwable.printStackTrace();
             }
         }
     }
 }

二、使用方式 

实际使用中，会通过直接在dubbo-admin中设置服务降级策略，这里使用dubbo用户手册中的方式来更清晰的看一下服务降级的配置（实际上就是进行配置覆盖）

配置规则：

1、使用自定义mock类（接口名+Mock）

• mock = default => DemoServiceMock
• mock = true => DemoServiceMock
• mock = fail => DemoServiceMock
• mock = force => DemoServiceMock

2、先普通执行，执行失败之后再执行相应的mock逻辑

• mock = fail:throw => throw new RpcException(" mocked exception for Service degradation. ");
• mock = fail:throw XxxException => throw new RpcException(RpcException.BIZ_EXCEPTION, XxxException);
• mock = fail:return => return null
• mock = fail:return xxx => return xxx
• mock = fail:return empty => return new Car()

3、直接执行相应的mock逻辑

• mock = force:throw => throw new RpcException(" mocked exception for Service degradation. ");
• mock = force:throw XxxException => throw new RpcException(RpcException.BIZ_EXCEPTION, XxxException);
• mock = force:return => return null
• mock = force:return xxx => return xxx
• mock = force:return empty => return new Car()

进行配置：

 public class DegradeTest {
     public static void main(String[] args) {
         RegistryFactory registryFactory = ExtensionLoader.getExtensionLoader(RegistryFactory.class).getAdaptiveExtension();
         Registry registry = registryFactory.getRegistry(URL.valueOf("zookeeper://10.211.55.5:2181"));
         // return null;
         registry.register(URL.valueOf("override://0.0.0.0/com.alibaba.dubbo.demo.DemoService?category=configurators&dynamic=false&application=demo-consumer&mock=force:return"));
         registry.register(URL.valueOf("override://0.0.0.0/com.alibaba.dubbo.demo.DemoService?category=configurators&dynamic=false&application=demo-consumer&mock=force:return+null"));
         // return 空对象;
         registry.register(URL.valueOf("override://0.0.0.0/com.alibaba.dubbo.demo.DemoService?category=configurators&dynamic=false&application=demo-consumer&mock=force:return+empty"));
         // return value;
         registry.register(URL.valueOf("override://0.0.0.0/com.alibaba.dubbo.demo.DemoService?category=configurators&dynamic=false&application=demo-consumer&mock=force:return+hello"));
         // throw exception
         registry.register(URL.valueOf("override://0.0.0.0/com.alibaba.dubbo.demo.DemoService?category=configurators&dynamic=false&application=demo-consumer&mock=force:throw"));
         // throw custom-msg exception
         registry.register(URL.valueOf("override://0.0.0.0/com.alibaba.dubbo.demo.DemoService?category=configurators&dynamic=false&application=demo-consumer&mock=force:throw+com.alibaba.dubbo.Test.MyRuntimeException"));
         // 执行mock类
         registry.register(URL.valueOf("override://0.0.0.0/com.alibaba.dubbo.demo.DemoService?category=configurators&dynamic=false&application=demo-consumer&mock=force:com.alibaba.dubbo.demo.DemoServiceMock"));
     }
 }

上述需要注意的是需要配置为“force:return+null”的格式而非“force:return null”。（实际上空格的url encode就是+号），上述代码的执行，实际上是在zk上创建configurators的子节点：

关于覆盖配置：http://dubbo.io/books/dubbo-user-book/demos/config-rule.html

• override:// 表示数据采用覆盖方式，支持 override 和 absent，可扩展，必填。
• 0.0.0.0 表示对所有 IP 地址生效，如果只想覆盖某个 IP 的数据，请填入具体 IP，必填。
• com.alibaba.dubbo.demo.DemoService表示只对指定服务生效，必填。
• category=configurators 表示该数据为动态配置类型，必填。
• dynamic=false 表示该数据为持久数据，当注册方退出时，数据依然保存在注册中心，必填。
• enabled=true 覆盖规则是否生效，可不填，缺省生效。
• application=demo-consumer 表示只对指定应用生效，可不填，表示对所有应用生效。
• mock=force:return+null表示将满足以上条件的 mock 参数的值覆盖为 force:return+null。如果想覆盖其它参数，直接加在 override 的 URL 参数上。

三、源码分析

 public class MockClusterInvoker<T> implements Invoker<T> {
     private final Directory<T> directory; //RegistryDirectory：存储invoker列表
     private final Invoker<T> invoker; //FailoverClusterInvoker:容错策略

     public Result invoke(Invocation invocation) throws RpcException {
         Result result = null;

         String value = directory.getUrl().getMethodParameter(invocation.getMethodName(), Constants.MOCK_KEY, Boolean.FALSE.toString()).trim();
         if (value.length() == 0 || value.equalsIgnoreCase("false")) {
             //no mock
             result = this.invoker.invoke(invocation);
         } else if (value.startsWith("force")) {
             ...
             //force:direct mock
             result = doMockInvoke(invocation, null);
         } else {
             //fail-mock
             try {
                 result = this.invoker.invoke(invocation);
             } catch (RpcException e) {
                 if (e.isBiz()) {
                     throw e;
                 } else {
                     ...
                     result = doMockInvoke(invocation, e);
                 }
             }
         }
         return result;
     }
 }

首先去获取mock参数，

• 如果没有配置，则直接使用FailoverClusterInvoker去正常的向provider发出请求；
• 如果配置为以force开头的，则直接执行doMockInvoke(Invocation invocation, RpcException e)，不再向provider发送请求；
• 如果配置为以fail开头的，则先使用FailoverClusterInvoker去正常的向provider发出请求，如果失败抛出了非业务异常，则执行doMockInvoke(Invocation invocation, RpcException e)；

     private Result doMockInvoke(Invocation invocation, RpcException e) {
         Result result = null;
         Invoker<T> minvoker;

         List<Invoker<T>> mockInvokers = selectMockInvoker(invocation); //获取mock类型的Invoker
         if (mockInvokers == null || mockInvokers.size() == 0) {
             minvoker = (Invoker<T>) new MockInvoker(directory.getUrl()); //如果没有配置mock类型的Invoker，则自己创建一个MockInvoker
         } else {
             minvoker = mockInvokers.get(0);
         }
         try {
             result = minvoker.invoke(invocation); //执行MockInvoker的invoke(Invocation invocation)方法
         } catch (RpcException me) {
             if (me.isBiz()) {
                 result = new RpcResult(me.getCause());
             } else { //非业务异常
                 throw new RpcException(me.getCode(), getMockExceptionMessage(e, me), me.getCause());
             }
         } catch (Throwable me) {
             throw new RpcException(getMockExceptionMessage(e, me), me.getCause());
         }
         return result;
     }

从RegistryDirectory中获取MockInvoker：

     /**
      * Return MockInvoker
      * Contract：
      * directory.list() will return a list of normal invokers if Constants.INVOCATION_NEED_MOCK is present in invocation, otherwise, a list of mock invokers will return.
      * if directory.list() returns more than one mock invoker, only one of them will be used.
      *
      * @param invocation
      * @return
      */
     private List<Invoker<T>> selectMockInvoker(Invocation invocation) {
         List<Invoker<T>> invokers = null;
         //TODO generic invoker？
         if (invocation instanceof RpcInvocation) {
             //Note the implicit contract (although the description is added to the interface declaration, but extensibility is a problem. The practice placed in the attachement needs to be improved)
             ((RpcInvocation) invocation).setAttachment(Constants.INVOCATION_NEED_MOCK, Boolean.TRUE.toString());
             //directory will return a list of normal invokers if Constants.INVOCATION_NEED_MOCK is present in invocation, otherwise, a list of mock invokers will return.
             try {
                 invokers = directory.list(invocation);
             } catch (RpcException e) {
                 if (logger.isInfoEnabled()) {
                     logger.info("Exception when try to invoke mock. Get mock invokers error for service:"
                             + directory.getUrl().getServiceInterface() + ", method:" + invocation.getMethodName()
                             + ", will contruct a new mock with 'new MockInvoker()'.", e);
                 }
             }
         }
         return invokers;
     }

首先使用RegistryDirectory获取出方法名为sayHello的Invoker列表，之后使用MockInvokersSelector（Router）选取出MockInvoker。

 public class MockInvokersSelector implements Router {

     public <T> List<Invoker<T>> route(final List<Invoker<T>> invokers,
                                       URL url, final Invocation invocation) throws RpcException {
         if (invocation.getAttachments() == null) {
             return getNormalInvokers(invokers);
         } else {
             String value = invocation.getAttachments().get(Constants.INVOCATION_NEED_MOCK);
             if (value == null)
                 return getNormalInvokers(invokers);
             else if (Boolean.TRUE.toString().equalsIgnoreCase(value)) {
                 return getMockedInvokers(invokers);
             }
         }
         return invokers;
     }

     private <T> List<Invoker<T>> getMockedInvokers(final List<Invoker<T>> invokers) {
         if (!hasMockProviders(invokers)) {
             return null;
         }
         List<Invoker<T>> sInvokers = new ArrayList<Invoker<T>>(1);
         for (Invoker<T> invoker : invokers) {
             if (invoker.getUrl().getProtocol().equals(Constants.MOCK_PROTOCOL)) {
                 sInvokers.add(invoker);
             }
         }
         return sInvokers;
     }

     private <T> boolean hasMockProviders(final List<Invoker<T>> invokers) {
         boolean hasMockProvider = false;
         for (Invoker<T> invoker : invokers) {
             if (invoker.getUrl().getProtocol().equals(Constants.MOCK_PROTOCOL)) {
                 hasMockProvider = true;
                 break;
             }
         }
         return hasMockProvider;
     }
 }

这里获取到的是空列表。

所以会先创建一个MockInvoker对象，之后执行其invoker方法。

MockInvoker：

     public Result invoke(Invocation invocation) throws RpcException {
         String mock = getUrl().getParameter(invocation.getMethodName() + "." + Constants.MOCK_KEY); //sayHello.mock
         if (invocation instanceof RpcInvocation) {
             ((RpcInvocation) invocation).setInvoker(this);
         }
         if (StringUtils.isBlank(mock)) {
             mock = getUrl().getParameter(Constants.MOCK_KEY); //mock
         }

         if (StringUtils.isBlank(mock)) {
             throw new RpcException(new IllegalAccessException("mock can not be null. url :" + url));
         }
         mock = normallizeMock(URL.decode(mock));
         if (Constants.RETURN_PREFIX.trim().equalsIgnoreCase(mock.trim())) { // return
             RpcResult result = new RpcResult();
             result.setValue(null);
             return result;
         } else if (mock.startsWith(Constants.RETURN_PREFIX)) { // return value(包括return null)
             mock = mock.substring(Constants.RETURN_PREFIX.length()).trim();
             mock = mock.replace('`', '"');
             try {
                 Type[] returnTypes = RpcUtils.getReturnTypes(invocation);
                 Object value = parseMockValue(mock, returnTypes);
                 return new RpcResult(value);
             } catch (Exception ew) {
                 throw new RpcException("mock return invoke error. method :" + invocation.getMethodName() + ", mock:" + mock + ", url: " + url, ew);
             }
         } else if (mock.startsWith(Constants.THROW_PREFIX)) { // throw xxx
             mock = mock.substring(Constants.THROW_PREFIX.length()).trim();
             mock = mock.replace('`', '"');
             if (StringUtils.isBlank(mock)) {// throw
                 throw new RpcException(" mocked exception for Service degradation. ");
             } else { // user customized class ： throw xxx
                 Throwable t = getThrowable(mock);
                 throw new RpcException(RpcException.BIZ_EXCEPTION, t);
             }
         } else { //impl mock: 自定义mock类
             try {
                 Invoker<T> invoker = getInvoker(mock);
                 return invoker.invoke(invocation);
             } catch (Throwable t) {
                 throw new RpcException("Failed to create mock implemention class " + mock, t);
             }
         }
     }

首先获取到mock配置，例如：mock=force:return+null，之后进行url解码为mock=force:return null，最后进行处理为mock=return null，然后根据规则走分支。

mock参数的处理函数：

     /**
      * 一、使用自定义mock类
      * mock = default => DemoServiceMock
      * mock = true => DemoServiceMock
      * mock = fail => DemoServiceMock
      * mock = force => DemoServiceMock
      *
      * 二、先普通执行，执行失败之后再执行相应的mock逻辑
      * mock = fail:throw => throw new RpcException(" mocked exception for Service degradation. ");
      * mock = fail:throw XxxException => throw new RpcException(RpcException.BIZ_EXCEPTION, XxxException);
      * mock = fail:return => return null
      * mock = fail:return xxx => return xxx
      *
      * 三、直接执行相应的mock逻辑
      * mock = force:throw => throw new RpcException(" mocked exception for Service degradation. ");
      * mock = force:throw XxxException => throw new RpcException(RpcException.BIZ_EXCEPTION, XxxException);
      * mock = force:return => return null
      * mock = force:return xxx => return xxx
      *
      * @param mock
      * @return
      */
     private String normallizeMock(String mock) {
         if (mock == null || mock.trim().length() == 0) {
             return mock;
         } else if (ConfigUtils.isDefault(mock) || "fail".equalsIgnoreCase(mock.trim()) || "force".equalsIgnoreCase(mock.trim())) {
             mock = url.getServiceInterface() + "Mock";
         }
         if (mock.startsWith(Constants.FAIL_PREFIX)) {
             mock = mock.substring(Constants.FAIL_PREFIX.length()).trim();
         } else if (mock.startsWith(Constants.FORCE_PREFIX)) {
             mock = mock.substring(Constants.FORCE_PREFIX.length()).trim();
         }
         return mock;
     }

我们这里来看一下自定义mock类。消费端编写：

 public class DemoServiceMock implements DemoService {

     @Override
     public Car sayHello(String name) {
         Car car = new Car();
         car.setCarNum("mock中");
         car.setGoMile(666);
         return car;
     }
 }

配置覆盖：

         registry.register(URL.valueOf("override://0.0.0.0/com.alibaba.dubbo.demo.DemoService?category=configurators&dynamic=false&application=demo-consumer&mock=force:com.alibaba.dubbo.demo.DemoServiceMock"));

MockInvoker.invoke

             try {
                 Invoker<T> invoker = getInvoker(mock);
                 return invoker.invoke(invocation);
             } catch (Throwable t) {
                 throw new RpcException("Failed to create mock implemention class " + mock, t);
             }

     private Invoker<T> getInvoker(String mockService) {
         Invoker<T> invoker = (Invoker<T>) mocks.get(mockService);
         if (invoker != null) {
             return invoker;
         } else {
             Class<T> serviceType = (Class<T>) ReflectUtils.forName(url.getServiceInterface());
             if (ConfigUtils.isDefault(mockService)) {
                 mockService = serviceType.getName() + "Mock";
             }

             Class<?> mockClass = ReflectUtils.forName(mockService);
             if (!serviceType.isAssignableFrom(mockClass)) {
                 throw new IllegalArgumentException("The mock implemention class " + mockClass.getName() + " not implement interface " + serviceType.getName());
             }

             if (!serviceType.isAssignableFrom(mockClass)) {
                 throw new IllegalArgumentException("The mock implemention class " + mockClass.getName() + " not implement interface " + serviceType.getName());
             }
             try {
                 T mockObject = (T) mockClass.newInstance(); // 获取自定义mock类实例
                 invoker = proxyFactory.getInvoker(mockObject, (Class<T>) serviceType, url); // 和普通类一样创建Invoker
                 if (mocks.size() < 10000) {
                     mocks.put(mockService, invoker);
                 }
                 return invoker;
             } catch (InstantiationException e) {
                 throw new IllegalStateException("No such empty constructor \"public " + mockClass.getSimpleName() + "()\" in mock implemention class " + mockClass.getName(), e);
             } catch (IllegalAccessException e) {
                 throw new IllegalStateException(e);
             }
         }
     }

上边看了return和自定义mock类，最后来看一下throw异常。

默认抛出RpcException，异常信息：mocked exception for Service degradation. 也可以自定义异常，例如：

 public class MyRuntimeException extends RuntimeException {
     private String msg;

     public MyRuntimeException(String msg){
         this.msg = msg;
     }
 }

自定义异常必须具有单参构造器且参数为String。

配置覆盖：

MockInvoker.invoke

 registry.register(URL.valueOf("override://0.0.0.0/com.alibaba.dubbo.demo.DemoService?category=configurators&dynamic=false&application=demo-consumer&mock=force:throw+com.alibaba.dubbo.Test.MyRuntimeException"));

     private Throwable getThrowable(String throwstr) {
         Throwable throwable = (Throwable) throwables.get(throwstr);
         if (throwable != null) {
             return throwable;
         } else {
             Throwable t = null;
             try {
                 Class<?> bizException = ReflectUtils.forName(throwstr);
                 Constructor<?> constructor;
                 constructor = ReflectUtils.findConstructor(bizException, String.class);
                 t = (Throwable) constructor.newInstance(new Object[]{" mocked exception for Service degradation. "});
                 if (throwables.size() < 1000) {
                     throwables.put(throwstr, t);
                 }
             } catch (Exception e) {
                 throw new RpcException("mock throw error :" + throwstr + " argument error.", e);
             }
             return t;
         }
     }

服务降级结束！！！