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！！！