[Dubbo优雅服务降级之Stub][Dubbo_Stub]
dubbo作为国内互联网最常用的Java开源服务治理框架，在提供了远程调用的同时也提供了服务降级功能。
首先可以考虑一下服务降级的需求===》考虑在系统服务调用失败时可以返回指定消息而不是异常
通常来说选用dubbo的Mock功能可以实现。
在上一篇中描述到MockClusterWrapper几乎是必须的操作，实质上Mock也是通过其生成的MockClusterInvoker来实现。
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")) {
if (logger.isWarnEnabled()) {
logger.info("force-mock: " + invocation.getMethodName() + " force-mock enabled , url : " + directory.getUrl());
}
//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 {
if (logger.isWarnEnabled()) {
logger.info("fail-mock: " + invocation.getMethodName() + " fail-mock enabled , url : " + directory.getUrl(), e);
}
result = doMockInvoke(invocation, e);
}
}
}
return result;
}
在判断对应URL中mock字段
- 不包含字段直接调用远程服务
- 包含MOck字段并且值为force打头那么直接调用Mock操作
- 否则优先执行远程调用，如果调用失败且返回的是非业务异常再调用Mock操作
调用Mock操作分为如下几步
@SuppressWarnings({ "unchecked", "rawtypes" })
private Result doMockInvoke(Invocation invocation,RpcException e){
Result result = null;
Invoker<T> minvoker ;
List<Invoker<T>> mockInvokers = selectMockInvoker(invocation);
if (mockInvokers == null || mockInvokers.size() == 0){
minvoker = (Invoker<T>) new MockInvoker(directory.getUrl());
} else {
minvoker = mockInvokers.get(0);
}
try {
result = minvoker.invoke(invocation);
} catch (RpcException me) {
if (me.isBiz()) {
result = new RpcResult(me.getCause());
} else {
throw new RpcException(me.getCode(), getMockExceptionMessage(e, me), me.getCause());
}
//
} catch (Throwable me) {
throw new RpcException(getMockExceptionMessage(e, me), me.getCause());
}
return result;
}
/**
* 返回MockInvoker
* 契约：
* directory根据invocation中是否有Constants.INVOCATION_NEED_MOCK，来判断获取的是一个normal invoker 还是一个 mock invoker
* 如果directorylist 返回多个mock invoker，只使用第一个invoker.
* @param invocation
* @return
*/
private List<Invoker<T>> selectMockInvoker(Invocation invocation){
//TODO generic invoker？
if (invocation instanceof RpcInvocation){
//存在隐含契约(虽然在接口声明中增加描述，但扩展性会存在问题.同时放在attachement中的做法需要改进
((RpcInvocation)invocation).setAttachment(Constants.INVOCATION_NEED_MOCK, Boolean.TRUE.toString());
//directory根据invocation中attachment是否有Constants.INVOCATION_NEED_MOCK，来判断获取的是normal invokers or mock invokers
List<Invoker<T>> invokers = directory.list(invocation);
return invokers;
} else {
return null ;
}
}
```
1. 首先找到对应的 可以调用的invoker，并且标记为mock
2. 如果无法找到invoker那么直接新建MockInvoker
3. 调用invoker，当调用MockInvoker发生下列行为
```java
public Result invoke(Invocation invocation) throws RpcException {
String mock = getUrl().getParameter(invocation.getMethodName()+"."+Constants.MOCK_KEY);
if (invocation instanceof RpcInvocation) {
((RpcInvocation) invocation).setInvoker(this);
}
if (StringUtils.isBlank(mock)){
mock = getUrl().getParameter(Constants.MOCK_KEY);
}
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())){
RpcResult result = new RpcResult();
result.setValue(null);
return result;
} else if (mock.startsWith(Constants.RETURN_PREFIX)) {
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)) {
mock = mock.substring(Constants.THROW_PREFIX.length()).trim();
mock = mock.replace('`', '"');
if (StringUtils.isBlank(mock)){
throw new RpcException(" mocked exception for Service degradation. ");
} else { //用户自定义类
Throwable t = getThrowable(mock);
throw new RpcException(RpcException.BIZ_EXCEPTION, t);
}
} else { //impl mock
try {
Invoker<T> invoker = getInvoker(mock);
return invoker.invoke(invocation);
} catch (Throwable t) {
throw new RpcException("Failed to create mock implemention class " + mock , t);
}
}
}
```
将mock参数对应的值进行解析，分为下列几种
//mock=fail:throw
//mock=fail:return
//mock=xx.Service
```java
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;
}
```
其实分为force或者fail打头的类型，如果不是则直接使用对应<Interface>Mock的类型为Mock类（方便自定义相关返回以及降级）。
为了便于操作提供了force或者fail指令
```java
public static Object parseMockValue(String mock, Type[] returnTypes) throws Exception {
Object value = null;
if ("empty".equals(mock)) {
value = ReflectUtils.getEmptyObject(returnTypes != null && returnTypes.length > 0 ? (Class<?>)returnTypes[0] : null);
} else if ("null".equals(mock)) {
value = null;
} else if ("true".equals(mock)) {
value = true;
} else if ("false".equals(mock)) {
value = false;
} else if (mock.length() >=2 && (mock.startsWith("\"") && mock.endsWith("\"")
|| mock.startsWith("\'") && mock.endsWith("\'"))) {
value = mock.subSequence(1, mock.length() - 1);
} else if (returnTypes !=null && returnTypes.length >0 && returnTypes[0] == String.class) {
value = mock;
} else if (StringUtils.isNumeric(mock)) {
value = JSON.parse(mock);
}else if (mock.startsWith("{")) {
value = JSON.parse(mock, Map.class);
} else if (mock.startsWith("[")) {
value = JSON.parse(mock, List.class);
} else {
value = mock;
}
if (returnTypes != null && returnTypes.length > 0) {
value = PojoUtils.realize(value, (Class<?>)returnTypes[0], returnTypes.length > 1 ? returnTypes[1] : null);
}
return value;
}
```
mock可以返回false，null等，常用mock=fail:return null
那么就可以实现在一些不重要的服务挂掉的时候强制返回null，如果客户端对应兼容此操作自然可以在客户端不报错，继续正常的业务流程（如果精细化控制可以使用Mock类）
```java
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();
invoker = proxyFactory.getInvoker(mockObject, (Class<T>)serviceType, url);
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);
}
}
}
```
Mock类的调用也很简单，直接调用对应类型名的对应Interface的构造函数===》此处没有依赖注入
至此Mock部分的分析结束了。
[Dubbo_Stub]: https://my.oschina.net/qixiaobo025/blog/1014845
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