调用Fallback有以下几种情况：
- 熔断器开启
- 信号量拒绝
- 线程池拒绝
- 执行方法失败
hystrix - @EnableCircuitBreaker那些事我们知道最终会调用HystrixCommand的execute方法，他这个方法就会调用queue方法。
public R execute() {
try {
return queue().get();
} catch (Exception e) {
throw Exceptions.sneakyThrow(decomposeException(e));
}
}queue方法如下，这里是拿到一个Future对象。重点还是toObservable方法。
public Future queue() {
// 其他略
final Future delegate = toObservable().toBlocking().toFuture();
// 其他略
} 这里会定义Observable，他会判断是否要从缓存取值，如果没有，afterCache就取applyHystrixSemantics，所以后面就会调用applyHystrixSemantics方法。
public Observable toObservable() {
//其他略
final Func0> applyHystrixSemantics = new Func0>() {
@Override
public Observable call() {
if (commandState.get().equals(CommandState.UNSUBSCRIBED)) {
return Observable.never();
}
return applyHystrixSemantics(_cmd);
}
};
//其他略
return Observable.defer(new Func0>() {
@Override
public Observable call() {
//其他略
final boolean requestCacheEnabled = isRequestCachingEnabled();
final String cacheKey = getCacheKey();
/* try from cache first */
if (requestCacheEnabled) {
HystrixCommandResponseFromCache fromCache = (HystrixCommandResponseFromCache) requestCache.get(cacheKey);
if (fromCache != null) {
isResponseFromCache = true;
return handleRequestCacheHitAndEmitValues(fromCache, _cmd);
}
}
Observable hystrixObservable =
Observable.defer(applyHystrixSemantics)
.map(wrapWithAllOnNextHooks);
Observable afterCache;
// put in cache
if (requestCacheEnabled && cacheKey != null) {
// wrap it for caching
HystrixCachedObservable toCache = HystrixCachedObservable.from(hystrixObservable, _cmd);
HystrixCommandResponseFromCache fromCache = (HystrixCommandResponseFromCache) requestCache.putIfAbsent(cacheKey, toCache);
if (fromCache != null) {
// another thread beat us so we'll use the cached value instead
toCache.unsubscribe();
isResponseFromCache = true;
return handleRequestCacheHitAndEmitValues(fromCache, _cmd);
} else {
// we just created an ObservableCommand so we cast and return it
afterCache = toCache.toObservable();
}
} else {
afterCache = hystrixObservable;
}
return afterCache
.doOnTerminate(terminateCommandCleanup) // perform cleanup once (either on normal terminal state (this line), or unsubscribe (next line))
.doOnUnsubscribe(unsubscribeCommandCleanup) // perform cleanup once
.doOnCompleted(fireOnCompletedHook);
}
});
} 熔断判断
这里是比较重要的代码，首先判断是否开启熔断，如果开启了，就调用handleShortCircuitViaFallback方法，如果没有开启熔断，他还会去判断是否能请求到信号量，请求不到就调用handleSemaphoreRejectionViaFallback方法。如果都正常，就调用executeCommandAndObserve方法。
private Observable applyHystrixSemantics(final AbstractCommand _cmd) {
//其他略
if (circuitBreaker.allowRequest()) {
//其他略
if (executionSemaphore.tryAcquire()) {
try {
/* used to track userThreadExecutionTime */
executionResult = executionResult.setInvocationStartTime(System.currentTimeMillis());
return executeCommandAndObserve(_cmd)
.doOnError(markExceptionThrown)
.doOnTerminate(singleSemaphoreRelease)
.doOnUnsubscribe(singleSemaphoreRelease);
} catch (RuntimeException e) {
return Observable.error(e);
}
} else {
return handleSemaphoreRejectionViaFallback();
}
} else {
//
return handleShortCircuitViaFallback();
}
} 熔断开启
我们先看看熔断开启后的调用，会通过getFallbackObservable方法获取fallbackExecutionChain，getFallbackObservable主要的作用就是调用getFallback方法。
private Observable handleShortCircuitViaFallback() {
// 其他略
return getFallbackOrThrowException(this, HystrixEventType.SHORT_CIRCUITED, FailureType.SHORTCIRCUIT,
"short-circuited", shortCircuitException);
// 其他略
}
private Observable getFallbackOrThrowException(final AbstractCommand _cmd, final HystrixEventType eventType, final FailureType failureType, final String message, final Exception originalException) {
// 其他略
fallbackExecutionChain = getFallbackObservable();
// 其他略
return fallbackExecutionChain
.doOnEach(setRequestContext)
.lift(new FallbackHookApplication(_cmd))
.lift(new DeprecatedOnFallbackHookApplication(_cmd))
.doOnNext(markFallbackEmit)
.doOnCompleted(markFallbackCompleted)
.onErrorResumeNext(handleFallbackError)
.doOnTerminate(singleSemaphoreRelease)
.doOnUnsubscribe(singleSemaphoreRelease);
}
// 其他略
}
@Override
final protected Observable getFallbackObservable() {
return Observable.defer(new Func0>() {
@Override
public Observable call() {
try {
return Observable.just(getFallback());
} catch (Throwable ex) {
return Observable.error(ex);
}
}
});
} getFallback是调用fallback方法的入口，MethodExecutionAction会通过发射，调用我们配置的方法。
@Override
protected Object getFallback() {
final CommandAction commandAction = getFallbackAction();
if (commandAction != null) {
try {
return process(new Action() {
@Override
Object execute() {
MetaHolder metaHolder = commandAction.getMetaHolder();
Object[] args = createArgsForFallback(metaHolder, getExecutionException());
return commandAction.executeWithArgs(metaHolder.getFallbackExecutionType(), args);
}
});
} catch (Throwable e) {
LOGGER.error(FallbackErrorMessageBuilder.create()
.append(commandAction, e).build());
throw new FallbackInvocationException(unwrapCause(e));
}
} else {
return super.getFallback();
}
}信号量隔离
我们看了上面熔断开启的代码，这边就比较简单，就是跟上面一样，调用getFallbackOrThrowException方法。
private Observable handleSemaphoreRejectionViaFallback() {
// 其他略
return getFallbackOrThrowException(this, HystrixEventType.SEMAPHORE_REJECTED, FailureType.REJECTED_SEMAPHORE_EXECUTION,
"could not acquire a semaphore for execution", semaphoreRejectionException);
} 线程池隔离
如果熔断没开，信号量又可以获取到，他就会调用executeCommandAndObserve方法。这个方法，handleFallback定义了几个异常，比如线程池异常处理，时间超时处理，请求异常处理，以及其他异常处理。然后调用executeCommandWithSpecifiedIsolation方法。
private Observable executeCommandAndObserve(final AbstractCommand _cmd) {
//其他略
final Func1> handleFallback = new Func1>() {
@Override
public Observable call(Throwable t) {
Exception e = getExceptionFromThrowable(t);
executionResult = executionResult.setExecutionException(e);
if (e instanceof RejectedExecutionException) {
return handleThreadPoolRejectionViaFallback(e);
} else if (t instanceof HystrixTimeoutException) {
return handleTimeoutViaFallback();
} else if (t instanceof HystrixBadRequestException) {
return handleBadRequestByEmittingError(e);
} else {
/*
* Treat HystrixBadRequestException from ExecutionHook like a plain HystrixBadRequestException.
*/
if (e instanceof HystrixBadRequestException) {
eventNotifier.markEvent(HystrixEventType.BAD_REQUEST, commandKey);
return Observable.error(e);
}
return handleFailureViaFallback(e);
}
}
};
//其他略
Observable execution;
if (properties.executionTimeoutEnabled().get()) {
execution = executeCommandWithSpecifiedIsolation(_cmd)
.lift(new HystrixObservableTimeoutOperator(_cmd));
} else {
execution = executeCommandWithSpecifiedIsolation(_cmd);
}
return execution.doOnNext(markEmits)
.doOnCompleted(markOnCompleted)
.onErrorResumeNext(handleFallback)
.doOnEach(setRequestContext);
} 这个方法主要是两件事情，一个是subscribeOn方法，这里是线程池隔离用的，另外一个就是正常情况下的调用，调用的是getUserExecutionObservable方法，这个方法在线程池后面讲。
private Observable executeCommandWithSpecifiedIsolation(final AbstractCommand _cmd) {
if (properties.executionIsolationStrategy().get() == ExecutionIsolationStrategy.THREAD) {
// mark that we are executing in a thread (even if we end up being rejected we still were a THREAD execution and not SEMAPHORE)
return Observable.defer(new Func0>() {
@Override
public Observable call() {
// 其他略
return getUserExecutionObservable(_cmd);
// 其他略
}
})// 其他略
}).subscribeOn(threadPool.getScheduler(new Func0() {
@Override
public Boolean call() {
return properties.executionIsolationThreadInterruptOnTimeout().get() && _cmd.isCommandTimedOut.get() == TimedOutStatus.TIMED_OUT;
}
}));
}
// 其他略
} 创建相关类
public Scheduler getScheduler(Func0 shouldInterruptThread) {
// 动态配置线程池信息
touchConfig();
// 创建一个HystrixContextScheduler对象，这里又会创建一个ThreadPoolScheduler
return new HystrixContextScheduler(HystrixPlugins.getInstance().getConcurrencyStrategy(), this, shouldInterruptThread);
}
这里创建一个HystrixContextScheduler对象，ThreadPoolScheduler对象用于创建ThreadPoolWorker，并赋值给HystrixContextSchedulerWorker
public Scheduler getScheduler(Func0 shouldInterruptThread) {
// 动态配置线程池信息
touchConfig();
// 创建一个HystrixContextScheduler对象，这里又会创建一个ThreadPoolScheduler
return new HystrixContextScheduler(HystrixPlugins.getInstance().getConcurrencyStrategy(), this, shouldInterruptThread);
}
public Worker createWorker() {
return new HystrixContextSchedulerWorker(actualScheduler.createWorker());
} 线程池判断的地方在HystrixContextSchedulerWorker的schedule方法，不够就抛RejectedExecutionException异常，异常的捕获上面已经讲了。
public Subscription schedule(Action0 action) {
if (threadPool != null) {
// 判断线程池线程的地方
if (!threadPool.isQueueSpaceAvailable()) {
throw new RejectedExecutionException("Rejected command because thread-pool queueSize is at rejection threshold.");
}
}
return worker.schedule(new HystrixContexSchedulerAction(concurrencyStrategy, action));
}正常调用
如果上面都正常，就会调用getUserExecutionObservable方法。这个方法最后会调用run方法。
private Observable getUserExecutionObservable(final AbstractCommand _cmd) {
//其他略
userObservable = getExecutionObservable();
//其他略
}
@Override
final protected Observable getExecutionObservable() {
return Observable.defer(new Func0>() {
@Override
public Observable call() {
try {
return Observable.just(run());
} catch (Throwable ex) {
return Observable.error(ex);
}
}
})//其他略
} 在这里，同样通过反射，调用到我们的方法。
protected Object run() throws Exception {
LOGGER.debug("execute command: {}", getCommandKey().name());
return process(new Action() {
@Override
Object execute() {
return getCommandAction().execute(getExecutionType());
}
});
}总结
上面讲了几个fallback调用的方法，以及正常的流程，流程图如下