回顾
为了防止服务之间的调用异常造成的连锁反应，在SpringCloud中提供了Hystrix组件来实现服务调用异常的处理，或对高并发情况下的服务降级处理 。简单回顾一下Hystrix的使用：
1.要使用 Hystrix熔断机制处理引入它本身的依赖之外，我们需要在主程序配置类上贴 @EnableHystrix 标签 开启Hystrix功能，如下
@EnableHystrix
@EnableEurekaClient
@SpringBootApplication
...
public class ConsumerApplication {
2.开启Hystrix熔断机制后，对方法进行熔断处理
@Service
public class HelloService {
@Autowired
private RestTemplate restTemplate;
//该注解对该方法创建了熔断器的功能，并指定了fallbackMethod熔断方法
@HystrixCommand(fallbackMethod = "hiError")
public String hiService(String name){
//调用接口进行消费
String result = restTemplate.getForObject("http://PRODUCER/hello?name="+name,String.class);
return result;
}
public String hiError(String name) {
return "hi,"+name+"error!";
}
}
当hiService方法第调用异常，会触发 fallbackMethod执行的hiError方法做成一些补救处理。
那么我们就沿着我们的使用方式来跟踪一下 Hystrix的 工作原理。
首先我们看一下标签：@EnableHystrix ，他的作用从名字就能看出就是开启Hystrix ，我们看一下它的源码
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@EnableCircuitBreaker
public @interface EnableHystrix {
}
它上面有一个注解：@ EnableCircuitBreaker ，翻译单词意思就是启用熔断器(断路器),那么@ EnableHystrix标签的本质其实是@ EnableCircuitBreaker ,我们看一下他的源码
/**
* Annotation to enable a CircuitBreaker implementation.
* http://martinfowler.com/bliki/CircuitBreaker.html
* @author Spencer Gibb
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(EnableCircuitBreakerImportSelector.class)
public @interface EnableCircuitBreaker {
}
@EnableCircuitBreaker标签引入了一个@Import(EnableCircuitBreakerImportSelector.class) 类，翻译类的名字就是 ， 开启熔断器的导入选择器 ，导入什么东西呢？看源码
/**
* Import a single circuit breaker implementation Configuration
* @author Spencer Gibb
*/
@Order(Ordered.LOWEST_PRECEDENCE - 100)
public class EnableCircuitBreakerImportSelector extends
SpringFactoryImportSelector<enablecircuitbreaker> {
@Override
protected boolean isEnabled() {
return getEnvironment().getProperty(
"spring.cloud.circuit.breaker.enabled", Boolean.class, Boolean.TRUE);
}
}
/<enablecircuitbreaker>
翻译类上的注释 “Import a single circuit breaker implementation Configuration”，其实EnableCircuitBreakerImportSelector的作用就是去导入熔断器的配置 。其实Spring中也有类似于JAVA SPI 的加载机制, 即会自动加载 jar包 spring-cloud-netflix-core 中的META-INF/spring.factories 中的Hystrix相关的自动配置类
注：SPI : 通过将服务的接口与实现分离以实现解耦，提高程序拓展性的机制，达到插拔式的效果 。
HystrixCircuitBreakerConfiguration 就是针对于 Hystrix熔断器的配置
/**
* @author Spencer Gibb
* @author Christian Dupuis
* @author Venil Noronha
*/
@Configuration
public class HystrixCircuitBreakerConfiguration {
@Bean
public HystrixCommandAspect hystrixCommandAspect() {
return new HystrixCommandAspect();
}
@Bean
public HystrixShutdownHook hystrixShutdownHook() {
return new HystrixShutdownHook();
}
@Bean
public HasFeatures hystrixFeature() {
return HasFeatures.namedFeatures(new NamedFeature("Hystrix", HystrixCommandAspect.class));
}
......
在该配置类中创建了 HystrixCommandAspect
/**
* AspectJ aspect to process methods which annotated with {@link HystrixCommand} annotation.
*/
@Aspect
public class HystrixCommandAspect {
private static final Map<hystrixpointcuttype> META_HOLDER_FACTORY_MAP;
static {
META_HOLDER_FACTORY_MAP = ImmutableMap.<hystrixpointcuttype>builder()
.put(HystrixPointcutType.COMMAND, new CommandMetaHolderFactory())
.put(HystrixPointcutType.COLLAPSER, new CollapserMetaHolderFactory())
.build();
}
//定义切点，切到 @HystrixCommand标签所在的方法
@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")
public void hystrixCommandAnnotationPointcut() {
}
@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCollapser)")
public void hystrixCollapserAnnotationPointcut() {
}
//针对切点：@hystrixCommand切点的处理
@Around("hystrixCommandAnnotationPointcut() || hystrixCollapserAnnotationPointcut()")
public Object methodsAnnotatedWithHystrixCommand(final ProceedingJoinPoint joinPoint) throws Throwable {
//获取到目标方法
Method method = getMethodFromTarget(joinPoint);
Validate.notNull(method, "failed to get method from joinPoint: %s", joinPoint);
//判断方法上不能同时存在@HystrixCommand标签和HystrixCollapser标签
if (method.isAnnotationPresent(HystrixCommand.class) && method.isAnnotationPresent(HystrixCollapser.class)) {
throw new IllegalStateException("method cannot be annotated with HystrixCommand and HystrixCollapser " +
"annotations at the same time");
}
MetaHolderFactory metaHolderFactory = META_HOLDER_FACTORY_MAP.get(HystrixPointcutType.of(method));
MetaHolder metaHolder = metaHolderFactory.create(joinPoint);
//把方法封装成 HystrixInvokable
HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);
ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?
metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();
Object result;
try {
// 通过CommandExecutor来执行方法
if (!metaHolder.isObservable()) {
result = CommandExecutor.execute(invokable, executionType, metaHolder);
} else {
result = executeObservable(invokable, executionType, metaHolder);
}
} catch (HystrixBadRequestException e) {
throw e.getCause() != null ? e.getCause() : e;
} catch (HystrixRuntimeException e) {
throw hystrixRuntimeExceptionToThrowable(metaHolder, e);
}
return result;
/<hystrixpointcuttype>/<hystrixpointcuttype>
HystrixCommandAspect 其实就是对 贴了@HystrixCommand标签的方法使用 Aop机制实现处理 。代码中通过把目标方法封装成 HystrixInvokable对象，通过CommandExecutor工具来执行目标方法。
HystrixInvokable是用来干嘛的？看源码知道，其实他是一个空行法的接口，他的目的只是用来标记可被执行，那么他是如何创建的我们看代码HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);的create方法
public HystrixInvokable create(MetaHolder metaHolder) {
HystrixInvokable executable;
...省略代码...
executable = new GenericCommand(HystrixCommandBuilderFactory.getInstance().create(metaHolder));
}
return executable;
}
其实是new了一个 GenericCommand 对象，很明显他们是实现关系，我们看一下关系图
跟踪 GenericCommand 的源码
@ThreadSafe
public class GenericCommand extends AbstractHystrixCommand<object> {
private static final Logger LOGGER = LoggerFactory.getLogger(GenericCommand.class);
public GenericCommand(HystrixCommandBuilder builder) {
super(builder);
}
protected Object run() throws Exception {
LOGGER.debug("execute command: {}", this.getCommandKey().name());
return this.process(new AbstractHystrixCommand<object>.Action() {
Object execute() {
return GenericCommand.this.getCommandAction().execute(GenericCommand.this.getExecutionType());
}
});
}
protected Object getFallback() {
final CommandAction commandAction = this.getFallbackAction();
if (commandAction != null) {
try {
return this.process(new AbstractHystrixCommand<object>.Action() {
Object execute() {
MetaHolder metaHolder = commandAction.getMetaHolder();
Object[] args = CommonUtils.createArgsForFallback(metaHolder, GenericCommand.this.getExecutionException());
return commandAction.executeWithArgs(metaHolder.getFallbackExecutionType(), args);
}
});
} catch (Throwable var3) {
LOGGER.error(FallbackErrorMessageBuilder.create().append(commandAction, var3).build());
throw new FallbackInvocationException(ExceptionUtils.unwrapCause(var3));
}
} else {
return super.getFallback();
}
}
}
/<object>/<object>/<object>
它本身对目标方法的正常执行和对 fallback方法的 执行做了实现 。
GenericCommand.this.getCommandAction().execute(...)获取到目标方法并执行，底层会交给 MethodExecutionAction 使用反射去执行方法，
回到 HystrixCommandAspect的methodsAnnotatedWithHystrixCommand方法中，我们看下 CommandExecutor.execute是如何执行的
public class CommandExecutor {
public CommandExecutor() {
}
public static Object execute(HystrixInvokable invokable, ExecutionType executionType, MetaHolder metaHolder) throws RuntimeException {
Validate.notNull(invokable);
Validate.notNull(metaHolder);
switch(executionType) {
//异步
case SYNCHRONOUS:
return castToExecutable(invokable, executionType).execute();
//同步
case ASYNCHRONOUS:
HystrixExecutable executable = castToExecutable(invokable, executionType);
if (metaHolder.hasFallbackMethodCommand() && ExecutionType.ASYNCHRONOUS == metaHolder.getFallbackExecutionType()) {
return new FutureDecorator(executable.queue());
}
return executable.queue();
case OBSERVABLE:
HystrixObservable observable = castToObservable(invokable);
return ObservableExecutionMode.EAGER == metaHolder.getObservableExecutionMode() ? observable.observe() : observable.toObservable();
default:
throw new RuntimeException("unsupported execution type: " + executionType);
}
}
private static HystrixExecutable castToExecutable(HystrixInvokable invokable, ExecutionType executionType) {
if (invokable instanceof HystrixExecutable) {
return (HystrixExecutable)invokable;
} else {
throw new RuntimeException("Command should implement " + HystrixExecutable.class.getCanonicalName() + " interface to execute in: " + executionType + " mode");
}
}
这里有两种执行方式 SYNCHRONOUS 异步 ，ASYNCHRONOUS同步 ，我们先看异步： castToExecutable(invokable, executionType).execute(); 这里代码把HystrixInvokable对象转成 HystrixExecutable并调用execute方法执行 ，跟踪execute方法进入HystrixCommand.execute方法中
public R execute() {
try {
return queue().get();
} catch (Exception e) {
throw Exceptions.sneakyThrow(decomposeException(e));
}
}
--------------
public Future queue() {
/*
* The Future returned by Observable.toBlocking().toFuture() does not implement the
* interruption of the execution thread when the "mayInterrupt" flag of Future.cancel(boolean) is set to true;
* thus, to comply with the contract of Future, we must wrap around it.
*/
final Future delegate = toObservable().toBlocking().toFuture();
final Future f = new Future() {
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
if (delegate.isCancelled()) {
return false;
}
if (HystrixCommand.this.getProperties().executionIsolationThreadInterruptOnFutureCancel().get()) {
/*
* The only valid transition here is false -> true. If there are two futures, say f1 and f2, created by this command
* (which is super-weird, but has never been prohibited), and calls to f1.cancel(true) and to f2.cancel(false) are
* issued by different threads, it's unclear about what value would be used by the time mayInterruptOnCancel is checked.
* The most consistent way to deal with this scenario is to say that if *any* cancellation is invoked with interruption,
* than that interruption request cannot be taken back.
*/
interruptOnFutureCancel.compareAndSet(false, mayInterruptIfRunning);
}
final boolean res = delegate.cancel(interruptOnFutureCancel.get());
if (!isExecutionComplete() && interruptOnFutureCancel.get()) {
final Thread t = executionThread.get();
if (t != null && !t.equals(Thread.currentThread())) {
t.interrupt();
}
}
return res;
}
....省略...
在 HystrixCommand.execute方法中 其实是Future 来异步执行，调用过程中会触发 GenericCommand来完成调用，执行完成后调用 Future.get()方法拿到执行结果 。
分享到:
閱讀更多 叩丁狼stef 的文章
