使用Spring Cloud时绕不开Hystrix，他帮助微服务实现断路器功能。该框架的目标在于通过控制那些访问远程系统、服务和第三方库的节点，从而对延迟和故障提供更强大的容错能力。Hystrix具备服务降级，服务熔断，线程和信号隔离，请求缓存，请求合并以及服务监控等强大功能。
关于Hystrix的介绍请参见：http://www.sohu.com/a/131568369_494947，本文部分介绍引自此文，建议先阅读此文了解Hystrix的使用场景和线程池概念。
@HystrixCommand的基础介绍请参见：http://blog.csdn.net/forezp/article/details/69934399，介绍的很详细，我这里主要说一说这个注解的各个参数的使用。
查看com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand的源代码如下：
/**
* Copyright 2012 Netflix, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.netflix.hystrix.contrib.javanica.annotation; import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target; /**
* This annotation used to specify some methods which should be processes as hystrix commands.
*/
@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface HystrixCommand { /**
* The command group key is used for grouping together commands such as for reporting,
* alerting, dashboards or team/library ownership.
* <p/>
* default => the runtime class name of annotated method
*
* @return group key
*/
String groupKey() default ""; /**
* Hystrix command key.
* <p/>
* default => the name of annotated method. for example:
* <code>
* ...
* @HystrixCommand
* public User getUserById(...)
* ...
* the command name will be: 'getUserById'
* </code>
*
* @return command key
*/
String commandKey() default ""; /**
* The thread-pool key is used to represent a
* HystrixThreadPool for monitoring, metrics publishing, caching and other such uses.
*
* @return thread pool key
*/
String threadPoolKey() default ""; /**
* Specifies a method to process fallback logic.
* A fallback method should be defined in the same class where is HystrixCommand.
* Also a fallback method should have same signature to a method which was invoked as hystrix command.
* for example:
* <code>
* @HystrixCommand(fallbackMethod = "getByIdFallback")
* public String getById(String id) {...}
*
* private String getByIdFallback(String id) {...}
* </code>
* Also a fallback method can be annotated with {@link HystrixCommand}
* <p/>
* default => see {@link com.netflix.hystrix.contrib.javanica.command.GenericCommand#getFallback()}
*
* @return method name
*/
String fallbackMethod() default ""; /**
* Specifies command properties.
*
* @return command properties
*/
HystrixProperty[] commandProperties() default {}; /**
* Specifies thread pool properties.
*
* @return thread pool properties
*/
HystrixProperty[] threadPoolProperties() default {}; /**
* Defines exceptions which should be ignored.
* Optionally these can be wrapped in HystrixRuntimeException if raiseHystrixExceptions contains RUNTIME_EXCEPTION.
*
* @return exceptions to ignore
*/
Class<? extends Throwable>[] ignoreExceptions() default {}; /**
* Specifies the mode that should be used to execute hystrix observable command.
* For more information see {@link ObservableExecutionMode}.
*
* @return observable execution mode
*/
ObservableExecutionMode observableExecutionMode() default ObservableExecutionMode.EAGER; /**
* When includes RUNTIME_EXCEPTION, any exceptions that are not ignored are wrapped in HystrixRuntimeException.
*
* @return exceptions to wrap
*/
HystrixException[] raiseHystrixExceptions() default {}; /**
* Specifies default fallback method for the command. If both {@link #fallbackMethod} and {@link #defaultFallback}
* methods are specified then specific one is used.
* note: default fallback method cannot have parameters, return type should be compatible with command return type.
*
* @return the name of default fallback method
*/
String defaultFallback() default "";
}
让我们来看一下这个注解的简单应用：
package com.example.demo.service; import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate; import com.netflix.hystrix.contrib.javanica.annotation.HystrixProperty;
import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand; @Service
public class ConsumerService { @Autowired
private RestTemplate restTemplate; @HystrixCommand(commandKey = "testCommand", groupKey = "testGroup", threadPoolKey = "testThreadKey",
fallbackMethod = "hiConsumerFallBack", ignoreExceptions = {NullPointerException.class},
threadPoolProperties = {
@HystrixProperty(name = "coreSize", value = "30"),
@HystrixProperty(name = "maxQueueSize", value = "101"),
@HystrixProperty(name = "keepAliveTimeMinutes", value = "2"),
@HystrixProperty(name = "queueSizeRejectionThreshold", value = "15"),
@HystrixProperty(name = "metrics.rollingStats.numBuckets", value = "12"),
@HystrixProperty(name = "metrics.rollingStats.timeInMilliseconds", value = "1440")
}
)
public String hiConsumer(String id) { //SERVICE_HI是服务端的spring.application.name，并且大写，hi为服务端提供的接口
return restTemplate.getForEntity("http://SERVICE_HI/hi", String.class).getBody();
} public String hiConsumerFallBack(String id, Throwable e) {
return "This is a error";
} }
让我们来逐个介绍下@HystrixCommand注解的各个参数：
1：commandKey：配置全局唯一标识服务的名称，比如，库存系统有一个获取库存服务，那么就可以为这个服务起一个名字来唯一识别该服务，如果不配置，则默认是@HystrixCommand注解修饰的函数的函数名。
2：groupKey：一个比较重要的注解，配置全局唯一标识服务分组的名称，比如，库存系统就是一个服务分组。通过设置分组，Hystrix会根据组来组织和统计命令的告、仪表盘等信息。Hystrix命令默认的线程划分也是根据命令组来实现。默认情况下，Hystrix会让相同组名的命令使用同一个线程池，所以我们需要在创建Hystrix命令时为其指定命令组来实现默认的线程池划分。此外，Hystrix还提供了通过设置threadPoolKey来对线程池进行设置。建议最好设置该参数，使用threadPoolKey来控制线程池组。
3：threadPoolKey：对线程池进行设定，细粒度的配置，相当于对单个服务的线程池信息进行设置，也可多个服务设置同一个threadPoolKey构成线程组。
4：fallbackMethod：@HystrixCommand注解修饰的函数的回调函数，@HystrixCommand修饰的函数必须和这个回调函数定义在同一个类中，因为定义在了同一个类中，所以fackback method可以是public/private均可。
5：commandProperties：配置该命令的一些参数，如executionIsolationStrategy配置执行隔离策略，默认是使用线程隔离，此处我们配置为THREAD，即线程池隔离。参见：com.netflix.hystrix.HystrixCommandProperties中各个参数的定义。
6：threadPoolProperties：线程池相关参数设置，具体可以设置哪些参数请见：com.netflix.hystrix.HystrixThreadPoolProperties
7：ignoreExceptions：调用服务时，除了HystrixBadRequestException之外，其他@HystrixCommand修饰的函数抛出的异常均会被Hystrix认为命令执行失败而触发服务降级的处理逻辑（调用fallbackMethod指定的回调函数），所以当需要在命令执行中抛出不触发降级的异常时来使用它，通过这个参数指定，哪些异常抛出时不触发降级（不去调用fallbackMethod），而是将异常向上抛出。
8：observableExecutionMode：定义hystrix observable command的模式；
9：raiseHystrixExceptions：任何不可忽略的异常都包含在HystrixRuntimeException中；
10：defaultFallback：默认的回调函数，该函数的函数体不能有入参，返回值类型与@HystrixCommand修饰的函数体的返回值一致。如果指定了fallbackMethod，则fallbackMethod优先级更高。
请求缓存功能：
|
注解
|
描述
|
属性
|
|
@CacheResult
|
该注解用来标记请求命令返回的结果应该被缓存，它必须与@HystrixCommand注解结合使用
|
cacheKeyMethod
|
|
@CacheRemove
|
该注解用来让请求命令的缓存失效，失效的缓存根据定义Key决定
|
commandKey,
cacheKeyMethod
|
|
@CacheKey
|
该注解用来在请求命令的参数上标记，使其作为缓存的Key值，如果没有标注则会使用所有参数。如果同事还是使用了@CacheResult和@CacheRemove注解的cacheKeyMethod方法指定缓存Key的生成，那么该注解将不会起作用
|
value
|
开启请求缓存功能：
开启请求缓存功能需要使用注解@CacheResult，该注解源码如下：
/**
* Copyright 2015 Netflix, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.netflix.hystrix.contrib.javanica.cache.annotation; import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target; /**
* Marks a methods that results should be cached for a Hystrix command.
* This annotation must be used in conjunction with {@link com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand} annotation.
*
* @author dmgcodevil
*/
@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface CacheResult { /**
* Method name to be used to get a key for request caching.
* The command and cache key method should be placed in the same class and have same method signature except
* cache key method return type, that should be <code>String</code>.
* <p/>
* cacheKeyMethod has higher priority than an arguments of a method, that means what actual arguments
* of a method that annotated with {@link CacheResult} will not be used to generate cache key, instead specified
* cacheKeyMethod fully assigns to itself responsibility for cache key generation.
* By default this returns empty string which means "do not use cache method".
*
* @return method name or empty string
*/
String cacheKeyMethod() default "";
}
从类签名上看，该注解标记结果需要被缓存，并且这个注解需要结合@HystrixCommand注解一起使用。该注解有一个参数cacheKeyMethod，用来指定获取cacheKey的方法名。Hystrix会根据获取到的cacheKey值来区分是否是重复请求，如果他们的cacheKey相同，那么该依赖服务只会在第一个请求到达时被真实的调用一次，另外一个请求则是直接从请求缓存中根据这个cacheKey来获取到的结果。所以开启请求缓存可以让我们实现的Hystrix命令具备下面几项好处：
- 减少重复的请求数，降低依赖服务的并发度；
- 在同一个用户请求的上下文，想同依赖服务的返回数据始终保持一致。
- 请求缓存在run()和contruct()执行之前生效，所以可以有效减少不必要的线程开销；
使用@CacheResult开启缓存代码示例：
package com.example.demo; import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate; import com.netflix.hystrix.contrib.javanica.cache.annotation.CacheResult;
import com.example.demo.entity.User;
import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand; @Service
public class CacheResultDemo { @Autowired
private RestTemplate restTemplate; @CacheResult(cacheKeyMethod = "getUserId")
@HystrixCommand(fallbackMethod = "hiConsumerFallBack")
public User hiConsumer(String id) { //SERVICE_HI是服务端的spring.application.name，并且大写，hi为服务端提供的接口
return restTemplate.getForEntity("http://SERVICE_HI/hi", User.class).getBody();
} public String hiConsumerFallBack(String id, Throwable e) {
return "This is a error";
} public String getUserId(String id) {
return id;
} }
使用CacheKey开启缓存代码示例：
package com.example.demo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;
importcom.netflix.hystrix.contrib.javanica.cache.annotation.CacheKey;
import com.example.demo.entity.User;
importcom.netflix.hystrix.contrib.javanica.annotation.HystrixCommand;
@Service
public class CacheKeyDemo {
@Autowired
private RestTemplate restTemplate;
@HystrixCommand(fallbackMethod = "hiConsumerFallBack")
public User hiConsumer(@CacheKey("id") String id) { //SERVICE_HI是服务端的spring.application.name，并且大写，hi为服务端提供的接口
return restTemplate.getForEntity("http://SERVICE_HI/hi", User.class).getBody();
} public String hiConsumerFallBack(String id, Throwable e) {
return "This is a error";
}
}
其中@CacheKey除了可以指定方法参数为缓存key之外，还可以指定对象中的属性作为缓存Key，如下：
package com.example.demo; import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate; import com.netflix.hystrix.contrib.javanica.cache.annotation.CacheKey;
import com.example.demo.entity.User;
import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand; @Service
public class CacheKeyDemo2 { @Autowired
private RestTemplate restTemplate; @HystrixCommand(fallbackMethod = "hiConsumerFallBack")
public User hiConsumer(@CacheKey("id") User user) { //SERVICE_HI是服务端的spring.application.name，并且大写，hi为服务端提供的接口
return restTemplate.getForEntity("http://SERVICE_HI/hi", User.class, user.getId()).getBody();
} public String hiConsumerFallBack(String id, Throwable e) {
return "This is a error";
} }
清理失效缓存功能：
使用请求缓存时，如果只是杜操作，那么不需要考虑缓存中的内容是否正确的问题，但是如果请求命令中还有更新数据的写操作，那么缓存中的数据就需要我们在进行写操作时进行及时处理，以防止读操作的请求命令获取到了失效的数据。
通过@CacheRemove注解来实现失效缓存清理功能：
package com.example.demo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;
importcom.netflix.hystrix.contrib.javanica.cache.annotation.CacheKey;
importcom.netflix.hystrix.contrib.javanica.cache.annotation.CacheRemove;
import com.example.demo.entity.User;
importcom.netflix.hystrix.contrib.javanica.annotation.HystrixCommand;
@Service
public class CacheRemoveDemo {
@Autowired
private RestTemplate restTemplate;
@CacheRemove(commandKey = "getUserId")
@HystrixCommand(fallbackMethod = "hiConsumerFallBack")
public void update(@CacheKey("id") User user) { //SERVICE_HI是服务端的spring.application.name，并且大写，hi为服务端提供的接口
restTemplate.postForObject("http://SERVICE_HI/hi", user, User.class);
return;
} public String hiConsumerFallBack(String id, Throwable e) {
return "This is a error";
} public String getUserId(String id) {
return id;
}
}