Spring Cloud @HystrixCommand和@CacheResult注解使用，参数配置

　　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：

1. 减少重复的请求数，降低依赖服务的并发度；
2. 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。
3. 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;
      }
}