Spring Cloud Hystrix使用

@EnableCircuitBreaker注解

使用@EnableCircuitBreaker注解开启断路器功能,另外@SpringCloudApplication注解也包含@EnableCircuitBreaker注解，以下为@SpringCloudApplication注解的源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootApplication
@EnableDiscoveryClient
@EnableCircuitBreaker
public @interface SpringCloudApplication {

}

引入Spring Cloud Hystrix

<dependency>
     <groupId>org.springframework.cloud</groupId>
     <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
</dependency>

方法一：实现HystrixCommand

public class HelloHystrix extends HystrixCommand{
    @Autowired
    RestTemplate restTemplate;

    protected HelloHystrix(String threadKeyName, int timeout, int queueSize, int coreSize) {
        super(Setter.withGroupKey(
                //设置命令组
                HystrixCommandGroupKey.Factory.asKey("springHealthGrooup"))
                //设置命令键
                .andCommandKey(HystrixCommandKey.Factory.asKey("interventionKey"))
                //设置线程池键
                .andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey(threadKeyName))
                //设置命令属性
                .andCommandPropertiesDefaults(
                        HystrixCommandProperties.Setter()
                        .withExecutionTimeoutInMilliseconds(timeout)
                )
                .andThreadPoolPropertiesDefaults(
                        HystrixThreadPoolProperties.Setter()
                        .withMaxQueueSize(queueSize)
                        .withCoreSize(coreSize)
                )
        );
    }

    @Override
    protected String run() throws Exception {
        return restTemplate.getForEntity("http://provider/hello",String.class).getBody();
    }
}

上述代码中的 HystrixCommandGroupKey 和 HystrixCommandKey 分别用来配置服务分组名称和服务名称，然后 HystrixThreadPoolKey 用来配置线程池的名称。

可以使用同步或异步的方式进行调用

//同步执行
Object result = new HelloHystrix("hystrix-thread", 5000, 5, 2).execute();
//异步执行
Future future = new HelloHystrix("hystrix-thread", 5000, 5, 2).queue();

除了传统的同步执行与异步执行之外， 我们还可以将  通过Observable 来实现响应式执行方式。

Observable ho = new HelloHystrix("hystrix-thread", 5000, 5, 2).observe();
Observable co = new HelloHystrix("hystrix-thread", 5000, 5, 2).toObservable();

        observe ()和toObservable ()虽然都返回了 Observable, 但是它们略有不同，前者返回的是一个 Hot Observable, 该命令会在 observe ()调用的时候立即执行， 当Observable 每次被订阅的时候会重放它的行为；而后者返回的是 一个 Cold Observable, toObservable ()执行之后，命令不会被立即执行，只有当所有订阅者都订阅它之后才会执行。

方法二：使用@HystrixCommand注解

@HystrixCommand源码如下：

@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 "";
}

示例：

@Service
public class HelloService {

    @Autowired
    RestTemplate restTemplate;

    @HystrixCommand(threadPoolKey = "hystrixTest",
            threadPoolProperties = {
                    @HystrixProperty(name = "coreSize", value = "2"),
                    @HystrixProperty(name = "maxQueueSize", value = "10")
            },
            fallbackMethod = "helloFallback")
    public String helloService() {
        return restTemplate.getForEntity("http://provider/hello", String.class).getBody();
    }


    public String helloFallback() {
        return "error";
    }
}

@HystrixCommand用于指定一些应该作为 hystrix 命令处理的方法。该注解中的fallbackMethod属性用于指定熔断时的回调函数，用来实现服务的降级处理逻辑。

当provider服务不可用时，就会触发断路器，然后直接调用回调函数helloFallback函数

@HystrixCommand还提供了

@HystrixCommand(commandProperties = {

            @HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "12000"),

            //一个滑动窗口内最小的请求数
            @HystrixProperty(name = "circuitBreaker.requestVolumeThreshold", value = "10"),

            //错误比率阈值
            @HystrixProperty(name = "circuitBreaker.errorThresholdPercentage", value = "75"),

           //触发熔断的时间值
        @HystrixProperty(name = "circuitBreaker.sleepWindowInMilliseconds", value = "7000"),

            //一个滑动窗口的时间长度
           @HystrixProperty(name = "metrics.rollingStats.timeInMilliseconds", value = "15000"),

            //一个滑动窗口被划分的数量
            @HystrixProperty(name = "metrics.rollingStats.numBuckets", value = "5") })

hystrix实现原理可参考：

Hystrix实现原理_skh2015java的博客-优快云博客https://skaygo.blog.youkuaiyun.com/article/details/120750565