Hystrix (容错，回退，降级，缓存)

Hystrix熔断机制就像家里的保险丝一样，若同时使用高功率的电器，就会烧坏电路，这时候保险丝自动断开就有效的保护了电路。而我们程序中也同样是这样。例如若此时数据库压力太大速度很慢，此时还有不断的请求访问后台，就会造成数据库崩溃。这时候hystrix容错机制，可以为客户端请求设置超时链接，添加回退的逻辑，减少集群压力。

1.1 导依赖

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.8.RELEASE</version> 
        <relativePath/>
    </parent>
    <!-- springCloud -->
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>Dalston.SR3</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>
    
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <!-- eureka客户端依赖 -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-eureka</artifactId>
        </dependency>
        <!-- hystrix -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-hystrix</artifactId>
        </dependency>

1.2 配置application.yml

# 指定端口
server:
  port: 8085
  context-path: /demo
# 服务名称
spring:
  application:
    name: hystrix

# eureka服务器地址
eureka:
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka/

1.3 配置Hystrix过滤器

import java.io.IOException; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.annotation.WebFilter; import com.netflix.hystrix.strategy.concurrency.HystrixRequestContext; /** * urlPatterns:拦截所有路径(拦截规则) * filterName:过滤器名称 */ @WebFilter(urlPatterns = "/*", filterName = "hystrixFilter") public class HystrixFilterConf implements Filter{ public void destroy() { } public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { // 初始化Hystrix上下文
        HystrixRequestContext ctx = HystrixRequestContext.initializeContext(); try { chain.doFilter(request, response); } catch (Exception e) { } finally { ctx.shutdown(); } } public void init(FilterConfig arg0) throws ServletException { } }

1.4 主函数入口

import org.springframework.boot.autoconfigure.SpringBootApplication; import org.springframework.boot.builder.SpringApplicationBuilder; import org.springframework.boot.web.servlet.ServletComponentScan; import org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker; import org.springframework.cloud.netflix.eureka.EnableEurekaClient; @SpringBootApplication @EnableEurekaClient //开启Eureka
@EnableCircuitBreaker//开启断路器
@ServletComponentScan//扫描servlet过滤器监听器
public class ServerMain { public static void main(String[] args) { new SpringApplicationBuilder(ServerMain.class).web(true).run(args); } }

二： hystrix 的 熔断，降级 机制。

2.1 回退机制案例

import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand; import com.netflix.hystrix.contrib.javanica.annotation.HystrixProperty; import org.springframework.stereotype.Service; import org.springframework.web.bind.annotation.RequestMapping; /** * Hystrix回退机制(熔断,降级) */ @Service public class RollbackService { /** * fallbackMethod: 指定回退方法 * * coreSize: 线程池最大数量 * maxQueueSize: 线程池最大队列,默认-1通过SynchronousQueue来实现; 否则使用LinkedBlockingQueue实现 * queueSizeRejectionThreshold: 当maxQueueSize是LinkedBlockingQueue时,即使没有达到最大列队也会据绝请求。 * * timeoutInMilliseconds: 超时时间 * requestVolumeThreshold: 单位时间内超过这个多个请求失败才执行熔断 */ @RequestMapping(value = "/testRollback") @HystrixCommand(fallbackMethod = "myRollback", threadPoolProperties = { @HystrixProperty(name = "coreSize", value = "30"), @HystrixProperty(name = "maxQueueSize", value = "-1"), @HystrixProperty(name = "queueSizeRejectionThreshold", value = "-1") }, commandProperties = { @HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "3000"), @HystrixProperty(name = "circuitBreaker.requestVolumeThreshold", value = "1") }) public String testRollback() { try {// 模拟请求阻塞
            Thread.sleep(4000); } catch (Exception e) { } return "恭喜你访问成功!"; } /** 回退方法 */
    public String myRollback() { return "服务器压力过大,明年再来吧!"; } }

2.2 编写接口， 调用测试。

import org.springframework.beans.factory.annotation.Autowired; import org.springframework.web.bind.annotation.RequestMapping; import org.springframework.web.bind.annotation.RestController; @RestController public class RollbackController { @Autowired private RollbackService rollbackService; @RequestMapping("/testRollback") public String testRollback() { return rollbackService.testRollback(); } }

三： hystrix 缓存机制：hystrix的缓存有点像mybatis默认开启的一级缓存：在session关闭之前(一次会话期间)，使用同样的参数调用同一个方法，实际只查询一次。

3.1 缓存逻辑

import org.springframework.stereotype.Service; import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand; import com.netflix.hystrix.contrib.javanica.cache.annotation.CacheRemove; import com.netflix.hystrix.contrib.javanica.cache.annotation.CacheResult; @Service public class CacheService { @CacheResult @HystrixCommand public void cacheMember(Integer id) { System.out.println("调用 cacheMember 方法"); } /** * commandKey:缓存的key * 获取和删除必须用同一个key,并必须是同一次请求。 */ @CacheResult @HystrixCommand(commandKey = "myCacheKey") public void getCache(Integer id) { System.out.println("执行查询方法"); } @CacheRemove(commandKey = "myCacheKey") @HystrixCommand public void removeCache(Integer id) { System.out.println("删除缓存方法"); } }

3.2 缓存接口开发

import org.springframework.beans.factory.annotation.Autowired; import org.springframework.http.MediaType; import org.springframework.web.bind.annotation.RequestMapping; import org.springframework.web.bind.annotation.RequestMethod; import org.springframework.web.bind.annotation.RestController; @RestController public class CacheController { @Autowired private CacheService cacheService; @RequestMapping(value = "/cache", method = RequestMethod.GET, produces = MediaType.APPLICATION_JSON_VALUE) public String cache() { for(int i = 0; i < 3; i++) { /** 在同一次请求里面调用了3次同一方法，会发现，控制台只 * 输出了一次，说明后2次走的缓存没调方法的逻辑 */ cacheService.cacheMember(1); } System.out.println("测试完毕"); return ""; } @RequestMapping(value = "/rc", method = RequestMethod.GET, produces = MediaType.APPLICATION_JSON_VALUE) public String testRemoveCache() { cacheService.getCache(1); cacheService.getCache(1); cacheService.removeCache(1); System.out.println("#########  分隔线   ###########"); cacheService.getCache(1); System.out.println("测试完毕"); return ""; } }