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

1    <parent>
2        <groupId>org.springframework.boot</groupId>
3        <artifactId>spring-boot-starter-parent</artifactId>
4        <version>1.5.8.RELEASE</version> 
5        <relativePath/>
6    </parent>
7    <!-- springCloud -->
8    <dependencyManagement>
9        <dependencies>
10            <dependency>
11                <groupId>org.springframework.cloud</groupId>
12                <artifactId>spring-cloud-dependencies</artifactId>
13                <version>Dalston.SR3</version>
14                <type>pom</type>
15                <scope>import</scope>
16            </dependency>
17        </dependencies>
18    </dependencyManagement>
19    
20    <dependencies>
21        <dependency>
22            <groupId>org.springframework.boot</groupId>
23            <artifactId>spring-boot-starter-web</artifactId>
24        </dependency>
25        <!-- eureka客户端依赖 -->
26        <dependency>
27            <groupId>org.springframework.cloud</groupId>
28            <artifactId>spring-cloud-starter-eureka</artifactId>
29        </dependency>
30        <!-- hystrix -->
31        <dependency>
32            <groupId>org.springframework.cloud</groupId>
33            <artifactId>spring-cloud-starter-hystrix</artifactId>
34        </dependency>

1.2 配置application.yml

1# 指定端口
2server:
3  port: 8085
4  context-path: /demo
5# 服务名称
6spring:
7  application:
8    name: hystrix
9
10# eureka服务器地址
11eureka:
12  client:
13    serviceUrl:
14      defaultZone: http://localhost:8761/eureka/

1.3 配置Hystrix过滤器

1import java.io.IOException;
2import javax.servlet.Filter;
3import javax.servlet.FilterChain;
4import javax.servlet.FilterConfig;
5import javax.servlet.ServletException;
6import javax.servlet.ServletRequest;
7import javax.servlet.ServletResponse;
8import javax.servlet.annotation.WebFilter;
9import com.netflix.hystrix.strategy.concurrency.HystrixRequestContext;
10/**
11 * urlPatterns:拦截所有路径(拦截规则)
12 * filterName:过滤器名称
13 */
14@WebFilter(urlPatterns = "/*", filterName = "hystrixFilter")
15public class HystrixFilterConf implements Filter{
16    public void destroy() { }
17
18    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
19            throws IOException, ServletException {
20        // 初始化Hystrix上下文
21        HystrixRequestContext ctx = HystrixRequestContext.initializeContext();
22        try {
23            chain.doFilter(request, response);
24        } catch (Exception e) {
25            
26        } finally {
27            ctx.shutdown();
28        }
29    }
30    public void init(FilterConfig arg0) throws ServletException { }
31}

1.4 主函数入口

1import org.springframework.boot.autoconfigure.SpringBootApplication;
2import org.springframework.boot.builder.SpringApplicationBuilder;
3import org.springframework.boot.web.servlet.ServletComponentScan;
4import org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker;
5import org.springframework.cloud.netflix.eureka.EnableEurekaClient;
6@SpringBootApplication
7@EnableEurekaClient  //开启Eureka
8@EnableCircuitBreaker//开启断路器
9@ServletComponentScan//扫描servlet过滤器监听器
10public class ServerMain {
11
12    public static void main(String[] args) {
13        new SpringApplicationBuilder(ServerMain.class).web(true).run(args);
14    }
15}

二： hystrix 的 熔断，降级 机制。

2.1 回退机制案例

1import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand;
2import com.netflix.hystrix.contrib.javanica.annotation.HystrixProperty;
3import org.springframework.stereotype.Service;
4import org.springframework.web.bind.annotation.RequestMapping;
5/**
6 * Hystrix回退机制(熔断,降级)
7 */
8@Service
9public class RollbackService {
10
11    /**
12     * fallbackMethod: 指定回退方法
13     * 
14     * coreSize: 线程池最大数量
15     * maxQueueSize: 线程池最大队列,默认-1通过SynchronousQueue来实现; 否则使用LinkedBlockingQueue实现
16     * queueSizeRejectionThreshold: 当maxQueueSize是LinkedBlockingQueue时,即使没有达到最大列队也会据绝请求。
17     * 
18     * timeoutInMilliseconds: 超时时间
19     * requestVolumeThreshold: 单位时间内超过这个多个请求失败才执行熔断
20     */
21    @RequestMapping(value = "/testRollback")
22    @HystrixCommand(fallbackMethod = "myRollback", 
23        threadPoolProperties = {
24            @HystrixProperty(name = "coreSize", value = "30"), 
25            @HystrixProperty(name = "maxQueueSize", value = "-1"),
26            @HystrixProperty(name = "queueSizeRejectionThreshold", value = "-1")
27        }, 
28        commandProperties = {
29            @HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "3000"),
30            @HystrixProperty(name = "circuitBreaker.requestVolumeThreshold", value = "1")
31        })
32    public String testRollback() {
33        try {// 模拟请求阻塞
34            Thread.sleep(4000);
35        } catch (Exception e) {
36        }
37        return "恭喜你访问成功!";
38    }
39    /** 回退方法  */
40    public String myRollback() {
41        return "服务器压力过大,明年再来吧!";
42    }
43}

2.2 编写接口， 调用测试。

1import org.springframework.beans.factory.annotation.Autowired;
2import org.springframework.web.bind.annotation.RequestMapping;
3import org.springframework.web.bind.annotation.RestController;
4@RestController
5public class RollbackController {
6
7    @Autowired
8    private RollbackService rollbackService;
9    
10    @RequestMapping("/testRollback")
11    public String testRollback() {
12        return rollbackService.testRollback();
13    }
14}

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

1import org.springframework.stereotype.Service;
2import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand;
3import com.netflix.hystrix.contrib.javanica.cache.annotation.CacheRemove;
4import com.netflix.hystrix.contrib.javanica.cache.annotation.CacheResult;
5@Service
6public class CacheService {
7
8    @CacheResult
9    @HystrixCommand
10    public void cacheMember(Integer id) {
11        System.out.println("调用 cacheMember 方法");
12    }
13    
14    /**
15     * commandKey:缓存的key
16     *       获取和删除必须用同一个key,并必须是同一次请求。
17     */
18    @CacheResult
19    @HystrixCommand(commandKey = "myCacheKey")
20    public void getCache(Integer id) {
21        System.out.println("执行查询方法");
22    }
23    @CacheRemove(commandKey = "myCacheKey")
24    @HystrixCommand
25    public void removeCache(Integer id) {
26        System.out.println("删除缓存方法");
27    }
28}

3.2 缓存接口开发

1import org.springframework.beans.factory.annotation.Autowired;
2import org.springframework.http.MediaType;
3import org.springframework.web.bind.annotation.RequestMapping;
4import org.springframework.web.bind.annotation.RequestMethod;
5import org.springframework.web.bind.annotation.RestController;
6@RestController
7public class CacheController {
8    
9    @Autowired
10    private CacheService cacheService;
11
12    @RequestMapping(value = "/cache", method = RequestMethod.GET, 
13            produces = MediaType.APPLICATION_JSON_VALUE)
14    public String cache() {
15        for(int i = 0; i < 3; i++) {
16            /** 在同一次请求里面调用了3次同一方法，会发现，控制台只
17             *  输出了一次，说明后2次走的缓存没调方法的逻辑    */
18            cacheService.cacheMember(1);
19        }
20        System.out.println("测试完毕");
21        return "";
22    }
23    @RequestMapping(value = "/rc", method = RequestMethod.GET, 
24            produces = MediaType.APPLICATION_JSON_VALUE)
25    public String testRemoveCache() {
26        cacheService.getCache(1);
27        cacheService.getCache(1);
28        
29        cacheService.removeCache(1);
30        System.out.println("#########  分隔线   ###########");
31        cacheService.getCache(1);
32        System.out.println("测试完毕");
33        return "";
34    }
35}