spring cloud 在初次使用 hystrix dashboard儀表盤的時候很容易出現hystrix dashboard Unable to connect to Command Metric Stream錯誤

如下圖：

首先檢視依賴時候新增全

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

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

        <dependency>
            <groupId>org.springframework.boot</
 
groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency> 

其次啟動程式註解是否新增

@EnableCircuitBreaker
@EnableHystrixDashboard

如果都沒問題那麼檢查下springboot 版本如果是2.0則需要新增 ServletRegistrationBean 因為springboot的預設路徑不是 "/hystrix.stream"，只要在自己的專案裡配置上下面的servlet就可以了

    @Bean
    
 
public ServletRegistrationBean getServlet() {
        HystrixMetricsStreamServlet streamServlet = new HystrixMetricsStreamServlet();
        ServletRegistrationBean registrationBean = new ServletRegistrationBean(streamServlet);
        registrationBean.setLoadOnStartup(1);
        registrationBean.addUrlMappings("/hystrix.stream");
        registrationBean.setName("HystrixMetricsStreamServlet");
        return registrationBean;
    }      

修改完成重啟服務問題便會解決

分析：

首先，檢視原始碼中 類 HystrixStreamEndpoint 從中可以看到 new EndpointServlet(HystrixMetricsStreamServlet.class）

package org.springframework.cloud.netflix.hystrix;
import com.netflix.hystrix.contrib.metrics.eventstream.HystrixMetricsStreamServlet;
import java.util.Map;
import java.util.function.Supplier;
import org.springframework.boot.actuate.endpoint.web.EndpointServlet;
import org.springframework.boot.actuate.endpoint.web.annotation.ServletEndpoint;

@ServletEndpoint(
    id = "hystrix.stream"
)
public class HystrixStreamEndpoint implements Supplier<EndpointServlet> {
    private final Map<String, String> initParameters;

    public HystrixStreamEndpoint(Map<String, String> initParameters) {
        this.initParameters = initParameters;
    }

    public EndpointServlet get() {
        return (new EndpointServlet(HystrixMetricsStreamServlet.class)).withInitParameters(this.initParameters);
    }
}

然後，我們再檢視 HystrixMetricsStreamServlet 這個類 在方法的註釋中我們可以看到

有提示 “Adding the following to web.xml”

而在springboot中是採用bean的形式配置就可以解決問題了

package com.netflix.hystrix.contrib.metrics.eventstream;

import com.netflix.config.DynamicIntProperty;
import com.netflix.config.DynamicPropertyFactory;
import com.netflix.hystrix.contrib.sample.stream.HystrixSampleSseServlet;
import com.netflix.hystrix.metric.consumer.HystrixDashboardStream;
import com.netflix.hystrix.serial.SerialHystrixDashboardData;
import rx.Observable;
import rx.functions.Func1;

import java.util.concurrent.atomic.AtomicInteger;

/**
 * Streams Hystrix metrics in text/event-stream format.
 * <p>
 * Install by:
 * <p>
 * 1) Including hystrix-metrics-event-stream-*.jar in your classpath.
 * <p>
 * 2) Adding the following to web.xml:
 * <pre>{@code
 * <servlet>
 *  <description></description>
 *  <display-name>HystrixMetricsStreamServlet</display-name>
 *  <servlet-name>HystrixMetricsStreamServlet</servlet-name>
 *  <servlet-class>com.netflix.hystrix.contrib.metrics.eventstream.HystrixMetricsStreamServlet</servlet-class>
 * </servlet>
 * <servlet-mapping>
 *  <servlet-name>HystrixMetricsStreamServlet</servlet-name>
 *  <url-pattern>/hystrix.stream</url-pattern>
 * </servlet-mapping>
 * } </pre>
 */
public class HystrixMetricsStreamServlet extends HystrixSampleSseServlet {

    private static final long serialVersionUID = -7548505095303313237L;

    /* used to track number of connections and throttle */
    private static AtomicInteger concurrentConnections = new AtomicInteger(0);
    private static DynamicIntProperty maxConcurrentConnections =
            DynamicPropertyFactory.getInstance().getIntProperty("hystrix.config.stream.maxConcurrentConnections", 5);

    public HystrixMetricsStreamServlet() {
        this(HystrixDashboardStream.getInstance().observe(), DEFAULT_PAUSE_POLLER_THREAD_DELAY_IN_MS);
    }

    /* package-private */ HystrixMetricsStreamServlet(Observable<HystrixDashboardStream.DashboardData> sampleStream, int pausePollerThreadDelayInMs) {
        super(sampleStream.concatMap(new Func1<HystrixDashboardStream.DashboardData, Observable<String>>() {
            @Override
            public Observable<String> call(HystrixDashboardStream.DashboardData dashboardData) {
                return Observable.from(SerialHystrixDashboardData.toMultipleJsonStrings(dashboardData));
            }
        }), pausePollerThreadDelayInMs);
    }

    @Override
    protected int getMaxNumberConcurrentConnectionsAllowed() {
        return maxConcurrentConnections.get();
    }

    @Override
    protected int getNumberCurrentConnections() {
        return concurrentConnections.get();
    }

    @Override
    protected int incrementAndGetCurrentConcurrentConnections() {
        return concurrentConnections.incrementAndGet();
    }

    @Override
    protected void decrementCurrentConcurrentConnections() {
        concurrentConnections.decrementAndGet();
    }
}