1、执行方式

HystrixCommand提供了3种执行方式：

1、同步执行

即一旦开始执行该命令，当前线程就得阻塞着直到该命令返回结果，然后才能继续执行下面的逻辑。当调用命令的execute()方法即为同步执行， 示例：

public class ThreadEchoCommand extends HystrixCommand<String> {

        private Logger logger = LoggerFactory.getLogger(ThreadEchoCommand.class);
        private String input;

    public ThreadEchoCommand(String input) {
            super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("Semaphore Echo"))
                    .andCommandKey(HystrixCommandKey.Factory.asKey("Echo"))
                    .andCommandPropertiesDefaults(HystrixCommandProperties.Setter()
                    .withExecutionIsolationStrategy(HystrixCommandProperties.ExecutionIsolationStrategy.SEMAPHORE)
                    .withExecutionIsolationSemaphoreMaxConcurrentRequests(5)));
            this.input = input;
        }

        @Override
        protected String run() throws Exception {
            logger.info("Run command with input: {}", input);
            Thread.sleep(100);
            return "Echo: " + input;

    }

    public static void main(String[] args) throws Exception {
        ThreadEchoCommand command = new ThreadEchoCommand("xianlinbox");
        System.out.println("command.execute()"+ command.execute());
    }
    //[main] INFO com.ThreadEchoCommand - Run command with input: xianlinbox
    //command.execute()Echo: xianlinbox

2、异步执行

命令开始执行会返回一个Future 的对象，不阻塞后面的逻辑，开发者自己根据需要去获取结果。当 调用HystrixCommand的queue()方法即为异步执行 ， 示例：

线程数==信号量

3、响应式执行

命令开始执行会返回一个Observable<T> 对象，开发者可以给给Obeservable对象注册上Observer或者Action1对象，响应式地处理命令执行过程中的不同阶段。当调用HystrixCommand的observe()方法，或使用Observable的工厂方法（just(),from()）即为响应式执行

这个功能的实现是基于Netflix的另一个开源项目RxJava（ https://github.com/Netflix/RxJava ）来的

更细节的用法可以参考： https://github.com/Netflix/Hystrix/wiki/How-To-Use#wiki-Reactive-Execution 。

2、异常总结

1、java.lang.UnsupportedOperationException: No fallback available.

@Override
    protected String run() throws Exception {
        logger.info("Run command with input: {}", input);
        Thread.sleep(2000);
        return "Echo: " + input;
}

主要原因如下：

1、 未设置hystrix超时时间，默认是1000ms上述代码执行了2000ms，执行超时；

2、java.lang.UnsupportedOperationException: No fallback available.

线程数>信号量

public class ThreadEchoCommand extends HystrixCommand<String> {

        private Logger logger = LoggerFactory.getLogger(ThreadEchoCommand.class);
        private String input;

    public ThreadEchoCommand(String input) {
            super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("Semaphore Echo"))
                    .andCommandKey(HystrixCommandKey.Factory.asKey("Echo"))
                    .andCommandPropertiesDefaults(HystrixCommandProperties.Setter()
                    .withExecutionIsolationStrategy(HystrixCommandProperties.ExecutionIsolationStrategy.SEMAPHORE)
                    .withExecutionIsolationSemaphoreMaxConcurrentRequests(2)));
            this.input = input;
        }

        @Override
        protected String run() throws Exception {
            logger.info("Run command with input: {}", input);
            Thread.sleep(100);
            return "Echo: " + input;

    }

    public static void main(String[] args) throws Exception {
        ThreadEchoCommand command = new ThreadEchoCommand("xianlinbox");
        System.out.println("command.execute()"+ command.execute());
    }
//[main] INFO com.ThreadEchoCommand - Run command with input: xianlinbox
//command.execute()Echo: xianlinbox
1:11:56.332 [Thread-2] DEBUG com.netflix.hystrix.AbstractCommand - HystrixCommand Execution Rejection by Semaphore.
01:11:56.332 [Thread-1] DEBUG com.netflix.hystrix.AbstractCommand - HystrixCommand Execution Rejection by Semaphore.
01:11:56.332 [Thread-3] DEBUG com.netflix.hystrix.AbstractCommand - HystrixCommand Execution Rejection by Semaphore.
01:11:56.341 [Thread-2] DEBUG com.netflix.hystrix.AbstractCommand - No fallback for HystrixCommand. 
java.lang.UnsupportedOperationException: No fallback available.

以下为fallback触发的情况说明：

引用：SpringCloud Hystrix超时：HystrixRuntimeException: xxx failed and no fallback available_年少青山的博客-CSDN博客

Hystrix使用介绍_赶路人儿-CSDN博客