Hystrix使用fallback机制很简单，继承HystrixCommand只需重写getFallback()，继承HystrixObservableCommand只需重写resumeWithFallback()，比如上篇文章的HelloWorldHystrixCommand加上下面代码片段：
@Override
protected String getFallback() {
return "fallback: " + name;
}
fallback实际流程是当run()/construct()被触发执行时或执行中发生错误时，将转向执行getFallback()/resumeWithFallback()。
结合下图，4种情况（出现异常，超时，熔断，线程池已满）将触发fallback：
①run()方法中出现异常（非HystrixBadRequestException异常）或者出现超时（）触发fallback()
public class HystrixFallbackNomal extends HystrixCommand<String>{
private final String name;
public HystrixFallbackNomal(String name) {
super(HystrixCommandGroupKey.Factory.asKey("ExampleGroup"));
this.name = name;
}
@Override
public String run() throws Exception {
/*---------------会触发fallback的case-------------------*/
// 无限循环，实际上属于超时
/* int j = 0;
while (true) {
j++;
}*/
Thread.sleep(1000);
// 除零异常
// int i = 1/0;
// 主动抛出异常
// throw new HystrixTimeoutException();
// throw new RuntimeException("this command will trigger fallback");
// throw new Exception("this command will trigger fallback");
// throw new HystrixRuntimeException(FailureType.BAD_REQUEST_EXCEPTION, commandClass, message, cause, fallbackException);
/*---------------不会触发fallback的case-------------------*/
// 被捕获的异常不会触发fallback
// try {
// throw new RuntimeException("this command never trigger fallback");
// } catch(Exception e) {
// e.printStackTrace();
// }
// HystrixBadRequestException异常由非法参数或非系统错误引起，不会触发fallback，也不会被计入熔断器
//throw new HystrixBadRequestException("HystrixBadRequestException is never trigger fallback");
//return name;
}
@Override
protected String getFallback() {
return "fallback: " + name;
}
}
}
②熔断触发fallback（）
/**
* 熔断机制相当于电路的跳闸功能，例如：我们可以配置熔断策略为当请求错误比例在10s内>50%时，该服务将进入熔断状态，后续请求都会进入fallback
* CircuitBreakerRequestVolumeThreshold设置为3，意味着10s内请求超过3次就触发熔断器(10s这个时间暂时不可配置)
* run()中无限循环使命令超时进入fallback，10s内请求超过3次，将被熔断，进入降级，即不进入run()而直接进入fallback
* 如果未熔断，但是threadpool被打满，仍然会降级，即不进入run()而直接进入fallback
*/
public class HystrixFallbackCircuitBreaker extends HystrixCommand<String>{
private final String name;
public HystrixFallbackCircuitBreaker(String name) {
super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("CircuitBreakerTestGroup"))
.andCommandKey(HystrixCommandKey.Factory.asKey("CircuitBreakerTestKey"))
.andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey("CircuitBreakerTest"))
.andThreadPoolPropertiesDefaults( // 配置线程池
HystrixThreadPoolProperties.Setter()
.withCoreSize(200) // 配置线程池里的线程数，设置足够多线程，以防未熔断却打满threadpool
)
.andCommandPropertiesDefaults( // 配置熔断器
HystrixCommandProperties.Setter()
.withCircuitBreakerEnabled(true)
.withCircuitBreakerRequestVolumeThreshold(3)
.withCircuitBreakerErrorThresholdPercentage(80)
// .withCircuitBreakerForceOpen(true) // 置为true时，所有请求都将被拒绝，直接到fallback
// .withCircuitBreakerForceClosed(true) // 置为true时，将忽略错误
// .withExecutionIsolationStrategy(ExecutionIsolationStrategy.SEMAPHORE) // 信号量隔离
// .withExecutionTimeoutInMilliseconds(5000)
)
);
this.name = name;
}
@Override
protected String run() throws Exception {
System.out.println("running run():" + name);
int num = Integer.valueOf(name);
if(num < 10) { // 直接返回
return name;
} else { // 无限循环模拟超时
int j = 0;
while (true) {
j++;
}
}
// return name;
}
@Override
protected String getFallback() {
return "CircuitBreaker fallback: " + name;
}
}
@Test
public void testFallbackCricuitBreaker() throws IOException {
for(int i = 0; i < 50; i++) {
try {
System.out.println("===========" + new HystrixFallbackCircuitBreaker(String.valueOf(i)).execute())；
} catch(Exception e) {
System.out.println("run()抛出HystrixBadRequestException时，被捕获到这里" + e.getCause());
}
}
System.out.println("------开始打印现有线程---------");
Map<Thread, StackTraceElement[]> map=Thread.getAllStackTraces();
for (Thread thread : map.keySet()) {
System.out.println(thread.getName());
}
System.out.println("thread num: " + map.size());
System.in.read();
}
③线程池已满，触发fallback()
public class HystrixThreadPoolFallback extends HystrixCommand<String>{
private final String name;
public HystrixThreadPoolFallback(String name) {
super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("ThreadPoolTestGroup"))
.andCommandKey(HystrixCommandKey.Factory.asKey("testCommandKey"))
.andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey("ThreadPoolTest"))
.andCommandPropertiesDefaults(
HystrixCommandProperties.Setter()
.withExecutionTimeoutInMilliseconds(5000)
)
.andThreadPoolPropertiesDefaults(
HystrixThreadPoolProperties.Setter()
.withCoreSize(3) // 配置线程池里的线程数
)
);
this.name = name;
}
@Override
protected String run() throws Exception {
System.out.println(name);
TimeUnit.MILLISECONDS.sleep(2000);
return name;
}
@Override
protected String getFallback() {
return "fallback: " + name;
}
}
@Test
public void testThreadPool() throws IOException {
for(int i = 0; i < 10; i++) {
try {
Future<String> future = new HystrixThreadPoolFallback("Hlx"+i).queue();
} catch(Exception e) {
System.out.println("run()抛出HystrixBadRequestException时，被捕获到这里" + e.getCause());
}
}
for(int i = 0; i < 20; i++) {
try {
System.out.println("===========" + new HystrixThreadPoolFallback("Hlx"+i).execute());
} catch(Exception e) {
System.out.println("run()抛出HystrixBadRequestException时，被捕获到这里" + e.getCause());
}
}
try {
TimeUnit.MILLISECONDS.sleep(2000);
}catch(Exception e) {}
System.out.println("------开始打印现有线程---------");
Map<Thread, StackTraceElement[]> map=Thread.getAllStackTraces();
for (Thread thread : map.keySet()) {
System.out.println(thread.getName());
}
System.out.println(map);
System.out.println("thread num: " + map.size());
System.in.read();
}
参考文献：http://www.jianshu.com/p/b9af028efebb