今天在做一个从Feign客户端到service服务端的POST请求，service服务端Controller类中方法测试正常，但却在Feign客户端返回时报错，错误信息如下：

Caused by: feign.RetryableException: Incomplete output stream executing POST http://服务名/路径/方法名
    at feign.FeignException.errorExecuting(FeignException.java:67)
    at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:104)
    at 
............异常太多省略...........
Caused by: java.io.IOException: Incomplete output stream
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1523)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1492)
    at java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:480)
    at feign.Client$Default.convertResponse(Client.java:152)
    at feign.Client$Default.execute(Client.java:74)
    at org.springframework.cloud.sleuth.instrument.web.client.feign.TracingFeignClient.execute(TracingFeignClient.java:88)
    at org.springframework.cloud.sleuth.instrument.web.client.feign.LazyTracingFeignClient.execute(LazyTracingFeignClient.java:54)
    at org.springframework.cloud.openfeign.ribbon.FeignLoadBalancer.execute(FeignLoadBalancer.java:89)
    at org.springframework.cloud.openfeign.ribbon.FeignLoadBalancer.execute(FeignLoadBalancer.java:55)
    at com.netflix.client.AbstractLoadBalancerAwareClient$1.call(AbstractLoadBalancerAwareClient.java:104)
    at com.netflix.loadbalancer.reactive.LoadBalancerCommand$3$1.call(LoadBalancerCommand.java:303)
    at com.netflix.loadbalancer.reactive.LoadBalancerCommand$3$1.call(LoadBalancerCommand.java:287)
    at rx.internal.util.ScalarSynchronousObservable$3.call(ScalarSynchronousObservable.java:231)
    at rx.internal.util.ScalarSynchronousObservable$3.call(ScalarSynchronousObservable.java:228)
    at rx.Observable.unsafeSubscribe(Observable.java:10327)
    at rx.internal.operators.OnSubscribeConcatMap$ConcatMapSubscriber.drain(OnSubscribeConcatMap.java:286)
    at rx.internal.operators.OnSubscribeConcatMap$ConcatMapSubscriber.onNext(OnSubscribeConcatMap.java:144)
    at com.netflix.loadbalancer.reactive.LoadBalancerCommand$1.call(LoadBalancerCommand.java:185)
    at com.netflix.loadbalancer.reactive.LoadBalancerCommand$1.call(LoadBalancerCommand.java:180)
    at rx.Observable.unsafeSubscribe(Observable.java:10327)
    at rx.internal.operators.OnSubscribeConcatMap.call(OnSubscribeConcatMap.java:94)
    at rx.internal.operators.OnSubscribeConcatMap.call(OnSubscribeConcatMap.java:42)
    at rx.Observable.unsafeSubscribe(Observable.java:10327)
    at rx.internal.operators.OperatorRetryWithPredicate$SourceSubscriber$1.call(OperatorRetryWithPredicate.java:127)
    at rx.internal.schedulers.TrampolineScheduler$InnerCurrentThreadScheduler.enqueue(TrampolineScheduler.java:73)
    at rx.internal.schedulers.TrampolineScheduler$InnerCurrentThreadScheduler.schedule(TrampolineScheduler.java:52)
    at rx.internal.operators.OperatorRetryWithPredicate$SourceSubscriber.onNext(OperatorRetryWithPredicate.java:79)
    at rx.internal.operators.OperatorRetryWithPredicate$SourceSubscriber.onNext(OperatorRetryWithPredicate.java:45)
    at rx.internal.util.ScalarSynchronousObservable$WeakSingleProducer.request(ScalarSynchronousObservable.java:276)
    at rx.Subscriber.setProducer(Subscriber.java:209)
    at rx.internal.util.ScalarSynchronousObservable$JustOnSubscribe.call(ScalarSynchronousObservable.java:138)
    at rx.internal.util.ScalarSynchronousObservable$JustOnSubscribe.call(ScalarSynchronousObservable.java:129)
    at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48)
    at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30)
    at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48)
    at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30)
    at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48)
    at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30)
    at rx.Observable.subscribe(Observable.java:10423)
    at rx.Observable.subscribe(Observable.java:10390)
    at rx.observables.BlockingObservable.blockForSingle(BlockingObservable.java:443)
    at rx.observables.BlockingObservable.single(BlockingObservable.java:340)
    at com.netflix.client.AbstractLoadBalancerAwareClient.executeWithLoadBalancer(AbstractLoadBalancerAwareClient.java:112)
    at org.springframework.cloud.openfeign.ribbon.LoadBalancerFeignClient.execute(LoadBalancerFeignClient.java:63)
    at org.springframework.cloud.sleuth.instrument.web.client.feign.TraceLoadBalancerFeignClient.access$001(TraceLoadBalancerFeignClient.java:36)
    at org.springframework.cloud.sleuth.instrument.web.client.feign.TraceLoadBalancerFeignClient.lambda$execute$0(TraceLoadBalancerFeignClient.java:50)
    at org.springframework.cloud.sleuth.instrument.web.client.feign.TracingFeignClient.execute(TracingFeignClient.java:88)
    at org.springframework.cloud.sleuth.instrument.web.client.feign.LazyTracingFeignClient.execute(LazyTracingFeignClient.java:54)
    at org.springframework.cloud.sleuth.instrument.web.client.feign.TraceLoadBalancerFeignClient.execute(TraceLoadBalancerFeignClient.java:50)
    at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:97)
    ... 77 common frames omitted

心想居然IO异常啦，但service端能正确返回数据啊，所以故障先锁定在Feign客户端服务上。根据异常调用链得知最终发生车祸的是sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1523) HttpURLConnection.getInputStream0方法1523行。经过搜索资料，原来feign采用jdk原生HttpURLConnection向下游服务发起http请求（源码详见feign.Client.Default），先到getInputStream0方法看看：

    private synchronized InputStream getInputStream0() throws IOException {
        if(!this.doInput) {
            throw new ProtocolException("Cannot read from URLConnection if doInput=false (call setDoInput(true))");
        } else if(this.rememberedException != null) {
            if(this.rememberedException instanceof RuntimeException) {
                throw new RuntimeException(this.rememberedException);
            } else {
                throw this.getChainedException((IOException)this.rememberedException);
            }
        } else if(this.inputStream != null) {
            return this.inputStream;
        } else {
            if(this.streaming()) {
                if(this.strOutputStream == null) {
                    this.getOutputStream();
                }

                this.strOutputStream.close();
                if(!this.strOutputStream.writtenOK()) {   
                    throw new IOException("Incomplete output stream"); // 1523行
                }
            }..........

        // this.strOutputStream.writtenOK()方法
        boolean writtenOK() {
            return this.closed && !this.error;
        }

看起来，this.error是true导致的，也就是说在OutputStream写入流时出错。

没办法，只能用Apache HttpClient替换掉原生HttpURLConnection。加入依赖：

<!-- 使用Apache HttpClient替换Feign原生httpclient -->
<!-- feign-httpclient内含Apache HttpClient -->
        <dependency>
            <groupId>com.netflix.feign</groupId>
            <artifactId>feign-httpclient</artifactId>
            <version>8.17.0</version>
        </dependency>

然后在application.yml中添加如下：

feign:
  httpclient:
    enabled: true

重新调用，一切恢复正常。噢耶！

另外补充一点：
引入Apache HttpClient包时，需要注意指定Content-Type，不然会产生以下异常：

Caused by: java.lang.IllegalArgumentException: MIME type may not contain reserved characters  

解决办法 ：

@RequestMapping(value = "/getUser", method = RequestMethod.GET, consumes = MediaType.APPLICATION_JSON_VALUE) 

注解中指定： Content-Type 即 指定 consumes 的属性值

feign通过jdk中的HttpURLConnection向下游服务发起http请求
更换Feign默认使用的HTTP Client
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