从线程堆栈可以看出，第1个问题是由于走到了第3步。开始时是有时会报504异常，刷新多次后会一直报504异常，经过跟踪调试发现前几次会成功获取到连接，而连接池满后，后面的请求会阻塞。正常情况下当前面的连接释放到连接池后，后面的请求会得到连接资源继续执行，可现实是后面的连接一直处于等待状态，猜想可能是由于连接一直未释放导致。
我们来看一下连接在什么时候会释放。
RestTemplate由于在调外部系统B时，使用的是RestTemplate的getForObject方法，从此入手跟踪调试看一看。
@Override public <T> T getForObject(String url, Class<T> responseType, Object... uriVariables) throws RestClientException { RequestCallback requestCallback = acceptHeaderRequestCallback(responseType); HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor<T>(responseType, getMessageConverters(), logger); return execute(url, HttpMethod.GET, requestCallback, responseExtractor, uriVariables); } @Override public <T> T getForObject(String url, Class<T> responseType, Map<String, ?> uriVariables) throws RestClientException { RequestCallback requestCallback = acceptHeaderRequestCallback(responseType); HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor<T>(responseType, getMessageConverters(), logger); return execute(url, HttpMethod.GET, requestCallback, responseExtractor, uriVariables); } @Override public <T> T getForObject(URI url, Class<T> responseType) throws RestClientException { RequestCallback requestCallback = acceptHeaderRequestCallback(responseType); HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor<T>(responseType, getMessageConverters(), logger); return execute(url, HttpMethod.GET, requestCallback, responseExtractor); }getForObject都调用了execute方法(其实RestTemplate的其它http请求方法调用的也是execute方法)
@Override public <T> T execute(String url, HttpMethod method, RequestCallback requestCallback, ResponseExtractor<T> responseExtractor, Object... uriVariables) throws RestClientException { URI expanded = getUriTemplateHandler().expand(url, uriVariables); return doExecute(expanded, method, requestCallback, responseExtractor); } @Override public <T> T execute(String url, HttpMethod method, RequestCallback requestCallback, ResponseExtractor<T> responseExtractor, Map<String, ?> uriVariables) throws RestClientException { URI expanded = getUriTemplateHandler().expand(url, uriVariables); return doExecute(expanded, method, requestCallback, responseExtractor); } @Override public <T> T execute(URI url, HttpMethod method, RequestCallback requestCallback, ResponseExtractor<T> responseExtractor) throws RestClientException { return doExecute(url, method, requestCallback, responseExtractor); }所有execute方法都调用了同一个doExecute方法
protected <T> T doExecute(URI url, HttpMethod method, RequestCallback requestCallback, ResponseExtractor<T> responseExtractor) throws RestClientException { Assert.notNull(url, "'url' must not be null"); Assert.notNull(method, "'method' must not be null"); ClientHttpResponse response = null; try { ClientHttpRequest request = createRequest(url, method); if (requestCallback != null) { requestCallback.doWithRequest(request); } response = request.execute(); handleResponse(url, method, response); if (responseExtractor != null) { return responseExtractor.extractData(response); } else { return null; } } catch (IOException ex) { String resource = url.toString(); String query = url.getRawQuery(); resource = (query != null ? resource.substring(0, resource.indexOf('?')) : resource); throw new ResourceAccessException("I/O error on " + method.name() + " request for \"" + resource + "\": " + ex.getMessage(), ex); } finally { if (response != null) { response.close(); } } }doExecute方法创建了请求，然后执行，处理异常，最后关闭。可以看到关闭操作放在finally中，任何情况都会执行到，除非返回的response为null。
InterceptingClientHttpRequest进入到request.execute()方法中，对应抽象类org.springframework.http.client.AbstractClientHttpRequest的execute方法
@Override public final ClientHttpResponse execute() throws IOException { assertNotExecuted(); ClientHttpResponse result = executeInternal(this.headers); this.executed = true; return result; }executeInternal方法是一个抽象方法，由子类实现(restTemplate内部的http调用实现方式有多种)。进入executeInternal方法，到达抽象类org.springframework.http.client.AbstractBufferingClientHttpRequest中
protected ClientHttpResponse executeInternal(HttpHeaders headers) throws IOException { byte[] bytes = this.bufferedOutput.toByteArray(); if (headers.getContentLength() < 0) { headers.setContentLength(bytes.length); } ClientHttpResponse result = executeInternal(headers, bytes); this.bufferedOutput = null; return result; }