HttpURLConnection InputStream 读取非200响应码，错误的getInputStream

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背景：

服务端项目中通过全局异常拦截器进行异常处理，但进行单元测试发现用java原生HttpURLConnection方法访问服务端时，无法读取到处理异常后的响应体中的内容，直接报错！在此记录下解决方案！

一、错误场景还原

先看看发生报错时的代码如何写的：

抛出自定义错误的代码：

try {
    service = (MoerService) SpringContextHolder.getBean(serviceImpl);
} catch (Exception e) {
    throw new SystemException(ErrorMessageConstants.NOT_BEAN_ERROR);
}

全局异常拦截器：

/**
 * 全局异常拦截器
 *
 * @Author ---
 * @Date: --
 */
@ControllerAdvice
public class GlobalExceptionHandler {

    private Logger logger = LoggerFactory.getLogger(GlobalExceptionHandler.class);


    /**
     * 拦截全局异常
     */
    @ExceptionHandler(Exception.class)
    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
    @ResponseBody
    public Result exception(Exception e) {
        logger.error("异常错误:", e);
        return Result.fail(ErrorMessageConstants.ABNORMAL_ERROR);
    }

    /**
     * 拦截系统异常
     */
    @ExceptionHandler(SystemException.class)
    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
    @ResponseBody
    public Result systemException(SystemException e) {
        logger.error("系统异常:", e);
        return Result.fail(e.getMessage());
    }

    /**
     * 拦截业务异常
     */
    @ExceptionHandler(BussinessException.class)
    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
    @ResponseBody
    public Result bussinessException(BussinessException e) {
        logger.error("业务异常:", e);
        return Result.fail(e.getMessage());
    }

HttpRequestUtil工具类方法（用来向服务端发起请求）：

public static String postJsonParams(String requestUrl, String requestMethod, String params) {
   StringBuilder sb = new StringBuilder();
   try {
      URL url = new URL(requestUrl);// 创建连接
      HttpURLConnection connection = (HttpURLConnection) url
            .openConnection();
      connection.setDoOutput(true);
      connection.setDoInput(true);
      connection.setUseCaches(false);
      connection.setInstanceFollowRedirects(true);
      connection.setRequestMethod(requestMethod); // 设置请求方式
      connection.setRequestProperty("Accept", "application/json"); // 设置接收数据的格式
      connection.setRequestProperty("Content-Type", "application/json"); // 设置发送数据的格式
      connection.connect();
      OutputStreamWriter out = new OutputStreamWriter(
            connection.getOutputStream(), "UTF-8"); // utf-8编码
      out.append(params);
      out.flush();
      out.close();
      if (connection != null) {
         BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream(), "UTF-8"));
         String line = null;
         while ((line = br.readLine()) != null) {
            sb.append(line);
            sb.append("\r\n");
         }
         br.close();
      }
      connection.disconnect();
   } catch (IOException e) {
      e.printStackTrace();
   }
   return sb.toString();
}

当服务端抛出 SystemException() 时，会被异常拦截器 GlobalExceptionHandler 所处理，然后return
Result.fail(e.getMessage())，返回自定义友好错误提示；

但通过postJsonParams方法发起请求时，却无法获取服务端响应内容，报错信息如下：

java.io.IOException: Server returned HTTP response code: 500 for URL: http://localhost:8885/moerService/access
	at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1839)
	at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1440)
	at com.moerlong.service_authorize.util.HttpRequest.postJsonParams(HttpRequest.java:94)
	at com.moerlong.service_authorize.controller.AccessController.test(AccessController.java:91)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205)
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:133)
......

二、错误定位

由错误信息可以看到：Server returned HTTP response code: 500，说明服务端响应码为500是错误的源头；

我们再看异常拦截器中的 @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR) 正好为response设置响应码为500，难道是这个问题？

之前通过HttpClient发送请求时没遇到这种问题，难道是HttpURLConnection API不能读取responseCode为500时的响应体内容么？

再看读取响应体的方法：

if (connection != null) {
         BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream(), "UTF-8"));
         String line = null;
         while ((line = br.readLine()) != null) {
            sb.append(line);
            sb.append("\r\n");
         }
         br.close();
      }

我们是通过connection.getInputStream()方法获取输入流来读取响应体内容的！但这个API只能读取
responseCode=200 时的响应内容！

我们翻阅 HttpURLConnection 的API文档，可以看到十分靠前的一个方法getErrorStream()：

Returns the error stream if the connection failed but the server sent
useful data nonetheless.

原来通过这个方法我们才能获取到responseCode不为200时(connection failed)的响应内容(useful data)！

三、解决方案

修改下HttpRequestUtil发送请求的工具类：

public static String postJsonParams(String requestUrl, String requestMethod, String params) {
   StringBuilder sb = new StringBuilder();
   try {
      URL url = new URL(requestUrl);// 创建连接
      HttpURLConnection connection = (HttpURLConnection) url
            .openConnection();
      connection.setDoOutput(true);
      connection.setDoInput(true);
      connection.setUseCaches(false);
      connection.setInstanceFollowRedirects(true);
      connection.setRequestMethod(requestMethod); // 设置请求方式
      connection.setRequestProperty("Accept", "application/json"); // 设置接收数据的格式
      connection.setRequestProperty("Content-Type", "application/json"); // 设置发送数据的格式
      connection.connect();
      OutputStreamWriter out = new OutputStreamWriter(
            connection.getOutputStream(), "UTF-8"); // utf-8编码
      out.append(params);
      out.flush();
      out.close();
      if (connection != null) {
         InputStream inputStream = null;
         //根据responseCode来获取输入流，此处错误响应码的响应体内容也要获取（看服务端的返回结果形式决定）
         if (HttpURLConnection.HTTP_OK == connection.getResponseCode()) {
            inputStream = connection.getInputStream();
         } else {
            inputStream = connection.getErrorStream();
         }
         BufferedReader br = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"));
         String line = null;
         while ((line = br.readLine()) != null) {
            sb.append(line);
            sb.append("\r\n");
         }
         br.close();
      }
      connection.disconnect();
   } catch (IOException e) {
      e.printStackTrace();
   }
   return sb.toString();
}

核心：

if (HttpURLConnection.HTTP_OK == connection.getResponseCode()) {
  inputStream = connection.getInputStream();
} else {
   inputStream = connection.getErrorStream();
}

根据 ResponseCode来选择getInputStream/getErrorStream API即可！

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