response content length_前端获取post请求返回数据

关于 reponse 返回类型 contentType 是 application/json;charset=ISO-8859-1 现象的阐述

现象发生描述：

在 Interceptor 的 preHandle 方法对 response 设置 contentType和charset

response.setContentType(MediaType.APPLICATION_JSON_VALUE);
response.setCharacterEncoding("UTF-8");

之后，发现了浏览器F12页面中的请求结果的 responseHeader 中 contentType 属性始终是 application/json;charset=ISO-8859-1 ，并不符合预期结果，最终结果的应该是 application/json;charset=UTF-8 才是理想的啊。

排查：

经过对上述代码代码以及上下文代码进行debug，最终通过源码逻辑获得到了编码始终不对的原因。

原因：

产生问题时的代码：

PrintWriter writer = response.getWriter();
response.setContentType(MediaType.APPLICATION_JSON_VALUE);
response.setCharacterEncoding("UTF-8");

处理之后的代码：

response.setContentType(MediaType.APPLICATION_JSON_VALUE);
response.setCharacterEncoding("UTF-8");
PrintWriter writer = response.getWriter();

对比代码可以看到，没错！就是 response.getWriter(); 这句代码的执行顺序导致的，它是在设置类型和编码之前还是之后！

那为什么先调用 response.getWriter(); 就会导致设置的编码类型不成功呢？来看源码：

org.apache.catalina.connector.Response#getWriter：

    @Override
    public PrintWriter getWriter()
        throws IOException { 
   

        if (usingOutputStream) { 
   
            throw new IllegalStateException
                (sm.getString("coyoteResponse.getWriter.ise"));
        }
				// 看这个if，这个是问题产生的核心内容
        if (ENFORCE_ENCODING_IN_GET_WRITER) { 
   
            /* * If the response's character encoding has not been specified as * described in <code>getCharacterEncoding</code> (i.e., the method * just returns the default value <code>ISO-8859-1</code>), * <code>getWriter</code> updates it to <code>ISO-8859-1</code> * (with the effect that a subsequent call to getContentType() will * include a charset=ISO-8859-1 component which will also be * reflected in the Content-Type response header, thereby satisfying * the Servlet spec requirement that containers must communicate the * character encoding used for the servlet response's writer to the * client). */
            setCharacterEncoding(getCharacterEncoding());
        }

        usingWriter = true;
        outputBuffer.checkConverter();
        if (writer == null) { 
   
            writer = new CoyoteWriter(outputBuffer);
        }
        return writer;
    }