java.lang.IllegalStateException:getOutputStream() has already been called for this response

Question

1. 系统使用的是spring3.1
2. tomcat6
3. 页面无 out 等输出，问题是随机出现

下面是错误提示信息，网上找了很多方法都没解决。

严重: Servlet.service() for servlet spring-mvc threw exception
java.lang.IllegalStateException: getOutputStream() has already been called for this response

严重: Servlet.service() for servlet spring-mvc threw exception
java.lang.IllegalStateException: getOutputStream() has already been called for this response
	at org.apache.catalina.connector.Response.getWriter(Response.java:611)
	at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
	at javax.servlet.ServletResponseWrapper.getWriter(ServletResponseWrapper.java:112)
	at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125)
	at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118)
	at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:180)

Answer 1

下面的代码是在实现HTTP协议封装Response时的部分代码 
SimpleHttpResponse.java 

OutputStream ops;
PrintWriter writer;
String ce = "utf-8";
public OutputStream getOutputStream() throws Exception {
		if(dateType==W)throw new Exception("getWriter() has already been called!");
		if(ops==null){
			ops = new ByteArrayOutputStream();
			dateType = O;
		}
		return ops;
	}
public PrintWriter getWriter() throws Exception {
		if(dateType==O)throw new Exception("getOutputStream() has already been called!");
		if(writer==null){
			ops = new ByteArrayOutputStream();
			writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(ops,ce)),true);
			dateType = W;
		}
		return writer;
	}

W和O表示已经定义流的模式。 
为什么getWriter()之后不能getOutputStream()？或者getOutputStream()之后不能getWriter()呢？
很简单writer是写入字符的，一行写完之后还可能会插入换行回车。显然是不能和outputStream重叠使用。 
原理已经给出了，请楼主自己根据实际情况理解错误的原因。