关于Servlet过滤器修改输出内容的疑惑

如题，先上代码，代码很简单，一个将jsp输出内容转为大写字母的过滤器：

```public class UpperCaseFilter implements Filter {
private HttpServletRequest request;
private HttpServletResponse response;

public void init(FilterConfig config) throws ServletException {}

public void destroy() {}

public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)
        throws ServletException, IOException {
    request = (HttpServletRequest) req;
    response = (HttpServletResponse) resp;

    ResponseWrapper responseWrapper = new ResponseWrapper(response);
    chain.doFilter(request, responseWrapper);
    String modifiedResponse = responseWrapper.toString().toUpperCase();
    PrintWriter out = response.getWriter();
    out.write(modifiedResponse);
    //response.getOutputStream().println(modifiedResponse);
}

}

对输出内容进行包裹并缓存的类：

public class ResponseWrapper extends HttpServletResponseWrapper {
private StringWriter stringWriter;

public ResponseWrapper(HttpServletResponse response) {
    super(response);
    stringWriter = new StringWriter();
}

@Override
public PrintWriter getWriter() {
    return (new PrintWriter(stringWriter));
}

@Override
public ServletOutputStream getOutputStream() {
    return new ServletStream(stringWriter);
}

class ServletStream extends ServletOutputStream {
    StringWriter stringWriter;

    public ServletStream(StringWriter str) {
        this.stringWriter = str;
    }

    @Override
    public void write(int c) {
        stringWriter.write(c);
    }
}

public String toString() {
    return (stringWriter.toString());
}

}

jsp文件的输出内容为几个小写字母，web.xml中注册了过滤器并映射到jsp文件，代码就不贴了。
现在问题来了，当我直接运行上面的代码的时候，能够输出大写字母；当我把ResponseWrapper类中的getOutputStream方法和ServletStream类注释掉后，仍然可以输出大写字母；当我把ResponseWrapper类中的getWriter方法注释掉，并且将UpperCaseFilter类中的最后两行换成后面的注释中的内容之后，运行会出现getWriter() has already been called for this response异常；当我把仅仅把UpperCaseFilter类中的最后两行换成后面的注释中的内容之后，又可以输出大写字母。
在上面的UpperCaseFilter中，我使用了两种方式获取输出对象，getWriter和getOutputStream，前面的描述的问题中可以看出，通过getWriter获得输出对象不依赖getOutputStream方法，反过来，通过getOutputStream获得输出对象要依赖getWriter方法。
现在，我就迷糊了，请问使用过滤器过滤输出内容的时候，执行过程具体是怎么样的？

http://blog.csdn.net/shuwei003/article/details/7986158