在一个servlet的请求中，响应的方式的通常有四式，response.getWriter()，response.getOutputStream()，
request.getRequestDispatcher("ajax.jsp").forward(request, response) 和 response.sendRedirect("ajax.jsp") 。
其中
这四种方式单独使用，大家应该都很熟悉，如果一个servlet里面写了两种响应，会出现什么情况呢，今天看一下写两个响应的现象分析。
getWriter 返回可将字符文本发送到客户端的
PrintWriter 对象。
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// 单独这一块，输出Welcome to Servlet
PrintWriter out = response.getWriter();
out.println("<body><h1>");
out.println("Welcome to Servlet");
out.println("</h1></body>");
}
| zh_cn |
ServletOutputStream。 /**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
OutputStream os = response.getOutputStream();
InputStream is = new FileInputStream(new File("D:\\pic\\P41004-154736.jpg"));
byte[] buf = new byte[1024];
while (is.read(buf) > 0) {
os.write(buf);
}
os.flush();
is.close();
os.close();
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.getRequestDispatcher("ajax.jsp").forward(request, response);
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.sendRedirect("ajax.jsp");
}
上面采用forward跳转的时候，地址栏显示的是res请求，使用redirect重定向之后，等于重新发送了一个ajax.jsp的请求，URL地址栏变为了ajax.jsp
一个servlet请求会有一个request请求和一个response响应，那如果一个response想响应两次呢？一般都是会出错的。代码中避免使用多个响应的情况。
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
PrintWriter out = response.getWriter();
out.println("<body><h1>");
out.println("Welcome to Servlet");
out.println("</h1></body>");
OutputStream os = response.getOutputStream();
InputStream is = new FileInputStream(new File("D:\\pic\\P41004-154736.jpg"));
byte[] buf = new byte[1024];
while (is.read(buf) > 0) {
os.write(buf);
}
os.flush();
is.close();
os.close();
}抛出 getWriter() has already been called for this response 异常信息
上面介绍了单独使用getWriter和 getOutputStream的情况，分别返回响应页面和二进制流文件。
这两个方法都有几个共同的异常信息，IOException 和 IllegalStateException。
getWriter的IllegalStateException异常描述：if thegetOutputStream method has already been called for this response object
getOutputStream的IllegalStateException异常描述：if thegetWriter method has been called on this response
这个IllegalStateException信息说明，这两个响应不可以同时使用，否则会出现IllegalStateException。
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// 单独这一块，输出Welcome to Servlet
PrintWriter out = response.getWriter();
out.println("<body><h1>");
out.println("Welcome to Servlet");
out.println("</h1></body>");
request.getRequestDispatcher("ajax.jsp").forward(request, response);
}
输出
没有输出Welcome to Servlet ，直接跳转到了ajax.jsp页面
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// 单独这一块，输出Welcome to Servlet
PrintWriter out = response.getWriter();
out.println("<body><h1>");
out.println("Welcome to Servlet");
out.println("</h1></body>");
// 带上response.sendRedirect 时，直接跳转到ajax.jsp
response.sendRedirect("ajax.jsp");
}
输出
通过上面的getWriter和forward的例子和这里的getWriter和redirect的例子可以看出，getWriter只向页面输出内容，但并没有提交（commit）该响应。后续有可以提交响应的响应代码时，会覆盖掉getWriter的响应输出。
将上面的代码做一点修改，添加一行代码out.flush();
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// 单独这一块，输出Welcome to Servlet
PrintWriter out = response.getWriter();
out.println("<body><h1>");
out.println("Welcome to Servlet");
out.println("</h1></body>");
out.flush();
request.getRequestDispatcher("ajax.jsp").forward(request, response);
}
同时，后台会出现异常
java.lang.IllegalStateException: Cannot forward after response has been committed at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:348) at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:338)
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
OutputStream os = response.getOutputStream();
InputStream is = new FileInputStream(new File("D:\\pic\\P41004-154736.jpg"));
byte[] buf = new byte[1024];
while (is.read(buf) > 0) {
os.write(buf);
}
os.flush();
is.close();
os.close();
request.getRequestDispatcher("ajax.jsp").forward(request, response);
}同时后台会出现
严重: Servlet.service() for servlet [com.test.ResTest] in context with path [/servletDemo] threw exception java.lang.IllegalStateException: Cannot forward after response has been committed at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:348) at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:338) at com.test.ResTest.doPost(ResTest.java:52) at com.test.ResTest.doGet(ResTest.java:34)
RequestDispatcher 类的forward方法有个IllegalStateException异常
异常信息这样描述：if the response was already committed ，如果响应已经被提交。
这里例子说明getOutputStream，对前台进行了响应，并且提交了该响应。
出现与getOutputStream和forward中同样的结果，前台展现，后台出错
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.getRequestDispatcher("ajax.jsp").forward(request, response);
response.sendRedirect("ajax.jsp");
}
这个异常信息与前面的相同，就不再做分析了。
经过上面的几个同时两个响应的例子，可以看出，一个servlet请求只能有一个响应，任何多个响应的代码都是错误的，都会出现IllegalStateException。也就是说，多个响应的场景是非法的，一个servlet一个响应就可以了，是它的标配。
上面的内容纯属个人总结，若有描述不详尽的地方或错误地方，烦请提出，谢谢。
原文：http://blog.csdn.net/magi1201/article/details/45271987
