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 the getOutputStream
method has already been called for this response object
getOutputStream的IllegalStateException异常描述: if the getWriter
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。