servlet的四种响应

逃离我推掉我的手 2022-08-08 00:44 119阅读 0赞

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

*  response.getWriter() 输出页面

*  response.getOutputStream() 输出二进制流文件

*  request.getRequestDispatcher("ajax.jsp").forward(request, response) 页面跳转

*  response.sendRedirect("ajax.jsp") 重定向

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。

# servlet的四种响应 #

## getWriter ##

getWriter 返回可将字符文本发送到客户端的 `PrintWriter` 对象。

输出

![Center][]

## getOutputStream ##

getOutputStream 返回适用于在响应中编写二进制数据的`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();
    		
    	}

输出

![Center 1][]

## getRequestDispatcher("ajax.jsp").forward(request, response)  ##

跳转到 ajax.jsp页面

![Center 2][]

## sendRedirect("ajax.jsp") ##

重定向到ajax.jsp页面

![Center 3][]

***       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***

# 两个响应情况 #

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。

## getWriter和getOutputStream ##

/**
    	 * @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();
    		
    	}

输出

![Center 4][]

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。

## getWriter和forward ##

/**
    	 * @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

![Center 5][]

没有输出Welcome to Servlet ，直接跳转到了ajax.jsp页面

## getWriter和redirect ##

/**
    	 * @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

![Center 6][]

***    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);
    
    	}

out.flush() 方法会提交掉该response响应，此时页面响应为

![Center 7][]

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)

## getOutputStream和forward ##

前台页面依然出现

![Center 8][]

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。

## getOutputStream和redirect ##

出现与getOutputStream和forward中同样的结果，前台展现，后台出错

## forward和redirect ##

/**
    	 * @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");
    	}

输出

![Center 9][]

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。***

[Center]: /images/20220805/811ed89f41bd4fa6a7c71b8eb40a6a17.png
[Center 1]: /images/20220805/936eafe1a76a4dc4a84f736e4d59f16a.png
[Center 2]: /images/20220805/c0fa7c93bb504e4badd944ecc23feb06.png
[Center 3]: /images/20220805/a4f40734c4f2468c964c389adcf1ed87.png
[Center 4]: /images/20220805/47e6a69811574f948573a88ffaef6543.png
[Center 5]: /images/20220805/9ef30fd1f7e3417cb3eca41a3b5addf5.png
[Center 6]: /images/20220805/6ee90fbf75e149efb716a2682dbf95b1.png
[Center 7]: /images/20220805/32795e85275341a09b04f3bcab5bc448.png
[Center 8]: /images/20220805/6427deee820e49d1b7953578b66dc1f5.png
[Center 9]: /images/20220805/1e033167c17f42a1af69a103b4ec312b.png