Java RequestDispatcher · ZetCode 中文系列教程

# Java `RequestDispatcher` 原文：http://zetcode.com/java/requestdispatcher/ Java `RequestDispatcher`教程显示了如何使用 Java `RequestDispatcher`将请求分派到资源。 ## `RequestDispatcher` `RequestDispatcher`从客户端接收请求，并将其发送到服务器上的资源（例如 Servlet，HTML 文件，JSP 文件，FreeMarker 或 Thymeleaf 模板）。 ## `RequestDispatcher`方法 `RequestDispatcher`有两种方法： * `forward()` - 将请求从 Servlet 转发到另一个资源 * `include()` - 在响应中包含资源的内容两种方法之间的区别在于，`forward()`方法将在调用后关闭输出流，而`include()`方法将打开输出流。 `include()`方法从另一个资源获取内容，并将其包含在 Servlet 中。 `forward()`方法将请求发送到另一个资源。 ## 获取`RequestDispatcher` `RequestDispatcher`可以从请求对象或 servlet 上下文中获得。 ```java RequestDispatcher dispatcher = request.getRequestDispatcher("greet.jsp"); dispatcher.forward(request, response); ``` 我们可以使用`getRequestDispatcher()`方法从请求对象中获取`RequestDispatcher`。 ```java RequestDispatcher dispatcher = getServletContext().getRequestDispatcher("/greet.jsp"); dispatcher.forward(request, response); ``` 在这里，我们从 servlet 上下文中获取`RequestDispatcher`。在这种情况下，路径必须以斜杠字符开头。 ## Java `RequestDispatcher`转发到 JSP 以下示例将来自客户端的请求发送到 JSP 页面。 `index.html` ```java <!DOCTYPE html> <html> <head> <title>Start Page</title> <meta charset="UTF-8"> </head> <body> <form action="MyServlet"> <label>Enter your name: <input type="text" name="name"> </label> <button type="submit">Submit</button> </form> </body> </html> ``` 在主页中，我们有一个简单的形式：它从用户那里获取一个值，并将其作为请求参数发送到`MyServlet`。 `MyServlet.java` ```java package com.zetcode.web; import java.io.IOException; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; @WebServlet(name = "MyServlet", urlPatterns = {"/MyServlet"}) public class MyServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=UTF-8"); request.getRequestDispatcher("greet.jsp").forward(request, response); } } ``` 在`MyServlet`中，我们使用`RequestDispatcher`转发到`greet.jsp`页面。 `greet.jsp` ```java <%@page contentType="text/html" pageEncoding="UTF-8"%> <!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <title>JSP Page</title> </head> <body> <p>Hello ${param.name}!</p> </body> </html> ``` 在`greet.jsp`页面中，我们显示`name`参数，该参数由用户在表单中设置。 ## Java `RequestDispatcher`转发到 Servlet 以下示例将来自客户端的请求发送到 Servlet，该 Servlet 将处理转发到另一个 Servlet。 `index.html` ```java <!DOCTYPE html> <html> <head> <title>Start Page</title> <meta charset="UTF-8"> </head> <body> <p> <a href="MyServlet">Call servlet</a> </p> </body> </html> ``` 主页包含一个调用`MyServlet`的链接。 `MyServlet.java` ```java package com.zetcode.web; import java.io.IOException; import java.time.Instant; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; @WebServlet(name = "MyServlet", urlPatterns = {"/MyServlet"}) public class MyServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=UTF-8"); request.setAttribute("now", Instant.now()); request.getRequestDispatcher("AnotherServlet").forward(request, response); } } ``` 该请求首先到达`MyServlet`。 ```java request.setAttribute("now", Instant.now()); ``` 我们为请求设置一个属性；这是当前时间。 ```java request.getRequestDispatcher("AnotherServlet").forward(request, response); ``` 包括新属性的请求被发送到`AnotherServlet`。 `AnotherServlet.java` ```java package com.zetcode.web; import java.io.IOException; import java.io.PrintWriter; import java.time.Instant; import java.time.ZoneId; import java.time.format.DateTimeFormatter; import java.time.format.FormatStyle; import java.util.Locale; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; @WebServlet(name = "AnotherServlet", urlPatterns = {"/AnotherServlet"}) public class AnotherServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/plain;charset=UTF-8"); PrintWriter out = response.getWriter(); DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT) .withLocale(Locale.ENGLISH) .withZone(ZoneId.of("UTC")); Instant now = (Instant) request.getAttribute("now"); String output = formatter.format(now); out.println(output); } } ``` `AnotherServlet`将即时对象格式化为简短的英语日期时间格式，并将其打印到输出流中。 ```java DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT) .withLocale(Locale.ENGLISH) .withZone(ZoneId.of("UTC")); ``` 我们使用`DateTimeFormatter`类格式化日期时间。 ```java Instant now = (Instant) request.getAttribute("now"); ``` 我们使用`getAttribute()`方法从请求中检索属性。 ```java String output = formatter.format(now); out.println(output); ``` 即时被格式化并打印到输出中。 ## Java `RequestDispatcher`包括下一个示例包括从另一个 servlet 到调用 servlet 的输出。 `index.html` ```java <!DOCTYPE html> <html> <head> <title>Start Page</title> <meta charset="UTF-8"> </head> <body> <p> <a href="MyServlet">Call servlet</a> </p> </body> </html> ``` 主页包含一个调用`MyServlet`的链接。 `MyServlet.java` ```java package com.zetcode.web; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; @WebServlet(name = "MyServlet", urlPatterns = {"/MyServlet"}) public class MyServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/plain;charset=UTF-8"); PrintWriter out = response.getWriter(); out.println("Hello from MyServlet"); request.getRequestDispatcher("AnotherServlet").include(request, response); } } ``` `MyServlet`将数据打印到输出流并转发到`AnotherServlet`。 `AnotherServlet.java` ```java package com.zetcode.web; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; @WebServlet(name = "AnotherServlet", urlPatterns = {"/AnotherServlet"}) public class AnotherServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/plain;charset=UTF-8"); PrintWriter out = response.getWriter(); out.println("Hello from AnotherServlet"); } } ``` `AnotherServlet`还将数据打印到输出流。最后，我们将两条消息都写入输出流并发送给客户端。在本教程中，我们介绍了 Java `RequestDispatcher`。我们已经介绍了`RequestDispatcher`，`forward`和`include()`方法。您可能还需要查看相关的教程： [Java servlet JSON 教程](/articles/javaservletjson/)，[从 Java servlet](/articles/javaservlettext/) ， [Java servlet 复选框教程](/articles/javaservletcheckbox/)，[提供纯文本 Java Servlet 图像教程](/articles/javaservletimage/)， [Java Servlet HTTP 标头](/articles/javaservlethttpheaders/)或 [Java 教程](/lang/java/)，