Java RequestDispatcher|极客教程

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。

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。

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

<!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

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

<%@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

<!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

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。

request.setAttribute("now", Instant.now());

我们为请求设置一个属性；这是当前时间。

request.getRequestDispatcher("AnotherServlet").forward(request, response);

包括新属性的请求被发送到AnotherServlet。

AnotherServlet.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。

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。

Instant now = (Instant) request.getAttribute("now");

我们使用getAttribute()方法从请求中检索属性。

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

<!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

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

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");
    }
}