Servlet – RequestDispatcher

RequestDispatcher 是javax.servlet包下的接口。使用这个接口，我们在收到请求后在 servlet 中得到一个对象。使用 RequestDispatcher 对象，我们向其他资源发送请求，其中包括（ servlet、HTML 文件或 JSP 文件）。 RequestDispatcher 对象可用于将请求转发到资源或将资源包含在响应中。资源可以是动态的或静态的。

如何创建 RequestDispatcher 的对象？

获取对象的三种方式：

1. RequestDispatcher requestDispatcher=ServletContext.getRequestDispatcher(String path);

Description:

• public interface ServletContext. Defines a set of methods that a servlet uses to communicate with its servlet container.
• path is a string specifying the pathname to the resource(servlet, HTML file, or JSP file).

2. RequestDispatcher requestDispatcher=ServletContext.getNamedDispatcher(String name);

Description:

• public interface ServletContext. Defines a set of methods that a servlet uses to communicate with its servlet container.
• name is a string specifying the name of a servlet to wrap.

3. RequestDispatcher requestDispatcher=request.getRequestDispatcher(“String path”);

Description:

• request is the HttpServletRequest type object.
• path is a string specifying the pathname to the resource. If it is relative, it must be relative to the current servlet.

方法及说明

该类包含两个方法：

1.前进

Syntax:

void forward(ServletRequest request,ServletResponse response) throws ServletException,IOException

Description:

• Modifier and Type:- void
• This method is used to forward a request from a servlet to another resource (servlet, JSP file, or HTML file) on the server.
• The method get called before the response has been sent to the client. If the response is already sent then the method will throws an IllegalStateException.
• The parameter request(HttpServletRequest type) and response(HttpServletResponse type) are the same objects as were passed to the calling servlet’s service method.
• This method sets the dispatcher type of the given request to DispatcherType.FORWARD.

例子：

Java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
public class GFG extends HttpServlet {
    public void doPost(HttpServletRequest request,
                       HttpServletResponse response)
    {
        // Perform all the work as per your
          // application's architecture
        try {
            RequestDispatcher requestDispatcher;
 
            // path is a string specifying the pathname to
            // the resource. If it is relative, it must be
            // relative against the current servlet
            requestDispatcher=request.getRequestDispatcher("path");
            requestDispatcher.forward(request, response);
        }
        catch (ServletException servletException) {
        }
        catch (IOException ioException) {
        }
        catch (IllegalStateException illegalStateException) {
        }
    }
}

Note: The above code will not run in online IDE this is server-side code.

2.包括

Syntax:

void include(ServletRequest request,ServletResponse response) throws ServletException,IOException

Description:

• Modifier and Type:- void
• This method is used to include the response of resource(for which the request passed servlet, JSP page, HTML file) in the current servlet response.
• The parameter request(HttpServletRequest type) and response(HttpServletResponse type) are the same objects as were passed to the calling servlet’s service method.
• This method sets the dispatcher type of the given request to DispatcherType.INCLUDE.

例子：

Java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
public class GFG extends HttpServlet {
    public void doPost(HttpServletRequest request,
                       HttpServletResponse response)
    {
        // Perform all the work as
        // per your application's architecture
        try {
            RequestDispatcher requestDispatcher;
 
            // path is a string specifying the pathname to
            // the resource. If it is relative, it must be
            // relative against the current servlet
            requestDispatcher=request.getRequestDispatcher("path");
            requestDispatcher.include(request, response);
        }
        catch (ServletException servletException) {
        }
        catch (IOException ioException) {
        }
    }
}

Note: The above code will not run in online IDE this is server-side code.

字段和描述

Type	Name of Field	Description
static String	FORWARD_REQUEST_URI	The string contains the name of the request attribute under which the original request URI is made available to the target of a forward.
static String	FORWARD_CONTEXT_PATH	The string contains the name of the request attribute under which the original context path is made available to the target of a forward.
static String	FORWARD_PATH_INFO	The string contains the name of the request attribute under which the original path info is made available to the target of a forward.
static String	FORWARD_SERVLET_PATH	The string contains the name of the request attribute under which the original servlet path is made available to the target of a forward.
static String	FORWARD_QUERY_STRING	The string contains the name of the request attribute under which the original query string is made available to the target of a forward.
static String	INCLUDE_REQUEST_URI	The string contains the name of the request attribute under which the request URI of the target of include is stored.
static String	INCLUDE_CONTEXT_PATH	The string contains the name of the request attribute under which the context path of the target of an include is stored.
static String	INCLUDE_PATH_INFO	The string contains the name of the request attribute under which the path info of the target of an include is stored.
static String	INCLUDE_SERVLET_PATH	The string contains the name of the request attribute under which the servlet path of the target of an include is stored.
static String	INCLUDE_QUERY_STRING	The string contains the name of the request attribute under which the query string of the target of an include is stored.
static String	ERROR_EXCEPTION	The string contains the name of the request attribute under which the exception object is propagated during an error dispatch.
static String	ERROR_EXCEPTION_TYPE	The string contains the name of the request attribute under which the type of the exception object is propagated during an error dispatch.
static String	ERROR_MESSAGE	The string contains the name of the request attribute under which the exception message is propagated during an error dispatch.
static String	ERROR_REQUEST_URI	The string contains the name of the request attribute under which the request URI whose processing caused the error is propagated during an error dispatch.
static String	ERROR_SERVLET_NAME	The string contains the name of the request attribute under which the name of the servlet in which the error occurred is propagated during an error dispatch.
static String	ERROR_STATUS_CODE	The string contains the name of the request attribute under which the response status is propagated during an error dispatch.