以下内容来自API文档：
public interface RequestDispatcherDefines an object that receives requests from the client and sends them to any resource (such as a servlet, HTML file, or JSP file) on the server. The servlet container creates the RequestDispatcher object, which is used as a wrapper around a server resource located at a particular path or given by a particular name.
This interface is intended to wrap servlets, but a servlet container can create RequestDispatcher objects to wrap any type of resource.
/*
参数：
request - a ServletRequest object that represents the request the client makes of the servlet
response - a ServletResponse object that represents the response the servlet returns to the client
异常：
ServletException - if the target resource throws this exception
IOException - if the target resource throws this exception
IllegalStateException - if the response was already committed*/
void forward(ServletRequest request,
ServletResponse response)
throws ServletException,
IOExceptionForwards a request from a servlet to another resource (servlet, JSP file, or HTML file) on the server. This method allows one servlet to do preliminary processing of a request and another resource to generate the response.
For a RequestDispatcher obtained via getRequestDispatcher(), the ServletRequest object has its path elements and parameters adjusted to match the path of the target resource.
forward should be called before the response has been committed to the client (before response body output has been flushed). If the response already has been committed, this method throws an IllegalStateException. Uncommitted output in the response buffer is automatically cleared before the forward.
The request and response parameters must be either the same objects as were passed to the calling servlet’s service method or be subclasses of the ServletRequestWrapper or ServletResponseWrapper classes that wrap them.
This method sets the dispatcher type of the given request to DispatcherType.FORWARD.