Javax.servlet.RequestDispatcher.forward(ServletRequest request, ServletResponse response)

Javax.servlet.http.HttpServletResponse.sendRedirect(String location)

forward

作用于服務器端，重定向后客戶端瀏覽器地址欄的URL不變，無法通過get方式傳遞參數，不過可以通過HttpServletResponse. setAttribute(key, values)來做：

JDK的DOC中的描述：

Forwards 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.

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.

這個方法允許一個servlet對一個請求作完處理后，并使用另一個資源作響應。

forward應該在向客戶端發出響應之前被調用（response的輸出流被刷新之前），如果已經向客戶端發出了響應，這個方法將拋出一個 “IllegalStateException”的異常。未發出響應response的輸出緩沖區在調用forward會被自動的清空。

得到RequestDispatcher引用的方法：

1、 javax.servlet.ServletContext.getRequestDispatcher(String path)

JDK的DOC中的描述：

The pathname must begin with a "/" and is interpreted as relative to the current context root. Use getContext to obtain a RequestDispatcher for resources in foreign contexts. This method returns null if the ServletContext cannot return a RequestDispatcher.

用的最多，最容易理解的方法，path必須是以“/”開頭，路徑是相對于全局上下文路徑，對應于web應用的根目錄

2、javax.servlet.ServletRequest.getRequestDispatcher(String path)

JDK的DOC中的描述：

The pathname specified may be relative, although it cannot extend outside the current servlet context. If the path begins with a "/" it is interpreted as relative to the current context root. This method returns null if the servlet container cannot return a RequestDispatcher.

可以是相對地址，以當前資源的根目錄，但不能超出當前資源根目錄。

可以是以“/”開頭的地址，當前root作為根目錄環境。

2、 javax.servlet.ServletContext.getNamedDispatcher(String name)

JDK的DOC中的描述：

Servlets (and JSP pages also) may be given names via server administration or via a web application deployment descriptor.

得到名為name的資源。

Servlets（也可是JSP 頁面）可能會有個給定的名字，該名字是服務器或者web應用的部署描述web.xml提供的。

sendRedirect

工作在客戶端，重定向后客戶端瀏覽器地址欄的URL變為新的資源URL，可以通過get方式傳遞參數。

JDK的DOC的描述：

Sends a temporary redirect response to the client using the specified redirect location URL. This method can accept relative URLs; the servlet container must convert the relative URL to an absolute URL before sending the response to the client. If the location is relative without a leading '/' the container interprets it as relative to the current request URI. If the location is relative with a leading '/' the container interprets it as relative to the servlet container root.

If the response has already been committed, this method throws an IllegalStateException. After using this method, the response should be considered to be committed and should not be written to.

這個方法可以接受相對的URL，在發送響應之前servlet容器會將其轉化為絕對的URL。如果相對的URL不以“/”開頭，容器將解釋為是相對當前請求的路徑URI，如果是以“/”開頭，則解釋為是相對于servlet容器的更目錄環境。

注意：如果已經向客戶端響應了請求，這個方法將拋出一個“IllegalStateException”的異常。調用了這個方法后，這個響應是否被提交或是否提交完畢。

sendRedirect比較簡單，參數location就是代表重定向目標新資源。

localtion可以有以下幾種情況：

可以是絕對地址，如:http：//localhost:8080/servlet/a.jsp

可以是相對地址，相對于當前訪問資源的根目錄，如：a.jsp

可以是以“/”開頭的地址，則認為是先對該web應用的根目錄，如：/a.jsp