【Java.Web】Servlet —— 请求的转发和包含
请求的转发和包含
Servlet的service()方法是由Servlet容器来调用的，用户不能在一个Servlet对象中直接调用另一个Servlet对象的servie()方法，因为一个Servlet对象无法得到另一个Servlet对象的引用。
但是web应用在响应客户端的一个请求时，有可能响应过程很复杂，需要多个web组件共同协作（参见上一节——动态生成图像）。尽管Servlet对象无法直接调用另一个Servlet对象的service()方法，但Servlet规范为Web组件之间的协作提供了两种途径：
- 请求转发
Servlet（源组件）先对客户请求做一些预处理操作，然后把请求转发给其他Web组件（目标组件）来完成包括生成响应结果在内的后续操作；
- 请求包含
Servlet（源组件）把其他Web组件（目标组件）生成的响应结果包含到自身的响应结果中；
- 请求转发 V.S. 请求包含
- 对于转发和包含，源组件和目标组件处理的都是同一个客户请求；源组件和目标组件共享同一个ServletRequest和ServletResponse对象
- 对于转发和包含，目标组件都可以为Servlet，JSP或HTML文档
- 对于转发和包含，都依赖与javax.servlet.RequestDispatcher接口
javax.servlet.RequestDispatcher接口
该接口表示请求分发器，
| Modifier and Type | Method and Description |
|---|---|
void |
forward(ServletRequest request, ServletResponse response)
Forwards a request from a servlet to another resource (servlet, JSP file, or HTML file) on the server.
|
void |
include(ServletRequest request, ServletResponse response)
Includes the content of a resource (servlet, JSP page, HTML file) in the response.
|
- forward()方法
- forward()方法的处理流程如下：（1）清空用于存放响应正文数据的缓冲区；（2）如果目标组件为Servlet或JSP，就调用它们的service()方法，把该方法产生的响应结果发送到客户端，如果目标组件为文件系统中的静态html文档，就读去文档中的数据并把它发送到客户端。
- 由于forward()方法先清空用于存放响应正文数据的缓冲区，因此servlet源组件生成的响应结果不会被发送到客户端，只有目标组件生成的结果才会被发送到客户端；
- 如果源组件在进行请求转发之前，已经提交了响应结果（例如调用了flushBuffer方法，或者close()方法），那么forward()方法会抛出IllegalStateException。为了避免该异常，不应该在源组件中提交响应结果。
- include()方法
- 包含与转发相比，源组件与被包含的目标组件的输出数据都会被添加到响应结果中
- 在目标组件中对响应状态代码或者响应头所做的修改都会被忽略
当Servlet源组件调用RequestDispatcher的forward或include方法时，都要把当前的ServletRequest对象和ServletResponse对象作为参数传给forward或include方法，这使得源组件和目标组件共享同一个ServletRequest对象和ServletResponse对象。
Servlet可以通过两个两种方式得到RequestDispatcher对象：
- 调用ServletContext的getRequestDispatcher(String path)方法，path参数指定目标组件的路径
- path参数必须为绝对路径；绝对路径，就是以符号“/”开头的路径，“/”表示当前Web应用的URL路口
- 调用ServletRequest的getRequestDispatcher(String path)方法，path参数指定目标组件的路径
- path参数既可以为绝对路径，也可以为相对路径；相对路径，就是相对于当前Servlet组件的路径，不以“/”开头
请求转发
如下实例：源servlet —— ForwardSourceServlet；目标servlet —— ForwardDestinationServlet；
package com.gof.test.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.http.HttpServlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletContext;
import javax.servlet.RequestDispatcher;
public class ForwardSourceServlet extends HttpServlet {
/**
*
*/
private static final long serialVersionUID = 7412912720309987937L;
@Override
public void service(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
String username = req.getParameter("username");
String message = null;
if (username == null){
message = "Please input username.";
}else{
message = "Hello " + username;
}
// add attribute to request
req.setAttribute("msg", message);
// forward the request to ForwardDestinationServlet
ServletContext context = getServletContext();
RequestDispatcher dispatcher = context.getRequestDispatcher("/forwarddest");
// try to output something into response
PrintWriter out = res.getWriter();
out.println("Output from ForwardSourceServlet before forwarding request");
if (dispatcher != null){
dispatcher.forward(req, res);
}
out.println("Output from ForwardSourceServlet after forwarding request");
out.close();
}
}
package com.gof.test.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.GenericServlet;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ForwardDestinationServlet extends HttpServlet {
/**
*
*/
private static final long serialVersionUID = 7412912720309987937L;
@Override
public void service(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
String message = (String)req.getAttribute("msg");
PrintWriter out = res.getWriter();
out.println(message);
out.close();
}
}
在web.xml中注册：
<!-- Forward request -->
<!-- test url = http://localhost:8080/base-webapp/forwardsrc?username=New User -->
<servlet>
<servlet-name>forwardsrc</servlet-name>
<servlet-class>com.gof.test.servlet.ForwardSourceServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>forwardsrc</servlet-name>
<url-pattern>/forwardsrc</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>forwarddest</servlet-name>
<servlet-class>com.gof.test.servlet.ForwardDestinationServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>forwarddest</servlet-name>
<url-pattern>/forwarddest</url-pattern>
</servlet-mapping>
访问如下URL:
http://localhost:8080/base-webapp/forwardsrc?username=New User
修改1：
在调用forward方法之前调用flush()/close()方法，将会抛出一个IllegalStateException异常；同时，目标组件中的内容不会输出：只会输出源组件前面输出的内容：
修改2：
将获取RequestDispatcher的方法参数path改为“forwarddest”，抛出IllegalArgumentException，提示参数没有以“/”开头。
修改3：
将获取RequestDispatcher的方法改为：
RequestDispatcher dispatcher = req.getRequestDispatcher("forwarddest");
可以正常运行。
请求包含
如下示例：源组件 —— IncludeSourceServlet；目标组件 —— includeheader.html，IncludeDestinationServlet，includefooter.html：
package com.gof.test.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.http.HttpServlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletContext;
import javax.servlet.RequestDispatcher;
public class IncludeSourceServlet extends HttpServlet {
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException{
resp.setContentType("text/html");
PrintWriter out = resp.getWriter();
out.println("<html><header><title>Include Test</title></header>");
out.println("<body>");
ServletContext context = getServletContext();
RequestDispatcher headDispatcher = context.getRequestDispatcher("/includeheader.html");
RequestDispatcher bodyDispatcher = context.getRequestDispatcher("/includedest");
RequestDispatcher footDispatcher = context.getRequestDispatcher("/includefooter.html");
headDispatcher.include(req, resp);
bodyDispatcher.include(req, resp);
footDispatcher.include(req, resp);
out.println("</body></html>");
out.close();
}
}
includeheader.html
<br/> This is from includeheader.html <br/>
package com.gof.test.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.http.HttpServlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletContext;
import javax.servlet.RequestDispatcher;
public class IncludeDestinationServlet extends HttpServlet {
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException{
PrintWriter out = resp.getWriter();
out.println("<br/>This is from IncludeDestinationServlet<br/>");
}
}
includefooter.html
<br/> This is from includefooter.html <br/>
在web.xml中注册：
<!-- Include request -->
<!-- test url = http://localhost:8080/base-webapp/includesrc -->
<servlet>
<servlet-name>includesrc</servlet-name>
<servlet-class>com.gof.test.servlet.IncludeSourceServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>includesrc</servlet-name>
<url-pattern>/includesrc</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>includedest</servlet-name>
<servlet-class>com.gof.test.servlet.IncludeDestinationServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>includedest</servlet-name>
<url-pattern>/includedest</url-pattern>
</servlet-mapping>
在浏览器中访问如下的URL：
http://localhost:8080/base-webapp/includesrc
结果如下：
