week hello2源码解析
hello2应用程序是一个Web模块，它使用Java Servlet技术来显示问候语和响应，源代码位置在_tut-install_/examples/web/servlet/hello2/
虽然该hello2应用程序的表现几乎相同的hello1 应用程序，但它使用Java Servlet技术，而不是JavaServer Faces技术实现的
cd到该目录，命令行打包（可参见上一篇博客步骤）
打开eclipse，点击GreetingServlet.java Run as Run on server
1.选择hello2文件夹并单击“打开项目”
2.点击pom.xml打开网页如图所示：
可见，跟hello1 不一样的是hello2是servlet的。
3.
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">//此标签可告知浏览器文档使用哪种HTML规范；DTD一套文本类型的定义；标准原文的链接
<html lang="en"
xmlns="http://www.w3.org/1999/xhtml"
xmlns:h="http://xmlns.jcp.org/jsf/html">//语言使用英文；xml采用名字空间声明，允许你通过一个网址来识别你的标记；jcp是java社区的执行委员会
<h:head>
<title>Facelets Hello Greeting</title>
</h:head>
<h:body>
<h:form>
<h:graphicImage url="#{resource['images:duke.waving.gif']}"
alt="Duke waving his hand"/>//作为资源形式出现的；alt加载不出来就进行替换
<h2>Hello, my name is Duke. What's yours?</h2>
<h:inputText id="username"
title="My name is: "
value="#{hello.name}"
required="true"
requiredMessage="Error: A name is required."
<p></p>
<h:commandButton id="submit" value="Submit" action="response">
</h:commandButton>
<h:commandButton id="reset" value="Reset" type="reset">
</h:commandButton>//Submit commandButton元素将操作指定为response，表示单击按钮时，将response.xhtml显示该页面
</h:form>
...
</h:body>
</html>
展开“源包”节点，然后展开javaeetutorial.hello2节点。双击该Hello.java文件以查看它。如图所示
点开两个后可参看代码：
greetingservelet.java
/**
* Copyright (c) 2014 Oracle and/or its affiliates. All rights reserved.
*
* You may not modify, use, reproduce, or distribute this software except in
* compliance with the terms of the License at:
* https://github.com/javaee/tutorial-examples/LICENSE.txt
*/
package javaeetutorial.hello2;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* This is a simple example of an HTTP Servlet. It responds to the GET method of
* the HTTP protocol.
*/
@WebServlet("/greeting")
public class GreetingServlet extends HttpServlet {
@Override
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
response.setBufferSize(8192);
try (PrintWriter out = response.getWriter()) {
out.println("<html lang=\"en\">"
+ "<head><title>Servlet Hello</title></head>");
// then write the data of the response
out.println("<body bgcolor=\"#ffffff\">"
+ "<img src=\"resources/images/duke.waving.gif\" "
+ "alt=\"Duke waving his hand\">"
+ "<form method=\"get\">"
+ "<h2>Hello, my name is Duke. What's yours?</h2>"
+ "<input title=\"My name is: \" type=\"text\" "
+ "name=\"username\" size=\"25\"/>"
+ "<p></p>"
+ "<input type=\"submit\" value=\"Submit\"/>"
+ "<input type=\"reset\" value=\"Reset\"/>"
+ "</form>");
String username = request.getParameter("username");
if (username != null && username.length() > 0) {
RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher("/response");
if (dispatcher != null) {
dispatcher.include(request, response);
}
}
out.println("</body></html>");
}
}
@Override
public String getServletInfo() {
return "The Hello servlet says hello.";
}
}
这个servlet覆盖了doGet方法，实现了HTTP的GET方法。servlet显示一个简单的HTML问候表单，它的Submit按钮，就像hello1那样，为其操作指定一个响应页面。下面的摘录从@WebServlet注释开始，它指定了相对于上下文根的URL模式:
responseservlet.java
/**
* Copyright (c) 2014 Oracle and/or its affiliates. All rights reserved.
*
* You may not modify, use, reproduce, or distribute this software except in
* compliance with the terms of the License at:
* https://github.com/javaee/tutorial-examples/LICENSE.txt
*/
package javaeetutorial.hello2;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* This is a simple example of an HTTP Servlet. It responds to the GET method of
* the HTTP protocol.
*/
@WebServlet("/greeting")
public class GreetingServlet extends HttpServlet {
@Override
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
response.setBufferSize(8192);
try (PrintWriter out = response.getWriter()) {
out.println("<html lang=\"en\">"
+ "<head><title>Servlet Hello</title></head>");
// then write the data of the response
out.println("<body bgcolor=\"#ffffff\">"
+ "<img src=\"resources/images/duke.waving.gif\" "
+ "alt=\"Duke waving his hand\">"
+ "<form method=\"get\">"
+ "<h2>Hello, my name is Duke. What's yours?</h2>"
+ "<input title=\"My name is: \" type=\"text\" "
+ "name=\"username\" size=\"25\"/>"
+ "<p></p>"
+ "<input type=\"submit\" value=\"Submit\"/>"
+ "<input type=\"reset\" value=\"Reset\"/>"
+ "</form>");
String username = request.getParameter("username");
if (username != null && username.length() > 0) {
RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher("/response");
if (dispatcher != null) {
dispatcher.include(request, response);
}
}
out.println("</body></html>");
}
}
@Override
public String getServletInfo() {
return "The Hello servlet says hello.";
}
}
要理解上面代码我们必须先要理解以下概念
（1）getParameter
String javax.servlet.ServletRequest.getParameter(String name) |
以String形式返回请求参数的值，如果参数不存在，则返回null。请求参数是随请求一起发送的额外信息。对于HTTP servlet，参数包含在查询字符串或发布的表单数据中。只有在确定参数只有一个值时才应使用此方法
（2）RequestDispatcher
javax.servlet.RequestDispatcher |
定义一个对象，该对象接收来自客户端的请求，并将它们发送到服务器上的任何资源（例如servlet，HTML文件或JSP文件）。 servlet容器创建RequestDispatcher对象，该对象用作位于特定路径或由特定名称给出的服务器资源的包装器。此接口旨在包装servlet，但servlet容器可以创建RequestDispatcher对象来包装任何类型的资源。
（3）getServletContext()
ServletContext javax.servlet.GenericServlet.getServletContext() |
提供该方法是为了方便起见。它从servlet的ServletConfig对象获取上下文。
返回：ServletContext由init方法传递给此servlet的ServletContext对象
（4）getRequestDispatcher()
RequestDispatcher javax.servlet.ServletContext.getRequestDispatcher(String path) |
返回一个RequestDispatcher对象，该对象充当位于给定路径的资源的包装器。 RequestDispatcher对象可用于将请求转发到资源或将资源包含在响应中。资源可以是动态的也可以是静态的。路径名必须以 / 开头，并且被解释为相对于当前上下文根。使用getContext为外部上下文中的资源获取RequestDispatcher。如果ServletContext无法返回RequestDispatcher，则此方法返回null。
(5) include(request, response)
void javax.servlet.RequestDispatcher.include(ServletRequest request, ServletResponse response) throws ServletException, IOException |
包含响应中的资源（servlet，JSP页面，HTML文件）的内容。从本质上讲，这种方法可以实现程序化服务器端包含。 ServletResponse对象的路径元素和参数与调用者保持不变。包含的servlet无法更改响应状态代码或设置标头;任何改变的尝试都会被忽略。请求和响应参数必须是传递给调用servlet的服务方法的相同对象，或者是包装它们的ServletRequestWrapper或ServletResponseWrapper类的子类。此方法将给定请求的调度程序类型设置为DispatcherType.INCLUDE。
参数：
Parameters:
-
request a ServletRequest object that contains the client's request
-
response a ServletResponse object that contains the servlet's response
运行后结果：
