Servlet的API有很多，这里只谈谈两个Servlet对象：ServletConfig对象和ServletContext对象。
1. ServletConfig对象
在Servlet的配置文件中，可以使用一个或多个<init-param>标签为servlet配置一些初始化参数，当Servlet配置了初始化参数后，web容器在创建servlet实例对象时，会自动将这些参数封装到ServletConfig对象中，并在调用Servlet的init方法时，将ServletConfig对象传递给Servlet。进而，程序员通过ServletConfig对象就可以得到当前Servlet的初始化参数信息。该对象的getInitParameter(String name)用来获得指定参数名的参数值，getInitParameterNames()用来获得所有参数名，我们测试一下：
在test工程的src下新建一个包servletConfig，然后新建一个ServletConfigDemo1类，在配置文件里进行如下配置：
- <servlet>
- <servlet-name>ServletConfigDemo1</servlet-name>
- <servlet-class>servletConfig.ServletConfigDemo1</servlet-class>
- <init-param>
- <param-name>category</param-name>
- <param-value>book</param-value>
- </init-param>
- <init-param>
- <param-name>school</param-name>
- <param-value>tongji</param-value>
- </init-param>
- <init-param>
- <param-name>name</param-name>
- <param-value>java</param-value>
- </init-param>
- </servlet>
- <servlet-mapping>
- <servlet-name>ServletConfigDemo1</servlet-name>
- <url-pattern>/ServletConfigDemo1</url-pattern>
- </servlet-mapping>
在ServletConfigDemo1.java中的代码如下：
- public class ServletConfigDemo1 extends HttpServlet {
- ServletConfig config = null;
- @Override
- protected void doGet(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- String value = config.getInitParameter("category");
- resp.getOutputStream().write((value + "<br/>").getBytes());
-
- Enumeration e = config.getInitParameterNames();
- while(e.hasMoreElements()){
- String name = (String) e.nextElement();
- value = config.getInitParameter(name);
- resp.getOutputStream().write((name + "=" + value + "<br/>").getBytes());
- }
- }
- @Override
- protected void doPost(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- doGet(req, resp);
- }
- @Override
- public void init(ServletConfig config) throws ServletException {
- this.config = config;
- }
- }
在浏览器中输入：http://localhost:8080/test/ServletConfigDemo1，即可在浏览器中显示读取参数的结果。
注：实际开发中，并不需要重写init方法，以上代码中重写init方法是为了说明config对象的传递过程。其实在父类的init方法中已经实现了该config的传递了，我们只要直接调用getServletConfig()就可以得到config对象，即在doGet方法中直接通过下面的调用方式获得ServletConfig对象：
- ServletConfig config = this.getServletConfig();
那么ServletConfig对象有什么作用呢？一般主要用于以下情况：
1）获得字符集编码；
2）获得数据库连接信息；
3）获得配置文件，查看struts案例的web.xml文件等。
2. ServletContext对象
web容器在启动时，它会为每个web应用程序都创建一个对应的ServletContext对象，它代表当前web应用(web工程)。在ServletConfig接口中有个getServletContext方法用来获得ServletContext对象；ServletContext对象中维护了ServletContext对象的引用，也可以直接获得ServletContext对象。所以开发人员在编写Servlet时，可以通过下面两种方式获得ServletContext对象：
- this.getServletConfig().getServletContext();
- this.getServletContext();
一般直接获得即可。
由于一个web应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯，ServletContext对象通常也被称为context域对象。有如下主要方法：
- getResource(String path);
- getResourceAsStream(String path);
- setAttribute(Sring name, Object obj);
- getAttribute(String name);
- getInitParameter(String name);
-
- getNamedeDispatcher(String name);
-
- getServletContextName();
ServletContext应用有哪些呢？
1）多个Servlet通过ServletContext对象实现数据共享（见下面的Demo1和Demo2）
2）获取web应用的初始化参数（见Demo3）
3）实现Servlet的转发（见Demo4和Demo5）
4）利用ServletContext对象读取资源文件（xml或者properties）（见Demo6）
下面对ServletContext对象写几个Demo测试一下：
Demo1：往context域中存入数据
- public class ServletContextDemo1 extends HttpServlet {
- @Override
- protected void doGet(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- String data = "adddfdf";
- ServletContext context = this.getServletConfig().getServletContext();
- context.setAttribute("data", data);
- }
- @Override
- protected void doPost(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- doGet(req, resp);
- }
- }
Demo2：从context域中读取数据
- public class ServletContextDemo2 extends HttpServlet {
- @Override
- protected void doGet(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- ServletContext context = this.getServletContext();
- String data = (String) context.getAttribute("data");
- resp.getOutputStream().write(data.getBytes());
- }
- @Override
- protected void doPost(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
-
- doGet(req, resp);
- }
- }
Demo3：获取整个web应用的初始化参数
- public class ServletContextDemo3 extends HttpServlet {
- @Override
- protected void doGet(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
-
- ServletContext context = this.getServletContext();
- String url = context.getInitParameter("url");
- resp.getOutputStream().write(url.getBytes());
- }
-
- @Override
- protected void doPost(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
-
- doGet(req, resp);
- }
- }
Demo4：实现转发
- public class ServletContextDemo5 extends HttpServlet {
- @Override
- protected void doGet(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- ServletContext context = this.getServletContext();
- RequestDispatcher rd = context.getRequestDispatcher("/ServletContextDemo5");
- rd.forward(req, resp);
- }
-
- @Override
- protected void doPost(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- doGet(req, resp);
- }
- }
Demo5：
- public class ServletContextDemo5 extends HttpServlet {
-
- @Override
- protected void doGet(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- resp.getOutputStream().write("ServletDemo5".getBytes());
- }
-
- @Override
- protected void doPost(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- doGet(req, resp);
- }
-
- }
Demo6：读取资源文件
- public class ServletContextDemo6 extends HttpServlet {
-
- @Override
- protected void doGet(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
-
-
-
-
- }
-
-
- private void test4() throws FileNotFoundException, IOException {
- String path = this.getServletContext().getRealPath("/WEB-INF/classes/db.properties");
- String filename = path.substring(path.lastIndexOf("\\")+1);
-
- InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
- byte buffer[] = new byte[1024];
- int len = 0;
-
- FileOutputStream out = new FileOutputStream("e:\\" + filename);
- while((len = in.read(buffer)) > 0){
- out.write(buffer, 0, len);
- }
- }
-
-
- private void test3(HttpServletResponse resp) throws IOException {
- ClassLoader loader = ServletContextDemo6.class.getClassLoader();
- InputStream in = loader.getResourceAsStream("db.properties");
- Properties prop = new Properties();
- prop.load(in);
- String driver = prop.getProperty("driver");
- resp.getOutputStream().write(driver.getBytes());
- }
-
- private void test2(HttpServletResponse resp) throws FileNotFoundException,
- IOException {
- String path = this.getServletContext().getRealPath("/WEB-INF/classes/db.properties");
- FileInputStream in = new FileInputStream(path);
-
- Properties prop = new Properties();
- prop.load(in);
- String driver = prop.getProperty("driver");
- resp.getOutputStream().write(driver.getBytes());
- }
-
-
- private void test1(HttpServletResponse resp) throws IOException {
- InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
-
- Properties prop = new Properties();
- prop.load(in);
- String driver = prop.getProperty("driver");
- String url = prop.getProperty("url");
- String username = prop.getProperty("username");
- String password = prop.getProperty("password");
- resp.getOutputStream().write(driver.getBytes());
- }
-
- @Override
- protected void doPost(HttpServletRequest req, HttpServletResponse resp)
- throws ServletException, IOException {
- doGet(req, resp);
- }
- }
ServletConfig对象和ServletContext对象就介绍这么多吧，如有错误之处，欢迎留言指正~
相关阅读：http://blog.csdn.net/column/details/servletandjsp.html
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