对于每个Servlet的设置信息，web容器会为其生成一个ServletConfig作为代表对象，可以从该对象取得Servlet初始参数，以及代表整个web应用程序的ServletContext对象。
一、Servlet、ServletConfig、与GenericServlet
在Servlet的接口上，定义了与Servlet生命周期以及请求服务相关的init()、service()、destroy()三个方法。
web容器启动后，会读取Servlet设置信息，将Servlet类加载并实例化，并为每个Servlet设置信息产生一个ServletConfig对象，而后调用Servlet接口的init()方法，并将产生的ServletConfig对象作为参数传入init()。如图
这个过程只在创建Servlet实例后发生一次，之后每次请求到来，调用Servlet实例的service()方法进行服务。
GenericServlet主要的目的是，将初始的Servlet调用init()方法传入的ServletConfig封装起来。
二、使用ServletConfig
ServletConfig设置了getInitParameter()、getInitParameterNames()方法，可以取得设置Servlet时的初始参数。如
...
@WebFilter(
urlPatterns = { "/*" },
initParams = {
@WebInitParam(name = "ENCODING", value = "UTF-8")
}) private String ENCODING; public void init(FilterConfig fConfig) throws ServletException {
// TODO Auto-generated method stub
ENCODING=fConfig.getInitParameter("ENCODING");//读取初始参数
}
三、使用ServletContext
ServletContext接口定义了运行Servlet的应用程序环境的一些行为与观点，可以用其实现对象来取得所请求资源的URL、设置与存储属性、应用程序初始参数，甚至动态设置Servlet实例。
当整个web应用程序加载web容器后，容器会生成一个ServletContext对象作为应用程序的代表，并设置给ServletConfig，只要通过ServletConfig的getServletContext()方法就可以取得ServletContext对象。
1、getRequestDispatcher()
用来取得RequestDispatcher实例，使用时路径的指定必须是"/"作为开头，这个斜杠代表应用程序的环境根目录。
context.getRequestDispatcher("/pages/some.jsp").forward(request,response);
2、getResourcePaths()
如果想知道web应用程序的某个目录有哪些文件，可以使用getResourcePaths()方法。如：
for ( String avater : getServletContext().getResourcePaths("/")){
//显示avatar文字
}
例1：取得图片的路径，并通过img标签显示图片。
package ServletAPI; import java.io.IOException;
import java.io.PrintWriter; import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse; /**
* Servlet implementation class Avatar
*/
@WebServlet(
urlPatterns = { "/avatar.view" },
initParams = {
@WebInitParam(name = "AVATAR_DIR", value = "/image")//根目录WebContent内部
})
public class Avatar extends HttpServlet {
private static final long serialVersionUID = 1L;
private String AVATAR_DIR;
/**
* @see HttpServlet#HttpServlet()
*/
public Avatar() {
super();
// TODO Auto-generated constructor stub
}
public void init() throws ServletException{
AVATAR_DIR=getInitParameter("AVATAR_DIR");
//AVATAR_DIR=(String) getServletContext().getAttribute("avatars");
} /**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
response.setContentType("text/html;charset=UTF-8");
PrintWriter out=response.getWriter();
out.println("<!DOCTYPE html>");
out.println("<html>");
out.println("<head>");
out.println("<meta content='text/html; charset=UTF-8' http-equiv='content-type'>");
out.println("<title>头像显示</title>");
out.println("</head>");
out.println("<body>");
for(String avatar:getServletContext().getResourcePaths(AVATAR_DIR)){//取得头像路径
avatar=avatar.replaceFirst("/", "");
out.println("<img src='"+avatar+"'>");
}
out.println("</body>");
out.println("</html>");
out.close();
} /**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
} }
3、getResourceAsStream()
如果想要在web应用程序中，读取某个文件的内容，可以使用该方法，使用时路径的指定必须是"/"作为开头，
例2：读取pdf文件
package cc.openhome; import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream; import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse; /**
* Servlet implementation class Download
*/
@WebServlet("/download.do")
public class Download extends HttpServlet {
private static final long serialVersionUID = 1L; /**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
processRequest(request, response);
} /**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
processRequest(request, response); } private void processRequest(HttpServletRequest request,
HttpServletResponse response) throws IOException {
// TODO Auto-generated method stub
//String passwd=request.getParameter("passwd");
//if("123456".equals(passwd)){
response.setContentType("application/pdf");//设置内容类型 InputStream in=getServletContext().getResourceAsStream("/WEB-INF/LaTeX2e.pdf");//取得输入串流
//File pdf = new File("E:/java web/workspace/FirstServlet/WebContent/WEB-INF/LaTeX2e.pdf");
//response.setContentLength((int) pdf.length());
// FileInputStream in = new FileInputStream(pdf);
OutputStream out=response.getOutputStream();//取得输出串流
writeBytes(in,out);//读取pdf并输出至浏览器
//}
} private void writeBytes(InputStream in, OutputStream out) throws IOException {
// TODO Auto-generated method stub
// TODO Auto-generated method stub
byte[] buffer=new byte[1024*1024];
int length=-1;
while((length=in.read(buffer))!=-1){
out.write(buffer,0,length);
}
in.close();
out.close();
}
67 }