Java EE 7 (1) -- Servlet

Java Servlet在是第一种Java网页应用的技术。Sun公司在1997年发布了第一个Java Servlet规范。Servlet是所有Java EE应用的基础，JSP和JSF框架最终都把页面编译成Java Servlet。虽然Servlet可以响应任何类型的请求，但它大多数是响应基于web的请求。Servlet必须布署在Java Servlet容器里，比如Oracle的GlassFish（其它的还有JBoss, Apache TomEE, WebLogic等）。

安装

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从下载安装GlassFish，用以下命令启动：

/PATH_TO_GLASSFISH/Glassfish/bin/asadmin start-domain domain1

之后访问可进入管理控制台。

java代码

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用文本编辑器新建一个名为SimpleServlet.java，然后敲入代码：

package zp.javaee;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class SimpleServlet extends HttpServlet {

    /**
     * Processes requests for both HTTP
     * GET and
     * POST methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        try {
           // Place page output here
            out.println("");
            out.println("");
            out.println("");
            out.println("");
            out.println("");
            out.println("

Servlet SimpleServlet at " + request.getContextPath() + "

");
            out.println(" ");
            out.println("");
        } finally {
            out.close();
        }
    }

   /**
     * Handles the HTTP GET
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Handles the HTTP POST
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Returns a short description of the servlet for documentation purposes.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }//
}

编译

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命令javac -cp c:\glassfish4\glassfish\modules\javax.servlet-api.jar SimpleServlet.java，编译出SimpleServlet.class。

web.xml

---

继续用文本编辑器，新建一个web.xml文件：

    xmlns:xsi=""
    xsi:schemaLocation=" /web-app_3_0.xsd"
    version="3.0">
   
        SimpleServlet
        zp.javaee.SimpleServlet
   
   
        SimpleServlet
        /SimpleServlet
   
   
        /SimpleServlet
   

打包布署

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新建一个名为SampleServlet的文件夹，在它下面再创建名为”WEB-INF"的文件夹，把web.xml拷贝到这里。在”WEB-INF"下分别建两个名为”classes"和"lib"的文件夹，在class里根据包名建立文件夹，拷贝SimpleServlt拷贝到WEB-INF/classes/zp/javaee下。最后把整个SampleServlet文件夹拷贝到C:\glassfish4\glassfish\domains\domain1\autodeploy下，访问，可以看到结果。

在熟悉了Servlet的底层逻辑后，就不再需要做这些繁琐的事情，而是让IDE去做。

去除web.xml

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Servelet 3.0引入@WebServlet注释。在java代码里做些修改：

...

import javax.servlet.annotation.WebServlet;

...

@WebServlet(name = "SimpleServlet", urlPatterns = {"/SimpleServlet"})
public class SimpleServlet extends HttpServlet {

...

在打包时只拷贝这个类的class文件，而不再需要web.xml文件。

注意：一个Servlet对象会被多个Client并发使用，所以它的成员变量在使用时需要进行线程保护。

Servlet的初始化参数

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@WebInitParam可以设置Servlet的初始化参数，而后通过Servlet的getServletConfig().getInitParameter方法可以访问这些参数：

...
import javax.servlet.annotation.WebInitParam;
...
@WebServlet (name="InitParamServlet" , urlPatterns={"/InitParamServlet" },initParams={ @WebInitParam(name= "name" , value="zp" ) })
public class InitParamServlet extends HttpServlet {
...
       protected void doGet(HttpServletRequest request, HttpServletResponse response)
                      throws ServletException, IOException {
...
                     out.println( "

" + getServletConfig().getInitParameter("name" ) + "

");
...

@WebInitParam等价于在web.xml中定义：

   
        InitParamServlet

        zp.javaee.InitParamServlet

       
            name
            zp
       
    . . .
   
    . . .

Filter

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Filter可以用来处理特定url的事件：

package zp.javaee;

import java.io.IOException;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.annotation.WebFilter;

@WebFilter("/*" )

public class SimpleFilter implements Filter {

        @Override

        public void destroy() {

               throw new UnsupportedOperationException("Not supported yet.");

       }

        @Override

        public void doFilter(ServletRequest request, ServletResponse response,

                     FilterChain chain) throws IOException, ServletException {

              String userAddy = request.getRemoteHost();

        filterConf.getServletContext().log("Vistor User IP: " + userAddy);

        chain.doFilter(request, response);

       }

        @Override

        public void init(FilterConfig filterConf) throws ServletException {

               this.filterConf = filterConf;

       }

        private FilterConfig filterConf ;

}

把Filter和SimpleServlet与InitParamServlet打包进同一个网站后，访问和都会在log中打印用户的IP。哪怕是访问不存在的类，Filter仍然会被触发。

@WebFilter注释等价于下面的web.xml片段：

    SimpleFilter

    SimpleFilter

    SimpleFilter

    /*

Listener

---

Listener可以监听网站的事件，比如：

package zp.javaee;

import javax.servlet.ServletContextEvent;

import javax.servlet.ServletContextListener;

import javax.servlet.annotation.WebListener;

@WebListener

public class SimpleListener implements ServletContextListener {

       

        @Override

        public void contextDestroyed(ServletContextEvent event) {

              System. out.println("Servlet shutdown. . ." );

        System. out.println(event.getServletContext().getServerInfo());

        System. out.println(System.currentTimeMillis());

       }

        @Override

        public void contextInitialized(ServletContextEvent event) {

              System. out.println("Servlet startup. . ." );

        System. out.println(event.getServletContext().getServerInfo());

        System. out.println(System.currentTimeMillis());

       }

}

每次在网站重新布署时，这两个Listener的两个方法就会被触发。

@WebListener等价于web.xml片段：

    org.javaeerecipes.chapter01.recipe01_07.StartupShutdownListener

监听Session事件：

package zp.javaee;

import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

@WebListener
public class SimpleSessionListener implements HttpSessionListener {

     @Override
     public void sessionCreated(HttpSessionEvent event) {
          event.getSession().setMaxInactiveInterval(60);
          event.getSession().setAttribute("name", "zp");
          System.out.println("Session created");
     }

     @Override
     public void sessionDestroyed(HttpSessionEvent event) {
          System.out.println("Session destoryed");
     }

}

提示：在Servelet的实现里可以调用HttpServletRequest的getSession(true)方法来创建session。

监听Session Attribute事件：

package zp.javaee;

import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

@WebListener
public class SimpleSessionAttributeListener implements
          HttpSessionAttributeListener {

     @Override
     public void attributeAdded(HttpSessionBindingEvent event) {
          System.out.println("Attribute added session: " + event.getSession().getId());
          System.out.println("Attribute added: " + event.getName() + " - " + event.getValue());
     }

     @Override
     public void attributeRemoved(HttpSessionBindingEvent event) {
          System.out.println("Attribute removed session: " + event.getSession().getId());
          System.out.println("Attribute removed: " + event.getName() + " - " + event.getValue());
     }

     @Override
     public void attributeReplaced(HttpSessionBindingEvent event) {
          System.out.println("Attribute replaced session: " + event.getSession().getId());
          System.out.println("Attribute replaced: " + event.getName() + " - " + event.getValue());
     }

}

下载文件

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package zp.javaee;

import java.io.IOException;

import java.io.InputStream;

import javax.servlet.ServletContext;

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;

@WebServlet(name="SimpleDownloadServlet" , urlPatterns="/SimpleDownloadServlet")

public class SimpleDownloadServlet extends HttpServlet {

        @Override

        protected void doGet(HttpServletRequest request, HttpServletResponse response)

                      throws ServletException, IOException {

              String fileToDownload = request.getParameter( "filename");

               final int BYTES = 1024;

        int                 length   = 0;

        ServletOutputStream outStream = response.getOutputStream();

        ServletContext      context  = getServletConfig().getServletContext();

 

        response.setContentType( (context.getMimeType( fileToDownload ) != null) ?

                context.getMimeType(fileToDownload) : "text/plain" );

        response.setHeader( "Content-Disposition", "attachment; filename=\"" + fileToDownload + "\"" );

        InputStream in = context.getResourceAsStream( "/" + fileToDownload);

        byte[] bbuf = new byte[BYTES];

        

        while ((in != null ) && ((length = in.read(bbuf)) != -1))

        {

            outStream.write(bbuf,0,length);

        }

        outStream.flush();

        outStream.close();

       }

}

将请求转接到别的Servlet

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package zp.javaee;

import java.io.IOException;

import javax.servlet.RequestDispatcher;

import javax.servlet.ServletContext;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

@WebServlet(name="SimpleDispatcherServlet" , urlPatterns="/SimpleDispatcherServlet" )

public class SimpleDispatcherServlet extends HttpServlet {

        @Override

        protected void doGet(HttpServletRequest request, HttpServletResponse response)

                      throws ServletException, IOException {

              ServletContext sc = getServletConfig().getServletContext();

              RequestDispatcher rd = sc.getRequestDispatcher("/SimpleDownloadServlet" );

              rd.forward(request, response);

       }

}

重定向到外网

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package zp.javaee;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(name="SimpleRedirectServlet", urlPatterns="/SimpleRedirectServlet")
public class SimpleRedirectServlet extends HttpServlet {

     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse response)
               throws ServletException, IOException {
          response.sendRedirect("");
     }

}

Cookie

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package zp.javaee;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.Cookie;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

@WebServlet (name="SimpleWriteCookieServlet" , urlPatterns="/SimpleWriteCookieServlet" )

public class SimpleWriteCookieServlet extends HttpServlet {

        @Override

        protected void doGet(HttpServletRequest request, HttpServletResponse response)

                      throws ServletException, IOException {

              response.setContentType( "text/html;charset=UTF-8" );

        PrintWriter out = response.getWriter();

        Cookie cookie = new Cookie("name" ,"zp" );

        cookie.setHttpOnly( true );

        cookie.setMaxAge(-30);

        response.addCookie(cookie);

        out.println( "Set Cookie");

        out.close();

       }

}

package zp.javaee;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.Cookie;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

@WebServlet (name="SimpleReadCookieServlet" , urlPatterns="/SimpleReadCookieServlet" )

public class SimpleReadCookieServlet extends HttpServlet {

        @Override

        protected void doGet(HttpServletRequest request,

                     HttpServletResponse response) throws ServletException, IOException {

              response.setContentType( "text/html;charset=UTF-8" );

              PrintWriter out = response.getWriter();

              out.println( "" );

              Cookie[] cookies = request.getCookies();

               for (Cookie cookie : cookies) {

                     out.println( "

" + cookie.getName() + " : " + cookie.getValue() + "

");

                     out.println( "" );

              }

              out.close();

       }

}

析构

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覆盖servlet的destroy方法，在里面释放资源。