J2EE应用服务器使用JNDI

在J2EE环境下使用JNDI是非常简单的事，因为所有的J2EE容器都要实现JNDI服务，所以，在J2EE环境下使用JNDI，与使用Hashtable也没有什么太大区别。只有一点限制，那就是绑定对象时，对象所属的类必须实现java.io.Serializable接口，这一点也实在一点也不困难，几乎所有用到的Java类都实现了这个接口，对于自定义的类，在接口实现列表里把这个接口加进去也就是了。

下面，我将演示一下如何在J2EE环境下使用JNDI，为了保证代码的通用性，我不使用struts之类的框架，而是直接使用标准JSP和Servlet实现。我将该项目的名称定为jndi_test

要使用JNDI，需要先到SUN的网站上去下载jndi.jar。

2.1 JSP
本项目包括5个JSP，功能说明如下：

index.jsp：首页
bind.jsp：用于在JNDI中绑定对象
bind_result.jsp：绑定对象后的返回页面
lookup.jsp：用于在JNDI中检索对象
lookup_result.jsp：用于显示检索对象
本节中用到的JSP代码如下，代码都简单地很，就不多做解释了。

2.1.1 index.jsp

< %...@ page language="java" contentType="text/html; charset=GB18030"
    pageEncoding="GB18030"%>
< !DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
< html>
< head>
< meta http-equiv="Content-Type" content="text/html; charset=GB18030">
< title>JNDI Test< /title>
< /head>
< body>
< a href="bind.jsp" target="_blank">http://www.51itcn.com/bind.jsp< /a>
< br />
< a href="lookup.jsp" target="_blank">http://www.51itcn.com/lookup.jsp< /a>
< /body>
< /html>

2.1.2 bind.jsp

< %...@ page language="java" contentType="text/html; charset=GB18030"
    pageEncoding="GB18030"%>
< !DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
< html>
< head>
< meta http-equiv="Content-Type" content="text/html; charset=GB18030">
< title>JNDI Test - Bind< /title>
< /head>
< body>
< a href="bind.do">bind an object< /a>
< /body>
< /html>

2.1.3 bind_result.jsp

< %...@ page language="java" contentType="text/html; charset=GB18030"
    pageEncoding="GB18030"%>
< !DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
< html>
< head>
< meta http-equiv="Content-Type" content="text/html; charset=GB18030">
< title>JNDI Test - Bind result< /title>
< /head>
< body>
< p>Binded successfully!< /p>
< /body>
< /html>

2.1.4 lookup.jsp

< %...@ page language="java" contentType="text/html; charset=GB18030"
    pageEncoding="GB18030"%>
< !DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
< html>
< head>
< meta http-equiv="Content-Type" content="text/html; charset=GB18030">
< title>JNDI Test - lookup< /title>
< /head>
< body>
< a href="lookup.do">lookup the binded object< /a>
< /body>
< /html>

2.1.5 lookup_result.jsp

< %...@ page language="java" contentType="text/html; charset=GB18030"
    pageEncoding="GB18030"%>
< !DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
< html>
< head>
< meta http-equiv="Content-Type" content="text/html; charset=GB18030">
< title>JNDI Test - Lookup result< /title>
< /head>
< body>
< %...
    Object o = request.getAttribute("found_jndi_obj");
    out.println(o);
%>
< /body>
< /html>

2.2 Servlet
本例包括两个Servlet，功能说明如下：

BindServlet：用于在JNDI服务中绑定一个对象
LookupServlet：用于在JNDI服务中取出一个对象

2.2.1 BindServlet.java

package lld.test.jndi;

import java.io.IOException;
import java.util.Date;

import javax.naming.Context;
import javax.naming.InitialContext;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.*;

public class BindServlet extends HttpServlet
...{

    private static final long serialVersionUID = 5219969790998794367L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException
    ...{
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException
    ...{
        try
        ...{
            Context jndi_ctx = new InitialContext();
            String key = "jndi_object";
            jndi_ctx.rebind(key, new Date());
        }catch(Exception ex)
        ...{
            ex.printStackTrace();
        }

        ServletContext context = this.getServletContext();
        RequestDispatcher dispatcher = context.getRequestDispatcher("/bind_result.jsp");
        dispatcher.forward(req, resp);
    }

}

使用rebind而不是bind绑定对象是因为，使用bind时，如果已经有对象绑定到该键值上，则会抛出异常。

因为只是示例代码，所以我只是绑定了一个最简单的日期对象。

2.2.2 LookupServlet.java

package lld.test.jndi;

import java.io.IOException;

import javax.naming.Context;
import javax.naming.InitialContext;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class LookupServlet extends HttpServlet
...{
    private static final long serialVersionUID = 6677219828267184673L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException
    ...{
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException
    ...{
        try
        ...{
            Context jndi_ctx = new InitialContext();
            String key = "jndi_object";
            Object o = jndi_ctx.lookup(key);
            req.setAttribute("found_jndi_obj", o);
        }catch(Exception ex)
        ...{
            ex.printStackTrace();
        }

        ServletContext context = this.getServletContext();
        RequestDispatcher dispatcher = context.getRequestDispatcher("/lookup_result.jsp");
        dispatcher.forward(req, resp);
    }

}

2.3 web.xml

在web.xml中，加入了servlet映射

< ?xml version="1.0" encoding="UTF-8"?>
< web-app id="WebApp_ID" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
    < display-name>jndi_test< /display-name>

    < servlet>
        < servlet-name>BindServlet< /servlet-name>
        < servlet-class>lld.test.jndi.BindServlet< /servlet-class>
    < /servlet>
    < servlet-mapping>
        < servlet-name>BindServlet< /servlet-name>
        < url-pattern>/bind.do< /url-pattern>
    < /servlet-mapping>

    < servlet>
        < servlet-name>LookupServlet< /servlet-name>
        < servlet-class>lld.test.jndi.LookupServlet< /servlet-class>
    < /servlet>

    < servlet-mapping>
        < servlet-name>LookupServlet< /servlet-name>
        < url-pattern>/lookup.do< /url-pattern>
    < /servlet-mapping>

    < welcome-file-list>
        < welcome-file>index.jsp< /welcome-file>
    < /welcome-file-list>
< /web-app>

OK，所有的代码都在这里了，部署到Tomcat下运行即可。这样就可以在J2EE下使用JNDI了。