资源注射是从JavaEE 5.0开始出现,实现了Tomcat在启动时自动将web.xml中的配置信息“注射”到Servlet中。资源注射是通过Annotation完成,Annotation是一种特殊的接口,以"@"为标志,用法如下:
- @Resource (name="messageNameInWebXml")
- private String message;
有时为了简洁,上面的两行代码可以缩写为一行:
- private @Resource(name="messageNameInWebXml") String message;
加入@Resource注释后,Tomcat会在Servlet运行时将变量message的值注入,这个值设置在web.xml中名为messageNameInWebXml的参数中。
下面用实例来说明:
- package com.hello.servlet;
-
- import java.io.*;
- import javax.annotation.Resource;
- import javax.servlet.*;
- import javax.servlet.http.*;
-
- public class InjectionServlet extends HttpServlet{
- private @Resource(name="hello") String hello;
- private @Resource(name="i") int i;
-
- @Resource(name="persons")
- private String persons;
-
- public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{
- response.setCharacterEncoding("UTF-8");
- request.setCharacterEncoding("UTF-8");
-
- response.setContentType("text/html");
- PrintWriter out = response.getWriter();
- out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional //EN\">");
- out.println("<HTML>");
- out.println("<HEAD><TITLE>资源注入</TITLE></HEAD>");
- out.println("<style>body{font-size:12px;}</style>");
-
- out.println("<b>注入的字符串</b>: <br/> - " + hello +"<br />");
- out.println("<b>注入的整数</b>: <br/> - " + i +"<br />");
- out.println("<b>注入的字符串数组</b>: <br/>");
- for(String person : persons.split(",")){
- out.println(" - " + person +"<br />");
- }
-
- out.println("<BODY>");
- out.println("</BODY>");
- out.println("</HTML>");
- out.flush();
- out.close();
- }
- }
下一步,在web.xml中使用标签<env-entry>来配置资源。<env-entry>仅能配置java.lang包下标准类型的变量,如String, Integer, Double等。配置后的代码如下:
- <servlet>
- <servlet-name>InjectionServlet</servlet-name>
- <servlet-class>com.hello.servlet.InjectionServlet</servlet-class>
- </servlet>
-
- <servlet-mapping>
- <servlet-name>InjectionServlet</servlet-name>
- <url-pattern>/servlet/InjectionServlet</url-pattern>
- </servlet-mapping>
-
- <!-- 下面是资源配置 -->
- <env-entry>
- <env-entry-name>hello</env-entry-name>
- <env-entry-type>java.lang.String</env-entry-type>
- <env-entry-value>Hello, Welcome to the JavaEE Resource Injection</env-entry-value>
- </env-entry>
-
- <env-entry>
- <env-entry-name>i</env-entry-name>
- <env-entry-type>java.lang.Integer</env-entry-type>
- <env-entry-value>14</env-entry-value>
- </env-entry>
-
- <env-entry>
- <env-entry-name>persons</env-entry-name>
- <env-entry-type>java.lang.String</env-entry-type>
- <env-entry-value>Helloween, Cobain, Roses, WLJ, </env-entry-value>
- </env-entry>
资源注射的原理:JNDI
InjectionServlet实例中使用<env-entry>配置了名为hello, i, persons的JNDI资源,然后使用@Resource将指定名称的JNDI资源注射到InjectionServlet里。如果不使用@Resource,直接通过JNDI也可以获取这三个参数,代码如下:
- Context ctx = new InitialContext();
- String message = (String)ctx.lookup("message");
- Integer i = (Integer)ctx.lookup("i");
- String persons = (String)ctx.lookup("persons");
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