Struts通过加载插件的形式初始化系统常量以及其他信息

一：通过插件的形式加载系统中定义的常量：

思路：首先定义一个常量类存放系统常量，或者定义一个xml或properties文件中然后进行读取，

然后把相应的常量放入map中

最后编写插件类，加载map类

最后把插件类放入struts-config.xml中启动的时候进行加载

这样在其他页面中就可以随意访问了。

1：首先定义在常量类中定义一些常量：

package com.topwqp.common.constants;

public class Globals {
   public static  final  String  AUTHOR = "topwqp";
   public static  final  String  PLUGINTEST = "puginTest";
}

2：把常量放在map中，

package com.topwqp.common.constants;

import java.util.HashMap;
import java.util.Map;


public class Constants {
	private Map  globals;
	
	public Constants(){
		globals = new HashMap();
		globals.put("AUTHOR", Globals.AUTHOR);
		globals.put("PLUGINTEST", Globals.PLUGINTEST);
	}

	public Map getGlobals() {
		return globals;
	}

	public void setGlobals(Map globals) {
		this.globals = globals;
	}
	
}

3：编写对应的插件类：

插件类需要实现PlugIn接口关于这个接口的介绍在下面

package com.topwqp.plugin;

import javax.servlet.ServletException;

import org.apache.struts.action.ActionServlet;
import org.apache.struts.action.PlugIn;
import org.apache.struts.config.ModuleConfig;

import com.topwqp.common.constants.Constants;

public class ConstantsPlugin implements PlugIn{

	@Override
	public void destroy() {
		// TODO Auto-generated method stub
		
	}

	@Override
	public void init(ActionServlet servlet, ModuleConfig config)
			throws ServletException {
		System.out.println("load constans success. then you  can access constans.......");
		// TODO Auto-generated method stub
		Constants  constants = new Constants();
		servlet.getServletContext().setAttribute("Constants", constants);
		
	}

}

关于PlugIn接口的源码如下：意思就是需要实现init(） 其中init方法用于系统启动的一些设置在这个方法中进行编写，

deploy方法是系统关闭时候的一些操作在这个方法中进行设置。

源码如下：

/*
 * $Id: PlugIn.java 471754 2006-11-06 14:55:09Z husted $
 *
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.struts.action;

import org.apache.struts.config.ModuleConfig;

import javax.servlet.ServletException;

/**
 * <p>A <strong>PlugIn</strong> is a configuration wrapper for a
 * module-specific resource or service that needs to be notified about
 * application startup and application shutdown events (corresponding to when
 * the container calls <code>init</code> and <code>destroy</code> on the
 * corresponding {@link ActionServlet} instance). <code>PlugIn</code> objects
 * can be configured in the <code>struts-config.xml</code> file, without the
 * need to subclass {@link ActionServlet} simply to perform application
 * lifecycle activities.</p>
 *
 * <p>Implementations of this interface must supply a zero-argument
 * constructor for use by {@link ActionServlet}. Configuration can be
 * accomplished by providing standard JavaBeans property setter methods, which
 * will all have been called before the <code>init()</code> method is
 * invoked.</p>
 *
 * <p>This interface can be applied to any class, including an Action
 * subclass. </p>
 *
 * @version $Rev: 471754 $ $Date: 2005-05-14 01:09:32 -0400 (Sat, 14 May 2005)
 *          $
 * @since Struts 1.1
 */
public interface PlugIn {
    /**
     * <p>Receive notification that our owning module is being shut down.</p>
     */
    void destroy();

    /**
     * <p>Receive notification that the specified module is being started
     * up.</p>
     *
     * @param servlet ActionServlet that is managing all the modules in this
     *                web application
     * @param config  ModuleConfig for the module with which this plug-in is
     *                associated
     * @throws ServletException if this <code>PlugIn</code> cannot be
     *                          successfully initialized
     */
    void init(ActionServlet servlet, ModuleConfig config)
        throws ServletException;
}

然后把编写的插件类在struts-config.xml中进行配置即可，当application在tomcat中启动 时候，首先加载web.xml的文件，web.xml中有相应的初始化设置加载指定的文件：

及struts-config.xml文件：

当然struts允许同时加载多个文件如下我的web.xml的配置如下：

<servlet>
    <servlet-name>action</servlet-name>
    <servlet-class>
        org.apache.struts.action.ActionServlet
    </servlet-class>
    <init-param>
        <param-name>config</param-name>
        <param-value>
         /WEB-INF/struts-config.xml, /WEB-INF/struts-config-1.xml, /WEB-INF/struts-config-2.xml
        </param-value>
    </init-param>
    
    <init-param>
        <param-name>config/admin</param-name>
        <param-value>
         /WEB-INF/struts-config-admin.xml
        </param-value>
    </init-param>
    <init-param>
        <param-name>config/common</param-name>
        <param-value>
         /WEB-INF/struts-config-common.xml
        </param-value>
    </init-param>
    
    <load-on-startup>1</load-on-startup>
  </servlet>
    
  <servlet-mapping>
       <servlet-name>action</servlet-name>
       <url-pattern>*.do</url-pattern>
  </servlet-mapping>

把编写的插件配置在struts-config.xml中即可：

 <plug-in className="com.topwqp.plugin.ConstantsPlugin"></plug-in>

在对应的jsp页面中访问：

<p>
Field name: AUTHOR<br />
Field value: <bean:write name="Constants" 
property="globals(AUTHOR)"/><br />
</p>
<p>
Field name: PluginTest<br />
Field value: <c:out value="${Constants.globals.PLUGINTEST}"/><br />
</p>

2：创建一个插件记录应用的启动时间以及已经运行了多久

思路：定义时间追踪类，该类中有应用的启动时间，以及方法获取距离启动时间多久，然后在插件中放入这个类的实例，最后在jsp页面中进行显示即可。

1：定义时间类：

package com.topwqp.plugin;

import java.util.Date;

public class TimeTracker {
	private long  startUpTimeMillis;
	private Date startupOn;
	
	public TimeTracker (){
		startUpTimeMillis = System.currentTimeMillis();
		startupOn = new Date();
	}
	
	//  how long the application  have started up;
	public  long getUptime(){
		return System.currentTimeMillis() - startUpTimeMillis;
	}
	// get  the  application  startup time
	public  Date getStartupOn(){
		return startupOn;
	}

}

这个类中有个默认的构造方法，当对象创建的时候对应的启动时间就产生了，

有一个getUptime方法，用去动态生成距离启动时间的毫秒数。

2：创建对应的插件类：

package com.topwqp.plugin;

import javax.servlet.ServletException;

import org.apache.struts.action.ActionServlet;
import org.apache.struts.action.PlugIn;
import org.apache.struts.config.ModuleConfig;

public class TimeTrackerPlugin implements PlugIn{
	
	private String contextKey ;
	
	public void  setContextKey(String contextKey){
		this.contextKey = contextKey;
	}

	@Override
	public void destroy() {
		// TODO Auto-generated method stub
		System.out.println("do nothing............");
	}

	@Override
	public void init(ActionServlet servlet, ModuleConfig config)
			throws ServletException {
		// TODO Auto-generated method stub
		servlet.getServletContext().setAttribute(contextKey, new TimeTracker());
		
	}

}

3：在struts-config.xml中配置插件：

 <plug-in className="com.topwqp.plugin.TimeTrackerPlugin">
      <set-property property="contextKey" value="timeTracker"/>
    </plug-in>

4：在jsp页面中获取对应的时间：

<h4> 系统启动日期： since 
<bean:write name="timeTracker" 
property="startupOn" 
format="MM/dd/yyyy HH:mm"/>
<br> 
已经运行时间：for
<bean:write name="timeTracker" 
property="uptime"/> milliseconds!
</h4>

然后启动应用访问即可：

两个插件的访问页面显示效果：