关于java static变量初始化的疑问

java中，类的static变量作为类变量，只需要被一次初始化，就可使用，但是，我在程序中遇到一个问题：

在web.xml中配置listener，服务器启动时，ServletContextListen初始化数据源（DataSource），第一次初始化成功，但是在Test类中，调用ConnectionManager.getConnection()时，DataSource却为null，意味着ServletContextListen初始化数据源失败了，大家帮忙解答下，到底什么原因造成的，谢谢！！！

以下是代码：
[code="java"]
package c3p0.connection;

import java.beans.PropertyVetoException;
import java.io.IOException;
import java.io.InputStream;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Properties;

import javax.sql.DataSource;

import com.mchange.v2.c3p0.ComboPooledDataSource;

public class ConnectionManager {
private static DataSource data_source=null;;
private static Properties props=null;
private static String porps_name=null;

/**
 * 创建数据库连接池
 * @return
 */
private static DataSource createDataSource(){
    load();
    data_source=initDB();
    return data_source;
}

/**
 * 加载配置文件
 */
private static void load(){
    try {
        props =  new Properties();
        InputStream in=Thread.currentThread().getContextClassLoader().getResourceAsStream(porps_name);
        props.load(in);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

/**
 * 初始化数据库
 */
private static  DataSource initDB(){
    System.out.println("开始初始化数据源...");
    ComboPooledDataSource pool_ds=new ComboPooledDataSource();
    try {
        pool_ds.setDriverClass(getProperty("c3p0.connection.driverClass"));
    } catch (PropertyVetoException e) {
        e.printStackTrace();
        throw new RuntimeException("数据库驱动加载失败");
    }
    pool_ds.setJdbcUrl(getProperty("c3p0.connection.url"));
    pool_ds.setUser(getProperty("c3p0.connection.user"));
    pool_ds.setPassword(getProperty("c3p0.connection.password"));
    pool_ds.setInitialPoolSize(Integer.parseInt(getProperty("c3p0.connection.initialPoolSize")));
    pool_ds.setMaxPoolSize(Integer.parseInt(getProperty("c3p0.connection.maxPoolSize")));
    pool_ds.setMinPoolSize(Integer.parseInt(getProperty("c3p0.connection.minPoolSize")));
    pool_ds.setMaxIdleTime(Integer.parseInt(getProperty("c3p0.connection.maxIdleTime")));
    pool_ds.setAcquireIncrement(Integer.parseInt(getProperty("c3p0.connection.acquireIncrement")));
    pool_ds.setAcquireRetryAttempts(Integer.parseInt(getProperty("c3p0.connection.acquireRetryAttempts")));
    pool_ds.setAcquireRetryDelay(Integer.parseInt(getProperty("c3p0.connection.acquireRetryDelay")));
    pool_ds.setTestConnectionOnCheckout(Boolean.parseBoolean(getProperty("c3p0.connection.testConnectionOnCheckout")));
    pool_ds.setTestConnectionOnCheckin(Boolean.parseBoolean(getProperty("c3p0.connection.testConnectionOnCheckin")));
    pool_ds.setIdleConnectionTestPeriod(Integer.parseInt(getProperty("c3p0.connection.idleConnectionTestPeriod")));
    pool_ds.setCheckoutTimeout(Integer.parseInt(getProperty("c3p0.connection.checkoutTimeout")));
    pool_ds.setAutomaticTestTable(getProperty("c3p0.connection.automaticTestTable"));

    System.out.println("数据源初始化完毕...");
    return pool_ds;
}

/**
 * 获取c3p0连接池文件配置信息
 * @param key
 * @return
 */
private static String getProperty(String key){
    if(key==null||"".equals(key)){
        throw new NullPointerException("key不能为空!!!请检查key的赋值!!!");
    }
    return props.getProperty(key);
}

public static Connection getConnection() throws SQLException{
    data_source=(data_source==null) ? createDataSource() : data_source;
    return data_source.getConnection();
}

public static void start(String porps_name) throws SQLException{
    ConnectionManager.porps_name=porps_name;
    Connection conn=getConnection();
    conn.close();
}

public static void stop(){
    data_source=null;
}

public static void release(Connection conn,PreparedStatement pstmt,ResultSet res){
        try {
            if(conn!=null){
                conn.close();
            }
            if(pstmt!=null){
                pstmt.close();
            }
            if(res!=null){
                res.close();
            }
        } catch (SQLException e) {

            e.printStackTrace();
        }

}

}

[/code]
[code="java"]
package mystruts.listener;

import java.sql.SQLException;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

import c3p0.connection.ConnectionManager;

/**

• ServletContext 上下文监听器
• 应用服务器一启动就产生该对象，服务器关闭即销毁
• 作用于全局，所有Servlet ,相当于静态变量

• @author tianly
  *
  /
  public class ServletContextListen implements ServletContextListener {
  /*

  • web服务器关闭时执行 / @Override public void contextDestroyed(ServletContextEvent event) { /*
    • 关闭数据源 */ ConnectionManager.stop(); }

  /**

  • web服务器启动时执行
    */
    @Override
    public void contextInitialized(ServletContextEvent event) {

    /**

    • 初始化数据源 */ String db_url=event.getServletContext().getInitParameter("DataBaseConfig"); try { ConnectionManager.start(db_url); } catch (SQLException e) { e.printStackTrace(); } }

}

[/code]
[code="java"]
package test.c3p0;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

import c3p0.connection.ConnectionManager;
import c3p0.util.DateUtil;

public class Test {

/**
 * @param args
 */
public static void main(String[] args) {
    String currdate=DateUtil.getDateTime();
    Connection conn=null;
    PreparedStatement pstmt=null;
    ResultSet res=null;
    try {
        String sql="insert into userinfo(username,password,sex,regdate) values(?,?,?,?)";
        String sql4ID="select LAST_INSERT_ID()";
        conn=ConnectionManager.getConnection();
        conn.setAutoCommit(false);
        pstmt=conn.prepareStatement(sql);
        pstmt.setString(1, "tly");
        pstmt.setString(2, "tly");
        pstmt.setInt(3, 0);
        pstmt.setString(4, currdate);   //mysql中无需进行日期格式转换

        pstmt.executeUpdate();
        pstmt=conn.prepareStatement(sql4ID);
        res=pstmt.executeQuery();
        if(res.next()){
            int ID=res.getInt(1);
            System.out.println("ID："+ID);
        }
        conn.commit();
    } catch (SQLException e) {
        e.printStackTrace();
        try {
            conn.rollback();
        } catch (SQLException e1) {
            e1.printStackTrace();
        }
    }finally{
        ConnectionManager.release(conn, pstmt, res);
    }

}

}

[/code]
[code="xml"]
<?xml version="1.0" encoding="UTF-8"?>

mysystem

index.html
index.htm
index.jsp
default.html
default.htm
default.jsp

<!-- 登录Servlet配置

LoginServlet
login.LoginServlet


LoginServlet
/login
-->

ActionDispatcher
mystruts.actionfilter.ActionDispatcher

ActionDispatcher
*.action

<!-- 数据库信息配置 -->

DataBaseConfig
c3p0/c3p0.properties


mystruts.listener.ServletContextListen

[/code]

在Test类中，调用ConnectionManager.getConnection()时，DataSource却为null，但我在debug时，跟踪查看，发现DataSource是被初始化了，不知什么原因造成初始化失败，请大家帮忙查看下，谢谢

没必要放到ServletContextListener。

直接放在ConnectionManager中的static块中，默认加载classpath下的配置文件。

如 static {
load();

data_source=initDB();

return data_source;

}
读取classpath下的资源可以通过
Thread.currentThread().getContextClassLoader().getResourceAsStream(name)

public static void stop(){  
    data_source=null;   //容器销毁了 自动垃圾回收了，不需要
} 

报的什么异常，能否全部贴出来

那是应为他们没有运行在同一个环境中，，服务器启动是在tomcat容器中的，而你自己写的test类 是在外面JDK里面跑的 和 tomcat没有关系，当然是空了

其它的 我就不多说了 ，开源中国 红薯大哥也是用的 c3p0，同时 他把它的数据库 工具类 已经贡献出来了

[code="java"]package my.db;

import java.sql.*;
import java.util.*;
import java.lang.reflect.*;

import javax.sql.DataSource;

import org.apache.commons.beanutils.BeanUtils;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**

• 数据库管理
• @author Winter Lau

• @date 2010-2-2 下午10:18:50
  */
  public class DBManager {

  private final static Log log = LogFactory.getLog(DBManager.class);
  private final static ThreadLocal conns = new ThreadLocal();
  private static DataSource dataSource;
  private static boolean show_sql = false;

  static {
  initDataSource(null);
  }

  /**

  • 初始化连接池
  • @param props

  • @param show_sql
    */
    private final static void initDataSource(Properties dbProperties) {
    try {
    if(dbProperties == null){
    dbProperties = new Properties();
    dbProperties.load(DBManager.class.getResourceAsStream("db.properties"));
    }
    Properties cp_props = new Properties();
    for(Object key : dbProperties.keySet()) {
    String skey = (String)key;
    if(skey.startsWith("jdbc.")){
    String name = skey.substring(5);
    cp_props.put(name, dbProperties.getProperty(skey));
    if("show_sql".equalsIgnoreCase(name)){
    show_sql = "true".equalsIgnoreCase(dbProperties.getProperty(skey));
    }
    }
    }
    dataSource = (DataSource)Class.forName(cp_props.getProperty("datasource")).newInstance();
    if(dataSource.getClass().getName().indexOf("c3p0")>0){
    //Disable JMX in C3P0
    System.setProperty("com.mchange.v2.c3p0.management.ManagementCoordinator",
    "com.mchange.v2.c3p0.management.NullManagementCoordinator");
    }
    log.info("Using DataSource : " + dataSource.getClass().getName());
    BeanUtils.populate(dataSource, cp_props);

    Connection conn = getConnection();
    DatabaseMetaData mdm = conn.getMetaData();
    log.info("Connected to " + mdm.getDatabaseProductName() + 
                      " " + mdm.getDatabaseProductVersion());
    closeConnection();
    

    } catch (Exception e) {
    throw new DBException(e);
    }
    }

  /**

  • 断开连接池 */ public final static void closeDataSource(){ try { dataSource.getClass().getMethod("close").invoke(dataSource); } catch (NoSuchMethodException e){ } catch (Exception e) { log.error("Unabled to destroy DataSource!!! ", e); } }

  public final static Connection getConnection() throws SQLException {
  Connection conn = conns.get();
  if(conn ==null || conn.isClosed()){
  conn = dataSource.getConnection();
  conns.set(conn);
  }
  return (show_sql && !Proxy.isProxyClass(conn.getClass()))?
  new _DebugConnection(conn).getConnection():conn;
  }

  /**

  • 关闭连接 */ public final static void closeConnection() { Connection conn = conns.get(); try { if(conn != null && !conn.isClosed()){ conn.setAutoCommit(true); conn.close(); } } catch (SQLException e) { log.error("Unabled to close connection!!! ", e); } conns.set(null); }

  /**

  • 用于跟踪执行的SQL语句

  • @author Winter Lau
    */
    static class _DebugConnection implements InvocationHandler {

    private final static Log log = LogFactory.getLog(_DebugConnection.class);

    private Connection conn = null;

    public _DebugConnection(Connection conn) {
    this.conn = conn;
    }

    /**

    • Returns the conn.
    • @return Connection */ public Connection getConnection() { return (Connection) Proxy.newProxyInstance(conn.getClass().getClassLoader(), conn.getClass().getInterfaces(), this); }

    public Object invoke(Object proxy, Method m, Object[] args) throws Throwable {
    try {
    String method = m.getName();
    if("prepareStatement".equals(method) || "createStatement".equals(method))
    log.info("[SQL] >>> " + args[0]);
    
    return m.invoke(conn, args);
    } catch (InvocationTargetException e) {
    throw e.getTargetException();
    }
    }

  }

}
[/code]

[url]http://www.oschina.net/code/snippet_12_8[/url]

要看你的tomcat启动了没的，如果没有启动，你的test又没连接数据库，如何会拿到值啊。
你的初始化是tomcat启动后才会去初始化的。

前面的几个哥们也都说了，如果配置在xml中那么你肯定得去启动这个xml才能生效。
如果你想测试你的代码用测试类中mian方法来测试，你首先要去执行xml文件。

楼上说得对。

你建一个Servlet,把那段测试代码放到Servlet里去，再用浏览器调Servlet.

兄弟你太幽默了。。。监听器 是运行在web容器里的。是一个jvm实例。

main方法是另外一个jvm实例。

本质是两个main方法。。能互相调用彼此的运行时变量么。。。