C3P0连接池使用小结

C3P0在最近的demo中也用了（我用的是0.9.2.1版本），因为单例很难应付大量并发。

用法详见文档：http://www.mchange.com/projects/c3p0/

基本的用法在http://www.mchange.com/projects/c3p0/#quickstart，

以及http://www.mchange.com/projects/c3p0/#using_c3p0中。

在项目中更为方便的做法是将配置写在配置文件中。

C0P0的配置文件名为c3p0-config.xml，详见http://www.mchange.com/projects/c3p0/#configuration_files

一个示例的配置文件如下：

<?xml version="1.0" encoding="utf-8"?>

<c3p0-config> 
  <default-config> 
    <property name="automaticTestTable">t_c3p0_test</property>  
    <property name="checkoutTimeout">10000</property>  
    <property name="idleConnectionTestPeriod">10</property>  
    <property name="initialPoolSize">5</property>  
    <property name="maxIdleTime">600</property>  
    <property name="maxPoolSize">20</property>  
    <property name="minPoolSize">2</property>  
    <property name="maxStatements">60</property>  
    <property name="maxStatementsPerConnection">3</property>  
    <user-overrides user="test-user"> 
      <property name="maxPoolSize">10</property>  
      <property name="minPoolSize">1</property>  
      <property name="maxStatements">0</property> 
    </user-overrides> 
  </default-config>  
  <named-config name="metkb"> 
    <property name="acquireIncrement">20</property>  
    <property name="initialPoolSize">10</property>  
    <property name="minPoolSize">10</property>  
    <property name="maxPoolSize">100</property>  
    <property name="maxStatements">0</property>  
    <property name="maxStatementsPerConnection">10</property>  
    <!-- he's important, but there's only one of him -->  
    <user-overrides user="master-of-the-universe"> 
      <property name="acquireIncrement">1</property>  
      <property name="initialPoolSize">1</property>  
      <property name="minPoolSize">1</property>  
      <property name="maxPoolSize">5</property>  
      <property name="maxStatementsPerConnection">50</property> 
    </user-overrides> 
  </named-config> 
</c3p0-config>

放在eclipse Web工程的src目录下即可（eclipse会将该目录下的配置文件复制到tomcat的webapps/XXX/WEB-INF/classes下）

一个C3P0的单例的示例代码如下（连接池的单例并不妨碍并发，因为从连接池中取连接并不费时，而且取出的连接是线程安全的）：

package cn.edu.ruc.metkb.util;

import java.beans.PropertyVetoException;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import com.mchange.v2.c3p0.ComboPooledDataSource;

public class PooledDBA implements DBA {
    private ComboPooledDataSource cpds = null;
    private static Log dblog = null;
    private static String DBClassName = null;
    private static String DBName = null;
    private static String DBUrl = null;
    private static String DBUser = null;
    private static String DBPassword = null;

    protected PooledDBA() {
        cpds = new ComboPooledDataSource("metkb");
        try {
            cpds.setDriverClass(DBClassName);
        } catch (PropertyVetoException e) {
            dblog.exception(e);
        }
        cpds.setJdbcUrl(DBUrl + DBName);
        cpds.setUser(DBUser);
        cpds.setPassword(DBPassword);
    }

    static {
        try {
            DBClassName = ConfigFactory.getInstance().get("db.classname");
            DBName = ConfigFactory.getInstance().get("db.name");
            DBUrl = ConfigFactory.getInstance().get("db.url");
            DBUser = ConfigFactory.getInstance().get("db.user");
            DBPassword = ConfigFactory.getInstance().get("db.password");
            dblog = LogFactory.getInstance().getLog("db");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public synchronized Connection getConnection() throws SQLException,
            ClassNotFoundException, InterruptedException {
        return cpds.getConnection();
    }

    @Override
    public synchronized void close(Connection conn) {
        try {
            if (conn != null) {
                conn.close();
                conn = null;
            }
        } catch (SQLException e) {
            dblog.exception(e);
        }
    }

    @Override
    public synchronized void close(Statement stat) {
        try {
            if (stat != null) {
                stat.close();
                stat = null;
            }
        } catch (SQLException e) {
            dblog.exception(e);
        }
    }

    @Override
    public synchronized void close(ResultSet rest) {
        try {
            if (rest != null) {
                rest.close();
                rest = null;
            }
        } catch (SQLException e) {
            dblog.exception(e);
        }
    }
}

转自：http://blog.csdn.net/bhq2010/article/details/9219947