import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

public class ConnectionPool {
 private List<Connection> pool;
 private static final String Url = "jdbc:oracle:thin:@localhost:1521:XE";
 private static final String USERNAME = "abing";
 private static final String PASSWORD = "abing";
 private static final String DRIVER = "oracle.jdbc.driver.OracleDriver";
 private int poolSize = 10;
 private static volatile ConnectionPool instance = null;

 private ConnectionPool() {
  init();
 }

 public void init() {
  pool = new ArrayList<Connection>(poolSize);
  addConnection();
 }

 public synchronized void release(Connection conn) {
  pool.add(conn);
 }

 public synchronized void closePool() {
  for (int i = 0; i < pool.size(); i++) {
   try {
    ((Connection) pool.get(i)).close();
   } catch (SQLException e) {
    e.printStackTrace();
   }
   pool.remove(i);
  }
 }

 public static ConnectionPool getInstance() {
  if (null == instance) {
   synchronized (ConnectionPool.class) {
    if (null == instance) {
     return new ConnectionPool();
    }
   }
  }
  return instance;
 }

 public synchronized Connection getConnection() {
  if (pool.size() > 0) {
   Connection conn = pool.get(0);
   if(conn!=null){
    System.out.println("Connection Pool Create Success");
   }
   pool.remove(conn);
   return conn;
  }else{
   return null;
  }
 }

 public void addConnection() {
  Connection conn = null;
  for (int i = 0; i < poolSize; i++) {
   try {
    Class.forName(DRIVER);
    conn = DriverManager.getConnection(Url, USERNAME, PASSWORD);
    pool.add(conn);
   } catch (Exception e) {
    e.printStackTrace();
   }

  }
 }

}

package com.abin.lee.collection.pool;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList;
import java.util.List;

import com.abin.lee.collection.threadlocal.UserBean;

public class TestConnectionPool {

 public static List<UserBean> findAll(){
  Connection conn=null;
  PreparedStatement ps=null;
  ResultSet rs=null;
  String sql="select * from userbean order by id desc";
  List<UserBean> list=new ArrayList<UserBean>();
  UserBean user=null;
  try {
   conn=ConnectionPool.getInstance().getConnection();
   ps=conn.prepareStatement(sql);
   rs=ps.executeQuery();
   while (rs.next()&&rs!=null) {
    user=new UserBean();
    user.setId(rs.getString("id"));
    user.setUsername(rs.getString("username"));
    user.setPassword(rs.getString("password"));
    list.add(user);
   }
  } catch (Exception e) {
   e.printStackTrace();
  }
  return list;
 }
 public static void main(String[] args) {
  List<UserBean> list=findAll();
  for(UserBean lst:list){
   System.out.println("id="+lst.getId());
   System.out.println("username="+lst.getUsername());
   System.out.println("id="+lst.getPassword());
  }
 }

}

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.LinkedList;
import java.util.List;

public class ConnectionManage {
 private static List<Connection> pool=new LinkedList<Connection>();;
 private static final String Url = "jdbc:oracle:thin:@localhost:1521:XE";
 private static final String USERNAME = "abing";
 private static final String PASSWORD = "abing";
 private static final String DRIVER = "oracle.jdbc.driver.OracleDriver";
 private static int initCount = 5;
 private static int maxCount = 10;
 private static int currentCount = 5;
 private static volatile ConnectionManage instance = null;
 private ConnectionManage(){
  init();
 }
 public static ConnectionManage getInstance(){
  if(null == instance){
   synchronized (ConnectionManage.class) {
    if(null == instance){
     return new ConnectionManage();
    }
   }
  }
  return instance;
 }
 public void init(){
  addConnection();
 }
 public static Connection createConnection() throws ClassNotFoundException {
  Connection conn=null;
  try {
   Class.forName(DRIVER);
   conn=DriverManager.getConnection(Url, USERNAME, PASSWORD);
  } catch (SQLException e) {
   e.printStackTrace();
  }
  return conn;
 }
 public static void addConnection(){
  for(int i=0;i<initCount;i++){
   try {
    pool.add(createConnection());
   } catch (ClassNotFoundException e) {
    e.printStackTrace();
   }
  }
 }
 
 public static void release(Connection conn){
  pool.add(conn);
 }
 public static Connection getConnection() throws SQLException, ClassNotFoundException{
  synchronized (pool) {
   if(pool.size()>0){
    System.out.println("Current Connection size is:"+pool.size());
    return pool.get(0);
   }else if(currentCount<maxCount){
    Class.forName(DRIVER);
    Connection conn=createConnection();
    pool.add(conn);
    currentCount++;
    return conn;
   }else{
    throw new SQLException("Current Connection is Zero");
   }
  }
 }

}

package com.abin.lee.collection.pool;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList;
import java.util.List;

import com.abin.lee.collection.threadlocal.UserBean;

public class TestConnectionPool {

 public static List<UserBean> findAll(){
  Connection conn=null;
  PreparedStatement ps=null;
  ResultSet rs=null;
  String sql="select * from userbean order by id desc";
  List<UserBean> list=new ArrayList<UserBean>();
  UserBean user=null;
  try {
   conn=ConnectionManage.getInstance().getConnection();
   ps=conn.prepareStatement(sql);
   rs=ps.executeQuery();
   while (rs.next()&&rs!=null) {
    user=new UserBean();
    user.setId(rs.getString("id"));
    user.setUsername(rs.getString("username"));
    user.setPassword(rs.getString("password"));
    list.add(user);
   }
  } catch (Exception e) {
   e.printStackTrace();
  }
  return list;
 }
 public static void main(String[] args) {
  List<UserBean> list=findAll();
  for(UserBean lst:list){
   System.out.println("id="+lst.getId());
   System.out.println("username="+lst.getUsername());
   System.out.println("id="+lst.getPassword());
  }
 }

}

下面是以前学java 的时候写的一个实现，没有考虑很多，比如同步处理等等。懒得改，你就看看吧。

package com.wangjia.tools;

import java.io.Serializable;
import java.sql.*;
import java.util.*;

/**
* 数据库连接池的简单实现
* @author wangjia
* @version 1.0
*@serial 2301410997974776106L
*/
public class DbConn implements Serializable{

/**
* 
*/
private static final long serialVersionUID = 2301410997974776106L;
private static List<Connection> connList=new ArrayList<Connection>();
//private static String driver="com.microsoft.jdbc.sqlserver.SQLServerDriver";//sql2000
private static String driver="com.microsoft.sqlserver.jdbc.SQLServerDriver";//sql2005
private static String url="jdbc:sqlserver://127.0.0.1:1433;DatabaseName=Forum";
private static String userName="sa";
private static String password="sa";
private static int maxConn=20;
private static int minConn=5;
private static int usedConn=0;
private static DbConn dbconn=null;
// private static int flag=1;
/**
* 设置数据库连接的驱动
* @param driver 数据库连接的驱动<br/>例如:"com.microsoft.sqlserver.jdbc.SQLServerDriver"
*/
public static void setDriver(String driver) {
DbConn.driver = driver;
}

/**
* 设置数据库连接池的最小数量
* @param maxConn 数据库连接池的最小数量
*/
public static void setMaxConn(int maxConn) {
DbConn.maxConn = maxConn;
}

/**
* 设置数据库连接池的最大数量
* @param minConn 数据库连接池的最大数量
*/
public static void setMinConn(int minConn) {
DbConn.minConn = minConn;
}

/**
* 设置数据库连接的URL
* @param url 数据库连接的URL,包括数据库的名称<br/>例如: "jdbc:sqlserver://127.0.0.1:1433;DatabaseName=master"
*/
public static void setUrl(String url) {
DbConn.url = url;
}

private DbConn() throws MyException{
try{
Class.forName(driver);
for(int i=0;i<minConn;i++){
connList.add(DriverManager.getConnection(url,userName.toString(),password.toString()));
}
}catch(ClassNotFoundException e){
throw new MyException("请设置正确的数据库连接驱动!\nusing: DbConn.setDriver(String driver)");

}catch(SQLException e){
throw new MyException("请设置正确的数据库连接URL以及用于连接的用户名和密码!\n" +
"using: DbConn.setUrl(String url),DbConn.setUserName(String userName),DbConn.setPassword(String password)");

}
}
/**
* 用于得到DbConn对象
* @return 返回一个DbConn对像
*/

public static Connection getConn() throws MyException{
// flag++;
if(dbconn==null)dbconn=new DbConn();
// System.out.println("falg="+flag);
// System.out.println("usedConn="+usedConn);
if(connList.size()==0){
throw new MyException("没有正确的设置相关信息,以至连接池未初始化");
}
if(usedConn==maxConn){

Thread thread=new Thread(new Runnable(){

@SuppressWarnings("static-access")
public void run() {
if(usedConn<maxConn){
try {
try {
Thread.currentThread().sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
getConn();
} catch (MyException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

}

});
thread.start();
}
if(usedConn>=minConn&&usedConn<maxConn){
try {
connList.add(DriverManager.getConnection(url,userName.toString(),password.toString()));
} catch (SQLException e) {

}
return connList.get(usedConn++);
}
if(usedConn<5){
return connList.get(usedConn++);
}
return null;
}

public static void close(){
try {
if(usedConn>5)
connList.get(--usedConn).close();
else usedConn--;
} catch (SQLException e) {
// TODO 自动生成 catch 块
e.printStackTrace();
}

}

/**
* 设置用于连接数据库的密码
* @param password 用于连接数据库的密码
*/
public static void setPassword(String password) {
DbConn.password = password;
}

/**
* 设置用于连接数据库的用户名
* @param userName 用于连接数据库的用户名
*/
public static void setUserName(String userName) {
DbConn.userName = userName;
}

}

在项目中用到连接池的话，一般会直接使用已有的开源连接池实现，比如用 apache 的common-dbcp.jar和common-pool.jar 相结合，很少去自己实现。