JAVA JDBC连接池

1.为什么要使用连接池

（1）对于开发者而言，通过JDBC获取数据连接，然后执行SQL，是很简单的事情

（2）对于JDBC Driver而言：具体到每一个数据库厂商的驱动实现

• 首先建立应用服务器到数据库服务器之间的连接也就是Socket Connection
• 接下来应用服务器和数据库服务器之间还有很多次数据交换，比如用户密码校验、权限认证等
• 最后数据库应用服务器要初始化连接会话句柄，记录联机日志，并为这一次连接分配进程和系统资源

也就是说：建立一次连接可能花费500毫秒，而你执行一个SQL，很可能只花费50毫秒！！！可以看出来，系统如此忙碌，如果开发者只是执行一个简单的SQL，然后就把此次连接抛弃关闭了，浪费了很多系统资源

 

2.连接池应该具备什么样的特性

（1）能满足分布式数据源，比如一个应用可能连接多个数据库，或者一个数据库不能的权限

（2）具备事务处理能力，每一个事务独占一个连接来实现，降低事务管理的复杂性

（3）连接池的分配与释放，提高连接的复用度，从而降低建立新连接的开销

（4）连接池的配置与维护，设置最小连接数（minConn）和最大连接数（maxConn）来控制连接池中的连接

 

3.最基本连接池实现

 

package com.yli.dal;

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

/**
 * 最基本连接池实现,本例使用静态单例,不使用多数据源<br>
 * 要使用多数据源,将单例改成普通模式即可
 * @author yli
 *
 */
public class ConnectionPool {

	private static ConnectionPool pool = null;

	private String poolName;

	private String driver;

	private String url;

	private String user;

	private String password;
	
	private final static int defaultMin = 5;
	
	private final static int defaultMax = 10;

	private int minConn = defaultMin;

	private int maxConn = defaultMax;

	private List<Connection> usedConnections = new ArrayList<Connection>();

	private List<Connection> freeConnections = new ArrayList<Connection>();

	/**
	 * 私有构造方法,初始化最小连接数
	 * @param poolName
	 * @param driver
	 * @param url
	 * @param user
	 * @param password
	 * @param minConn
	 * @param maxConn
	 */
	private ConnectionPool(String poolName, String driver, String url,
			String user, String password, int minConn, int maxConn) {
		this.poolName = poolName;
		this.driver = driver;
		this.url = url;
		this.user = user;
		this.password = password;
		this.minConn = minConn;
		this.maxConn = maxConn;
		initDriver();
		initPool();
	}

	/**
	 * 单例模式,构造连接池,使用默认的最小最大连接数
	 * @param poolName
	 * @param driver
	 * @param url
	 * @param user
	 * @param password
	 * @return
	 */
	public static ConnectionPool newInstanceDefault(String poolName,
			String driver, String url, String user, String password) {
		return newInstanceFull(poolName, driver, url, user, password, defaultMin, defaultMax);
	}

	/**
	 * 单例模式,构造连接池,指定最小最大连接数
	 * @param poolName
	 * @param driver
	 * @param url
	 * @param user
	 * @param password
	 * @param minConn
	 * @param maxConn
	 * @return
	 */
	public static ConnectionPool newInstanceFull(String poolName,
			String driver, String url, String user, String password,
			int minConn, int maxConn) {
		if (null == pool) {
			pool = new ConnectionPool(poolName, driver, url, user, password, minConn, maxConn);
		}
		return pool;
	}

	/**
	 * 获取连接
	 * @return
	 */
	public synchronized Connection getConnection() {
		Connection conn = null;
		if (freeConnections.size() > 0) {
			System.out.println("======================>当前闲置连接数为：[" + freeConnections.size() + "]个");
			System.out.println("======================>从闲置连接中获取一个连接");
			conn = freeConnections.get(0);
			
			freeConnections.remove(0);
			System.out.println("======================>当前闲置连接减少一个,还剩余[" + freeConnections.size() + "]个");
			
			System.out.println("======================>将获取的闲置连接加入到使用中列表");
			usedConnections.add(conn);
			System.out.println("======================>当前使用中连接数为：[" + usedConnections.size() + "]个");
		} else {
			if (usedConnections.size() >= maxConn) {
				System.out.println("======================>警告:线程池连接数已经达到最大[" + maxConn + "]个！");
				System.out.println("======================>不允许再创建新连接");
			} else {
				System.out.println("======================>当前使用中连接数为：[" + usedConnections.size() + "]个");
				System.out.println("======================>已经没有闲置连接,重新创建一个连接");
				conn = newConnection();
				
				System.out.println("======================>将新创建的连接状态置为使用中");
				usedConnections.add(conn);
				System.out.println("======================>当前使用中连接数为：[" + freeConnections.size() + "]个");
			}
		}
		return conn;
	}

	/**
	 * 获取连接,指定超时时间
	 * @param time
	 * @return
	 */
	public synchronized Connection getConnection(long time) {
		Connection conn = null;
		if (freeConnections.size() > 0) {
			conn = freeConnections.get(0);
			freeConnections.remove(0);
			usedConnections.add(conn);
			this.notifyAll();
		} else {
			if (usedConnections.size() >= maxConn) {
				System.out.println("======================>警告:线程池连接数已经达到最大[" + maxConn + "]个！");
				System.out.println("======================>不允许再创建新连接");
				try {
					this.wait(time);
					conn = getConnection();
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			} else {
				conn = newConnection();
				usedConnections.add(conn);
				this.notifyAll();
			}
		}
		return conn;
	}

	/**
	 * 释放连接
	 * 
	 * @param conn
	 */
	public synchronized void releaseConnection(Connection conn) {
		System.out.println("======================>释放当前连接");
		usedConnections.remove(conn);
		System.out.println("======================>使用中连接减少一个");

		freeConnections.add(conn);
		System.out.println("======================>闲置连接增加一个");
	}

	/**
	 * 关闭连接：一旦关闭,使用中的连接数应该减少一个
	 * 
	 * @param conn
	 */
	public synchronized void closeConnection(Connection conn) {
		try {
			if (usedConnections.size() > 0) {
				usedConnections.remove(conn);
			}
			if (null != conn) {
				conn.close();
			}
		} catch (SQLException e) {
			e.printStackTrace();
		}
	}

	/**
	 * 关闭连接池
	 */
	public synchronized void closePool() {
		System.out.println("======================>开始初关闭连接池");
		System.out.println("======================>当前使用中连接数总共有[" + usedConnections.size() + "]个");
		clearListConnection(usedConnections);
		
		System.out.println("======================>当前闲置连接数总共有[" + freeConnections.size() + "]个");
		clearListConnection(freeConnections);
		System.out.println("======================>关闭连接池成功");
	}

	private void clearListConnection(List<Connection> list) {
		Connection itemConn;
		try {
			for (int i = 0; i < list.size(); i++) {
				itemConn = list.get(i);
				list.remove(i);
				if (null != itemConn) {
					itemConn.close();
				}
			}
		} catch (SQLException e) {
			e.printStackTrace();
		}
	}

	private void initDriver() {
		try {
			Class.forName(driver);
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		}
	}

	private void initPool() {
		System.out.println("======================>开始初始化最少个数连接数");
		for (int i = 0; i < minConn; i++) {
			Connection conn = newConnection();
			freeConnections.add(conn);
		}
		System.out.println("======================>初始化成功，当前闲置连接数剩余[" + minConn + "]个");
	}

	private Connection newConnection() {
		Connection conn = null;
		try {
			conn = DriverManager.getConnection(url, user, password);
		} catch (SQLException e) {
			e.printStackTrace();
		}
		return conn;
	}

	public String getPoolName() {
		return poolName;
	}

	public String getDriver() {
		return driver;
	}

	public String getUrl() {
		return url;
	}

	public String getUser() {
		return user;
	}

	public String getPassword() {
		return password;
	}

	public int getMinConn() {
		return minConn;
	}

	public int getMaxConn() {
		return maxConn;
	}
}

 

4.连接池简单测试

package com.yli.dal;

import java.sql.Connection;

/**
 * 连接池简单测试
 * @author yli
 *
 */
public class PoolTest implements Runnable {

	private ConnectionPool pool;

	public PoolTest(ConnectionPool pool) {
		this.pool = pool;
	}

	@Override
	public void run() {
		Connection conn = pool.getConnection();
		// 超时机制获取
		// Connection conn = pool.getConnection(200);
		int threadName = Integer.valueOf(Thread.currentThread().getName());
		// 测试释放链接
		if (threadName >= 8) {
			pool.releaseConnection(conn);
		}
	}

	public static void main(String[] args) {
		String poolName = "MySQL-Connection-Pool";
		String driver = "com.mysql.jdbc.Driver";
		String url = "jdbc:mysql://localhost:3306/world";
		String user = "root";
		String password = "123456";
		ConnectionPool pool = ConnectionPool.newInstanceDefault(poolName,
				driver, url, user, password);
		System.out.println();

		for (int i = 0; i < 20; i++) {
			PoolTest poolTarget = new PoolTest(pool);
			new Thread(poolTarget, "" + i).start();
			System.out.println();
		}
		
		try {
			// 等待5秒再关闭连接池
			Thread.sleep(5*1000);
			pool.closePool();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

	}
}