几种Java数据库连接池实现（二)

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

/**
* 管理类DBConnectionManager支持对一个或多个由属性文件定义的数据库连接
* 池的访问.客户程序可以调用getInstance()方法访问本类的唯一实例.
*/
public class DBConnectionManager {
    private static DBConnectionManager instance; // 唯一实例

    private static int clients;

    private Vector<Driver> drivers = new Vector<Driver>();

    private PrintWriter log;

    private Hashtable<String, DBConnectionPool> pools = new Hashtable<String, DBConnectionPool>();

    /**
    * 返回唯一实例.如果是第一次调用此方法,则创建实例
    *
    * @return DBConnectionManager 唯一实例
    */
    public static synchronized DBConnectionManager getInstance() {
        if (instance == null) {
            instance = new DBConnectionManager();
        }
        clients++;
        return instance;
    }

    /**
    * 建构函数私有以防止其它对象创建本类实例
    */
    private DBConnectionManager() {
        init();
    }

    /**
    * 读取属性完成初始化
    */
    private void init() {
        InputStream is = getClass().getResourceAsStream("/db.properties");
        Properties dbProps = new Properties();
        try {
            dbProps.load(is);
        } catch (Exception e) {
            System.err.println("不能读取属性文件. "
                    + "请确保db.properties在CLASSPATH指定的路径中");
            return;
        }
        String logFile = dbProps.getProperty("logfile",
                "DBConnectionManager.log");
        try {
            log = new PrintWriter(new FileWriter(logFile, true), true);
        } catch (IOException e) {
            System.err.println("无法打开日志文件: " + logFile);
            log = new PrintWriter(System.err);
        }
        loadDrivers(dbProps);
        createPools(dbProps);
    }

    /**
    * 将连接对象返回给由名字指定的连接池
    *
    * @param name
    *            在属性文件中定义的连接池名字
    * @param con
    *            连接对象
    */
    public void freeConnection(String name, Connection con) {
        DBConnectionPool pool = (DBConnectionPool) pools.get(name);
        if (pool != null) {
            pool.freeConnection(con);
        }
    }

    /**
    * 获得一个可用的(空闲的)连接.如果没有可用连接,且已有连接数小于最大连接数 限制,则创建并返回新连接
    *
    * @param name
    *            在属性文件中定义的连接池名字
    * @return Connection 可用连接或null
    */
    public Connection getConnection(String name) {
        DBConnectionPool pool = (DBConnectionPool) pools.get(name);
        if (pool != null) {
            return pool.getConnection();
        }
        return null;
    }

    /**
    * 获得一个可用连接.若没有可用连接,且已有连接数小于最大连接数限制, 则创建并返回新连接.否则,在指定的时间内等待其它线程释放连接.
    *
    * @param name
    *            连接池名字
    * @param time
    *            以毫秒计的等待时间
    * @return Connection 可用连接或null
    */
    public Connection getConnection(String name, long time) {
        DBConnectionPool pool = (DBConnectionPool) pools.get(name);
        if (pool != null) {
            return pool.getConnection(time);
        }
        return null;
    }

    /**
    * 关闭所有连接,撤销驱动程序的注册
    */
    public synchronized void release() {
        // 等待直到最后一个客户程序调用
        if (--clients != 0) {
            return;
        }

        Enumeration allPools = pools.elements();
        while (allPools.hasMoreElements()) {
            DBConnectionPool pool = (DBConnectionPool) allPools.nextElement();
            pool.release();
        }
        Enumeration allDrivers = drivers.elements();
        while (allDrivers.hasMoreElements()) {
            Driver driver = (Driver) allDrivers.nextElement();
            try {
                DriverManager.deregisterDriver(driver);
                log("撤销JDBC驱动程序 " + driver.getClass().getName() + "的注册");
            } catch (SQLException e) {
                log(e, "无法撤销下列JDBC驱动程序的注册: " + driver.getClass().getName());
            }
        }
    }

    /**
    * 根据指定属性创建连接池实例.
    *
    * @param props
    *            连接池属性
    */
    private void createPools(Properties props) {
        Enumeration propNames = props.propertyNames();
        while (propNames.hasMoreElements()) {
            String name = (String) propNames.nextElement();
            if (name.endsWith(".url")) {
                String poolName = name.substring(0, name.lastIndexOf("."));
                String url = props.getProperty(poolName + ".url");
                if (url == null) {
                    log("没有为连接池" + poolName + "指定URL");
                    continue;
                }
                String user = props.getProperty(poolName + ".user");
                String password = props.getProperty(poolName + ".password");
                String maxconn = props.getProperty(poolName + ".maxconn", "");
                int max;
                try {
                    max = Integer.valueOf(maxconn).intValue();
                } catch (NumberFormatException e) {
                    log("错误的最大连接数限制: " + maxconn + " .连接池: " + poolName);
                    max = 0;
                }
                DBConnectionPool pool = new DBConnectionPool(poolName, url,
                        user, password, max);
                pools.put(poolName, pool);
                log("成功创建连接池" + poolName);
            }
        }
    }

    /**
    * 装载和注册所有JDBC驱动程序
    *
    * @param props
    *            属性
    */
    private void loadDrivers(Properties props) {
        String driverClasses = props.getProperty("drivers");
        StringTokenizer st = new StringTokenizer(driverClasses);
        while (st.hasMoreElements()) {
            String driverClassName = st.nextToken().trim();
            try {
                Driver driver = (Driver) Class.forName(driverClassName)
                        .newInstance();
                DriverManager.registerDriver(driver);
                drivers.addElement(driver);
                log("成功注册JDBC驱动程序" + driverClassName);
            } catch (Exception e) {
                log("无法注册JDBC驱动程序: " + driverClassName + ", 错误: " + e);
            }
        }
    }

    /**
    * 将文本信息写入日志文件
    */
    private void log(String msg) {
        log.println(new Date() + ": " + msg);
    }

    /**
    * 将文本信息与异常写入日志文件
    */
    private void log(Throwable e, String msg) {
        log.println(new Date() + ": " + msg);
        e.printStackTrace(log);
    }

    /**
    * 此内部类定义了一个连接池.它能够根据要求创建新连接,直到预定的最大连接数为止.在返回连接给客户程序之前,它能够验证连接的有效性.
    */
    class DBConnectionPool {
        private int checkedOut;// 当前连接数

        private Vector<Connection> freeConnections = new Vector<Connection>();// 保存所有可用连接

        private int maxConn;// 此连接池允许建立的最大连接数

        private String name;// 连接池名字

        private String password;// 密码或null

        private String URL;// 数据库的JDBC URL

        private String user;// 数据库账号或null

        /**
        * 创建新的连接池
        *
        * @param name
        *            连接池名字
        * @param URL
        *            数据库的JDBC URL
        * @param user
        *            数据库帐号,或 null
        * @param password
        *            密码,或 null
        * @param maxConn
        *            此连接池允许建立的最大连接数
        */
        public DBConnectionPool(String name, String URL, String user,
                String password, int maxConn) {
            this.name = name;
            this.URL = URL;
            this.user = user;
            this.password = password;
            this.maxConn = maxConn;
        }

        /**
        * 创建新的连接
        */
        private Connection newConnection() {
            Connection con = null;
            try {
                if (user == null) {
                    con = DriverManager.getConnection(URL);
                } else {
                    con = DriverManager.getConnection(URL, user, password);
                }
                log("连接池" + name + "创建一个新的连接");
            } catch (SQLException e) {
                log(e, "无法创建下列URL的连接: " + URL);
                return null;
            }
            return con;
        }

        /**
        * 将不再使用的连接返回给连接池
        *
        * @param con
        *            客户程序释放的连接
        */
        public synchronized void freeConnection(Connection con) {
            // 将指定连接加入到向量末尾
            freeConnections.addElement(con);
            checkedOut--;
            notifyAll();
        }

        /**
        * 从连接池获得一个可用连接.如没有空闲的连接且当前连接数小于最大连接 数限制,则创建新连接.如原来登记为可用的连接不再有效,则从向量删除之,
        * 然后递归调用自己以尝试新的可用连接.
        */
        public synchronized Connection getConnection() {
            Connection con = null;
            if (freeConnections.size() > 0) {
                // 获取向量中第一个可用连接
                con = (Connection) freeConnections.firstElement();
                freeConnections.removeElementAt(0);
                try {
                    if (con.isClosed()) {
                        log("从连接池" + name + "删除一个无效连接");
                        // 递归调用自己,尝试再次获取可用连接
                        con = getConnection();
                    }
                } catch (SQLException e) {
                    log("从连接池" + name + "删除一个无效连接");
                    // 递归调用自己,尝试再次获取可用连接
                    con = getConnection();
                }
            } else if (maxConn == 0 || checkedOut < maxConn) {
                con = newConnection();
            }
            if (con != null) {
                checkedOut++;
            }
            return con;
        }

        /**
        * 从连接池获取可用连接.可以指定客户程序能够等待的最长时间 参见前一个getConnection()方法.
        *
        * @param timeout
        *            以毫秒计的等待时间限制
        */
        public synchronized Connection getConnection(long timeout) {
            long startTime = new Date().getTime();
            Connection con;
            while ((con = getConnection()) == null) {
                try {
                    wait(timeout);
                } catch (InterruptedException e) {
                }
                if ((new Date().getTime() - startTime) >= timeout) {
                    // wait()返回的原因是超时
                    return null;
                }
            }
            return con;
        }

        /**
        * 关闭所有连接
        */
        public synchronized void release() {
            Enumeration allConnections = freeConnections.elements();
            while (allConnections.hasMoreElements()) {
                Connection con = (Connection) allConnections.nextElement();
                try {
                    con.close();
                    log("关闭连接池" + name + "中的一个连接");
                } catch (SQLException e) {
                    log(e, "无法关闭连接池" + name + "中的连接");
                }
            }
            freeConnections.removeAllElements();
        }

    }
}