一個很不錯的數據庫連接池實現，備份之

import java.sql.*;
import java.io.*;
import java.util.*;
public class DBConnectionManager {

??private static int clients = 0;
??private static DBConnectionManager instance;
??private Vector drivers = new Vector();
??private PrintWriter log;
??private Hashtable pools = new Hashtable();


??public DBConnectionManager() {
????init();
??}
??private void log(String msg) {
????log.println(new java.util.Date() + ": " + msg);
??}

??/**
??* 將文本信息與異常寫入日志文件
??*/
??private void log(Throwable e, String msg) {
????log.println(new java.util.Date() + ": " + msg);
????e.printStackTrace(log);
??}

??public static synchronized??DBConnectionManager getInstance() {
????if (instance == null) {
??????instance = new DBConnectionManager();
????}
????clients++;
????return instance;
??}
??private void init() {
????InputStream is = getClass().getResourceAsStream("/db.properties");
????Properties dbProps = new Properties();
????try {
??????dbProps.load(is);
????}
????catch (Exception e) {
????????System.err.println("Can not read the properties file. " +
????????"Make sure db.properties is in the CLASSPATH");
????????return;
????}
????loadDrivers(dbProps);
????createPools(dbProps);
??}
??public void freeConnection(String name, Connection con) {
????DBConnectionPool pool = (DBConnectionPool) pools.get(name);
????if (pool != null) {
??????pool.freeConnection(con);
????}
??}
??public Connection getConnection(String name) {
????DBConnectionPool pool = (DBConnectionPool) pools.get(name);
????if (pool != null) {
??????return pool.getConnection();
????}
????return null;
??}
??public Connection getConnection(String name, long time) {
????DBConnectionPool pool = (DBConnectionPool) pools.get(name);
????if (pool != null) {
??????return pool.getConnection(time);
????}
????return null;
??}
??public synchronized void release() {
????// 等待直到最后一個客戶程序調用
????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("撤銷JDBC驅動程序 " + driver.getClass().getName()+"的注冊");
??????}
??????catch (SQLException e) {
??????log(e, "無法撤銷下列JDBC驅動程序的注冊: " + driver.getClass().getName());
??????}
????}
??}


/*
drivers=sun.jdbc.odbc.JdbcOdbcDriver jdbc.idbDriver

logfile=D:\\user\\src\\java\\DBConnectionManager\\log.txt

idb.url=jdbc:idb:c:\\local\\javawebserver1.1\\db\\db.prp

idb.maxconn=2

access.url=jdbc:odbc:demo

access.user=demo

access.password=demopw

*/
??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.log("Registered JDBC driver " + driverClassName);
??????}
??????catch (Exception e) {
??????//??Log.log("Can not register JDBC driver: " + driverClassName + ", Exception: " + e.toString()));
??????}
????}
??}
??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.log("No URL specified for " + poolName);
??????????continue;
????????}
????????String user = props.getProperty(poolName + ".user");
????????String password = props.getProperty(poolName + ".password");
????????String maxconn = props.getProperty(poolName + ".maxconn", "0");
????????int max;
????????try {
??????????max = Integer.valueOf(maxconn).intValue();
????????}
????????catch (NumberFormatException e) {
??????//????Log.log("Invalid maxconn value " + maxconn + " for " +???poolName);
??????????max = 0;
????????}
????????DBConnectionPool pool =
??????????new DBConnectionPool(poolName, url, user, password, max);
????????pools.put(poolName, pool);
??????//??Log.log("Initialized pool " + poolName);
??????}
????}
??}

??class DBConnectionPool {
????private int checkedOut;
????private Vector freeConnections = new Vector();
????private int maxConn;
????private String name;
????private String password;
????private String URL;
????private String user;

????/**
????* 創建新的連接池
????*
????* @param name 連接池名字
????* @param URL 數據庫的JDBC URL
????* @param user 數據庫帳號,或 null
????* @param password 密碼,或 null
????* @param maxConn 此連接池允許建立的最大連接數
????*/
????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;
????}

??/**
??* 將不再使用的連接返回給連接池
??*
??* @param con 客戶程序釋放的連接
??*/
????public synchronized void freeConnection(Connection con) {
????// 將指定連接加入到向量末尾
??????freeConnections.addElement(con);
??????checkedOut--;
??????notifyAll();
????}

????/**
?????* 從連接池獲得一個可用連接.如沒有空閑的連接且當前連接數小于最大連接
?????* 數限制,則創建新連接.如原來登記為可用的連接不再有效,則從向量刪除之,
?????* 然后遞歸調用自己以嘗試新的可用連接.
?????*/
????public synchronized Connection getConnection()
????{
??????Connection con = null;
??????if (freeConnections.size() > 0)
??????{
??????// 獲取向量中第一個可用連接
????????con = (Connection) freeConnections.firstElement();
????????freeConnections.removeElementAt(0);
????????try {
??????????if (con.isClosed())
??????????{
????????????log("從連接池" + name+"刪除一個無效連接");
????????????// 遞歸調用自己,嘗試再次獲取可用連接
????????????con = getConnection();
??????????}
????????}
????????catch (SQLException e)
????????{
??????????log("從連接池" + name+"刪除一個無效連接");
??????????// 遞歸調用自己,嘗試再次獲取可用連接
??????????con = getConnection();
????????}
??????}
??????else if (maxConn == 0 || checkedOut < maxConn)
??????{
????????con = newConnection();
??????}
??????if (con != null) {
????????checkedOut++;
??????}
??????return con;
????}

??/**
??* 從連接池獲取可用連接.可以指定客戶程序能夠等待的最長時間
??* 參見前一個getConnection()方法.
??*
??* @param timeout 以毫秒計的等待時間限制
??*/
????public synchronized Connection getConnection(long timeout)
????{
??????long startTime = new java.util.Date().getTime();
??????Connection con;
??????while ((con = getConnection()) == null)
??????{
????????try {
??????????wait(timeout);
????????}
????????catch (InterruptedException e) {}
????????if ((new java.util.Date().getTime() - startTime) >= timeout)
????????{
????????// wait()返回的原因是超時
??????????return null;
????????}
??????}
??????return con;
????}

??/**
??* 關閉所有連接
??*/
????public synchronized void release()
????{
??????Enumeration allConnections = freeConnections.elements();
??????while (allConnections.hasMoreElements())
??????{
????????Connection con = (Connection) allConnections.nextElement();
????????try {
????????con.close();
??????????log("關閉連接池" + name+"中的一個連接");
????????}
????????catch (SQLException e) {
??????????log(e, "無法關閉連接池" + name+"中的連接");
????????}
??????}
??????freeConnections.removeAllElements();
????}

??/**
??* 創建新的連接
??*/
????private Connection newConnection()
????{
??????Connection con = null;
??????try {
????????if (user == null) {
??????????con = DriverManager.getConnection(URL);
????????}
????????else {
??????????con = DriverManager.getConnection(URL, user, password);
????????}
????????log("連接池" + name+"創建一個新的連接");
??????}
??????catch (SQLException e) {
????????log(e, "無法創建下列URL的連接: " + URL);
????????return null;
??????}
??????return con;
????}
??}
}