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一个 JDBC 连接池例子


  import java.io.*;
  import java.sql.*;
  import java.util.*;
  import java.util.Date;
  
  /**
  * This class is a Singleton that provides access to one or many
  * connection pools defined in a Property file. A client gets
  * access to the single instance through the static getInstance()
  * method and can then check-out and check-in connections from a pool.
  * When the client shuts down it should call the release() method
  * to close all open connections and do other clean up.
  */
  public class DBConnectionManager {
   static private DBConnectionManager instance; // The single instance
   static private int clients;
  
   private Vector drivers = new Vector();
   private PrintWriter log;
   private Hashtable pools = new Hashtable();
  
   /**
   * Returns the single instance, creating one if it's the
   * first time this method is called.
   *
   * @return DBConnectionManager The single instance.
   */
   static synchronized public DBConnectionManager getInstance() {
   if (instance == null) {
   instance = new DBConnectionManager();
   }
   clients++;
   return instance;
   }
  
   /**
   * A private constructor since this is a Singleton
   */
   private DBConnectionManager() {
   init();
   }
  
   /**
   * Returns a connection to the named pool.
   *
   * @param name The pool name as defined in the properties file
   * @param con The Connection
   */
   public void freeConnection(String name, Connection con) {
   DBConnectionPool pool = (DBConnectionPool) pools.get(name);
   if (pool != null) {
   pool.freeConnection(con);
   }
   }
  
   /**
   * Returns an open connection. If no one is available, and the max
   * number of connections has not been reached, a new connection is
   * created.
   *
   * @param name The pool name as defined in the properties file
   * @return Connection The connection or null
   */
   public java.sql.Connection getConnection(String name) {
   DBConnectionPool pool = (DBConnectionPool) pools.get(name);
   if (pool != null) {
   return pool.getConnection();
   }
   return null;
   }
  
   /**
   * Returns an open connection. If no one is available, and the max
   * number of connections has not been reached, a new connection is
   * created. If the max number has been reached, waits until one
   * is available or the specified time has elapsed.
   *
   * @param name The pool name as defined in the properties file
   * @param time The number of milliseconds to wait
   * @return Connection The connection or null
   */
   public java.sql.Connection getConnection(String name, long time) {
   DBConnectionPool pool = (DBConnectionPool) pools.get(name);
   if (pool != null) {
   return pool.getConnection(time);
   }
   return null;
   }
  
   /**
   * Closes all open connections and deregisters all drivers.
   */
   public synchronized void release() {
   // Wait until called by the last client
   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("Deregistered JDBC driver " + driver.getClass().getName());
   }
   catch (SQLException e) {
   log(e, "Can't deregister JDBC driver: " + driver.getClass().getName());
   }
   }
   }
  
   /**
   * Creates instances of DBConnectionPool based on the properties.
   * A DBConnectionPool can be defined with the following properties:
   *

   * <poolname>.url The JDBC URL for the database
   * <poolname>.user A database user (optional)
   * <poolname>.password A database user password (if user specified)
   * <poolname>.maxconn The maximal number of connections (optional)
   *

   *
   * @param props The connection pool properties
   */
   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("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("Invalid maxconn value " + maxconn + " for " + poolName);
   max = 0;
   }
   DBConnectionPool pool =
   new DBConnectionPool(poolName, url, user, password, max);
   pools.put(poolName, pool);
   log("Initialized pool " + poolName);
   }
   }
   }
  
   /**
   * Loads properties and initializes the instance with its values.
   */
   private void init() {
   InputStream is = getClass().getResourceAsStream("/db.properties");
   Properties dbProps = new Properties();
   try {
   dbProps.load(is);
   }
   catch (Exception e) {
   System.err.println("Can't read the properties file. " +
   "Make sure db.properties is in the CLASSPATH");
   return;
   }
   String logFile = dbProps.getProperty("logfile", "DBConnectionManager.log");
   try {
   log = new PrintWriter(new FileWriter(logFile, true), true);
   }
   catch (IOException e) {
   System.err.println("Can't open the log file: " + logFile);
   log = new PrintWriter(System.err);
   }
   loadDrivers(dbProps);
   createPools(dbProps);
   }
  
   /**
   * Loads and registers all JDBC drivers. This is done by the
   * DBConnectionManager, as opposed to the DBConnectionPool,
   * since many pools may share the same driver.
   *
   * @param props The connection pool properties
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