dbconnectionmanager.java

数据库连接包括连接池。很有用的资料

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

public class DBConnectionManager {
    static private DBConnectionManager instance;       // The single instance

    private Vector drivers = new Vector();
    private PrintWriter log;
    private Hashtable pools = new Hashtable();

    private Hashtable clients = new Hashtable();

    /**
     * A private constructor since this is a Singleton
     */
    private DBConnectionManager()
    throws SQLException {
        init();
    }

    /**
     * Returns the single instance, creating one if it's the
     * first time this method is called.
     *
     * @return DBConnectionManager The single instance.
     */
    public static synchronized DBConnectionManager getInstance() 
    throws SQLException
    {
        if (instance == null) {
            instance = new DBConnectionManager();
        }
        return instance;
    }

   /** 
     * 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 Connection getConnection(String name,String url,
	      String username,String password) throws SQLException {
	if (!clients.containsKey(name))
	{
	   createPools(name,url,username,password,0);
	   clients.put(name,new Integer(0));
	}

	int iCurr = ((Integer)clients.get(name)).intValue();
	iCurr++;
	clients.put(name,new Integer(iCurr));

        DBConnectionPool pool = (DBConnectionPool) pools.get(name);
        if (pool != null) {
            return pool.getConnection();
        }
        return null;
    }
    
    /**
     * Closes all open connections and deregisters all drivers.
     */
    public synchronized void release(String name) throws SQLException {
        // Wait until called by the last client
	if (!clients.containsKey(name)) return;

        int iCurr = ((Integer)clients.get(name)).intValue();
	iCurr--;
	clients.put(name,new Integer(iCurr));

	if (iCurr > 0) return;
	clients.remove(name);

        DBConnectionPool pool = (DBConnectionPool)pools.get(name);
        pool.release();

        Enumeration allDrivers = drivers.elements();
        while (allDrivers.hasMoreElements()) {
            Driver driver = (Driver) allDrivers.nextElement();
            DriverManager.deregisterDriver(driver);
        }
    }
    
    private void createPools(String poolName,String url,
	      String username,String password,int max) {
       DBConnectionPool pool = 
          new DBConnectionPool(poolName,url,username,password, max);
       pools.put(poolName, pool);
       log("Initialized pool " + poolName);
    }
    
    /**
     * Loads properties and initializes the instance with its values.
     */
    private void init() throws SQLException {
        log = new PrintWriter(System.err);
        loadDrivers();
        //createPools(0);
    }
    
    /**
     * 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
     */
    private void loadDrivers() throws SQLException {
       DriverManager.registerDriver(new oracle.jdbc.driver.OracleDriver());
    }
    
    /**
     * Writes a message to the log file.
     */
    private void log(String msg) {
        log.println(new Date() + ": " + msg);
    }
    
    /**
     * Writes a message with an Exception to the log file.
     */
    private void log(Throwable e, String msg) {
        log.println(new Date() + ": " + msg);
        e.printStackTrace(log);
    }
    
    /**
     * This inner class represents a connection pool. It creates new
     * connections on demand, up to a max number if specified.
     * It also makes sure a connection is still open before it is
     * returned to a client.
     */
    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;
        
        /**
         * Creates new connection pool.
         *
         * @param name The pool name
         * @param URL The JDBC URL for the database
         * @param user The database user, or null
         * @param password The database user password, or null
         * @param maxConn The maximal number of connections, or 0
         *   for no limit
         */
        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;
        }
        
        /**
         * Checks in a connection to the pool. Notify other Threads that
         * may be waiting for a connection.
         *
         * @param con The connection to check in
         */
        public synchronized void freeConnection(Connection con) {
            // Put the connection at the end of the Vector
            freeConnections.addElement(con);
            checkedOut--;
            notifyAll();
        }
        
        /**
         * Checks out a connection from the pool. If no free connection
         * is available, a new connection is created unless the max
         * number of connections has been reached. If a free connection
         * has been closed by the database, it's removed from the pool
         * and this method is called again recursively.
         */
        public synchronized Connection getConnection() throws SQLException {
            Connection con = null;
            if (freeConnections.size() > 0) {
                // Pick the first Connection in the Vector
                // to get round-robin usage
                con = (Connection) freeConnections.firstElement();
                freeConnections.removeElementAt(0);
                if (con.isClosed()) {
                    log("Removed bad connection from " + name);
                    // Try again recursively
                    con = getConnection();
                }
            }
            else if (maxConn == 0 || checkedOut < maxConn) {
                con = newConnection();
            }
            if (con != null) {
                checkedOut++;
            }
            return con;
        }
        
        /**
         * Closes all available connections.
         */
        public synchronized void release() throws SQLException {
            Enumeration allConnections = freeConnections.elements();
            while (allConnections.hasMoreElements()) {
                Connection con = (Connection) allConnections.nextElement();
                con.close();
            }
            freeConnections.removeAllElements();
        }
        
        /**
         * Creates a new connection, using a userid and password
         * if specified.
         */
        private Connection newConnection() throws SQLException {
            Connection con = null;
            if (user == null) {
               con = DriverManager.getConnection(URL);
            }
            else {
               con = DriverManager.getConnection(URL, user, password);
            }
            log("Created a new connection in pool " + name);
            return con;
        }
    }
}