作为一名初级Java程序员都很羡慕Windows ADO ，只需要new Connection 就可以直接从数据库连接池中返回Connection。并且 ADO Connection 是线程安全的，多个线程可以共用一个Connection，所以ASP程序一般都把getConnection 放在 Global.asa 文件中，在 IIS 启动时建立数据库连接。ADO 的Connection 和Result 都有很好的缓冲，并且很容易使用。
其实我们可以自己写一个JDBC数据库连接池。
写JDBC connection pool 的注意事项有：
1. 有一个简单的函数从连接池中得到一个 Connection。
2. close 函数必须将connection 放回 数据库连接池。
3. 当数据库连接池中没有空闲的connection，数据库连接池必须能够自动增加connection 个数。
4. 当数据库连接池中的connection 个数在某一个特别的时间变得很大，但是以后很长时间只用其中一小部分，应该可以自动将多余的connection 关闭掉。
5. 如果可能，应该提供debug 信息报告没有关闭的new Connection 。
如果要new Connection 就可以直接从数据库连接池中返回Connection， 可以这样写( Mediator pattern ) (以下代码中使用了中文全角空格)：
代码如下：
public class EasyConnection implements java.sql.Connection{ private Connection m_delegate = null; public EasyConnection(){ m_delegate = getConnectionFromPool(); } public void close(){ putConnectionBackToPool(m_delegate); } public PreparedStatement prepareStatement(String sql) throws SQLException{ m_delegate.prepareStatement(sql); } //...... other method } 看来并不难。不过不建议这种写法，因为应该尽量避免使用Java Interface, 关于Java Interface 的缺点我另外再写文章讨论。大家关注的是Connection Pool 的实现方法。下面给出一种实现方法。
import java.sql.*; import java.lang.reflect.*; import java.util.*; import java.io.*; public class SimpleConnetionPool { private static LinkedList m_notUsedConnection = new LinkedList(); private static HashSet m_usedUsedConnection = new HashSet(); private static String m_url = ""; private static String m_user = ""; private static String m_password = ""; static final boolean DEBUG = true; static private long m_lastClearClosedConnection = System.currentTimeMillis(); public static long CHECK_CLOSED_CONNECTION_TIME = 4 * 60 * 60 * 1000; //4 hours static { initDriver(); } private SimpleConnetionPool() { } private static void initDriver() { Driver driver = null; //load mysql driver try { driver = (Driver) Class.forName("com.mysql.jdbc.Driver").newInstance(); installDriver(driver); } catch (Exception e) { } //load postgresql driver try { driver = (Driver) Class.forName("org.postgresql.Driver").newInstance(); installDriver(driver); } catch (Exception e) { } } public static void installDriver(Driver driver) { try { DriverManager.registerDriver(driver); } catch (Exception e) { e.printStackTrace(); } } public static synchronized Connection getConnection() { clearClosedConnection(); while (m_notUsedConnection.size() > 0) { try { ConnectionWrapper wrapper = (ConnectionWrapper) m_notUsedConnection.removeFirst(); if (wrapper.connection.isClosed()) { continue; } m_usedUsedConnection.add(wrapper); if (DEBUG) { wrapper.debugInfo = new Throwable("Connection initial statement"); } return wrapper.connection; } catch (Exception e) { } } int newCount = getIncreasingConnectionCount(); LinkedList list = new LinkedList(); ConnectionWrapper wrapper = null; for (int i = 0; i < newCount; i++) { wrapper = getNewConnection(); if (wrapper != null) { list.add(wrapper); } } if (list.size() == 0) { return null; } wrapper = (ConnectionWrapper) list.removeFirst(); m_usedUsedConnection.add(wrapper); m_notUsedConnection.addAll(list); list.clear(); return wrapper.connection; } private static ConnectionWrapper getNewConnection() { try { Connection con = DriverManager.getConnection(m_url, m_user, m_password); ConnectionWrapper wrapper = new ConnectionWrapper(con); return wrapper; } catch (Exception e) { e.printStackTrace(); } return null; } static synchronized void pushConnectionBackToPool(ConnectionWrapper con) { boolean exist = m_usedUsedConnection.remove(con); if (exist) { m_notUsedConnection.addLast(con); } } public static int close() { int count = 0; Iterator iterator = m_notUsedConnection.iterator(); while (iterator.hasNext()) { try { ( (ConnectionWrapper) iterator.next()).close(); count++; } catch (Exception e) { } } m_notUsedConnection.clear(); iterator = m_usedUsedConnection.iterator(); while (iterator.hasNext()) { try { ConnectionWrapper wrapper = (ConnectionWrapper) iterator.next(); wrapper.close(); if (DEBUG) { wrapper.debugInfo.printStackTrace(); } count++; } catch (Exception e) { } } m_usedUsedConnection.clear(); return count; } private static void clearClosedConnection() { long time = System.currentTimeMillis(); //sometimes user change system time,just return if (time < m_lastClearClosedConnection) { time = m_lastClearClosedConnection; return; } //no need check very often if (time - m_lastClearClosedConnection < CHECK_CLOSED_CONNECTION_TIME) { return; } m_lastClearClosedConnection = time; //begin check Iterator iterator = m_notUsedConnection.iterator(); while (iterator.hasNext()) { ConnectionWrapper wrapper = (ConnectionWrapper) iterator.next(); try { if (wrapper.connection.isClosed()) { iterator.remove(); } } catch (Exception e) { iterator.remove(); if (DEBUG) { System.out.println("connection is closed, this connection initial StackTrace"); wrapper.debugInfo.printStackTrace(); } } } //make connection pool size smaller if too big int decrease = getDecreasingConnectionCount(); if (m_notUsedConnection.size() < decrease) { return; } while (decrease-- > 0) { ConnectionWrapper wrapper = (ConnectionWrapper) m_notUsedConnection.removeFirst(); try { wrapper.connection.close(); } catch (Exception e) { } } } /** * get increasing connection count, not just add 1 connection * @return count */ public static int getIncreasingConnectionCount() { int count = 1; int current = getConnectionCount(); count = current / 4; if (count < 1) { count = 1; } return count; } /** * get decreasing connection count, not just remove 1 connection * @return count */ public static int getDecreasingConnectionCount() { int count = 0; int current = getConnectionCount(); if (current < 10) { return 0; } return current / 3; } public synchronized static void printDebugMsg() { printDebugMsg(System.out); } public synchronized static void printDebugMsg(PrintStream out) { if (DEBUG == false) { return; } StringBuffer msg = new StringBuffer(); msg.append("debug message in " + SimpleConnetionPool.class.getName()); msg.append("\r\n"); msg.append("total count is connection pool: " + getConnectionCount()); msg.append("\r\n"); msg.append("not used connection count: " + getNotUsedConnectionCount()); msg.append("\r\n"); msg.append("used connection, count: " + getUsedConnectionCount()); out.println(msg); Iterator iterator = m_usedUsedConnection.iterator(); while (iterator.hasNext()) { ConnectionWrapper wrapper = (ConnectionWrapper) iterator.next(); wrapper.debugInfo.printStackTrace(out); } out.println(); } public static synchronized int getNotUsedConnectionCount() { return m_notUsedConnection.size(); } public static synchronized int getUsedConnectionCount() { return m_usedUsedConnection.size(); } public static synchronized int getConnectionCount() { return m_notUsedConnection.size() + m_usedUsedConnection.size(); } public static String getUrl() { return m_url; } public static void setUrl(String url) { if (url == null) { return; } m_url = url.trim(); } public static String getUser() { return m_user; } public static void setUser(String user) { if (user == null) { return; } m_user = user.trim(); } public static String getPassword() { return m_password; } public static void setPassword(String password) { if (password == null) { return; } m_password = password.trim(); } } class ConnectionWrapper implements InvocationHandler { private final static String CLOSE_METHOD_NAME = "close"; public Connection connection = null; private Connection m_originConnection = null; public long lastAccessTime = System.currentTimeMillis(); Throwable debugInfo = new Throwable("Connection initial statement"); ConnectionWrapper(Connection con) { this.connection = (Connection) Proxy.newProxyInstance( con.getClass().getClassLoader(), con.getClass().getInterfaces(), this); m_originConnection = con; } void close() throws SQLException { m_originConnection.close(); } public Object invoke(Object proxy, Method m, Object[] args) throws Throwable { Object obj = null; if (CLOSE_METHOD_NAME.equals(m.getName())) { SimpleConnetionPool.pushConnectionBackToPool(this); } else { obj = m.invoke(m_originConnection, args); } lastAccessTime = System.currentTimeMillis(); return obj; } } 使用方法
public class TestConnectionPool{ public static void main(String[] args) { SimpleConnetionPool.setUrl(DBTools.getDatabaseUrl()); SimpleConnetionPool.setUser(DBTools.getDatabaseUserName()); SimpleConnetionPool.setPassword(DBTools.getDatabasePassword()); Connection con = SimpleConnetionPool.getConnection(); Connection con1 = SimpleConnetionPool.getConnection(); Connection con2 = SimpleConnetionPool.getConnection(); //do something with con ... try { con.close(); } catch (Exception e) {} try { con1.close(); } catch (Exception e) {} try { con2.close(); } catch (Exception e) {} con = SimpleConnetionPool.getConnection(); con1 = SimpleConnetionPool.getConnection(); try { con1.close(); } catch (Exception e) {} con2 = SimpleConnetionPool.getConnection(); SimpleConnetionPool.printDebugMsg(); } } 最后一点就是 运行测试程序后打印JDBC数据库连接池中Connection状态，以及正在使用的没有关闭Connection信息。 end,动手练习一下。