Java 的JDBC 数据库连接池实现方法

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Java 的JDBC 数据库连接池实现方法

关键字: Java, JDBC, Connection Pool, Database, 数据库连接池, sourcecode

  虽然 J2EE 程序员一般都有现成的应用服务器所带的JDBC 数据库连接池,不过对于开发一般的 Java Application 、 Applet 或者 JSP、velocity 时,我们可用的JDBC 数据库连接池并不多,并且一般性能都不好。 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):

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) {
    Class[] interfaces = {java.sql.Connection.class};
    this.connection = (Connection) Proxy.newProxyInstance(
      con.getClass().getClassLoader(),
      interfaces, 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();

  }
}

运行测试程序后打印连接池中 Connection 状态, 以及正在使用的没有关闭 Connection 信息。

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