JDBC 数据库连接池的简单实现

连接池代码：

public class MyDataSource2{
    private static String url = "jdbc:mysql://localhost:3306/jdbc";
    private static String user = "root";
    private static String password = "";

    private static int initCount = 1;
    private static int maxCount = 1;
    int currentCount = 0;

    LinkedList<Connection> connectionsPool = new LinkedList<Connection>();

    public MyDataSource2() {
        try {
            for (int i = 0; i < initCount; i++) {
                this.connectionsPool.addLast(this.createConnection());
                this.currentCount++;
            }
        } catch (SQLException e) {
            throw new ExceptionInInitializerError(e);
        }
    }

    public Connection getConnection() throws SQLException {
        synchronized (connectionsPool) {
            if (this.connectionsPool.size() > 0)
                return this.connectionsPool.removeFirst();

            if (this.currentCount < maxCount) {
                this.currentCount++;
                return this.createConnection();
            }

            throw new SQLException("已没有链接");
        }
    }

    public void free(Connection conn) {
        this.connectionsPool.addLast(conn);
    }

    private Connection createConnection() throws SQLException {
        Connection realConn = DriverManager.getConnection(url, user, password);
         MyConnection myConnection = new MyConnection(realConn, this);
         return myConnection;
    }

 

 

//自建Connection类，重写了close方法，并且限定了同一个Connection的最大使用次数，其它的都使用realConnection的方法

public class MyConnection implements Connection {
    private Connection realConnection;
    private MyDataSource2 dataSource;
    private int maxUseCount = 5;
    private int currentUserCount = 0;

    MyConnection(Connection connection, MyDataSource2 dataSource) {
        this.realConnection = connection;
        this.dataSource = dataSource;
    }

 

    public void close() throws SQLException {
        this.currentUserCount++;
        if (this.currentUserCount < this.maxUseCount)
            this.dataSource.connectionsPool.addLast(this);
        else {
            this.realConnection.close();
            this.dataSource.currentCount--;//这段是限定同一个链接的使用次数，如果这个MyConnection对象被重复使用过好多次了，就释放了它。
            //但是这么做有什么意义？同一个链接使用了很多次会造成什么严重后果吗？这块我真是百思不得其解
        }
    }

 

       ……
}

 

 

 

public final class JdbcUtils {
    private static MyDataSource2 myDataSource = null;

    private JdbcUtils() {
    }

    static {
        try {
            Class.forName("com.mysql.jdbc.Driver");
             myDataSource = new MyDataSource2();//放在这个地方，要加载后才能初始化
            } catch (Exception e) {
            throw new ExceptionInInitializerError(e);
        }
    }

    public static DataSource getDataSource() {
        return myDataSource;
    }

    public static Connection getConnection() throws SQLException {
        return myDataSource.getConnection();
    }

    public static void free(ResultSet rs, Statement st, Connection conn) {
        try {
            if (rs != null)
                rs.close();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (st != null)
                    st.close();
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                if (conn != null)
                    try {
                        conn.close();
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
            }
        }
    }
}