自己手动写代码实现数据库连接池

//抽取连接池架构接口
public interface MyPool {
//对外提供可复用连接包装内
PooledConnection getConnection();
//对内创建连接
void createConnections(int count);
}

//表示繁忙标志     复用的标志 线程安全
private boolean busy = false;
//真正的sql 连接connection(java.sql.Connection)
private Connection con;
//只是用来测试当前connectionName,便于观察
private String connName;

// 将该连接置为不可用,而不是真正关掉连接
public void close() {
    this.busy = false;
}

public ResultSet queryBySql(String sql) {
    Statement sttm = null;
    ResultSet rs = null;
    try {
        sttm = con.createStatement();
        rs = sttm.executeQuery(sql);
        //System.out.println("当前连接编号是:" + connName);
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return rs;
}

private static String driver = null;
private static String url = null;
private static String user = null;
private static String password = null;
/**连接池中管道参数**/
private static int initCount = 5;
private static int stepSize = 10;
private static int poolMaxSize = 55;
private static int expandTime = 0;

/**线程安全集合,用来放(可复用)数据库连接管道(集合之前用Vector,但在测试的时候发现多线程并发出错了ConcurrentModificationException)这个还要仔细研究一下**/
//连接池
private static CopyOnWriteArrayList<PooledConnection> pooledCons = new CopyOnWriteArrayList<PooledConnection>();

private void init() {
    InputStream inStream = this.getClass().getClassLoader().getResourceAsStream("jdbc.properties");
    Properties properties = new Properties();
    try {
        properties.load(inStream);
    } catch (IOException e) {
        //若这里抛出异常则下面不运行
        e.printStackTrace();
    }
    this.driver = properties.getProperty("jdbc_driver");
    this.url = properties.getProperty("jdbc_url");
    this.user = properties.getProperty("jdbc_username");
    this.password = properties.getProperty("jdbc_password");
    if(Integer.valueOf(properties.getProperty("initCount")) > 0) {
        this.initCount = Integer.valueOf(properties.getProperty("initCount"));
    }else if(Integer.valueOf(properties.getProperty("stepSize")) > 0) {
        this.stepSize = Integer.valueOf(properties.getProperty("stepSize"));
    }else if(Integer.valueOf(properties.getProperty("poolMaxSize")) > 0) {
        this.poolMaxSize = Integer.valueOf(properties.getProperty("poolMaxSize"));
    }
    //准备创建DriverManager
    try {
        Driver dbDriver = (Driver) Class.forName(this.driver).newInstance();
        DriverManager.registerDriver(dbDriver);
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    //获取连接,用create方法获取
    /**DriverManager.getConnection(url, user, password);**/
    this.createConnections(initCount);
    }

@Override
public void createConnections(int count) {
    //this.expandTime++;
    //System.out.println("第"+expandTime+"次扩容,扩容量为:"+count);
    if((pooledConnections.size() + count) <= poolMaxSize) {
        for(int i = 0;i < count ;i++) {
            try {
                //获取连接放入线程安全的连接池中
                Connection conn = DriverManager.getConnection(url, user, password);
                PooledConnection pooledConnection = new PooledConnection(conn,false,String.valueOf(i));
                this.pooledConnections.add(pooledConnection);
                //System.out.println("初始化"+(i + 1) + "个连接");
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
    System.out.println("当前连接池连接数量:"+ pooledConnections.size());
    System.out.println("最大连接池数量为:"+ this.poolMaxSize);
}

@Override
public PooledConnection getConnection() {
    //spring思想要拿连接,先判断管道集合中是否有连接
    if(this.pooledConnections.size() == 0) {
        System.out.println("连接池没有连接！");
        //如果没有就手动再建一把连接池
        this.createConnections(initCount);
    }
    PooledConnection connection = getRealConnection();
    //如果还是没有拿到,说明全部线程都处于busy状态,得扩容
    while(connection == null) {
        this.createConnections(stepSize);
        connection = getRealConnection();
        try {//拿到连接等待一会,防止连接又被别的线程抢夺
            Thread.sleep(50);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    return connection;
}

//同步方法,真正的获取连接(连接包装内包括:connection和标志位busy)
private synchronized PooledConnection getRealConnection() {
    for(PooledConnection conn:pooledConnections) {
        //判断该连接是否已被占用
        if(!conn.isBusy()) {//false为可用(空闲),true为占用(繁忙)
            Connection connection = conn.getConnection();
            try {
                //判断该连接是否在设定时间连接通数据库(连接通为true)
                if(!connection.isValid(2000)) {
                    connection = DriverManager.getConnection(url, user, password);
                }
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            conn.setBusy(true);
            return conn;
        }
    }
    //如果连接池中的连接都被占用,则返回null,由调用函数处理(即扩容)
    return null;
}

public class PoolManager {
private PoolManager(){}
/**
 * 内部类单利模式 伪造类加载器加载原理:实现线程安全问题(类加载器加载内部类实例是互斥特性)
 */
private static class createPool {
    private static MyPoolImpl poolImpl = new MyPoolImpl();
}
public static MyPool getInstace() {
    return createPool.poolImpl;
}
}

//测试2000条线程 jdk1.8 内部类用lambda表达示
public static void main(String[] args) {
    for(int i =0; i < 2000; i++) {
        new Thread(() -> selectDate()).start();
    }
}

成员变量
public static MyPool myPool = PoolManager.getInstace();
//方法加了synchronized之后,连接池不扩容了???
public static void selectDate() {       
    PooledConnection connection = myPool.getConnection();
    String sql = "select * from t_area";
    ResultSet rs = connection.queryBySql(sql);
    try {
        while(rs.next()) {
            String name = rs.getString("name");
            Integer id = rs.getInt("id");
            //System.out.println("当前线程:"+ Thread.currentThread().getName() +",id:"+ id + ",name" + name);
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }
    connection.close();
}