简单的数据库连接示例 基于等待超时模式

描述：使用等待超时模式来构造一个简单的数据库连接池，在示例中模拟从连接池中获取、使用和释放连接的过程。客户端获取连接的过程被设计成等待超时模式

/*
连接池的定义。通过构造函数初始化连接的最大上限，通过双向队列维护连接。调用方需要先调用fetchConnection（long）
方法来指定在多少毫秒内超时获取连接，当连接使用完时，需要调用releaseConnection（Connection）方法来将来凝结放回
线程池
 */
class ConnectionPool{
    private LinkedList<Connection> pool = new LinkedList<Connection>();
    public ConnectionPool(int initialSize){
        if (initialSize > 0){
            for (int i = 0; i < initialSize; i++){
                pool.addLast(ConnectionDriver.creatConnection());
            }
        }
    }
    public void releaseConnection(Connection connection){
        if(connection != null){
            synchronized (pool){
                //连接释放后需要进行通知，这样其他消费者能够感知到连接池中已经归还了一个连接
                pool.addLast(connection);///?
                pool.notifyAll();
            }
        }
    }

    //在mills内无法获得连接，将会返回null
    public Connection fetchConnection(long mills) throws InterruptedException{
        synchronized (pool){
            //完全超时
            if (mills <= 0){
                while (pool.isEmpty()){
                    pool.wait();
                }
                return pool.removeFirst();
            }else {
                long future = System.currentTimeMillis() + mills;
                long remaining = mills;
                while (pool.isEmpty() && remaining > 0){
                    pool.wait(remaining);
                    remaining = future - System.currentTimeMillis();
                }
                Connection result = null;
                if (!pool.isEmpty()){
                    result = pool.removeFirst();
                }
                return result;
            }
        }
    }
}
/*
由于java.sql.Connection是一个接口，最终实现是由数据库驱动提供方案来实现的，这里通过动态代理构造了一个Connection，
该Connection的代理实现仅仅实在commit方法调用时休眠100毫秒
 */
class ConnectionDriver{
    static class ConnectionHander implements InvocationHandler{
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable{
            if (method.getName().equals("commit")){
                TimeUnit.MILLISECONDS.sleep(100);
            }
            return null;
        }
    }
    //创建一个Connection的代理，在commit时休眠100毫秒
    public static final Connection creatConnection(){
        return (Connection) Proxy.newProxyInstance(ConnectionHander.class.getClassLoader(),
                new Class<?>[] {Connection.class},new ConnectionHander());
    }
}

/*
测试，客户端模拟ConnectionRunner获取、使用、释放连接的过程
 */
class ConnectionPoolTest{
    static ConnectionPool pool = new ConnectionPool(10);
    //保证所有的ConnectionRunner能同时开始
    static CountDownLatch start = new CountDownLatch(1);
    //main线程将会等待所有ConnectionRunner结束后才能继续执行
    static CountDownLatch end;
    public static void main(String[] args) throws Exception{
        //线程数量，可以修改线程数量进行观察
        int threadCount = 20;
        end = new CountDownLatch(threadCount);
        int count = 20;
        AtomicInteger got = new AtomicInteger();
        AtomicInteger notGot = new AtomicInteger();
        for (int i = 0; i < threadCount; i++){
            Thread thread = new Thread(new ConnectionRunner(count, got, notGot), "ConnectionRunnerThread");
            thread.start();
        }
        start.countDown();//只要计数器为0，那么线程就可以结束阻塞往下执行,进而保证线程同时开始
        end.await();
        System.out.println("total invoke: " + (threadCount * count));
        System.out.println("got connection: " + got);
        System.out.println("not got connection: " + notGot);
    }

    static class ConnectionRunner implements Runnable{
        int count;
        AtomicInteger got;
        AtomicInteger notGot;
        public ConnectionRunner(int count, AtomicInteger got, AtomicInteger notGot){
            this.count = count;
            this.got = got;
            this.notGot = notGot;
        }
        @Override
        public void run(){
            try{
                start.await();
            }catch (Exception ex){
            }
            while (count > 0){
                try {
                    //从线程池中获取连接，如果1000ms内无法获取到，将会返回null
                    //分别统计连接获取的数量got和未获取到的数量notGot
                    Connection connection = pool.fetchConnection(1000);
                    if (connection != null){
                        try {
                            connection.createStatement();
                            connection.commit();
                        }finally {
                            pool.releaseConnection(connection);
                            got.getAndIncrement();
                        }
                    }else {
                        notGot.getAndIncrement();
                    }
                }catch (Exception ex){
                }finally {
                    count--;
                }
            }
            end.countDown();
        }
    }
}

输出结果：

total invoke: 400
got connection: 398
not got connection: 2