一个简单的数据库连接池示例

连接池的定义

import java.sql.Connection;
import java.util.LinkedList;

/**
 * 数据库连接池
 *
 * @author xiaoshu
 */
public class ConnectionPool {
    private LinkedList<Connection> pool = new LinkedList<>();

    public ConnectionPool(int initialSize) {
        if (initialSize > 0) {
            for (int i = 0; i < initialSize; i++) {
                pool.addLast(ConnectionDriver.createConnection());
            }
        }
    }

    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();
            }
        }
    }

    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;
            }
        }
    }
}

连接驱动

由于java.sql.Connection是一个接口，最终的实现是由数据库驱动提供方来实现的，考虑到只是个示例，我们通过动态代理构造一个Connection，该Connection的代理实现仅仅是在commit()方法调用时休眠100毫秒。

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.sql.Connection;
import java.util.concurrent.TimeUnit;

/**
 * 连接驱动
 *
 * @author xiaoshu
 */
public class ConnectionDriver {
    static class ConnectionHandler implements InvocationHandler {
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            if (method.getName().equals("commit")) {
                TimeUnit.MILLISECONDS.sleep(100);
            }
            return null;
        }
    }

    public static final Connection createConnection() {
        return (Connection) Proxy.newProxyInstance(ConnectionDriver.class.getClassLoader(), new Class[]{Connection.class}, new ConnectionHandler());
    }
}

测试程序

下面通过一个示例来测试简易数据库连接池的工作情况。

import java.sql.Connection;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 连接池测试
 *
 * @author xiaoshu
 */
public class ConnectionPoolTest {
    static ConnectionPool pool = new ConnectionPool(10);
    /**
     * 保证所有ConnectionRunner能够同时开始
     */
    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 InterruptedException {
        //线程数量
        int threadCount = 100;
        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();
        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, 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 (InterruptedException e) {
            }

            while (count > 0) {
                try {
                    Connection connection = pool.fetchConnection(1000);
                    if (connection != null) {
                        try {
                            connection.createStatement();
                            connection.commit();
                        } finally {
                            pool.releaseConnection(connection);
                            got.incrementAndGet();
                        }
                    } else {
                        notGot.incrementAndGet();
                    }
                } catch (Exception e) {

                } finally {
                    count--;
                }
            }
            end.countDown();
        }
    }
}

参考： 《Java并发编程的艺术》