java 实现 数据库连接池
使用到 代理模式、LinkedList、await、notifyAll、CountDownLatch

• ConnectionDriver (jdbc驱动器)
• ConnectionPool (连接池类)
• MainTest (测试类)

ConnectionDriver (jdbc驱动器)

代理模式使用

• 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，
• 本类模拟数据库驱动，通过动态代理构造一个Connection，
• 该Connection的代理实现仅仅是在commit()方法调用休眠100毫秒

package test.connectPoolDemo;

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

/**
 * @author xuzhihua
 * @date 2019/2/23 11:01 AM
 */
public class ConnectionDriver {
    static class ConnectionHandler implements InvocationHandler {
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            if ("commit".equals(method.getName())){
                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 createConnection() {
        return (Connection) Proxy.newProxyInstance(ConnectionDriver.class.getClassLoader(), new Class[]{Connection.class}, new ConnectionHandler());
    }
}

ConnectionPool (连接池类)

• 连接池定义：
• 通过构造函数初始化连接的最大上限，通过一个双向链表LinkedList来维护链接
• fetchConnection(long):调用方需要先调用该方法来指定在多少毫秒内超时获取连接
• releaseConnection(Connection):当连接使用完后，需要调用该方法将连接放回线程池

package test.connectPoolDemo;

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

/**
 * @author xuzhihua
 * @date 2019/2/23 10:56 AM
 */
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());
            }
        }
    }

    // 释放连接
    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();
            }
        }
    }

    // 获取连接，在mills内无法获取到连接，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;
            }
        }
    }
}

MainTest (测试类)

• 数据库连接池测试类

package test.connectPoolDemo;

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

/**
 * @author xuzhihua
 * @date 2019/2/23 11:13 AM
 */
public class MainTest {
    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，可以修改线程数量进行观察
        int threadCount = 10;
        int count = 20;
        end = new CountDownLatch(threadCount);
        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("thread num: " + threadCount);
        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 (InterruptedException e) {
                e.printStackTrace();
            }
            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 未获取的数量notGot
                    Connection connection = pool.fetchConnection(1000);
                    if (connection != null) {
                        try {
                            connection.createStatement();
                            connection.commit();
                        } catch (SQLException e) {
                            e.printStackTrace();
                        } finally {
                            pool.releaseConnection(connection);
                            got.incrementAndGet();
                        }
                    } else {
                        notGot.incrementAndGet();
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    count --;
                }
            }
            end.countDown();
        }
    }
}