java并发实战：连接池实现

摘要：池化技术简介在我们使用数据库的过程中，我们往往使用数据库连接池而不是直接使用数据库连接进行操作，这是因为每一个数据库连接的创建和销毁的代价是昂贵的，而池化技术则预先创建了资源，这些资源是可复用的这样就保证了在多用户情况下只能使用指定数目的资

在我们使用数据库的过程中，我们往往使用数据库连接池而不是直接使用数据库连接进行操作，这是因为每一个数据库连接的创建和销毁的代价是昂贵的，而池化技术则预先创建了资源，这些资源是可复用的,这样就保证了在多用户情况下只能使用指定数目的资源，避免了一个用户创建一个连接资源，造成程序运行开销过大。

这里只实现一个简易的连接池，更多复杂的需求可根据该连接池进行改进，该连接池主要参数如下：

一个繁忙队列busy

一个空闲队列idle

连接池最大活动连接数maxActive

连接池最大等待时间maxWait

连接池的活动连接数activeSize

程序流程图如下：

泛型接口ConnectionPool.java

public interface ConnectionPool {

    /**
     * 初始化池资源
     * @param maxActive 池中最大活动连接数
     * @param maxWait 最大等待时间
     */
    void init(Integer maxActive, Long maxWait);

    /**
     * 从池中获取资源
     * @return 连接资源
     */
    T getResource() throws Exception;

    /**
     * 释放连接
     * @param connection 正在使用的连接
     */
    void release(T connection) throws Exception;

    /**
     * 释放连接池资源
     */
    void close();


}

以zookeeper为例，实现zookeeper连接池，ZookeeperConnectionPool.java

public class ZookeeperConnectionPool implements ConnectionPool {
    //最大活动连接数
    private Integer maxActive; 
    //最大等待时间
    private Long maxWait; 
    //空闲队列
    private LinkedBlockingQueue idle = new LinkedBlockingQueue<>();
    //繁忙队列
    private LinkedBlockingQueue busy = new LinkedBlockingQueue<>();
    //连接池活动连接数
    private AtomicInteger activeSize = new AtomicInteger(0);
    //连接池关闭标记
    private AtomicBoolean isClosed = new AtomicBoolean(false);
    //总共获取的连接记数
    private AtomicInteger createCount = new AtomicInteger(0);
    //等待zookeeper客户端创建完成的计数器
    private static ThreadLocal latchThreadLocal = ThreadLocal.withInitial(() -> new CountDownLatch(1));

    public ZookeeperConnectionPool(Integer maxActive, Long maxWait) {
        this.init(maxActive, maxWait);
    }

    @Override
    public void init(Integer maxActive, Long maxWait) {
        this.maxActive = maxActive;
        this.maxWait = maxWait;
    }

    @Override
    public ZooKeeper getResource() throws Exception {
        ZooKeeper zooKeeper;
        Long nowTime = System.currentTimeMillis();
        final CountDownLatch countDownLatch = latchThreadLocal.get();
        
        //空闲队列idle是否有连接
        if ((zooKeeper = idle.poll()) == null) {
            //判断池中连接数是否小于maxActive
            if (activeSize.get() < maxActive) {
                //先增加池中连接数后判断是否小于等于maxActive
                if (activeSize.incrementAndGet() <= maxActive) {
                    //创建zookeeper连接
                    zooKeeper = new ZooKeeper("localhost", 5000, (watch) -> {
                        if (watch.getState() == Watcher.Event.KeeperState.SyncConnected) {
                            countDownLatch.countDown();
                        }
                    });
                    countDownLatch.await();
                    System.out.println("Thread:" + Thread.currentThread().getId() + "获取连接：" + createCount.incrementAndGet() + "条");
                    busy.offer(zooKeeper);
                    return zooKeeper;
                } else {
                    //如增加后发现大于maxActive则减去增加的
                    activeSize.decrementAndGet();
                }
            }
            //若活动线程已满则等待busy队列释放连接
            try {
                System.out.println("Thread:" + Thread.currentThread().getId() + "等待获取空闲资源");
                Long waitTime = maxWait - (System.currentTimeMillis() - nowTime);
                zooKeeper = idle.poll(waitTime, TimeUnit.MILLISECONDS);
            } catch (InterruptedException e) {
                throw new Exception("等待异常");
            }
            //判断是否超时
            if (zooKeeper != null) {
                System.out.println("Thread:" + Thread.currentThread().getId() + "获取连接：" + createCount.incrementAndGet() + "条");
                busy.offer(zooKeeper);
                return zooKeeper;
            } else {
                System.out.println("Thread:" + Thread.currentThread().getId() + "获取连接超时，请重试！");
                throw new Exception("Thread:" + Thread.currentThread().getId() + "获取连接超时，请重试！");
            }
        }
        //空闲队列有连接，直接返回
        busy.offer(zooKeeper);
        return zooKeeper;
    }

    @Override
    public void release(ZooKeeper connection) throws Exception {
        if (connection == null) {
            System.out.println("connection 为空");
            return;
        }
        if (busy.remove(connection)){
            idle.offer(connection);
        } else {
            activeSize.decrementAndGet();
            throw new Exception("释放失败");
        }
    }

    @Override
    public void close() {
        if (isClosed.compareAndSet(false, true)) {
            idle.forEach((zooKeeper) -> {
                try {
                    zooKeeper.close();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
            busy.forEach((zooKeeper) -> {
                try {
                    zooKeeper.close();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }
    }
}

这里创建20个线程并发测试连接池，Test.java

public class Test {

    public static void main(String[] args) throws Exception {
        int threadCount = 20;
        Integer maxActive = 10;
        Long maxWait = 10000L;
        ZookeeperConnectionPool pool = new ZookeeperConnectionPool(maxActive, maxWait);
        CountDownLatch countDownLatch = new CountDownLatch(20);
        for (int i = 0; i < threadCount; i++) {
            new Thread(() -> {
                countDownLatch.countDown();
                try {
                    countDownLatch.await();
                    ZooKeeper zooKeeper = pool.getResource();
                    Thread.sleep(2000);
                    pool.release(zooKeeper);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (Exception e) {
                    e.printStackTrace();
                }

            }).start();
        }
        while (true){

        }
    }
}