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.MICROSECONDS.sleep(100);
      }
      return null;
    }
  }

  /**
   * 創(chuàng)建一個 Connection 的代理，在 commit 時(shí)休眠 100 毫秒
   */
  public static Connection createConnection() {
    return (Connection) Proxy.newProxyInstance(ConnectionDriver.class.getClassLoader(),
        new Class<?>[]{Connection.class}, new ConnectionHandler());
  }
}

public class ConnectionPool {

  private final LinkedList<Connection> pool = new LinkedList<>();

  public ConnectionPool(int initialSize) {
    // 初始化連接的最大上限
    if (initialSize > 0) {
      for (int i = 0; i < initialSize; i++) {
        pool.addLast(ConnectionDriver.createConnection());
      }
    }
  }

  public void releaseConnection(Connection connection) {
    if (connection != null) {
      synchronized (pool) {
        /* 連接釋放后需要進(jìn)行通知
         * 這樣其他消費(fèi)者就能知道連接池已經(jīng)歸還了一個連接
         */
        pool.addLast(connection);
        pool.notifyAll();
      }
    }
  }

  /**
   * 在給定毫秒時(shí)間內(nèi)獲取連接
   */
  public Connection fetchConnection(long mills) throws InterruptedException {
    synchronized (pool) {
      // 完全超時(shí)
      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;
      }
    }
  }
}

public class ConnectionPoolTest {

  static ConnectionPool pool = new ConnectionPool(10);
  static CountDownLatch start = new CountDownLatch(1);
  static CountDownLatch end;

  public static void main(String[] args) throws InterruptedException {
    // 線程數(shù)量
    int threadCount = 200;
    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;
    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 e) {
        e.printStackTrace();
      }
      while (count > 0) {
        try {
          // 從線程池中獲取連接，如果 1000ms 內(nèi)無法獲取到，將返回 null
          // 分別統(tǒng)計(jì)獲取連接的數(shù)量 got 和未獲取到的數(shù)量 notGot
          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) {
          e.printStackTrace();
        } finally {
          count--;
        }
      }
      end.countDown();
    }
  }
}