用java多线程模拟数据库连接池

模拟一个ConnectionDriver，用于创建Connection

package tread.demo.threadpool;

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

public class ConnectionDriver {
    static class ConnectionHandler implements InvocationHandler {

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            return null;
        }
    }

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

线程池的实现：

package tread.demo.threadpool;

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

public 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.createConnection());
            }
        }
    }

    public void releaseConnection(Connection connection) {
        if (connection != null) {
            synchronized (pool) {
                pool.addLast(connection);//将Connection还回给Pool
                pool.notifyAll();//通知等待的线程
            }
        }
    }

    public Connection fetchConnection(long mills) throws Exception {
        synchronized (pool) {
            if (mills <= 0) {
                while (pool.isEmpty()) {
                    pool.wait();//一直等带release-》Notify
                }
                return pool.removeFirst();//得到一个connection
            } else {
                long future = System.currentTimeMillis() + mills;
                long remaining = mills;
                while (pool.isEmpty() && remaining > 0) {//基于时间进行等待，一直到超时。
                    pool.wait();
                    remaining = future - System.currentTimeMillis();
                }
                Connection result = null;
                if (!pool.isEmpty()) {
                    result = pool.removeFirst();
                }
                return result;
            }
        }
    }
}

两点：

1. 对象的wait和notify
2. 基于超时时间的等待。

测试：

package tread.demo.threadpool;

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

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 Exception {
        int threadCount = 1000;
        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();//tart的CountDown为0,保证了所有线程同时执行。
        end.await();//等待所有线程执行完毕，
        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;
        }

        public void run() {
            try {
                start.await();//等待start的CountDown为0.
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            while (count > 0) {
                try {
                    Connection connection = pool.fetchConnection(1);//超时时间
                    if (connection != null) {
                        try {
                            connection.createStatement();
                        } finally {
                            pool.releaseConnection(connection);
                            got.incrementAndGet();
                        }
                    } else {
                        notGot.incrementAndGet();
                    }
                } catch (Exception ex) {
                } finally {
                    count--;
                }
            }
            end.countDown();
        }
    }
}

继续巧用了CatdownLatch

结果：

total invoke: 20000
got connection: 11914
not got connection: 8086

如果调整超时时间，调整为100ms

结果如下（大部分时候都能得到connection）

total invoke: 20000
got connection: 19050
not got connection: 950