数据库连接池示例

学习多线程期间写的小demo，使用到了一些多线程的知识，countDownLatch、wait、notify、AtomicInteger，以及动态代理（模拟数据库连接）。

在测试的时候可以通过增加获取/释放线程的数量，来模拟实际业务中，对数据库压力逐渐增加的场景，能够观察到随着线程的增加，获取成功的数量会下降的现象。

首先是测试类，其中start这个CountDownLatch是为了让所有需要获取/释放数据库连接的线程同时启动任务。end是为了让所有的线程都完成自己获取/释放的任务数量，保证公平性。

java 复制代码

package com.practice.thread;

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

/**
 * 连接池的资源是有限的10
 * 当获取的线程逐渐增加的情况下，未在限定时间内获取到的情况会逐渐增加
 */
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 = 30;
        end = new CountDownLatch(threadCount);
        int count = 20;
        AtomicInteger got = new AtomicInteger();
        AtomicInteger noGot = new AtomicInteger();
        for (int i = 0; i < threadCount; i++) {
            Thread thread = new Thread(new ConnectionRunner(count, got, noGot), "ConnectionRunnerThread");
            thread.start();
        }
        start.countDown();
        end.await();
        System.out.println("total invoke:" + (threadCount * count));
        System.out.println("got connection:" + got);
        System.out.println("no got connection:" + noGot);
    }

    static class ConnectionRunner implements Runnable {
        int count;
        AtomicInteger got;
        AtomicInteger noGot;

        public ConnectionRunner(int count, AtomicInteger got, AtomicInteger noGot) {
            this.count = count;
            this.got = got;
            this.noGot = noGot;
        }

        @Override
        public void run() {
            try {
                start.await();
            } catch (Exception ex) {
            }
            while (count > 0) {
                try {
                    Connection connection = pool.fetchConnection(1000);
                    if (connection != null) {
                        try {
                            connection.createStatment();
                            connection.commit();
                        } finally {
                            pool.releaseConnection(connection);
                            got.incrementAndGet();
                        }
                    } else {
                        noGot.incrementAndGet();
                    }
                } catch (Exception exception) {
                } finally {
                    count--;
                }
            }
            end.countDown();
        }
    }
}

连接池实现类，初始化没什么好说的。释放链接需要注意需要加锁，该类使用的是synchronized同步锁，释放完成，需要通知所有在等待的线程。获取连接方法使用了超时等待的设计，这一设计可以防止在获取连接上阻塞过长的时间，影响系统的吞吐量。

java 复制代码

package com.practice.thread;

import java.util.LinkedList;

public class ConnectionPool {
    private final 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) {
                pool.addLast(connection);
                pool.notifyAll();
            }
        }
    }

    public Connection fetchConnection(long mills) throws InterruptedException {
        synchronized (pool) {
            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;
            }
        }
    }
}

连接接口和代理类

java 复制代码

package com.practice.thread;

public interface Connection {
    void createStatment();

    void commit();
}

java 复制代码

package com.practice.thread;

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

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(ConnectionPool.class.getClassLoader(), new Class<?>[]{Connection.class},new ConnectionHandler());
    }
}