ReentrantLock实现PV操作-模拟多线程竞争数据库连接池资源场景

ConnectionPool.class（连接池）

package demo.lock.db;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.stream.Collectors;

import org.apache.commons.collections4.CollectionUtils;

/**
 * 模拟数据库连接池
 */
public class ConnectionPool {
    /** 连接池最大连接数量 */
    private static final int POOL_SIZE = 5;
    public static List<Connection> connections;
    /** 信号量 */
    public static volatile int signal = POOL_SIZE;
    /** 锁 */
    public static ReentrantLock lock = new ReentrantLock();
    /** 等待条件 */
    public static Condition condition = lock.newCondition();

    /**
     * 

     * 初始化数据库连接池
     * 
     */
    public static void init() {
        connections = new ArrayList<>(POOL_SIZE);
        for (int i = 0; i < POOL_SIZE; i++) {
            connections.add(new Connection("connection-" + i));
        }
    }

    /**
     * 

     * 使用ReentrantLock将获取数据连接逻辑包装为一个P操作原语
     * 

     * 抢占到锁后，如果检测到信号量不大于0，将自身加入Condition对象的阻塞队列中，并释放锁
     * 
     * 

     * 继续持有锁或被唤醒后重新持有锁时，尝试自旋来获取数据库连接
     * 
     * 
     * @return 获取到的数据库连接对象
     * @throws Exception
     */
    public static Connection getConnection() throws Exception {
        // 进行P操作，首先获取锁对象
        lock.lock();
        try {
            // 此时抢占到锁对象，但由于信号量小于等于0，也就是没有资源可用，所以阻塞自身，同时释放锁
            if (signal <= 0) {
                condition.await();
            }
            // 持有或重新持有锁对象时，进行自旋操作尝试获取数据库连接
            while (true) {
                if (signal > 0) {
                    // 从连接池中获取一个空闲连接
                    List<Connection> freeConnections = connections.stream()
                        .filter(x -> x.state.equals(ConnectionState.FREE)).collect(Collectors.toList());
                    Connection currConnection =
                        CollectionUtils.isEmpty(freeConnections) ? null : freeConnections.get(0);
                    if (null == currConnection) {
                        return null;
                    }
                    currConnection.state = ConnectionState.BUSY;
                    // 获取连接成功，信号量自减1
                    signal--;
                    System.out.println("当前线程：" + Thread.currentThread().getName() + " 抢到数据库连接："
                        + currConnection.getName() + " 当前连接池中空闲连接数：" + signal);
                    return currConnection;
                }
            }
        } finally {
            // 释放锁对象
            lock.unlock();
        }

    }

    /**
     * 

     * 使用ReentrantLock将归还数据连接逻辑包装为一个V原语
     * 
     * 

     * 在归还连接成功，并将信号量自增之后，唤醒Condition的等待队列中的一个线程
     * 
     * 
     * @param connection 数据库连接对象
     */
    public static void repayConnection(Connection connection) {
        if (null == connection) {
            return;
        }
        // 进行V操作,首先获取锁对象
        lock.lock();
        try {
            connections.forEach(x -> x.state = connection.equals(x) ? ConnectionState.FREE : x.state);
            // 信号量自增1，表示归还1个资源
            signal++;
            System.out.println("当前线程：" + Thread.currentThread().getName() + " 归还数据库连接：" + connection.getName()
                + " 当前连接池中空闲连接数：" + signal);
            // 唤醒该condition的等待队列中的一个线程
            condition.signal();
        } finally {
            // 释放锁资源
            lock.unlock();
        }
    }
}

Connection.class（连接对象）

package demo.lock.db;

/**
 * 数据库连接对象
 */
public class Connection {
    /**
     * 连接名
     */
    String name;
    /**
     * 当前状态
     */
    ConnectionState state = ConnectionState.FREE;

    public Connection(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "当前连接：" + ",状态：" + state.state;
    }
}

ConnectionState.class（连接状态枚举）

package demo.lock.db;

/**
 * 数据库连接当前状态枚举
 */
public enum ConnectionState {

    FREE(0, "空闲"), BUSY(1, "忙碌");

    /**
     * 状态码 0-空闲 1-忙碌
     */
    int code;
    /**
     * 状态名
     */
    String state;

    ConnectionState(int code, String state) {
        this.code = code;
        this.state = state;
    }
}

Application.class（程序入口）

package demo.lock;

import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

import demo.lock.db.Connection;
import demo.lock.db.ConnectionPool;

public class Application {

    ThreadPoolExecutor executor =
        new ThreadPoolExecutor(10, 10, 30, TimeUnit.MILLISECONDS, new LinkedBlockingDeque<>());

    public void execute() {
        for (int i = 0; i < 10; i++) {
            executor.submit(new Runnable() {
                @Override
                public void run() {
                    try {
                        Connection connection = ConnectionPool.getConnection();
                        // 随机占用0-1秒时间后归还连接
                        Thread.sleep((long)(Math.random() * 1000));
                        ConnectionPool.repayConnection(connection);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            });
        }

    }

    public void shutdown() {
        if (null == executor) {
            return;
        }
        executor.shutdown();
    }

    public static void main(String[] args) throws InterruptedException {
        ConnectionPool.init();
        Application application = new Application();
        application.execute();
        Thread.sleep(10000L);
        application.shutdown();
    }
}

运行结果