某软件公司开发人员要创建一个数据库连接池,将指定个数的(如两个或3个)数据库连接对象存储在连接池中,客户端代码可以从池中随机取一个连接对象来连接数据库。试通过对单例类进行改造,设计一个能够自行提供指定个数实例对象的数据库连接类并用Java代码编程模拟。

实现方式:
1. 单例模式中增加一个池大小参数，用来控制池中连接对象的数量
2. 创建连接池的时候，在单例的构造函数中初始化连接对象列表，并按照池大小参数创建连接对象
3. 客户端请求连接对象的时候，从连接对象列表中随机获取一个连接对象
4. 连接对象使用完毕后，归还给连接对象列表，即可重复利用

下面是Java代码实现:

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class ConnectionPool {
    private static ConnectionPool instance;
    private List<Connection> connectionList;
    private int poolSize;

    private ConnectionPool(int poolSize) {
        this.connectionList = new ArrayList<>();
        this.poolSize = poolSize;
        for (int i = 0; i < poolSize; i++) {
            Connection connection = new Connection();
            connectionList.add(connection);
        }
    }

    public static ConnectionPool getInstance(int poolSize) {
        if (instance == null) {
            instance = new ConnectionPool(poolSize);
        }
        return instance;
    }

    public Connection getConnection() {
        Random random = new Random();
        int index = random.nextInt(poolSize);
        return connectionList.get(index);
    }

    public void releaseConnection(Connection connection) {
        //重置连接对象状态，即可重复利用
        connection.reset();
    }
}

class Connection {
    private boolean isUsed = false;

    public void connect() {
        //连接到数据库
    }

    public void reset() {
        isUsed = false;
        //重新连接到数据库
    }

    public boolean isUsed() {
        return isUsed;
    }

    public void setUsed(boolean used) {
        isUsed = used;
    }
}

class Main {
    public static void main(String[] args) {
        ConnectionPool connectionPool = ConnectionPool.getInstance(2);
        Connection connection1 = connectionPool.getConnection();
        Connection connection2 = connectionPool.getConnection();
        Connection connection3 = connectionPool.getConnection();
        //连接1和连接2已经被占用，连接3获取连接时会从连接1和连接2中随机选择一个
        System.out.println("连接1被占用: " + connection1.isUsed());
        System.out.println("连接2被占用: " + connection2.isUsed());
        System.out.println("连接3被占用: " + connection3.isUsed());

        connectionPool.releaseConnection(connection1);
        connectionPool.releaseConnection(connection2);
        connectionPool.releaseConnection(connection3);
        //连接1、2、3已经被归还，均为未被占用状态
        System.out.println("连接1被占用: " + connection1.isUsed());
        System.out.println("连接2被占用: " + connection2.isUsed());
        System.out.println("连接3被占用: " + connection3.isUsed());
    }
}