A Simple Guide to Connection Pooling in Java
一篇关于Java连接池指导，个人比较关注是实现一个简单连接池内容。

文章提供一段连接池的代码

public class BasicConnectionPool 
  implements ConnectionPool {
 
    private String url;
    private String user;
    private String password;
    private List<Connection> connectionPool;
    private List<Connection> usedConnections = new ArrayList<>();
    private static int INITIAL_POOL_SIZE = 10;
     
    public static BasicConnectionPool create(
      String url, String user, 
      String password) throws SQLException {
  
        List<Connection> pool = new ArrayList<>(INITIAL_POOL_SIZE);
        for (int i = 0; i < INITIAL_POOL_SIZE; i++) {
            pool.add(createConnection(url, user, password));
        }
        return new BasicConnectionPool(url, user, password, pool);
    }
     
    // standard constructors
     
    @Override
    public Connection getConnection() {
        Connection connection = connectionPool
          .remove(connectionPool.size() - 1);
        usedConnections.add(connection);
        return connection;
    }
     
    @Override
    public boolean releaseConnection(Connection connection) {
        connectionPool.add(connection);
        return usedConnections.remove(connection);
    }
     
    private static Connection createConnection(
      String url, String user, String password) 
      throws SQLException {
        return DriverManager.getConnection(url, user, password);
    }
     
    public int getSize() {
        return connectionPool.size() + usedConnections.size();
    }
 
    // standard getters
}

然后按照最大线程池数量进行优化

@Override
public Connection getConnection() throws SQLException {
    if (connectionPool.isEmpty()) {
        if (usedConnections.size() < MAX_POOL_SIZE) {
            connectionPool.add(createConnection(url, user, password));
        } else {
            throw new RuntimeException(
              "Maximum pool size reached, no available connections!");
        }
    }
 
    Connection connection = connectionPool
      .remove(connectionPool.size() - 1);
    usedConnections.add(connection);
    return connection;
}

新增一个正常关闭连接池的方法

public void shutdown() throws SQLException {
    usedConnections.forEach(this::releaseConnection);
    for (Connection c : connectionPool) {
        c.close();
    }
    connectionPool.clear();
}

这个只是一个简单连接池，并存在以下两个问题
1、并发获取连接时，getConnection(）会存在问题。尤其ArrayList还是线程非安全集合。
2、getConnection(）发现连接池的连接到了最大数量，也直接报错。

然后参考一下这篇分析druid源码逻辑进一步了解连接逻辑
Druid-connection pool source code analysis
完整架构图，重点分析DruidDataSrouce ，其中分析获取和归还连接的代码

image.png