How would you test a Connection Pool

Question

I have implemented a very simple ConnectionPool in Java. It has no fancy features, just get/release connection methods.

How can I test it is working?

I know there are plenty of connection Pools ready to use out there, which are much more reliable than what I'll do, but I am just trying to practice to understand how connections Pool work.

Thank you!

Here is the code in case it helps:

public class ConnectionPoolImpl implements ConnectionPool {
    private Vector<PooledConnection> connections; // The connections container
    String url;
    String username; 
    String password;

    /**
     * Instanciates a new MySQLConnectionPool
     * @param nbConnectionsMax
     */
    public ConnectionPoolImpl(String DBUrl, String username, String password){
        this.connections = new Vector<PooledConnection>();
        this.url = DBUrl;
        this.username = username;
        this.password = password;
    }

    /**
     * Returns a connection from the pool, if some are available, or create a new one.
     * 
     * @return the connection.
     */
    public Connection getConnection() throws SQLException {
        synchronized(this.connections){
            // Checking if there is an available connection to return
            for(PooledConnection c : this.connections){
                if(!c.isUsed()){
                    c.setUsed();
                    return c.getConnection();
                }
            }

            // If there are none, open a new one and return it
            Connection conn = DriverManager.getConnection(url, username, password);
        PooledConnection pConn = new PooledConnection(conn);
        pConn.setUsed();
        connections.add(pConn);
        return pConn.getConnection();
        }
    }

    /**
     * Releases a connection to the pool.
     * 
     * @param con the connection to release.
     */
    public void releaseConnection(Connection con) throws SQLException {
        synchronized(this.connections){
            for(PooledConnection c : this.connections){
                if(c.getConnection().equals(con)){
                    c.setFree();
                    return;
                }
            }
        }
      }
}

And my PooledConnection.java:

public class PooledConnection {
    private Connection conn;
    private boolean used;

    public PooledConnection(Connection conn){
        this.conn = conn;
        this.used = false;
    }

    public void setUsed(){
        this.used = true;
    }

    public void setFree(){
        this.used = false;
    }

    public boolean isUsed(){
        return this.used;
    }

    public Connection getConnection(){
        return this.conn;
    }
}

Answer 1

You could test that

• getting a connection when the pool is empty gives you a connection
• getting a connection when a connection has already been got and not released gives you another, different connection
• releasing a connection doesn't throw any exception
• getting a connection after one has been released gives you the same connection

Note that such a unit test would need a real database, with a real user and password to test. You could make your connection pool depend on a DataSource, and build your ConnectionPool using a mock DataSource returning mock Connections, in order to be able to test the class without depending on a real database.