享元模式

public interface Pool {
    void connect();
}

public class PoolObj implements Pool{

    private String db;

    public PoolObj(String db) {
        this.db = db;
    }

    @Override
    public void connect() {
        System.out.println("连接 "+db);
    }
}

public class PoolFactory {
    private HashMap<String,Pool> pools=new HashMap<>();

    public Pool getPool(String db){
        if (pools.get(db)!=null){
            return pools.get(db);
        }else{
            Pool pool=new PoolObj(db);
            pools.put(db,pool);
            return pool;
        }
    }

    public void printMapCount(){
        System.out.println(pools.size());
    }
}

@Test
public void testFlyWeight(){
    PoolFactory poolFactory=new PoolFactory();
    Pool pool1=poolFactory.getPool("mysql");
    Pool pool2=poolFactory.getPool("oracle");
    pool1.connect();
    pool2.connect();
    Pool pool3=poolFactory.getPool("mysql");
    Pool pool4=poolFactory.getPool("oracle");
    pool3.connect();
    pool4.connect();

    poolFactory.printMapCount();
}