使用double check 读写锁 读多写少场景 记录下

//来源 jdbc 中的查询连接信息  
//public abstract class ConnectionUrl implements DatabaseUrlContainer 

public static ConnectionUrl getConnectionUrlInstance(String connString, Properties info) {
        if (connString == null) {
            throw ExceptionFactory.createException(WrongArgumentException.class, Messages.getString("ConnectionString.0"));
        }
        String connStringCacheKey = buildConnectionStringCacheKey(connString, info);
        ConnectionUrl connectionUrl;

        rwLock.readLock().lock();
        connectionUrl = connectionUrlCache.get(connStringCacheKey);
        if (connectionUrl == null) {
            rwLock.readLock().unlock();
            rwLock.writeLock().lock();
            try {
                // Check again, in the meantime it could have been cached by another thread.
                connectionUrl = connectionUrlCache.get(connStringCacheKey);
                if (connectionUrl == null) {
                    ConnectionUrlParser connStrParser = ConnectionUrlParser.parseConnectionString(connString);
                    connectionUrl = Type.getConnectionUrlInstance(connStrParser, info);
                    connectionUrlCache.put(connStringCacheKey, connectionUrl);
                }
                rwLock.readLock().lock();
            } finally {
                rwLock.writeLock().unlock();
            }
        }
        rwLock.readLock().unlock();
        return connectionUrl;
    }