DCL双检锁实现线程池单例模式

@Slf4j  
public class ThreadPool {  
    private static ThreadPoolExecutor instance;  
  
    private ThreadPool() {  
    }  
    public static ThreadPoolExecutor getInstance(){  
        if (instance != null) return instance;  
        synchronized(ThreadPool.class){  
            if (instance != null) return instance;  
            instance = new ThreadPoolExecutor(
			            coresize, 
			            maxPoolSize,
			            keepAlive,
			            timeUnit,
			            workQueue,
			            handler  
                    );  
        }        
        return instance;  
    }  
}