java单例模式的两种实现方式

1.饿汉式

public class ThreadPoolManager {

    private ThreadPoolManager() {

    }

    private static ThreadPoolManager threadPoolManager = new ThreadPoolManager();

    public static ThreadPoolManager getInstance() {
        return threadPoolManager;
    }
}

2.懒汉式

public class ThreadPoolManager {

    private ThreadPoolManager() {

    }

    private static volatile ThreadPoolManager threadPoolManager;

    public static ThreadPoolManager getInstance() {
        if (threadPoolManager == null) {
            synchronized (ThreadPoolManager.class) {
                if (threadPoolManager == null) {
                    threadPoolManager = new ThreadPoolManager();
                }

            }
        }
        return threadPoolManager;
    }
}

3.两者相比较而言，讲究性能和节省资源选择懒汉式，如果是平时开发而且这种单例类较少，我建议用饿汉式，因为饿汉式较为简单而且线程安全，一个类的实例也不会占用多少资源。