单例java

public class Singleton {
/**
* 1.私有构造函数,只能在该类中new
* 2.获取单例要加锁，保证只有一个实例存在
* 3.加锁之前要判断是否为空，是为了减少频繁加锁，提高性能
*/
private Singleton() {};
private static Singleton singleton = null;

public static Singleton getSingleton() {
    if (singleton == null)
    {
        synchronized (Singleton.class) {
            if (singleton == null) {
                return new Singleton();
            }
        }
    }
    return singleton;
}

}