单件模式-Singleton

package singleton;
import java.util.concurrent.locks.Lock;
public class Singleton {
    private Singleton() {
    }
    static volatile Singleton singleton;
    /**
     * 创建单实例
     * @return
     */
    static Singleton getInstance() {
        if (singleton == null) {
            Lock lock;
            if (singleton == null) {
                singleton = new Singleton();
            }
        }
        return singleton;
    }
}