单例

/**
 * 懒汉式
 * 1.用时创建对象
 */
public class Single {
    private Single(){}
    public static Single instance = null;
    public static Single getInstance() {
        if (null == instance) {
            //多个线程判断instance都为null时，
                        / /在执行new操作时多线程会出现重复情况
            instance = new Single();
        }
        return instance;
    }
}

/**
 * 双重检测,懒汉式模式添加同步，效率低
 * 1.用时创建对象
 */
public class Single {
    private Single(){}
    public static Single instance = null;
    public static Single getInstance() {
        if (null == instance) {
            synchronized (Single.class) {
                if (null == instance) {
                    instance = new Single();
                }
            }
        }
        return instance;
    }
}