关于设计模式:单例设计模式

1. 本人写的单例模式

// 双重测验锁 双重测验锁模式（double checked locking pattern），// 是一种应用同步块加锁的办法。程序员称其为双重查看锁，// 因为会有两次查看 instance == null，一次是在同步块外，一次是在同步块内。// 为什么在同步块内还要再测验一次？因为可能会有多个线程一起进入同步块外的 if，如果在同步块内不进行二次测验的话就会生成多个实例了。public class SingletonDemo04 {
    private SingletonDemo04 singletonDemo04;
    private static boolean flag = false;

    private SingletonDemo04() {if (flag == false) {flag = !flag;} else {throw new RuntimeException("单例模式被进犯！");
        }
    }

    public SingletonDemo04 getInstance() {if (singletonDemo04 == null) {synchronized (this) {if (singletonDemo04 == null) {singletonDemo04 = new SingletonDemo04();
                }
            }
        }
        return singletonDemo04;
    }

    public static void main(String[] args) {System.out.println("main");
        SingletonDemo04 s1 = new SingletonDemo04().getInstance();
        SingletonDemo04 s2 = new SingletonDemo04().getInstance();
        System.out.println(s1 == s2);

    }

}