多线程实例

class Instance {

    
}
/*
 *     instance=new Singleton()
    memory = allocate();　　// 1：分配对象的内存空间
    ctorInstance(memory);　 // 2：初始化对象
    instance = memory;　　 // 3：设置instance指向刚分配的内存地址
 */
public class SafeDoubleCheckedLocking {
    private volatile static Instance instance;//2,3之间不会重排序
    public static Instance getInstance() {
    if (instance == null) {
        synchronized (SafeDoubleCheckedLocking.class) {
        if (instance == null)
            instance = new Instance(); // instance为volatile，现在没问题了
        }
    }
    return instance;
    }
}

class Instance{
    
}
public class InstanceFactory {
    private static class InstanceHolder {
        public static Instance instance = new Instance();
    }
    public static Instance getInstance() {
        return InstanceHolder.instance;　　// 这里将导致InstanceHolder类被初始化
    }
}