单例模式：
1.饥饿无锁式线程安全型
2.懒惰双重检测带锁线程安全型
1.饥饿无锁式线程安全型
// ∵依赖于Java对静态成员的初始化，基于 classloader 机制避免了多线程的同步问题
public class OnlyOne {
private static OnlyOne instance = new OnlyOne();
private OnlyOne() {};
public static OnlyOne getInstance() {
return instance;
}
}
2.懒惰双重检测带锁线程安全型
public class OnlyOne {
private static OnlyOne instance;
private OnlyOne() {};
public static OnlyOne getInstance() {
if(instance == null) {
synchronized (OnlyOne.class) {
if(instance == null)
instance = new OnlyOne();
}
}
return instance;
}
}
