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| /**
* @author tang
* @version 1.0
* @date 2019-03-04 11:07
* singleton mode (4 ways to do it)
*/
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| public class Singleton {
/** 懒汉模式 */
LazySingletion lazySingletion = LazySingletion.getInstance();
/** 饿汉模式 */
HungrySingletion hungrySingletion = HungrySingletion.getInstance();
/** 静态内部类 */
StaticInnerSingletion staticInnerSingletion = StaticInnerSingletion.getInstance();
/** 静态内部类 */
EnumSingleton enumSingleton = EnumSingleton.getInstance();
}
/**
* 懒汉模式
*/
class LazySingletion {
private static final LazySingletion instance = new LazySingletion();
static LazySingletion getInstance() {
return instance;
}
}
/**
* 饿汉模式 双重检查锁
*
* 一定要加 volatile 因为有指令重排的坑
*
* 创建对象的过程 1.allocate 分配内存-> 2.初始化对象 -> 3.设置对象指向刚分配的内存 2 3 可能顺序反了 可能会出现 对象初始化2次
*/
class HungrySingletion {
private static volatile HungrySingletion hungrySingletion;
static HungrySingletion getInstance() {
if (hungrySingletion == null) {
synchronized (HungrySingletion.class) {
hungrySingletion = new HungrySingletion();
}
}
return hungrySingletion;
}
}
/**
* 静态内部类 虚拟机会保证一个类的<clinit>()方法在多线程环境中被正确地加锁、同步 同一个加载器下，一个类型只会初始化一次
*/
class StaticInnerSingletion {
public static class InnerSingletion {
static StaticInnerSingletion instance = new StaticInnerSingletion();
}
static StaticInnerSingletion getInstance() {
return InnerSingletion.instance;
}
}
/**
* 枚举单例
*/
class EnumSingleton {
public static EnumSingleton getInstance() {
return Singleton.INSTANCE.getInstance();
}
private static enum Singleton {
INSTANCE;
private EnumSingleton singleton;
//JVM会保证此方法绝对只调用一次
private Singleton() {
singleton = new EnumSingleton();
}
public EnumSingleton getInstance() {
return singleton;
}
}
}
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