单例常用实现
- 懒汉
线程不安全 - 饿汉
基于static线程安全
a. 静态成员变量
b. 静态块儿
c. 静态内部类 - 枚举：
基于static线程安全 - 锁
a. AtomicReferenceCAS乐观锁
优点：不需要使用传统的锁机制来保证线程安全，CAS是一种基于忙等待的算法，依赖底层硬件的实现，相对于锁它没有线程切换和阻塞的额外消耗，可以支持较大的并行度。
缺点：如果忙等待一直执行不成功(一直在死循环中)，会对CPU造成较大的执行开销。
b. synchronized排他锁
- ThreadLocal
线程安全单例有点牵强
破坏单例： 反射、序列化
- 反射
原理：通过反射获取默认构造函数实例化 - 反序列化
原理：根据 Object 构造函数反射生成。
步骤：- readObject() - readObject0(false) - readOrdinaryObject() // 关键方法 obj = desc.isInstantiable() ? desc.newInstance() : null; - newInstance() Object ob = in.readObject(); // 反序列化为 Object Target target = (Target) ob; // 强转为目标对象 Target
阻止破坏单例：
- 针对反射的情况：
构造函数中增加标志位，使构造函数仅能被调用一次 - 针对反序列化的情况：
在单例类中实现readResolve()方法。参考 readOrdinaryObject() 实现。
多种单例模式、破坏单例模式、组织破坏单例模式
public class ModeSingleton {
/**
* 通过反射，破坏单例模式
*/
static void destroySingletonByReflect() throws Exception {
SingletonOfHungary singleton = SingletonOfHungary.getInstance();
SingletonOfHungary newInstance;
Class<SingletonOfHungary> clazz = SingletonOfHungary.class;
// 获取默认构造函数
Constructor<SingletonOfHungary> declaredConstructor = clazz.getDeclaredConstructor();
// 默认构造函数可访问
declaredConstructor.setAccessible(true);
// 创建对象
newInstance = declaredConstructor.newInstance();
System.out.println("singleton.hashCode: " + singleton.hashCode());
System.out.println("newInstance.hashCode: " + newInstance.hashCode());
}
/**
* 通过反序列化，破坏单例模式
*/
static void destroySingletonByDeSerialize() throws Exception {
SingletonOfHungary singleton = SingletonOfHungary.getInstance();
SingletonOfHungary newInstance;
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("tmp"));
out.writeObject(singleton);
File file = new File("tmp");
ObjectInputStream in = new ObjectInputStream(new FileInputStream(file));
//调用readObject()反序列化
newInstance = (SingletonOfHungary) in.readObject();
System.out.println("singleton.hashCode: " + singleton.hashCode());
System.out.println("newInstance.hashCode: " + newInstance.hashCode());
}
/**
* 阻止 反射破坏单例。加构造函数标志位
*/
static void stopReflectDestroy() throws Exception {
SingletonOfInnerStatic singleton = SingletonOfInnerStatic.getInstance();
SingletonOfInnerStatic newInstance;
Class<SingletonOfInnerStatic> clazz = SingletonOfInnerStatic.class;
// 获取默认构造函数
Constructor<SingletonOfInnerStatic> declaredConstructor = clazz.getDeclaredConstructor();
// 默认构造函数可访问
declaredConstructor.setAccessible(true);
// 创建对象
newInstance = declaredConstructor.newInstance();
System.out.println("singleton.hashCode: " + singleton.hashCode());
System.out.println("newInstance.hashCode: " + newInstance.hashCode());
}
/**
* 阻止 反序列化破坏单例。实现 readResolve()
*/
static void stopDeSerializeDestroy() throws Exception{
SingletonOfInnerStatic singleton = SingletonOfInnerStatic.getInstance();
SingletonOfInnerStatic newInstance;
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("tmp"));
out.writeObject(singleton);
File file = new File("tmp");
ObjectInputStream in = new ObjectInputStream(new FileInputStream(file));
//调用readObject()反序列化
newInstance = (SingletonOfInnerStatic) in.readObject();
System.out.println("singleton.hashCode: " + singleton.hashCode());
System.out.println("newInstance.hashCode: " + newInstance.hashCode());
}
public static void main(String[] args) throws Exception {
System.out.println("反射，破坏单例模式：");
destroySingletonByReflect();
System.out.println("反序列化，破坏单例模式：");
destroySingletonByDeSerialize();
System.out.println("阻止 反射破坏单例：");
stopReflectDestroy();
System.out.println("阻止 反序列化破坏单例");
stopDeSerializeDestroy();
}
}
/**
* 懒汉 延迟初始化 - 不安全
*/
class SingletonOfLazy {
private static SingletonOfLazy instance;
private SingletonOfLazy() {
}
public static SingletonOfLazy getInstance() {
if (instance == null) {
instance = new SingletonOfLazy();
}
return instance;
}
}
/**
* 饿汉.静态成员变量 类加载时初始化 - 线程安全
*/
class SingletonOfHungary implements Serializable {
private static SingletonOfHungary instance = new SingletonOfHungary();
private SingletonOfHungary() {
}
public static SingletonOfHungary getInstance() {
return instance;
}
}
/**
* 饿汉.静态块初始化静态成员变量 类加载时初始化 - 线程安全
*/
class SingletonOfInnerStatic implements Serializable{
private static SingletonOfInnerStatic instance;
//申明一个标志位，用于标志构造函数是否被调用过
private static boolean alreadyExist;
static {
instance = new SingletonOfInnerStatic();
alreadyExist = true;
}
private SingletonOfInnerStatic() {
if (alreadyExist) {
System.out.println("单例模式不能破坏，已存在此单例！");
}
}
public static SingletonOfInnerStatic getInstance() {
return instance;
}
public Object readResolve() {
return instance;
}
}
/**
* 饿汉.静态内部类 类加载时初始化 - 线程安全
*/
class SingletonOfInnerClass {
private SingletonOfInnerClass() {
}
private static class SingletonHolder {
private static final SingletonOfInnerClass INSTANCE = new SingletonOfInnerClass();
}
public static SingletonOfInnerClass getInstance() {
return SingletonHolder.INSTANCE;
}
}
/**
* 枚举.枚举类的成员变量均为static 在静态代码块中实例化 - 线程安全
*/
enum SingletonOfEnum {
INSTANCE;
}
/**
* 乐观锁 - 线程安全
*/
class SingletonOfCAS {
public static final AtomicReference<SingletonOfCAS> INSTANCE = new AtomicReference<>();
private SingletonOfCAS() {
}
public static SingletonOfCAS getInstance() {
for (; ; ) {
SingletonOfCAS singleton = INSTANCE.get();
if (null != singleton) {
return singleton;
}
singleton = new SingletonOfCAS();
if (INSTANCE.compareAndSet(null, singleton)) {
return singleton;
}
}
}
}
/**
* ThreadLocal - 线程安全
* ThreadLocal是线程隔离的，会为每一个线程提供一个独立的变量副本，从而隔离了多个线程对数据的访问冲突。
* 对于多线程资源共享的问题，同步机制(synchronized)采用了“以时间换空间”的方式，而ThreadLocal采用了“以空间换时间”的方式。
*/
class SingletonOfThreadLocal {
private SingletonOfThreadLocal() {
}
private static final ThreadLocal<SingletonOfThreadLocal> INSTANCE = ThreadLocal.withInitial(SingletonOfThreadLocal::new);
public static SingletonOfThreadLocal getInstance() {
return INSTANCE.get();
}
}
测试结果:
反射，破坏单例模式：
singleton.hashCode: 1163157884
newInstance.hashCode: 1956725890
反序列化，破坏单例模式：
singleton.hashCode: 1163157884
newInstance.hashCode: 325040804
阻止 反射破坏单例：
单例模式不能破坏，已存在此单例！
singleton.hashCode: 1173230247
newInstance.hashCode: 856419764
阻止 反序列化破坏单例
singleton.hashCode: 1173230247
newInstance.hashCode: 1173230247
推荐 设计模式——单例模式的破坏 很详细，参考了[抱拳.jpg]
