public class Singleton {
private volatile static Singleton uniqueInstance;
private Singleton(){};
public static Singleton getInstance(){
if(uniqueInstance == null){
synchronized(Singleton.class){
if(uniqueInstance == null){
uniqueInstance = new Singleton();
}
}
}
return uniqueInstance;
}
}
uniqueInstance = new Singleton() 这行代码虽然会发生指令重排序， 但 synchronized 代码块已经加锁了，每次只有一个线程进入代码块，为啥还要加 volatile ？
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