双重检查锁定
Published:
“聪明”的单例模式
public class Singleton {
private static Singleton instance;
private Singleton() {}
public static Singleton getInstance() {
if(instance == null) {
synchronized(Singleton.class) {
if(instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}
问题根源
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
代码2和代码3可能发生重排序，导致访问到一个还没初始化的对象
基于volatile的解决方案(JDK5或更高版本，volatile语义被加强)
public class Singleton {
private volatile static Singleton instance;
private Singleton() {}
public static Singleton getInstance() {
if(instance == null) {
synchronized(Singleton.class) {
if(instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}
## 基于类初始化的解决方案(类初始化锁)
public class Singleton {
private Singleton(){}
private static class InstanceHolder {
public static Singleton instance = new Singleton();
}
public static Singleton getInstance() {
return InstanceHolder.instance;
}
}