代码如下：
public class Singleton{
private Singleton(){}; //私有化构造方法
private static volatile Singleton = null;
public static SingletongetInstance(){
//第一次校验
if(Singleton==null){
synchronized(Singleton.class){
//第二次校验
if(Singleton==null){
Singleton=new Singleton();
}
}
}
return Singleton;
}
在Spring中有用到这个单例模式，Spring依赖注入Bean实例默认是单例的。在AbstractBeanFactory的getBean里。getBean的doGetBean方法调用getSingleton进行bean的创建。
/**
* Return the (raw) singleton object registered under the given name.
* <p>Checks already instantiated singletons and also allows for an early
* reference to a currently created singleton (resolving a circular reference).
* @param beanName the name of the bean to look for
* @param allowEarlyReference whether early references should be created or not
* @return the registered singleton object, or {@code null} if none found
*/
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
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