Spring获取单例getSingleton（1）

标签： spring源码学习

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spring获取单例首先是从缓存去找，如果找不到就要从头开始bean的加载过程，spring中主要通过getSingleton的重载方法实现单例bean的加载过程

要创建一个bean的单例，首先了解下单例是什么样子的，

    public static synchronized Singleton getInstance() {  
             if (single == null) {    
                 single = new Singleton();  
             }    
            return single;  
    }  

spring创建单例bean主要查看 org.springframework.beans.factory.support.DefaultSingletonBeanRegistry类的getSingleton(String beanName, ObjectFactory

public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
        Assert.notNull(beanName, "'beanName' must not be null");
        //同步 开始创建单例
        synchronized (this.singletonObjects) {
            Object singletonObject = this.singletonObjects.get(beanName);
            //判断单例是否被创建 如果已经创建则不在重复创建
            if (singletonObject == null) {
                if (this.singletonsCurrentlyInDestruction) {
                    throw new BeanCreationNotAllowedException(beanName,
                            "Singleton bean creation not allowed while the singletons of this factory are in destruction " +
                            "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
                }
                //把当前正在创建的bean记录在缓存中，对循环依赖进行检测
                beforeSingletonCreation(beanName);
                boolean newSingleton = false;
                boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = new LinkedHashSet();
                }
                try {
                //使用回调方法 创建单例bean
                    singletonObject = singletonFactory.getObject();
                    newSingleton = true;
                }
                catch (IllegalStateException ex) {
                    singletonObject = this.singletonObjects.get(beanName);
                    if (singletonObject == null) {
                        throw ex;
                    }
                }
                catch (BeanCreationException ex) {
                    if (recordSuppressedExceptions) {
                        for (Exception suppressedException : this.suppressedExceptions) {
                            ex.addRelatedCause(suppressedException);
                        }
                    }
                    throw ex;
                }
                finally {
                    if (recordSuppressedExceptions) {
                        this.suppressedExceptions = null;
                    }
                    //移除缓存中对该bean正在加载的状态
                    afterSingletonCreation(beanName);
                }
                if (newSingleton) {
                    //将新创建的bean加入缓存，并且删除加载bean过程中所记录的各种辅助状态
                    //这些辅助状态主要是在回调方法创建bean时候引入的
                    addSingleton(beanName, singletonObject);
                }
            }
            return (singletonObject != NULL_OBJECT ? singletonObject : null);
        }
    }

现在继续去找创建bean的回调方法

sharedInstance = getSingleton(beanName, new ObjectFactory