Spring IoC 中的（Singleton）单例对象创建过程探索

前言

　　之前将spring framework 源码导入了idea，后来折腾调试了一下，于是研究了一下最简单的singleton对象在spring中是如何创建的。这里所谓的简单，就是指无属性注入，无复杂构造函数的对象。

测试代码

　　spring配置：

　　<bean id="userService" class="UserService" scope="singleton"></bean>
　 测试类：

public class UserService {

   private final Log logger = LogFactory.getLog(getClass());

    public UserService(){
        logger.info("UserService created");
        id = UUID.randomUUID().toString();
    }

    private String id;
    public String getId(){
        return id;
    }
    public String getUserName(){
        return "xiaopanzi";
    }
}

　　

 public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
        context.start();
        testNormalSingleton(context);
    }

    private static void testCircleSingleton(ClassPathXmlApplicationContext context){
        SingletonAService aService = context.getBean("aService",SingletonAService.class);
        SingletonBService bService = context.getBean("bService",SingletonBService.class);

        aService.getbService().print();

        bService.getaService().print();
    }

调试详情

　　首先在ApplicationContext 初始化过程，在 refresh 方法中会调用  finishBeanFactoryInitialization 方法，注释上也写的很明白：Instantiate all remaining （not-lazy-init） signletons。（初始化剩余的非懒加载的单例对象）。那么这里就是入口点。

　　然后在调用 beanFactory.preInstantiateSingletons().

　　后续调用链如下：

　　DefaultListBeanFactory.getBean(beanName)

　　AbstractBeanFactory.doGetBean(name,requiredType,args,typeCheckOnly)

　　AbstractAutowireCapableBeanFactory.createBean(String beanName,RootBeanDefinition mbd,Object[] args)

       AbstractAutowireCapableBeanFactory.doCreateBean(String beanName,RootBeanDefinition mbd,Object[] args)

       AbstractAutowireCapableBeanFactory.instantiateBean(final String beanName, final RootBeanDefinition mbd)

       getInstantiationStrategy().instantiate(mbd, beanName, parent); (this.instantiationStrategy=CglibSubclassingInstantiationStrategy)

　   SimpleInstantiationStrategy.instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) 

　　BeanUtils.instantiateClass(Constructor<T> ctor, Object... args)

　　通过上述代码调用链我们可以看出，最终的示例创建是由 BeanUtils.instantiateClass 方法完成的，也就是这个方法：

　  

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
		Assert.notNull(ctor, "Constructor must not be null");
		try {
			ReflectionUtils.makeAccessible(ctor);
			return ctor.newInstance(args);
		}
		catch (InstantiationException ex) {
			throw new BeanInstantiationException(ctor, "Is it an abstract class?", ex);
		}
		catch (IllegalAccessException ex) {
			throw new BeanInstantiationException(ctor, "Is the constructor accessible?", ex);
		}
		catch (IllegalArgumentException ex) {
			throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", ex);
		}
		catch (InvocationTargetException ex) {
			throw new BeanInstantiationException(ctor, "Constructor threw exception", ex.getTargetException());
		}
	}

　　到此为止单例就创建完毕了。但是创建完成之后，还有后续的处理。

　　DefaultSingletonBeanRegistry.addSingletonFactory(String beanName,ObjectFactory<?> singletonFactory);

　　也就是将该示例放入到  singletonObjects 中，作为缓存方便后续取值。

　　this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));

　　当我们在次调用getBean的时候，那么在 DefaultSingletonBeanRegistry.getSingleton 方法中直接从 singletonObjects 中获取即可。

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);
	}

　　

总结

　　上述内容记录的很少，基本就是一个轮廓的记录，要真正理解详情内容，还得自己去慢慢调试啊！！！