Spring IoC 中的（Singleton）單例物件建立過程探索

前言

　　之前將spring framework 原始碼匯入了idea，後來折騰除錯了一下，於是研究了一下最簡單的singleton物件在spring中是如何建立的。這裡所謂的簡單，就是指無屬性注入，無複雜建構函式的物件。

測試程式碼

　　spring配置：

　　<bean id="userService" class="UserService" scope="singleton"></bean>
　 測試類：

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();
    }

除錯詳情

　　首先在ApplicationContext 初始化過程，在 refresh 方法中會呼叫  finishBeanFactoryInitialization 方法，註釋上也寫的很明白：Instantiate all remaining （not-lazy-init） signletons。（初始化剩餘的非懶載入的單例物件）。那麼這裡就是入口點。

　　然後在呼叫 beanFactory.preInstantiateSingletons().

　　後續呼叫鏈如下：

　　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)

 

　　通過上述程式碼呼叫鏈我們可以看出，最終的示例建立是由 BeanUtils.instantiateClass 方法完成的，也就是這個方法：

　  

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());
		}
	}

　　到此為止單例就建立完畢了。但是建立完成之後，還有後續的處理。

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

　　也就是將該示例放入到  singletonObjects 中，作為快取方便後續取值。

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

　　當我們在次呼叫getBean的時候，那麼在 DefaultSingletonBeanRegistry.getSingleton 方法中直接從 singletonObjects 中獲取即可。

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);
	}

　　

總結

　　上述內容記錄的很少，基本就是一個輪廓的記錄，要真正理解詳情內容，還得自己去慢慢除錯啊！！！