Spring BeanFactory getBean 源码剖析

首先看一张时序图



最开始,一切都来自这里:

public static void main(String[] args) {
		ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext3.xml");
		context.getBean("newsBean"); 

	}

如果我们是通过BeanFactory来构造IoC容器的话,那就是直接从上面的时序图的第五步开始

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public static void main(String[] args){



Resource resource=new ClassPathResource("applicationContext2.xml");

BeanFactory factory=new DefaultListableBeanFactory();

BeanDefinitionReader bdr=new XmlBeanDefinitionReader((BeanDefinitionRegistry) factory);

bdr.loadBeanDefinitions(resource);



String url=((FXNewsBean) factory.getBean("newsBean2")).getUrl();

System.out.println(url);

}

当然,ApplicatonContext有缓存机制,在容器启动的时候,就加载了所有的bean,并缓存之,如果有了显式或隐式的调用,直接从缓存里拿就是了

********************
我们从上面的代码很容易追踪到AbstractBeanFactory的doGetBean方法

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@SuppressWarnings("unchecked")

protected <T> T doGetBean(

final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)

throws BeansException {

//*************************1

final String beanName = transformedBeanName(name);

Object bean;





// Eagerly check singleton cache for manually registered singletons.

//*************************2

Object sharedInstance = getSingleton(beanName);

if (sharedInstance != null && args == null) {

if (logger.isDebugEnabled()) {

if (isSingletonCurrentlyInCreation(beanName)) {

logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +

"' that is not fully initialized yet - a consequence of a circular reference");

}

else {

logger.debug("Returning cached instance of singleton bean '" + beanName + "'");

}

}

//判断要获得的bean是否实现了FactoryBean,是获得工厂本身还是获得工厂生产的产品

//如果要获得工厂本身 beanName就要以&开头

bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);

}





else {

// Fail if we're already creating this bean instance:

// We're assumably within a circular reference.

//下面这个是关于循环依赖的处理

if (isPrototypeCurrentlyInCreation(beanName)) {

throw new BeanCurrentlyInCreationException(beanName);

}





// Check if bean definition exists in this factory.

//如果存在父容器,且当前容器中不存在要获取的对象 那就去父容器去找

BeanFactory parentBeanFactory = getParentBeanFactory();

if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {

// Not found -> check parent.

String nameToLookup = originalBeanName(name);

if (args != null) {

// Delegation to parent with explicit args.

return (T) parentBeanFactory.getBean(nameToLookup, args);

}

else {

// No args -> delegate to standard getBean method.

return parentBeanFactory.getBean(nameToLookup, requiredType);

}

}





//typeCheckOnly参数 调用是就是false

//标明这个beanname已经被创建了

//后面会用到

if (!typeCheckOnly) {

markBeanAsCreated(beanName);

}





//*************************3

final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);

checkMergedBeanDefinition(mbd, beanName, args);





// Guarantee initialization of beans that the current bean depends on.

//*************************4

String[] dependsOn = mbd.getDependsOn();

if (dependsOn != null) {

for (String dependsOnBean : dependsOn) {

getBean(dependsOnBean);

registerDependentBean(dependsOnBean, beanName);

}

}





// Create bean instance.

//*************************5

if (mbd.isSingleton()) {

sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {

public Object getObject() throws BeansException {

try {

return createBean(beanName, mbd, args);

}

catch (BeansException ex) {

// Explicitly remove instance from singleton cache: It might have been put there

// eagerly by the creation process, to allow for circular reference resolution.

// Also remove any beans that received a temporary reference to the bean.

destroySingleton(beanName);

throw ex;

}

}

});

bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);

}





else if (mbd.isPrototype()) {

// It's a prototype -> create a new instance.

Object prototypeInstance = null;

try {

beforePrototypeCreation(beanName);

prototypeInstance = createBean(beanName, mbd, args);

}

finally {

afterPrototypeCreation(beanName);

}

bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);

}





else {

String scopeName = mbd.getScope();

final Scope scope = this.scopes.get(scopeName);

if (scope == null) {

throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");

}

try {

Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {

public Object getObject() throws BeansException {

beforePrototypeCreation(beanName);

try {

return createBean(beanName, mbd, args);

}

finally {

afterPrototypeCreation(beanName);

}

}

});

bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);

}

catch (IllegalStateException ex) {

throw new BeanCreationException(beanName,

"Scope '" + scopeName + "' is not active for the current thread; " +

"consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",

ex);

}

}

}





// Check if required type matches the type of the actual bean instance.

if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {

try {

return getTypeConverter().convertIfNecessary(bean, requiredType);

}

catch (TypeMismatchException ex) {

if (logger.isDebugEnabled()) {

logger.debug("Failed to convert bean '" + name + "' to required type [" +

ClassUtils.getQualifiedName(requiredType) + "]", ex);

}

throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());

}

}

return (T) bean;

}

代码1处,转化有两个步骤，首先处理beanName为&XXX的格式，这里表示要取指定name的factoryBean。在这里先把&符号取消，先获取bean再处理。然后，针对bean的alias机制，这里传入的参数可能是一个bean别名，那么我们先获取这个bean的主要id，只需要根据id值取bean就可以了。

代码2处,就是依次检查缓存。如果缓存里有,就直接拿出来。

spring的缓存有3种

之前已经获取过的bean

手动注入的bean

手动注入的ObjectFactory

第一种缓存好理解,看一下下面的代码,大家就知道手动注入bean与ObjectFctory是怎么回事了。

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public static void main(String[] args){



Resource resource=new ClassPathResource("applicationContext2.xml");

BeanFactory factory=new DefaultListableBeanFactory();

BeanDefinitionReader bdr=new XmlBeanDefinitionReader((BeanDefinitionRegistry) factory);

bdr.loadBeanDefinitions(resource);





FXNewsBean fx=new FXNewsBean();

fx.setUrl("Thank glt");

//同时还有一个方法叫addSingletonFactory

((DefaultSingletonBeanRegistry) factory).registerSingleton("newsBean2",fx);

FXNewsBean news=(FXNewsBean) factory.getBean("newsBean2");

System.out.println(news.getUrl()); //打印出Thank glt

}

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final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);

checkMergedBeanDefinition(mbd, beanName, args);

代码3处,麻烦的是是getMergedLocalBeanDefinition(beanName)。这个处理的主要是bean的present信息与scope信息。

说实话,这部分我没有看。

下面就是检查bean了。

首先,这个bean不能是abstract的。其次,如果getBean时还传递了要获取bean的参数(这里指构造方法的参)并且这个bean是singleton,那就得报错。

为什么?你想呀

factory.getBean("myBean","aaaa");

factory.getBean("myBean","bbbb");

如果myBean这个对象是singleton,上面的两行代码获得的对象能相等吗?

所以只能报错了。

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String[] dependsOn = mbd.getDependsOn();

if (dependsOn != null) {

for (String dependsOnBean : dependsOn) {

getBean(dependsOnBean);

registerDependentBean(dependsOnBean, beanName);

}

}

代码4处,如果BeanA中引用了BeanB,那么再getBean("BeanA")时就会先getBean("BeanB");

之后,注册依赖关系。
这个依赖关系有什么用呢?

似乎是在销毁类时用的,我不是很清楚。

代码5

如果要获取的bean是单例的

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if (mbd.isSingleton()) {

sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {

public Object getObject() throws BeansException {

try {

return createBean(beanName, mbd, args);

}

catch (BeansException ex) {

// Explicitly remove instance from singleton cache: It might have been put there

// eagerly by the creation process, to allow for circular reference resolution.

// Also remove any beans that received a temporary reference to the bean.

destroySingleton(beanName);

throw ex;

}

}

});

}

注意上面的代码其实只有一行。

即调用getSingleton方法。

只不过getSingleton的第二个参数是一个匿名内部类。

这个内部类有一个getObject方法。,里面调用的是外部类的方法createBean。(外部类是谁?是AbstractBeanFactory)

我们先看getSingleton

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public Object getSingleton(String beanName, ObjectFactory singletonFactory) {

singletonObject = singletonFactory.getObject();

addSingleton(beanName, singletonObject);//添加到已成功创建列表中 这是缓存

return (singletonObject != NULL_OBJECT ? singletonObject : null);

}

getSingleton省略了一些代码,主要是验证

getSingleton里面调用getObject就跑到了匿名类里面,最后的是AbstractBeanFactory的createBean方法。

不过AbstractBeanFactory中creatBean是abstract的。

实现在它的子类----AbstractAutowireCapableBeanFactory

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@Override

protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)

throws BeanCreationException {





if (logger.isDebugEnabled()) {

logger.debug("Creating instance of bean '" + beanName + "'");

}

// Make sure bean class is actually resolved at this point.

//*************************5.1 保证RootBeanDefinition里面有beanclass

resolveBeanClass(mbd, beanName);





// Prepare method overrides.

try {

//*************************5.2 方法注入

mbd.prepareMethodOverrides();

}

catch (BeanDefinitionValidationException ex) {

throw new BeanDefinitionStoreException(mbd.getResourceDescription(),

beanName, "Validation of method overrides failed", ex);

}





try {

// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.

//*************************5.3

Object bean = resolveBeforeInstantiation(beanName, mbd);

if (bean != null) {

return bean;

}

}

catch (Throwable ex) {

throw new BeanCreationException(mbd.getResourceDescription(), beanName,

"BeanPostProcessor before instantiation of bean failed", ex);

}

//*************************5.4

Object beanInstance = doCreateBean(beanName, mbd, args);

if (logger.isDebugEnabled()) {

logger.debug("Finished creating instance of bean '" + beanName + "'");

}

return beanInstance;

}

关于5.2处的代码,它处理的类似这样的bean。 更多信息可查看 http://blog.csdn.net/dlf123321/article/details/47862175
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<bean id="mockPersister" class="..impl.MockNewsPersister">

<lookup-method name="getNewsBean" bean="newsBean"/>

</bean>

关于5.3处的代码,得提到一个接口InstantiationAwareBeanPostProcessor,它本身也继承了BeanPostProcessor接口。

我们看到了在5.3代码的下面一旦resolveBeforeInstantiation的返回值不是null,那么直接就return。

resolveBeforeInstantiation类似于一个"短路器",执行了resolveBeforeInstantiation后(我是指getBean的对象实现了InstantiationAwareBeanPostProcessor接口,并且postProcessBeforeInstantiation方法的返回值不为null)下面的流程就不走了,直接返回bean。

这个方法一般情况下返回的都是null,通常情况下都是Spring容器内部使用这种特殊类型的BeanPostProcessor做一些动态对象代理等工作，我们使用普通的BeanPostProcessor实现就可以。这里简单提及一下，目的是让大家有所了解。

历尽千辛万苦,我们终于到了5.4了

让我歇会,下一节我们聊doCreatBean。

参考资料

http://michael-softtech.iteye.com/blog/816469 spring源码分析之——spring bean的获取

http://www.iflym.com/index.php/code/201208290001.html Spring中获取一个bean的流程-1

<<spring揭秘>> 第四章 79页

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