简话bean加载
首先看示例代码:
<!--no-lazy-init scope=singleton-->
<bean class="com.lios.service.test.LiosTestA" id="liosTestA"/>
<bean class="com.lios.service.test.LiosTestB" id="liosTestB"/>
<bean class="com.lios.service.test.LiosServiceServiceImpl" id="liosServiceService"/>
ClassPathXmlApplicationContext resource = new ClassPathXmlApplicationContext("app.xml");
BeanFactory beanFactory = resource.getBeanFactory();
LiosServiceServiceImpl liosServiceService = (LiosServiceServiceImpl) beanFactory.getBean("liosServiceService");
liosServiceService.t();LiosServiceServiceImpl:
@Service
public class LiosServiceServiceImpl {
private int i = 1;
@Autowired
LiosTestA liosTestA;
public void t(){
try {
liosTestA.testA();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("===========>");
}
}LiosTestA:
@Service
public class LiosTestA {
@Autowired
LiosTestB liosTestB;
public void testA(){
liosTestB.testB();
}
}以上代码就是LiosServiceServiceImpl类中引用了LiosTestA,LiosTestA类中引用了LiosTestB,今天的问题是LiosServiceServiceImpl如何引用LiosTestA,LiosTestA如何引用LiosTestB? 看过源码的同学肯定知道, org.springframework.context.support.AbstractApplicationContext#refresh是spring解析xml、初始化bean的入口,该方法里会调用:
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);这个方法会实例化所有的non-lazy-init单例的bean,毫无疑问,这个方法是分析问题的入口,紧跟进去:
// Instantiate all remaining (non-lazy-init) singletons.
beanFactory.preInstantiateSingletons();再跟进去,调用了 org.springframework.beans.factory.support.DefaultListableBeanFactory#preInstantiateSingletons方法,再跟进去 getBean(beanName);方法,调用了 org.springframework.beans.factory.support.AbstractBeanFactory#getBean(java.lang.String)方法:
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}进入doGetBean方法中,绕了那么多,这个方法才是会真正干事:
//获取beanName
final String beanName = transformedBeanName(name);
//从缓存里获取bean,第一次时毫无疑问,sharedInstance为null
Object sharedInstance = getSingleton(beanName);
//获取RootBeanDefinition,其实用BeanDefinition初始化
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
创建bean
createBean(beanName, mbd, args)下面分析 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean(java.lang.Class<T>)方法,明显这是把具体实现委托给子类实现了,继续跟: org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean(java.lang.String, org.springframework.beans.factory.support.RootBeanDefinition, java.lang.Object[]),该方法里有一段这样的代码:
instanceWrapper = createBeanInstance(beanName, mbd, args);继续跟:
// Make sure bean class is actually resolved at this point.
Class<?> beanClass = resolveBeanClass(mbd, beanName);
//.... 后面省略继续跟 resolveBeanClass()方法,其实该方法里利用反射创建了bean实例,createBeanInstance()方法主要是返回BeanWrapper对象,该对象用bean实例初始化,getWrappedInstance()即可返回bean对象:
/**
* Return the bean instance wrapped by this object, if any.
* @return the bean instance, or {@code null} if none set
*/
Object getWrappedInstance();所以上面的LiosServiceServiceImpl、LiosTestA、LiosTestB都是通过resolveBeanClass()方法创建实例,但是里面的引用的属性如何创建呢,那回到 doCreateBean()方法,继续看下面的代码:
// Initialize the bean instance.
Object exposedObject = bean;
try {
//初始化bean属性的值
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}首先分析 populateBean(beanName,mbd,instanceWrapper),为了直接点,直接看后置处理器BeanPostProcessor:
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}再看InstantiationAwareBeanPostProcessor的实现类, org.springframework.context.annotation.CommonAnnotationBeanPostProcessor#postProcessPropertyValues:
InjectionMetadata metadata = findResourceMetadata(beanName, bean.getClass(), pvs);
metadata.inject(bean, beanName, pvs);一直跟进去到 org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject:
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);再跟进去:
result = doResolveDependency(descriptor, beanName, autowiredBeanNames, typeConverter);继续跟进去:
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);doResolveDependency方法会根据属性的属性类型去获取引用,继续跟,可以看到 org.springframework.beans.factory.support.DefaultListableBeanFactory#findAutowireCandidates方法里的这段代码:
result.put(candidateName, getBean(candidateName));getBean(candidateName)这个不是调用 org.springframework.beans.factory.support.AbstractBeanFactory#getBean(java.lang.String)么,没错,是的!
回到 org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject中:
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}这段代码会设置bean中的属性的值,一个真正的bean已经完成了. 再回到 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean方法中,看这句代码:
initializeBean(beanName, exposedObject, mbd);跟进后再进入 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#invokeInitMethods方法:
boolean isInitializingBean = (bean instanceof InitializingBean);
if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
if (logger.isDebugEnabled()) {
logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
}
if (System.getSecurityManager() != null) {
try {
AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
@Override
public Object run() throws Exception {
((InitializingBean) bean).afterPropertiesSet();
return null;
}
}, getAccessControlContext());
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
((InitializingBean) bean).afterPropertiesSet();
}
}看到InitializingBean、afterPropertiesSet()是否会想起什么呢,如果一个bean实现了InitializingBean接口后,在bean被容器加载时,自动调用afterPropertiesSet()方法,现在明白是咋回事了吧. 说了那么多,总结下LiosServiceServiceImpl类的加载过程,首先容器会加载LiosServiceServiceImpl或者LiosTestA或者LiosTestB,默认是没有明确顺序之分,如果按照先加载LiosTestA的话,会先创建LiosTestA实例,里面的属性LiosTestB值还是为空,然后设置其属性的值,其实就是调用getBean方法,完成LiosTestA的实例创建后,创建LiosServiceServiceImpl套路完全一样,其属性LiosTestA已经在缓存有了,直接获取即可. 最后,bean的加载远远不止这么复杂,文中有错误之处,麻烦指正!