Spring加载Bean的流程（源码分析）
2018-08-06 10:23:47 食鱼酱 阅读数 204 文章标签： SpringBean加载Spring源码Spring Bean加载流程 更多
版本
4.3.10
入口
public class Application {
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
Person person = context.getBean("person", Person.class);
System.out.println(person.toString());
}
}对上面的getBean方法进行跟踪，最终到达AbstractBeanFactory类下面的doGetBean()方法
Bean加载源码分析
public abstract class AbstractBeanFactory
extends FactoryBeanRegistrySupport
implements ConfigurableBeanFactory {
//以下三个方法是重写父类的getBean方法
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
@Override
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
return doGetBean(name, requiredType, null, false);
}
@Override
public Object getBean(String name, Object... args) throws BeansException {
return doGetBean(name, null, args, false);
}
//这个方法是bean加载的关键
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
/*
1、转换beanName(别名转换)
平时开发中传入的参数name可能只是别名，也可能是FactoryBean，所以需要进行解析转换：
（1）消除修饰符，比如name="&test"，会去除&使name="test"；
（2）解决spring中alias标签的别名问题
*/
final String beanName = transformedBeanName(name);
Object bean;
//2、尝试从缓存中加载实例，如果获取不到就从singletonFactories中加载
Object sharedInstance = getSingleton(beanName);
//如果缓存中存在对应的bean
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 + "'");
}
}
//从缓存中获取的bean是原始状态的bean,需要在这里对bean进行bean实例化
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// 如果缓存中没有对应bean
//4、循环依赖检查
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 5、如果缓存中没有数据，就会转到父类工厂去加载
//获取父工厂
BeanFactory parentBeanFactory = getParentBeanFactory();
/*
!containsBeanDefinition(beanName)就是检测如果当前加载的xml配置文件中不包含beanName所对应的
配置，就只能到parentBeanFacotory去尝试加载bean。
*/
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);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
//6、存储XML配置文件的GernericBeanDefinition转换成RootBeanDefinition
/*
XML配置文件中读取到的bean信息是存储在GernericBeanDefinition中的，但Bean的后续处理是针
对于RootBeanDefinition的，所以需要转换后才能进行后续操作。
*/
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//7、初始化依赖的bean
//bean中可能依赖了其他bean属性，在初始化bean之前会先初始化这个bean所依赖的bean属性。
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
getBean(dep);
}
}
//8、创建bean
//下面这一大段是Spring容器根据不同scope创建bean实例。
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
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 name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
@Override
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);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw 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;
}
}总结
1、转换beanName(别名转换)
要知道平时开发中传入的参数name可能只是别名，也可能是FactoryBean，所以需要进行解析转换，一般会进行以下解析：
（1）消除修饰符，比如name=”&test”，会去除&使name=”test”；
（2）解决spring中alias标签的别名问题
2、尝试从缓存中加载实例
实例在Spring的同一个容器中只会被创建一次，后面再想获取该bean时，就会尝试从缓存中获取；如果获取不到的话再从singletonFactories中加载，再存入到缓存中。
3、实例化bean（构建实例）
缓存中记录的bean一般只是最原始的bean状态，这时就需要对bean进行实例化。如果得到的是bean的原始状态，但又要对bean进行处理，这时真正需要的是工厂bean中定义的factory-method方法中返回的bean，上面源码中的getObjectForBeanInstance就是来完成这个工作的。
4、循环依赖检查
5、检测parentBeanFactory
从源码可以看出如果缓存中没有数据会转到父类工厂去加载，源码中的!containsBeanDefinition(beanName)就是检测如果当前加载的xml配置文件中不包含beanName所对应的配置，就只能到parentBeanFacotory去尝试加载bean。
6、存储XML配置文件的GernericBeanDefinition转换成RootBeanDefinition；
XML配置文件中读取到的bean信息是存储在GernericBeanDefinition中的，但Bean的后续处理是针对于RootBeanDefinition的，所以需要转换后才能进行后续操作。
7、初始化依赖的bean
这里应该比较好理解，就是bean中可能依赖了其他bean属性，在初始化bean之前会先初始化这个bean所依赖的bean属性。
8、创建bean
Spring容器根据不同scope创建bean实例。