开篇
在Spring 加载BeanDefinition文章中我们已经Spring如何加载xml文件并扫描注解class文件生成BeanDefinition，在这篇文章中我们着重梳理下GetBean过程。
文章的整体思路在于ClassPathXmlApplicationContext通过到DefaultListableBeanFactory查找Bean对应的BeanDefinition后创建Bean对象。
时序图
GetBean
说明：
- 核心调用链路使ClassPathXmlApplicationContext -> DefaultListableBeanFactory。
- DefaultListableBeanFactory保存的是从xml文件加载的BeanDefinition对象。
ClassPathXmlApplicationContext
说明：
- ClassPathXmlApplicationContext的类依赖图，用于补充说明上图的调用链。
DefaultListableBeanFactory
说明：
- DefaultListableBeanFactory的类依赖图，用于补充说明上图的调用链。
Bean获取源码分析
public static void main(String[] args) {
ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean.xml");
SimpleBean simpleBean = applicationContext.getBean(SimpleBean.class);
}
说明：
- 1、通过创建ClassPathXmlApplicationContext对象加载xml文件。
- 2、通过applicationContext.getBean()返回bean对象。
public abstract class AbstractApplicationContext extends DefaultResourceLoader
implements ConfigurableApplicationContext {
public Object getBean(String name) throws BeansException {
assertBeanFactoryActive();
return getBeanFactory().getBean(name);
}
}
说明：
- 3、ClassPathXmlApplicationContext.getBean()通过基类AbstractApplicationContext来实现。
- 4、AbstractBeanFactory.getBean()方法继续跟进Bean对象。
public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
protected T doGetBean(final String name, @Nullable final Class requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// 处理单例 情况 Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
//合并BeanDefinition信息
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
// 标记beanName正在创建
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 处理依赖的bean率先创建
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);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// 区分不同的类型的Bean实例并进行创建
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
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);
try {
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
}
}
}
catch (BeansException ex) {
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
}
}
return (T) bean;
}
}
说明：
- 5、AbstractBeanFactory.getBean()负责返回Bean对象，按照一定的逻辑进行获取返回。
- 6、如果单例Bean已经存在那么就直接返回单例Bean。
- 7、如果Bean不存在就先合并BeanDefinition信息。
- 8、先创建Bean依赖的Bean对象
- 9、区分不同的Bean类型创建Bean对象。
单例(singleton) IOC容器中有且只有一个对象，多次调用getBean返回同一个对象
多例(prototype) 每次调用getBean返回一个新对象
HTTP Request 每次HTTP请求共用同一个对象
HTTP Session 每个HTTP会话共用同一个对象
Application ServletContext共用一个对象
单例Bean获取逻辑
/** Cache of singleton objects: bean name --> bean instance */
private final Map singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name --> ObjectFactory */
private final Map> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name --> bean instance */
private final Map earlySingletonObjects = new HashMap<>(16);
public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
@Nullable
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;
}
}
说明：
10、getSingleton() 整个过程如下：首先从一级缓存 singletonObjects 获取，如果没有且当前指定的 beanName 正在创建，就再从二级缓存中 earlySingletonObjects 获取，如果还是没有获取到且运行 singletonFactories 通过 getObject() 获取，则从三级缓存 singletonFactories 获取，如果获取到则，通过其 getObject() 获取对象，并将其加入到二级缓存 earlySingletonObjects 中 从三级缓存 singletonFactories 删除
11、Bean创建完成了初始化的第一步并且将自己提前曝光到singletonFactories中，用于解决相互依赖问题。
singletonObjects：单例对象的cache
singletonFactories ： 单例对象工厂的cache
earlySingletonObjects ：提前曝光的单例对象的Cache
参考文章
Spring源码初探-IOC(4)-Bean的初始化-循环依赖的解决