接上回，AbstractApplicationContext#refresh调用AbstractApplicationContext#finishBeanFactoryInitialization来初始化所有的非懒加载单例 Bean。在该AbstractApplicationContext#finishBeanFactoryInitialization方法内部通过调用AbstractBeanFactory#doGetBean来获取 Spring 容器所管理的 Bean。
AbstractBeanFactory#doGetBean
AbstractBeanFactory#doGetBean源码如下：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// 如果这个 name 是 FactoryBean 的beanName (&+beanName),就删除& , 返回beanName ,传入的name也可以是别名,也需要做转换
// 注意 beanName 和 name 变量的区别,beanName是经过处理的,经过处理的beanName就直接对应singletonObjects中的key
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 根据beanName尝试从singletonObjects获取Bean
// 获取不到则再尝试从earlySingletonObjects,singletonFactories 从获取Bean
// 这段代码和解决循环依赖有关
Object sharedInstance = getSingleton(beanName);
// 第一次进入sharedInstance肯定为null
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
} else {
logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
// 如果sharedInstance不为null,也就是非第一次进入
// 为什么要调用 getObjectForBeanInstance 方法,判断当前Bean是不是FactoryBean,如果是,那么要不要调用getObject方法
// 因为传入的name变量如果是(&+beanName),那么beanName变量就是(beanName),也就是说,程序在这里要返回FactoryBean
// 如果传入的name变量(beanName),那么beanName变量也是(beanName),但是,之前获取的sharedInstance可能是FactoryBean,需要通过sharedInstance来获取对应的Bean
// 如果传入的name变量(beanName),那么beanName变量也是(beanName),获取的sharedInstance就是对应的Bean的话,就直接返回Bean
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
} else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 判断是否循环依赖
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 获取父BeanFactory,一般情况下,父BeanFactory为null,如果存在父BeanFactory,就先去父级容器去查找
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);
}
}
// 创建的Bean是否需要进行类型验证,一般情况下都不需要
if (!typeCheckOnly) {
// 标记 bean 已经被创建
markBeanAsCreated(beanName);
}
try {
// 获取其父类Bean定义,子类合并父类公共属性
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 获取当前Bean依赖的Bean的名称 ,@DependsOn
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 + "'");
}
// 如果当前Bean依赖其他Bean,把被依赖Bean注册给当前Bean
registerDependentBean(dep, beanName);
try {
// 先去创建所依赖的Bean
getBean(dep);
} catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
if (mbd.isSingleton()) {
// 创建单例Bean
sharedInstance = getSingleton(beanName, () -> {
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.
// 创建prototype Bean,每次都会创建一个新的对象
Object prototypeInstance = null;
try {
// 回调beforePrototypeCreation方法，注册当前创建的原型对象
beforePrototypeCreation(beanName);
// 创建对象
prototypeInstance = createBean(beanName, mbd, args);
} finally {
// 回调 afterPrototypeCreation 方法，告诉容器该Bean的原型对象不再创建
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
} else {
// 如果既不是单例Bean,也不是prototype,则获取其Scope
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 {
// 创建对象
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) {
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.
// 对创建的Bean进行类型检查
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) {
if (logger.isTraceEnabled()) {
logger.trace("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
主要流程都已经在上述的源码中增加了注释，读者可以自己查阅。其中，我觉得最主要是要明白以下几点：
Object sharedInstance = getSingleton(beanName);
Object sharedInstance = getSingleton(beanName);这段代码是解决循环依赖的关键，先大概看一下，留个印象，具体的等后面有空讲一下循环依赖吧。
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
//getSingleton(beanName, true);源码
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//singletonObjects 就是Spring内部用来存放单例Bean的对象池,key为beanName，value为Bean
Object singletonObject = this.singletonObjects.get(beanName);
// singletonsCurrentlyInCreation 存放了当前正在创建的bean的BeanName
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
// earlySingletonObjects 是早期单例Bean的缓存池,此时Bean已经被创建(newInstance),但是还没有完成初始化
// key为beanName，value为Bean
singletonObject = this.earlySingletonObjects.get(beanName);
//是否允许早期依赖
if (singletonObject == null && allowEarlyReference) {
//singletonFactories 单例工厂的缓存,key为beanName,value 为ObjectFactory
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//获取早期Bean
singletonObject = singletonFactory.getObject();
//将早期Bean放到earlySingletonObjects中
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
transformedBeanName 和 getObjectForBeanInstance 的作用
调用doGetBean方法时会传入name变量，表明需要从容器中获取那个 Bean。transformedBeanName除了在处理别名之外，这里会有以下几种情况：
第一种情况，直接往容器中注册 Bean。
@Bean
public UserBean userBean() {
return new UserBean("shen", 111);
}
在这种情况下，name变量如果为userBean，那么就是要从容器中获取UserBean对象。
调用transformedBeanName方法返回beanName对象的取值userBean。
然后根据beanName去容器中获取相应的Bean，而获取到的就是UserBean对象。
最后调用getObjectForBeanInstance方法，返回的还是UserBean对象。
第二种情况，通过FactoryBean往容器中注册 Bean
@Bean
public FactoryBean userBean() {
return new FactoryBean<UserBean>() {
@Override
public UserBean getObject() throws Exception {
return new UserBean("shen", 111);
}
@Override
public Class<?> getObjectType() {
return UserBean.class;
}
};
}
这里又可以分为两种情况：
第一种情况：
如果name变量如果为userBean，那么也要从容器中获取UserBean对象。
调用transformedBeanName方法返回beanName对象的取值为userBean。
然后根据beanName去容器中获取相应的Bean，而获取到的是FactoryBean对象。
最后调用getObjectForBeanInstance方法，发现是从容器中获取UserBean对象，于是调用FactoryBean#getObject返回UserBean对象。
第二种情况：
name变量如果为&userBean，那么就是要从容器中获取FactoryBean对象本身。
调用transformedBeanName方法返回beanName对象的取值为userBean。
然后根据beanName去容器中获取相应的Bean，而获取到的是FactoryBean对象，
最后调用getObjectForBeanInstance方法，就是要获取FactoryBean对象，于是方法返回FactoryBean对象。
@DependsOn
通过源码可以知道，Spring 在创建 Bean 之前，首先会创建当前 Bean 所有依赖的 Bean。
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 + "'");
}
// 如果当前Bean依赖其他Bean,把被依赖Bean注册给当前Bean
registerDependentBean(dep, beanName);
try {
// 先去创建所依赖的Bean
getBean(dep);
} catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
那什么是当前 Bean 所有依赖的 Bean 呢？也就是说，String[] dependsOn = mbd.getDependsOn();什么情况下这个数组中会有值呢？
在 Spring 中有这个一个注解：@DependsOn。这个注解一般用的很少(碰巧最近项目中用到了这个注解，哈哈哈)。来看一下 Spring 文档中是怎么描述的：
Beans on which the current bean depends. Any beans specified are guaranteed to be created by the container before this bean. Used infrequently in cases where a bean does not explicitly depend on another through properties or constructor arguments, but rather depends on the side effects of another bean's initialization.
A depends-on declaration can specify both an initialization-time dependency and, in the case of singleton beans only, a corresponding destruction-time dependency. Dependent beans that define a depends-on relationship with a given bean are destroyed first, prior to the given bean itself being destroyed. Thus, a depends-on declaration can also control shutdown order.机器翻译：当前 bean 所依赖的 bean。任何指定的 bean 都保证在此 bean 之前由容器创建。当一个 bean 不是通过属性或构造函数参数显式依赖于另一个 bean，而是依赖于另一个 bean 初始化的副作用时，很少使用。依赖项声明既可以指定初始化时间依赖项，也可以指定(在只有单例 bean 的情况下)对应的销毁时间依赖项。在销毁给定 bean 之前，首先销毁定义与给定 bean 的依赖关系的依赖 bean。因此，依赖声明也可以控制关机顺序。
举个例子
@Service
public class OrderService {
public OrderService() {
System.out.println("OrderService create");
}
@PreDestroy
public void destroy() {
System.out.println("OrderService destroy");
}
}
@DependsOn("orderService")
@Service
public class UserService {
public UserService() {
System.out.println("UserService create");
}
@PreDestroy
public void destroy() {
System.out.println("UserService destroy");
}
}
@Configuration
@ComponentScan
public class DependsOnMain {
public static void main(String[] args) {
AnnotationConfigApplicationContext context =
new AnnotationConfigApplicationContext(DependsOnMain.class);
context.close();
}
}
//运行结果
OrderService create
UserService create
UserService destroy
OrderService destroy
createBean(beanName, mbd, args);
在 Spring 中存在者多种 scope，Spring 会根据不用的 scope 选用不同的初始化方式。但是，不管怎么样，Spring 在底层创建 Bean 的时候都是通过调用AbstractAutowireCapableBeanFactory#createBean方法来创建对象的。
那 Spring 究竟是怎么创建 Bean 的呢？AbstractAutowireCapableBeanFactory#createBean方法内部到底做了什么事情呢？循环依赖该怎么解决呢？
未完待续...
源码注释 GITHUB 地址：https://github.com/shenjianeng/spring-code-study
