Spring 源码学习 – 单例bean的实例化过程
本文作者：geek，一个聪明好学的同事
1. 简介
开发中我们常用@Commpont，@Service，@Resource等注解或者配置xml去声明一个类，使其成为spring容器中的bean，以下我将用从源码角度看以AnnotationConfigApplicationContext为例看spring如何把带有注解的类生成spring中bean。
2. 示例代码
public class TestContext {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
SingleBean singleBean = context.getBean(SingleBean.class);
System.out.println("<=====>"+singleBean.getTestStr());
}
}
@ComponentScan("com.geek")
public class AppConfig {
}
@Component
public class SingleBean {
private String testStr = "testStr";
public String getTestStr() {
return testStr;
}
}
注意：以上代码仅需要引入spring-context依赖即可。
3. 源码分析
上面的demo在调用AnnotationConfigApplicationContext构造函数的时候，AppConfig类会被注册到AnnotatedBeanDefinitionReader，由这个reader把AppConfig解释为beanDefination，从而被spring获取到要实例化的类信息，以下为bean生产的源码及其注释。（源码基于springFramework 5.1.X）
3.1 创建入口
单例bean的创建的入口为DefaultListableBeanFactory.java#preInstantiateSingletons，下面源码可见创建的bean条件为非抽象，非@LazyInit注解，Scope为singleTon（默认为singleTon）。
@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
//所有可能需要去实例化的class（lazy，scope）
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
/**
* 非抽象，非懒初始化，单例bean
*/
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
//非工厂bean实例化
else {
getBean(beanName);
}
}
}
/**
* 实例化完成后触发实现了SmartInitializingSingleton方法的bean
* 的afterSingletonsInstantiated方法
*/
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
3.2 创建前doGetBean代码逻辑
getBean方法进来后便是直接调用doGetBean，doGetBean执行的源码解释如下：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
/**
* 获取beanName
* 1，去掉factortoryBean前缀&
* 2，带有别名的bean转换为原来名字
*/
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
/**
* 从DefaultSingletonBeanRegistry的singletonObjects
* （spring内部用来缓存单例bean的currentHashMap）检查是否存在该bean，不存在则创建
* 涉及单例模式下的循环依赖解决
*/
Object sharedInstance = getSingleton(beanName);
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 + "'");
}
}
/**
*获取给定bean实例的对象，如果是Factory Bean，
* 则可以是bean实例本身或其创建的对象。
*/
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assembly within a circular reference.
/**
* 原型模式下的bean存在循环依赖则会抛异常
*/
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
/**
* 找不到则从父容器中查找
*/
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);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
/**
* 父容器中也找不到该bean，则需要重新实例化
* 1，获取要实例化bean的beanDefinition,
* 2，检查bean的实例化需要依赖的其他bean
*/
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
/**
* 若给定的依赖 bean 已经注册为依赖给定的bean
*/
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
try {
/**
* 递归调用getBean，优先创建依赖的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()) {
/**
* 通过调用DefaultSingletonBeanRegistry的getSingleton，从而调用核心方法createBean创建
*/
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.
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, () -> {
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;
}
}
/**
* 检查需要的bean类型是否符合
*/
// 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) {
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;
}
doGetBean的操作流程如下：
1，执行transformedBeanName方法转换beanName
传递的参数可能是bean的alias或者为FactoryBean，transformedBeanName执行的操作:(1)若传进来的FactoryBean（FactoryBean以&作为前缀标记），去掉&修饰符。（2）经过（1）的处理后，有alias的bean则从aliasMap中获取bean的原始beanName。
2，从容器的缓存中获取bean
getSingleton先从spring三级缓存中的第一级singletonObjects（Map结构）中获取，若不存在，则检查该bean是否正在创建isSingletonCurrentlyInCreation(beanName)) ，正在创建的bean会从二级缓存earlySingletonObjects（Map结构）获取。获取到缓存的bean后会调用AbstractBeanFactory#getObjectForBeanInstance转换bean的实例本身返回。因为从缓存中拿到的可能是factoryBean，所以getObjectForBeanInstance需要把是通过从缓存factoryBeanObjectCache获取或通过factory.getObject()获得相应的bean返回。
3，bean实例化前检查
（1）先检查是否原型模式下的bean是否存在循环依赖，是则会抛异常。
（2）检查父类工厂（parentBeanFactory）是否存在，存在则从parentBeanFactory中递归调用doGetBean。
（3）获取改bean的beanDefinition，检查该bean实例化过程中是否涉及依赖了其他的bean，若是则递归调用getBean，优先创建依赖的bean。（涉及单例下的循环以来解决，下篇文章详细介绍）。
（4）对创建bean代码加synchronized和执行beforeSingletonCreation(beanName)前置处理。
3.3 创建前createBean逻辑
经过前面的doGetBean的一轮检查与准备后，便在AbstractAutowireCapableBeanFactory#createBean中开始bean的创建。
/**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* 解析指定 BeanDefinition 的 class
* 处理 override 属性
* 实例化的前置处理
* 创建 bean
* @see #doCreateBean
*/
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isTraceEnabled()) {
logger.trace("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
/**
* 解释bean的class，看beanDefinition是否有class，否则load class
*/
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
/**
* 对bean不存在lookup-method 和 replace-method
* 标记其方法的overloaded为false
*/
// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
/**
* 调用实现实现BeanPostProcessor的bean后置处理生成代理对象，
* 有代理对象则直接返回代理对象
*/
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
/**
* 无需代理的bean实例化
*/
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
createBean的操作流程如下：
1，resolveBeanClass
解释beanDefinition的class，并且保存在beanDefinition中。
2，prepareMethodOverrides
处理bean中的lookup-method （在单例bean用 @Lookup注解标记的方法，注解的方法返回的对象是原型）和 replace-method（
3，resolveBeforeInstantiation
调用实现实现BeanPostProcessor的bean后置处理生成代理对象，有代理对象则直接返回代理对象。（spring AOP则是基于此处实现）
3.4 bean的真正实例化createBeanInstance
在AbstractAutowireCapableBeanFactory#createBeanInstance中，真正创建bean，源码及注释如下：
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
Class<?> beanClass = resolveBeanClass(mbd, beanName);
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
/**
* 通过提供supplier回调方法创建
*/
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
/**
* 通过工厂方法创建 bean 实例，可以是静态工厂方法或者实例工厂
*/
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
/**
* 查找已经bean已经缓存解析的构造函数或者工厂方法
*/
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
/**
* 已经有缓存的构造函数或者工厂方法，直接实例化
*/
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
/**
* 通过实现BeanPostProcessor的代理类autowired的构造函数实例化
*/
// Candidate constructors for autowiring?
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
/**
* 通过本身带有autowired的构造函数实例化，通过调用反射newInstance实现
*/
// Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
/**
* 无参构造函数实例化，通过调用反射newInstance实现
*/
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
由上述源码可以看出，实例化bean操作流程如下：
1，如果存在 Supplier 回调，则通过提供supplier回调方法创建，如以下方式定义的bean：
@RunWith(SpringRunner.class)
@SpringBootTest(classes = Spring5Application.class)
public class BeanRegistrationTest {
@Autowired
private GenericWebApplicationContext context;
context.registerBean(A.class, () -> new A());
}
2，如果存在工厂方法，则通过工厂方法创建 bean 实例，可以是静态工厂方法或者实例工厂，如以下方式定义的bean：
public class AFactory implements FactoryBean<A> {
@Override
public A getObject() throws Exception {
return new A();
}
@Override
public Class<?> getObjectType() {
return A.class;
}
}
//或：
@Configuration
public class BeanConfigration {
@Bean
public A a(){
return new A();
}
}
3，已经有缓存的构造函数或者工厂方法，直接实例化。
4，以上三点都不存在，则使用带参构造函数与无参构造函数实例化。如以下方式定义的bean：
@Commponet
public class A{}
4.总结
spring单例bean的实例化流程大概就是这样，很多细节地方，包括循环依赖处理，bean属性填充等细节点下一章介绍。
参考
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