介绍
前面一章说了AbstractApplicationContext中的refresh方法中的invokeBeanFactoryPostProcessors。主要是调用BeanFactoryPostProcessor。其中也有获取bean的过程，就是beanFactory.getBean的方法。这一章就说下getBean这个方法。由于spring中获取bean的方法比较复杂，涉及到的流程也非常多，这一章就先说下整个大体的流程。其中的细节会在后面也会慢慢说。
源码
直接看源码吧
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
@Override
public <T> T getBean(String name, @Nullable 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);
}
public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
throws BeansException {
return doGetBean(name, requiredType, args, false);
}
@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// 把name转化成beanName，也就是把FactoryBean的名称转化成beanName如果有别名则用别名
final String beanName = transformedBeanName(name);
Object bean;
// 从缓存中获取实例
// 可能是需要的Bean实例，也可能是FactoryBean
Object sharedInstance = getSingleton(beanName);
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 + "'");
}
}
// 获取需要的bean或者FactoryBean
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// 判断prototype类型的bean是否存在循环引用
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 校验父类BeanFactory
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// 父类去获取bean
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 {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
// 标记成已创建
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 把原来BeanDefinition转换成RootBeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 获取依赖的bean，也就是通过@DependsOn注入进来的bean
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
// 校验dependsOn的bean是否存在循环应用
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 加入到引用的缓存中，由于校验dependsOn循环引用
registerDependentBean(dep, beanName);
// 获取@dependsOn的bean
getBean(dep);
}
}
// 创建单例的bean
if (mbd.isSingleton()) {
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;
}
});
// 获取需要的bean或者FactoryBean
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 创建prototype的bean
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);
}
// 创建其他的bean，比如session，request等
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;
}
}
// 如果要求的类型不是这个bean的实例类型，则进行转换
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.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;
}
获取bean的整体流程就像上面源码所示，这里再梳理下spring获取bean的整个流程
1.先转换bean的名称，转换成beanName。这里意思就是，我们在获取bean的时候，可能是FactoryBean的名称（&开头），这里转成不带&开头的名称，如果有别名，再获取别名。
2.从缓存中获取bean，这里的缓存分为一二三级缓存，也就是我们常常被问到了spring三级缓存，具体逻辑下面再说。
3.根据获取的到对象再去获取我们想要的bean，因为这里获取到的对象可能是我们需要的bean，也可能是FactoryBean。
4.如果缓存中没有，那么我们就要自己去创建bean了。
5.查看有没有父类的BeanFactory，如果有，那么就父类去创建bean。
6.获取要创建的bean对象的@DependsOn注解上的名称，先去创建DependsOn的bean，并且校验是否存在循环引用
7.创建bean，根据类型创建不同的bean，比如singleton，prototype，request，session等。
8.如果需要转换类型，则进行类型转换。
整体的获取bean的流程就是这样了，其中有些具体流程接着分析。
从缓存中获取bean对象
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 从一级缓存中获取
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
// 从二级缓存中获取
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
// 从三级缓存中获取
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
spring通过三级缓存来解决循环依赖的问题。简单来介绍下三级缓存。
1. singletonObjects为一级缓存，我们实例化的bean都在这个map里，侠义的说singletonObjects才是我们真正的spring容器，存放bean的地方。
2. earlySingletonObjects为二级缓存，是存放未完成的bean的缓存，如果有代理的话，存放的是代理对象。
3. singletonFactories为三级缓存，存放的是一个ObjectFactory，数据通过getObject方法获得。
从BeanInstance中获取对象
接下来看getObjectForBeanInstance方法。
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// name是不是factoryBean的name（&开头的）
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
// 如果是FactoryBeanName,但是获取到的bean不是FactoryBean，则抛异常
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
}
// 如果bean不是FactoryBean，或者名称是FactoryBeanName，直接返回BeanInstace
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
// 从缓存中获取
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// 这里可以确定beanInstance是FactoryBean了
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
// 通过FactoryFBean中获取需要的beanInstance
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
这里是通过BeanInstance获取我们想要的bean，这里也简单说下流程
1. 首先判断name是不是FactoryBean的name，也就是&开头的name，如果是去判断beanInstance是不是FactoryBean，如果beanInstance不是FactoryBean则抛异常。
2. 由于上面已经判断过，如果name是FactoryBeanName，但是BeanInstance不是FactoryBean的话，就会抛出异常。所以如果BeanInstance如果不是FactoryBean的话，那么name一定不是FactoryBeanName。那么就直接返回BeanInstance就是我们需要的了。
如果name是FactoryBeanName，那么我们需要获取的就是FactoryBean，也直接返回就可以了。
3. 如果都没有返回，那么已经可以确定我们此时的已经可以确定BeanInstance是FactoryBean了，因为如果不是FactoryBean的话，在!(beanInstance instanceof FactoryBean)就已经返回了。
4. 通过FactoryBean的getObject方法获取我们需要的bean实例。
创建bean
根据@dependsOn查找依赖的bean并且加到依赖里面去没有什么好说的，代码逻辑也很简单，接下来看创建单例bean。其他类型的bean的创建也都差别不大。看源码
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
// 直接从一级缓存中取
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
// 在没创建bean之前的处理
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
// 获取创建的bean
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
// 创建结束之后的工作
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 加到一级缓存中，其实也就是真正的容器中了
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
对于创建单例bean的主要流程就是如此，传入一个beanName，和一个ObjectFactory。ObjectFactory中具体实现了创建bean的逻辑。在看具体创建bean的逻辑之前，我们还需要去看下getSingleton中的创建bean之前的工作和创建bean之后的工作。这里面就是查找bean的循环依赖的方法（和dependsOn不同）。主要是查找根据filed，set，构造器方法的循环依赖。
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
protected void afterSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
}
}
可以看到代码非常的简单，就是创建bean之前，如果没有排除依赖检查，那么就加入到正在创建的Set中，如果加入不进去，说明之前已经加过，这就产生了循环依赖，从而抛出异常。
如果在创建bean之后，没有排除检查依赖，并且移除失败，说明已经不在Set中，也会抛出异常。
好了，既然明白了spring是如何校验循环依赖的，也看到了三级缓存，后面再说为什么不能解决构造器依赖就很好说了。接着看创建bean的方法。
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// 获取要创建bean的class
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
// 如果没有beanclass，设置beanclass
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// 配置方法重载
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// 那些beanPostProcessor如果能产生代理，则直接返回bean
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 {
// 创建bean
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException ex) {
// A previously detected exception with proper bean creation context already...
throw ex;
}
catch (ImplicitlyAppearedSingletonException ex) {
// An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
这里其实还是没有到创建bean的过程，还是在创建bean的一些准备工作。其实我们可以发现，spring中，真正做事的都是do开头的方法。
这边的流程就是设置beanClass，后面需要根据反射来创建bean。然后会根据spring里面的beanPostProcessor，看看有没有能产生代理bean的，如果有就返回，没有就去创建bean。
看真正的doCreateBean方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// 装饰Bean的对象
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
// 通过缓存获取
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// 创建bean
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
// 调用MergedBeanDefinitionPostProcessor的方法
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// 加入到三级缓存中去
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
// 填充依赖的bean，field注入，和方法注入的bean
populateBean(beanName, mbd, instanceWrapper);
// 调用初始化的方法
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
// 获取二级缓存的值
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
// 如果一致，则直接使用二级缓存的对象
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
// 如果调用初始化后的bean和之前的bean不一致，并且有依赖
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
// 查找循环依赖
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
// 如果有循环依赖并且在创建中，则抛出异常
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// 注册disposableBean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
真正创建bean这里还是有点复杂的。这里再进行一个简单梳理。
1. 根据class还有bean以及参数创建bean。
2. 调用beanPostprocessor的方法，调用属于MergedBeanDefinitionPostProcessor的方法。对bean进行一些处理，比如找到那些依赖的bean的field和method。
3. 将bean加入到三级缓存中去。
4. 填充bean需要注入的其他bean。
5. 调用初始化方法，先去调用@PostConstruct注解方法，然后调用InitializingBean的afterPropertiesSet，以及自定义的init-method方法。在bean调用初始化方法之后，再去调用后置接口看看是否需要生成Aop代理。
6. 接着进行校验。这里稍微比较复杂一点。如果从二级缓存能取到，那就说明之前被别人从三级缓存拿出来过了。可能是因为循环依赖，也可能是因为别的地方调用了getBean方法了。从三级缓存拿出来的时候有个getEarlyBeanReference的方法，就是查看是否要生成代理的bean。如果生成过了，那么在调用第五步的时候，就不会在生成代理了。这样exposedObject ==bean，直接只用代理返回。
如果不相等：这里的情况就是如果是spring自己的@Async，在从二级缓存生成代理之后，再去调用第五步时候一样会生成代理。所以exposedObject !=bean，所以在再往下发现有循环调用，并且bean还在创建时，就会抛出异常了。所以一般慎用spring的@Async。但是一般也可以使用@Lazy进行处理。至于原理后面再说。
到这里spring的创建bean就结束了。然后返回时候就到了入口方法getBean的getObjectForBeanInstance的方法，到底需要的bean还是FactoryBean。
最后就是如果requiredType和实例不一样就要进行类型转换了。
总结
本篇大概说下spring获取bean和加到容器里面的流程。其实广义上来说Bean的容器是BeanFactory或者applicationContext。狭义上说就是一个map。也就是一级缓存SingletonObjects。我们获取的真正需要的bean也就是从中获取的。本篇只是简要的说了下bean获取和加入容器的整个流程，具体的根据无参构造器创建bean，有参数构造器创建bean。还有对于创建bean中依赖的bean的查找还有创建，三级缓存如何解决循环依赖还有为何不能解决构造器依赖，以及bean调用初始化的等等操作都没有说。因为一篇说起来确实太长了。后面都会一一去分析。
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