doGetBean源码详解
1. doGetBean方法源码解析
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
/**
* 通过name获取BeanName,这里不能使用name作为beanName：
* 1. name可能是别名，通过方法转换为具体的实例名称
* 2. name可能会以&开头，表明调用者想获取FactoryBean本身，而非FactoryBean创建bean
* FactoryBean 的实现类和其他的 bean 存储方式是一致的，即 <beanName, bean>，
* beanName 中是没有 & 这个字符的。所以我们需要将 name 的首字符 & 移除，这样才能从
* 缓存里取到 FactoryBean 实例。
*
*/
final String beanName = transformedBeanName(name);
Object bean;
// 从缓存中获取bean
Object sharedInstance = getSingleton(beanName);
/*
* 如果 sharedInstance = null，则说明缓存里没有对应的实例，表明这个实例还没创建。
*( BeanFactory 并不会在一开始就将所有的单例 bean 实例化好，而是在调用 getBean 获取bean 时再实例化，也就是懒加载)。
* getBean 方法有很多重载，比如 getBean(String name, Object... args)，我们在首次获取
* 某个 bean 时，可以传入用于初始化 bean 的参数数组（args），BeanFactory 会根据这些参数
* 去匹配合适的构造方法构造 bean 实例。当然，如果单例 bean 早已创建好，这里的 args 就没有
* 用了，BeanFactory 不会多次实例化单例 bean。
*/
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 是普通的单例 bean，下面的方法会直接返回。但如果
* sharedInstance 是 FactoryBean 类型的，则需调用 getObject 工厂方法 获取真正的
* bean 实例。如果用户想获取 FactoryBean 本身，这里也不会做特别的处理，直接返回
* 即可。毕竟 FactoryBean 的实现类本身也是一种 bean，只不过具有一点特殊的功能而已。
*/
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
/*
* 如果上面的条件不满足，则表明 sharedInstance 可能为空，此时 beanName 对应的 bean
* 实例可能还未创建。这里还存在另一种可能，如果当前容器有父容器，beanName 对应的 bean 实例
* 可能是在父容器中被创建了，所以在创建实例前，需要先去父容器里检查一下。
*/
else {
// BeanFactory 不缓存 Prototype 类型的 bean，无法处理该类型 bean 的循环依赖问题
//判断是否存在循环依赖
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 如果 sharedInstance = null，则到父容器中查找 bean 实例
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 {
// 合并父 BeanDefinition 与子 BeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 检查是否有 dependsOn 依赖，如果有则先初始化所依赖的 bean
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
/*
* 检测是否存在 depends-on 循环依赖，若存在则抛异常。比如 A 依赖 B，
* B 又依赖 A，他们的配置如下：
* <bean id="beanA" class="BeanA" depends-on="beanB">
* <bean id="beanB" class="BeanB" depends-on="beanA">
*
* beanA 要求 beanB 在其之前被创建，但 beanB 又要求 beanA 先于它
* 创建。这个时候形成了循环，对于 depends-on 循环，Spring 会直接
* 抛出异常
*/
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 注册依赖记录
registerDependentBean(dep, beanName);
try {
// 加载 depends-on 依赖
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// 创建 bean 实例
if (mbd.isSingleton()) {
/*
* 这里并没有直接调用 createBean 方法创建 bean 实例，而是通过
* getSingleton(String, ObjectFactory) 方法获取 bean 实例。
* getSingleton(String, ObjectFactory) 方法会在内部调用
* ObjectFactory 的 getObject() 方法创建 bean，并会在创建完成后，
* 将 bean 放入缓存中。
*/
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;
}
});
// 如果 bean 是 FactoryBean 类型，则调用工厂方法获取真正的 bean 实例。否则直接返回 bean 实例
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 创建 prototype 类型的 bean 实例
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);
}
// 创建其他类型的 bean 实例
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;
}
}
// 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;
}
从源码分析一下 doGetBean 的执行流程如下：
-
转换beanName
传入的name参数可能是别名，也可能是FactoryBean，索引还需要进行一系列解析
- 去除FactoryBean的修饰符，也就是如果name=也就是如果name = "&factoryBean"，那么会首先去除&而使name = "factoryBean"
- 将别名alias转换为最终指向的beanName，比如别名A执行名称为B的bean，而B没有指向任何其他的bean，即为最终的bean，则返回B;但是如果B又指向C，而是C是最终的bean，则返回C。
-
尝试从缓存中获取原始单例
这就是Spring的IOC，首先尝试从缓存中加载单例模式
-
检测是否为FactoryBean并获取Bean以及初始化后处理
在doGetBean方法中频繁出现getObjectForBeanInstance方法，它主要完成对获取的Bean Instance进行检测是否为FactoryBean，如果是FactoryBean则通过工厂方法获取Bean以及初始化后处理
-
创建单例Bean
如果缓存中没有单例Bean的缓存，则需要从头开始创建单例Bean，这主要是重载getSingleton的重载方法来实现单例Bean的加载。
-
原型模式的依赖检查
只有单例模式才会尝试解决循环依赖，如果存在A中有B的属性，B中有A的属性，那么当依赖注入的时候看，就会产生当A还未创建完的时候因为对于B的创建再次返回创建A，造成循环依赖，也就是情况：isPrototypeCurrentlyInCreation(beanName)判断为true
-
处理 depends-on 依赖
-
创建并缓存 bean
-
调用 getObjectForBeanInstance 方法，并按 name 规则返回相应的 bean 实例
-
按需转换 bean 类型，并返回转换后的 bean 实例
2. 方法的源码解析
接下来逐步分析每一个方法
2.1 transformedBeanName
transformedBeanName(name) beanName 的转换，之前分析过由于 name 可能是 FactoryBean 或者普通的 Bean的别名所以需要转换。
name可能是FactoryBean或者是beanName的别名，用当前这个方法来进行转换成真正的beanName来进行后续的操作
protected String transformedBeanName(String name) {
//BeanFactoryUtils.transformedBeanName(name)处理 FactoryBean 类型
return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}
//FactoryBean转换
public static String transformedBeanName(String name) {
//判空
Assert.notNull(name, "'name' must not be null");
//判断是否已 & 开头是否为FactoryBean
if (!name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
return name;
}
return transformedBeanNameCache.computeIfAbsent(name, beanName -> {
do {
beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
}
while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
return beanName;
});
}
//将别名处理成BeanName
public String canonicalName(String name) {
String canonicalName = name;
// Handle aliasing...
String resolvedName;
do {
resolvedName = this.aliasMap.get(canonicalName);
if (resolvedName != null) {
canonicalName = resolvedName;
}
}
while (resolvedName != null);
return canonicalName;
}
从上面可以看出来 transformedBeanName 方法主要是用来处理beanName。
2.2 getSingleton(beanName)
getSingleton(beanName) 主要从缓存中获取bean。
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//从缓存Map中获取
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
//从earlySingletonObjects获取bean
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;
}
2.3 AbstractBeanFactory#getObjectForBeanInstance方法
通过给定的bean实例获取对象，返回的bean为自己或者是FactoryBean创建的对象。
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
//判断name是否为Bean FactoryBean的引用
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
}
//现在beanInstance可能是普通的bena或者FactoryBean，如果是普通的Bean直接返回实例
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
//如果是FactoryBean，使用FactoryBean来创建一个bean实例
Object object = null;
if (mbd == null) {
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
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());
//这里从FactoryBean中获取创建的Bean
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
FactoryBean创建的Bean的名称FactoryBean本身作为一个Bean在Spring容器中是用是否包含
&前缀来区分的。
2.4 创建单例Bean DefaultSingletonBeanRegistry#getSingleton
在bean的创建过程，通过判断 scope 来判断创建的Bean的类型
if (mbd.isSingleton()) {
//创建单例
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
通过上面可以看出主要是通过调用 DefaultSingletonBeanRegistry#getSingleton 来创建Bean
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
//从缓存中获取Bean
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
//创建Bean
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
//从缓存中获取
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;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
上面的主要一行代码 singletonObject = singletonFactory.getObject(); 来获取，也就是代码中执行的 createBean(beanName, mbd, args)
sharedInstance = getSingleton(beanName, () -> {
try {
//这里才是真正的调用创建Bean
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
destroySingleton(beanName);
throw ex;
}
});
createbean 是 AbstractBeanFactory 类的一个抽象方法，实现看具体的子类。 抽象子类 AbstractAutowireCapableBeanFactory 对 createBean 方法进行了实现。
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
RootBeanDefinition mbdToUse = mbd;
//确定并加载bean的class
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// 准备需要的方法
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// 给InstantiationAwareBeanPostProcessor返回一个代理对象而不是真正的对象
// 这里主要处理的InstantiationAwareBeanPostProcessor，处理对象的实例化
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 {
//创建真正的bean
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
Object bean = resolveBeforeInstantiation(beanName, mbdToUse) 上面的这段代码是需要关注的，这个主要用来处理实现了InstantiationAwareBeanPostProcessor接口的类
在对于不是实现代理类似通过调用 doCreateBean 方法来创建对象的：
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// 实例化 bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// 下面开始处理 Bean实现的 BeanPostProcessor
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//处理实现了MergedBeanDefinitionPostProcessor接口的
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
//循环引用的处理
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// 初始化Bean
Object exposedObject = bean;
try {
//处理InstantiationAwareBeanPostProcessor的postProcessAfterInstantiation
populateBean(beanName, mbd, instanceWrapper);
//初始化Bean -- 包含处理 BeanPostProcessor的实现以及init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) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
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.");
}
}
}
}
// 处理实现了DisposableBean接口的bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
exposedObject = initializeBean(beanName, exposedObject, mbd)这个方法主要执行了我们实现的 BeanPostProcessor。BeanPostProcessor的Bean被单独保存对象的私有变量