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技术文章第一时间送达！
那什么是依赖注入呢?
那什么时候会触发依赖注入呢?
通过getBean()方法获取Bean对象。
给Bean配置了懒加载，ApplicationContext启动完成后调用getBean()来实例化对象。
好了，介绍完这些就开始我们的DI之旅。
1. BeanFactory
// 如果myJndiObject时FactoryBean， 则 &myJndiObject 将返回工厂而不是返回实例。
String FACTORY_BEAN_PREFIX = "&";
// 获取bean实例
Object getBean(String name) throws BeansException;
// 判断一个bean是否时单例
boolean isSingleton(String name) throws NoSuchBeanDefinitionException;
// 判断一个bean是否是原型
boolean isPrototype(String name) throws NoSuchBeanDefinitionException;
// 检查bean的name和type是否匹配
boolean isTypeMatch(String name, Class targetType) throws NoSuchBeanDefinitionException;
// 获取bean类型
Class> getType(String name) throws NoSuchBeanDefinitionException;
// 获取bean别名
String[] getAliases(String name);
getBean()方法有很多重载方法，上面只总结了一个。这个方法是DI的入口方法，接下来会从这个方法开始往下研究。
2. AbstractBeanFactory
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
doGetBean()方法也是该类中的方法。
// 依赖注入 从这里开始发生
private <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 根据指定名字获取被管理Bean的名称
// 如果是别名， 则转换为真正的bean名
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 先从缓存中取单例 bean
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
// 如果有，则直接返回该bean
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 bean 的实例对象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 如果不是单例对象， 而且 缓存中有原型模式bean， 就抛异常
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 检查 BeanDefinition 是否再当前的factory中， 如果不在则委托父类容器取查找
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
// 委托父类容器取找(名字+参数)
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// 委托父类容器取找(名称+类型)
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
// 标记 bean 被创建
markBeanAsCreated(beanName);
}
// 根据bean名称获取 父类的 beanDefinition， 合并继承公共属性
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 获取当前bean 所有依赖Bean 的集合
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
// 递归调用， 获取当前Bean的依赖Bean
getBean(dependsOnBean);
// 把依赖Bean注册给当前的Bean
registerDependentBean(dependsOnBean, beanName);
}
}
// Create bean instance.
// 创建bean 实例
if (mbd.isSingleton()) {
// 创建 bean 实例对象， 并且注册给所依赖的对象
sharedInstance = getSingleton(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
try {
// 创建一个指定bean 实例对象
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.
// 清除该单例
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);
}
// 如果创建的bean 不是单例也不是原型， 则根据声明周期选择实例化bean的方法
// 如 request session 等不同范围的实例
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
// 如果 scope 是空， 则抛异常
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
// 否则
try {
// 获取一个指定了scope的bean实例
Object scopedInstance = scope.get(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
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);
}
}
}
// Check if required type matches the type of the actual bean instance.
// 检查是否需要类型检测
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return (T) bean;
}
总结以下它都做了什么事情:
1、根据传来的 bean的 name(有可能是别名)来获取真正的 bean名称: beanName。
2、根据 beanName获取单例实例，如果有直接获取到 bean实例并返回，DI完成。
3、如果根据 beanName没有获得到单例实例：
3.7 检查是否需要类型检测
3.8 返回`3.1-3.7 `生成的实例。
然后我们再看看 createBean()方法的实现。
protected abstract Object createBean(String beanName, RootBeanDefinition mbd, Object[] args)
throws BeanCreationException;
3. AbstractAutowireCapableBeanFactory.java
// 创建bean 实例
@Override
protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
// Make sure bean class is actually resolved at this point.
// 解析和确定 bean 可以实例化
resolveBeanClass(mbd, beanName);
// Prepare method overrides.
// 准备方法覆盖
try {
mbd.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
// 给 Bean处理器 一个机会， 返回一个目标bean实例
Object bean = resolveBeforeInstantiation(beanName, mbd);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
Object beanInstance = doCreateBean(beanName, mbd, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
总结以下它都做了什么:
确定
beanName和RootBeanDefinition可以被实例化。执行方法覆盖
看
BeanPostProcessors能否再解析之前获取到bean，如果能则直接返回，否则下一步。调用
doCreateBean()方法，获取bean实例.
doCreateBean()方法也是该类中的。
// 真正创建bean实例
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// Instantiat|e the bean.
// 封装bean
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
// 如果是单例模式的bean，从容器中获取同名bean
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
// 如果没有同名bean， 则创建bean实例
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
// 如果有同名bean， 则获取到封装实例
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
// 获取实例化对象类型
Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
// Allow post-processors to modify the merged bean definition.
// 调用后置处理器
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
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, new ObjectFactory() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
// bean对象初始化， 依赖注入开始，exposedObject就是完成后的bean
Object exposedObject = bean;
try {
// 将bean 实例封装， 并且 bean 定义中配置的属性值赋值给实例对象
populateBean(beanName, mbd, instanceWrapper);
// 初始化 bean对象
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);
}
}
// 如果指定名称bean已经注册单例模式
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
// 如果两个对象相等， bean初始化完成
exposedObject = earlySingletonReference;
}
// 如果不相等， 则找出当前bean的依赖bean
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
// 检查依赖bean (是否继承接口，是否是父子关系。。)
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.");
}
}
}
}
// Register bean as disposable.
// 注册完成依赖注入的bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
同样，总结以下它干的事情：
根据
beanName获取beanWrapper对象。如果beanWrapper对象是空，则调用createBeanInstance()方法创建bean实例。否则，下一步通过
beanWrapper对象获取bean实例和class类型。允许
postProcessors调整组合BeanDefinition。如果
RootBeanDefinition是单例并且允许循环引用并且beanName正在进行单例创建，将beanName添加到单例工厂。调用
populateBean()方法给bean的属性值赋值，然后初始化bean对象并返回创建的bean实例，如果抛异常，则下一步。如果该
beanName对象已经注册单例模式，则从单例中获取，并判断获取到的bean实例(B)与BeanWrapper中的bean实例(A)是同一个实例，如果是，则返回A或者B，如果不是，则递归找出它的依赖bean。返回
1-6产生的bean实例。
我们首次获取bean实例的时候，bean工厂是肯定没有的，所以我们首次获取到的BeanWrapper应该是空对象，但是它调用了 createBeanInstance()方法后，可以看到spring是很确定它能拿到对象，那么我们看看这个方法的实现。它仍然是这个类中的方法。
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// Make sure bean class is actually resolved at this point.
// 确保bean可实例化(不能是抽象类等)
Class beanClass = resolveBeanClass(mbd, beanName);
// 如果这个bean 不是public 修饰符或者不被允许公共访问， 抛出异常
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());
}
if (mbd.getFactoryMethodName() != null) {
// 通过工厂方法实例化
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
// 是否有构造器
if (mbd.resolvedConstructorOrFactoryMethod != null && args == null) {
if (mbd.constructorArgumentsResolved) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
// Need to determine the constructor...
// 需要确认构造器
Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
// 自动装配，调用匹配的构造方法进行实例化
return autowireConstructor(beanName, mbd, ctors, args);
}
// No special handling: simply use no-arg constructor.
// 使用默认无参构造
return instantiateBean(beanName, mbd);
}
这个类用来创建 Bean实例，然后返回 BeanWrapper对象。注释写的很详细了。其中有个 instantiateBean()方法，当没有参数和构造方法的时候，就会调用该方法来实例化bean。
// 使用默认无参构造方法实例化bean
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
try {
Object beanInstance;
final BeanFactory parent = this;
// 获取JDK安全管理
if (System.getSecurityManager() != null) {
// 根据实例化策略实例化对象
beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
public Object run() {
return getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
}, getAccessControlContext());
}
else {
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
// 对实例化对象进行封装
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
这个方法是使用默认无参构造方法实例化bean的，它的核心代码是 getInstantiationStrategy().instantiate(mbd,beanName,parent);,因为它，我们可以得到一个bean实例对象，然后封装成 BeanWrapper并返回。
4. SimpleInstantiationStrategy.java
// 使用初始化策略 实例化bean
public Object instantiate(
RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
// 如果beanDefinition 中没有方法覆盖， 就用jdk，否则用cglib
if (beanDefinition.getMethodOverrides().isEmpty()) {
// 获取对象的构造方法和工厂方法
Constructor constructorToUse = (Constructor) beanDefinition.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
// 如果 没有构造方法和工厂方法， 使用JDK反射， 判断实例化的bean是不是接口
final Class clazz = beanDefinition.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
// 使用反射获取bean构造方法
constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor>() {
public Constructor run() throws Exception {
return clazz.getDeclaredConstructor((Class[]) null);
}
});
} else {
constructorToUse = clazz.getDeclaredConstructor((Class[]) null);
}
beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Exception ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
// 使用beanUtils实例化 构造方法.newInstance(arg) 来实例化
return BeanUtils.instantiateClass(constructorToUse);
}
else {
//如果 有覆盖或者重写， 则用CGLIB来实例化对象
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(beanDefinition, beanName, owner);
}
}
总结它的步骤：
如果BeanDefinition的覆盖方法不为空，则交给CGLIB来实例化对象，否则获取构造方法和工厂方法，下一步。
如果没有构造方法和工厂方法，则使用JDK反射，判断实例化的bean是不是接口，如果是，抛出异常，如果不是，则使用反射来获取bean的构造方法，最后，用
构造器.newInstance()的方法(BeanUtils.instantiateClass()方法底层实现)来实例化并返回。
那cglib是如何实例化呢，我们来看下 instantiateWithMethodInjection(beanDefinition,beanName,owner);方法源码：
@Override
protected Object instantiateWithMethodInjection(
RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {
// Must generate CGLIB subclass.
return new CglibSubclassCreator(beanDefinition, owner).instantiate(null, null);
}
然后再跟进 CglibSubclassCreator(beanDefinition,owner).instantiate(null,null);方法:
// 使用cglib 来进行bean实例化
public Object instantiate(Constructor ctor, Object[] args) {
// cglib
Enhancer enhancer = new Enhancer();
// bean本身作为基类
enhancer.setSuperclass(this.beanDefinition.getBeanClass());
enhancer.setCallbackFilter(new CallbackFilterImpl());
enhancer.setCallbacks(new Callback[] {
NoOp.INSTANCE,
new LookupOverrideMethodInterceptor(),
new ReplaceOverrideMethodInterceptor()
});
// 生成实例对象
return (ctor == null) ?
enhancer.create() :
enhancer.create(ctor.getParameterTypes(), args);
}
从上面代码可以看到，这就是CGLIB动态代理中创建代理的过程代码，不熟悉的可以往前翻 彻底搞懂动态代理章节的内容。
好了，到了这里，Spring就完成了bean实例的创建，但是此时就能拿着这个实例去使用吗，显然是不可以，因为属性还没有被赋入，下一章再继续介绍如何将属性依赖关系注入到Bean实例对象。
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