目录
这里主要是对于spring容器的理解和关于其源码的手撕笔记，源码学习是十分必要的，故此处为学习笔记，以便加深理解。
以下是学习的博客，放最前面贴出来：
一、核心概念：
1.1、IOC（DI）：
Spring容器来实现某些相互依赖对象的创建、协调的工作。对象只需要关注业务逻辑本身就可以了。即，对象如何获取其他对象的责任被反转了（控制反转），交给了spring容器处理。
PS：这里的IOC和DI是对于同一件事情的不同角度的描述，DI：控制的什么东西反转了；答：获取依赖对象的方式反转了（依赖注入）
1.2、IOC和DI的联系：
IOC（Inversion of Control，控制倒转），即是spring的核心，对于spring框架来说，就是由spring来负责控制对象的生命周期和对象间的关系的维护。
为了方便理解很多人都会举类似工厂和婚介所的概念模型出来，总得来说就是当对象A想要获取其他对象时候，就不需要亲力亲为了，只需要告诉spring容器他的需要就可以了，spring容器会自动处理产生并管理好一个A需要的对象给他调用，大大降低了其中的耦合。
IOC在系统的运行过程中，动态的向某个对象提供他所需要的其他对象，而这就是通过DI（Dependency Injection，依赖注入）来完成的。
这里引用大佬的一个例子便于理解。如对象A需要操作数据库，以前我们总是要在A中自己编写代码来获得一个Connection对象，有了 spring我们就只需要告诉spring，A中需要一个Connection，至于这个Connection怎么构造，何时构造，A不需要知道。在系统运行时，spring会在适当的时候制造一个Connection，然后像打针一样，注射到A当中，这样就完成了对各个对象之间关系的控制。A需要依赖 Connection才能正常运行，而这个Connection是由spring注入到A中的，依赖注入的名字就这么来的。
那么DI是如何实现的呢？ Java 1.3之后一个重要特征是反射（reflection），它允许程序在运行的时候动态的生成对象、执行对象的方法、改变对象的属性，spring就是通过反射来实现注入的。
二、源码解析
2.1、引入
IOC容器是spring最核心的模块之一，spring其他模块中，都需要用到IOC容器的功能。spring框架为我们提供了多种IOC容器：
DefaultableBeanFactory、FileSystemXmlApplicationContext、ClassPathXmlApplicationContext、XmlWebApplicationContext等
我们平时很少在项目中使用这种硬编码的方式来获取IOC容器和获取IOC容器中的bean，但是研究这些IOC容器的源码，对我们理解IOC容器的原理还是很有必要的。BeanFactory这个接口是spring所有IOC容器最上层的接口，getBean()这个方法就是在这个接口中定义的。
public interface BeanFactory {
Object getBean(String name) throws BeansException;
<T> T getBean(String name, Class<T> requiredType) throws BeansException;
Object getBean(String name, Object... args) throws BeansException;
<T> T getBean(Class<T> requiredType) throws BeansException;
<T> T getBean(Class<T> requiredType, Object... args) throws BeansException;
boolean containsBean(String name);
boolean isSingleton(String name) throws NoSuchBeanDefinitionException;
boolean isPrototype(String name) throws NoSuchBeanDefinitionException;
boolean isTypeMatch(String name, ResolvableType typeToMatch) throws NoSuchBeanDefinitionException;
boolean isTypeMatch(String name, Class<?> typeToMatch) throws NoSuchBeanDefinitionException;
Class<?> getType(String name) throws NoSuchBeanDefinitionException;
String[] getAliases(String name);
}
其中定义了获取bean的多种方式（getBean方法），和各种对bean的判断，以及获取bean的类型和别名的方法。BeanFactory这个接口是spring框架IOC容器的入口。下面以FileSystemXmlApplicationContext为例，深入源码探究IOC容器的实现原理。
2.2、深入分析
IOC容器的初始化过程分为三个阶段：1.定位、2.载入、3.注册。
2.2.1、XML的定位
我们经常会使用以下代码获取IOC容器，从而得到想要的bean：
FileSystemXmlApplicationContext context = new FileSystemXmlApplicationContext("bean.xml");
进入FileSystemXmlApplicationContext这个类，发现它定义了各种构造器，但最终都会调用下面这个构造器：
public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh(); //IOC容器的启动方法
}
}
FileSystemXmlApplicationContext的构造器中有个重要的方法refresh()，这是IOC容器的启动方法，在它的父类AbstractXmlApplicationContext中有实现，其代码如下：
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
//准备要进行刷新的上下文对象
//例如对系统环境进行准备和验证
prepareRefresh();
// Tell the subclass(子类) to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
进入obtainFreshBeanFactory()方法，作用大概告诉子类刷新内部的 bean factory，其代码如下：
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (logger.isDebugEnabled()) {
logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
}
return beanFactory;
}
继续跟，进入refreshBeanFactory()方法，在父类AbstractRefreshableApplicationContext中有实现，其代码如下：
@Override
protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
//创建DefaultListableBeanFactory的IOC容器
DefaultListableBeanFactory beanFactory = createBeanFactory();
//指定序列化的id，所以，如果需要反序列化这个BeanFactory,则可以直接根据这个id来进行反序列化
beanFactory.setSerializationId(getId());
//定制化
customizeBeanFactory(beanFactory);
//初始化DocumentReader,读取XML（调用方法对XML进行定位和加载）
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
这段代码可以看到：
1、首先，创建了一个DefaultListableBeanFactory的IOC容器；
2、对容器进行了一些设置；
3、调用loadBeanDefinitions()方法对XML文件进行定位和加载。
所以，进入loadBeanDefinitions()方法继续探索，在类AbstractXmlApplicationContext中有实现，它是FileSystemXmlApplicationContext的父类，其代码如下：
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// Create a new XmlBeanDefinitionReader for the given BeanFactory.
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// Configure the bean definition reader with this context's
// resource loading environment.
beanDefinitionReader.setEnvironment(this.getEnvironment());
beanDefinitionReader.setResourceLoader(this);
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
// Allow a subclass to provide custom initialization of the reader,
// then proceed with actually loading the bean definitions.
initBeanDefinitionReader(beanDefinitionReader);
loadBeanDefinitions(beanDefinitionReader);
}
这个方法中，使用XmlBeanDefinitionReader类来加载XML文件，最后经过一系列的设置，调用了loadBeanDefinitions(beanDefinitionReader)这个方法，进入：
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
Resource[] configResources = getConfigResources();
if (configResources != null) {
reader.loadBeanDefinitions(configResources);
}
String[] configLocations = getConfigLocations();
if (configLocations != null) {
reader.loadBeanDefinitions(configLocations);
}
}
跟到这里，到底是走哪个方法呢？我们再回过头看一下，FileSystemXmlApplicationContext的那个构造器，其中有个setConfigLocations(configLocations)方法，通过这个方法将我们配置的XML文件的路径设置进来了，跟代码，发现它调用的是父类的方法，并将路径赋给了AbstractRefreshableConfigApplicationContext类中的configLocations成员变量，而getConfigLocations()方法也是AbstractRefreshableConfigApplicationContext类中的，它正好获取了configLocations的值，所以configLocations一定不为null，上面方法应该走下面的loadBeanDefinitions()方法。跟进，其代码如下：
public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
Assert.notNull(locations, "Location array must not be null");
int counter = 0;
for (String location : locations) {
counter += loadBeanDefinitions(location);
}
return counter;
}
public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
return loadBeanDefinitions(location, null);
}
public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {
ResourceLoader resourceLoader = getResourceLoader();
if (resourceLoader == null) {
throw new BeanDefinitionStoreException(
"Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
}
if (resourceLoader instanceof ResourcePatternResolver) {
// Resource pattern matching available.
try {
Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
int loadCount = loadBeanDefinitions(resources);
if (actualResources != null) {
for (Resource resource : resources) {
actualResources.add(resource);
}
}
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
}
return loadCount;
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"Could not resolve bean definition resource pattern [" + location + "]", ex);
}
}
else {
// Can only load single resources by absolute URL.
Resource resource = resourceLoader.getResource(location);
int loadCount = loadBeanDefinitions(resource);
if (actualResources != null) {
actualResources.add(resource);
}
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
}
return loadCount;
}
}
这里先得到一个ResourceLoader对象。在类AbstractXmlApplicationContext中的loadBeanDefinitions()方法中有beanDefinitionReader.setResourceLoader(this)这段代码，而DefaultListableBeanFactory又是继承了DefaultResourceLoader的，所以，这里的resourceLoader对象是DefaultResourceLoader类型的，所以走了最下面的逻辑。首先，获取一个resource 对象，getResource方法在DefaultResourceLoader中有实现，其代码如下：
public Resource getResource(String location) {
Assert.notNull(location, "Location must not be null");
for (ProtocolResolver protocolResolver : this.protocolResolvers) {
Resource resource = protocolResolver.resolve(location, this);
if (resource != null) {
return resource;
}
}
if (location.startsWith("/")) {
return getResourceByPath(location);
}
else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
}
else {
try {
// Try to parse the location as a URL...
URL url = new URL(location);
return new UrlResource(url);
}
catch (MalformedURLException ex) {
// No URL -> resolve as resource path.
//如果都不是，则使用子类重写的方法，例如子类FileSystemXMLApplicationContext中就重写了这个方法
return getResourceByPath(location);
}
}
}
根据不同的情况，生成一个ResourceLoader对象，这样就完成了对配置的xml文件的定位。
经过这么一长条的跟踪，终于完成了XML资源的定位工作。在上面的分析中，spring使用了很多的模板方法，比如getResource方法，还有就是单一职责原则，每个类很清晰，每个方法中都是一个一个方法的调用，而不是代码的堆砌。
代码部分全部是引用的，把我绕晕了，不过多少有点概念了
2.2.2、加载和注册
开始之前，首先我们先来了解一下IOC容器所使用的数据结构-------BeanDefinition，它是一个上层接口，有很多实现类，分别对应不同的数据载体。我们平时开发的时候，也会定义很多pojo类，来作为获取数据的载体。最常见的就是，从数据库中获取数据之后，使用一个定义的pojo来装载，然后我们就可以在程序中使用这个pojo类来编写各种业务逻辑。同样，IOC容器首先会读取配置的XML中各个节点，即各个标签元素，然后根据不同的标签元素，使用不同的数据结构来装载该元素中的各种属性的值。比如我们最熟悉的
先回到资源的定位部分，代码如下：
public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {
ResourceLoader resourceLoader = getResourceLoader();
if (resourceLoader == null) {
throw new BeanDefinitionStoreException(
"Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
}
if (resourceLoader instanceof ResourcePatternResolver) {
// Resource pattern matching available.
try {
Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
int loadCount = loadBeanDefinitions(resources);
if (actualResources != null) {
for (Resource resource : resources) {
actualResources.add(resource);
}
}
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
}
return loadCount;
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"Could not resolve bean definition resource pattern [" + location + "]", ex);
}
}
else {
// 定位到资源之后，封装成一个resource对象
Resource resource = resourceLoader.getResource(location);
int loadCount = loadBeanDefinitions(resource);
if (actualResources != null) {
actualResources.add(resource);
}
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
}
return loadCount;
}
}
进入loadBeanDefinitions(resource)方法，正式开始加载源码的跟踪：
@Override
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
return loadBeanDefinitions(new EncodedResource(resource));
}
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (logger.isInfoEnabled()) {
logger.info("Loading XML bean definitions from " + encodedResource);
}
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet<EncodedResource>(4);
this.resourcesCurrentlyBeingLoaded.set(currentResources);
}
if (!currentResources.add(encodedResource)) {
throw new BeanDefinitionStoreException(
"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
}
try {
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
}
finally {
inputStream.close();
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"IOException parsing XML document from " + encodedResource.getResource(), ex);
}
finally {
currentResources.remove(encodedResource);
if (currentResources.isEmpty()) {
this.resourcesCurrentlyBeingLoaded.remove();
}
}
}
进入doLoadBeanDefinitions(inputSource, encodedResource.getResource())方法：
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
Document doc = doLoadDocument(inputSource, resource);
return registerBeanDefinitions(doc, resource);
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (SAXParseException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
}
catch (SAXException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"XML document from " + resource + " is invalid", ex);
}
catch (ParserConfigurationException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Parser configuration exception parsing XML from " + resource, ex);
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"IOException parsing XML document from " + resource, ex);
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Unexpected exception parsing XML document from " + resource, ex);
}
}
继续进入registerBeanDefinitions(doc, resource)方法：
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
//此时documentReader已经是DefaultBeanDefinitionDocumentReader类了
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
int countBefore = getRegistry().getBeanDefinitionCount();
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
//返回当前注册的beanDefinition的个数
return getRegistry().getBeanDefinitionCount() - countBefore;
}
进入registerBeanDefinitions(doc, createReaderContext(resource))方法：
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
logger.debug("Loading bean definitions");
Element root = doc.getDocumentElement();
doRegisterBeanDefinitions(root);
}
进入doRegisterBeanDefinitions(root)方法：
protected void doRegisterBeanDefinitions(Element root) {
// Any nested <beans> elements will cause recursion in this method. In
// order to propagate and preserve <beans> default-* attributes correctly,
// keep track of the current (parent) delegate, which may be null. Create
// the new (child) delegate with a reference to the parent for fallback purposes,
// then ultimately reset this.delegate back to its original (parent) reference.
// this behavior emulates a stack of delegates without actually necessitating one.
BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createDelegate(getReaderContext(), root, parent);
if (this.delegate.isDefaultNamespace(root)) {
//profile属性平时使用非常少，该属性可以用于配置数据库的切换（常用），使用时，需要在web.xml中配置context-parm
//<context-parm>
// <parm-name>Spring.profiles.active</parm-name>
// <parm-value>dev(在applicationContext.xml中配置的profile属性的beans的profile属性值)</parm-name>
//</context-parm>
//在applicationContext.xml中的配置
//<beans profile="dev"> </beans>
//<beans profile="produce"> </beans>
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
if (logger.isInfoEnabled()) {
logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
"] not matching: " + getReaderContext().getResource());
}
return;
}
}
}
preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;
}
这里也用到了模板方法，preProcessXml(root)和postProcessXml(root)这两个方法都是空实现，是留给客户来实现自己的逻辑的。重点研究一下parseBeanDefinitions(root, this.delegate)方法：
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
if (delegate.isDefaultNamespace(root)) {
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (node instanceof Element) {
Element ele = (Element) node;
if (delegate.isDefaultNamespace(ele)) {
parseDefaultElement(ele, delegate);
}
else {
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}
parseCustomElement(root)方法不需要怎么研究，我们平时几乎不会用到自定义的标签，所以只跟踪parseDefaultElement(ele, delegate)里面的代码：
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
//import标签
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
//alias标签
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
//bean标签
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
//beans标签
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}
可以看到，对于不同的标签，spring采用不同的策略进行处理，重点跟踪一下处理bean标签的方法processBeanDefinition(ele, delegate)：
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
//委托给delegate去进行各种标签的解析，parseBeanDefinitionElement方法中包含了各种标签元素的解析，
//并将解析好的内容封装成BeanDefinitionHolder对象
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// Send registration event.
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}
在这个方法中，delegate.parseBeanDefinitionElement(ele)是解析bean元素中各种属性的方法，registerBeanDefinition(bdHolder, getReaderContext().getRegistry())是将封装好的数据进行存储的方法。先看一下解析的方法：
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
//获取bean标签的id属性的值
String id = ele.getAttribute(ID_ATTRIBUTE);
//获取bean标签上name属性的值
String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
List<String> aliases = new ArrayList<String>();
if (StringUtils.hasLength(nameAttr)) {
//将name的值进行分割，并将它们当作别名存到aliases中
String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
aliases.addAll(Arrays.asList(nameArr));
}
String beanName = id;
//如果bean标签的id没有值，但是name属性有值，则将name属性的第一个值当作id的值，并从aliases中将第一个别名移除掉
if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
beanName = aliases.remove(0);
if (logger.isDebugEnabled()) {
logger.debug("No XML 'id' specified - using '" + beanName +
"' as bean name and " + aliases + " as aliases");
}
}
if (containingBean == null) {
//检查bean的唯一性
checkNameUniqueness(beanName, aliases, ele);
}
//这里已经是将XML中bean元素中的所有属性都封装到beanDefinition对象中了
AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
if (beanDefinition != null) {
if (!StringUtils.hasText(beanName)) {
try {
if (containingBean != null) {
beanName = BeanDefinitionReaderUtils.generateBeanName(
beanDefinition, this.readerContext.getRegistry(), true);
}
else {
beanName = this.readerContext.generateBeanName(beanDefinition);
// Register an alias for the plain bean class name, if still possible,
// if the generator returned the class name plus a suffix.
// This is expected for Spring 1.2/2.0 backwards compatibility.
String beanClassName = beanDefinition.getBeanClassName();
if (beanClassName != null &&
beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
!this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
aliases.add(beanClassName);
}
}
if (logger.isDebugEnabled()) {
logger.debug("Neither XML 'id' nor 'name' specified - " +
"using generated bean name [" + beanName + "]");
}
}
catch (Exception ex) {
error(ex.getMessage(), ele);
return null;
}
}
String[] aliasesArray = StringUtils.toStringArray(aliases);
//最后将封装好的beanDefinition、它的id、以及它的别名一起封装成BeanDefinitionHolder对象返回
return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
}
return null;
}
我们可以得到如下信息：
- 获取bean标签的id属性和name属性的值
- name属性是可以有多个值的，以逗号或者分号分割
- 如果id没有赋值，则取name的第一个值作为id的值。所以，我们一般都会给id赋值，这样效率高一些
- 检查以这个id标识的bean是不是唯一的
- 进行其他属性的解析，并最终封装成AbstractBeanDefinition对象，也就是我们前文中提到的数据结构
- 最后封装成BeanDefinitionHolder对象之后返回。
PS: 进入parseBeanDefinitionElement(ele, beanName, containingBean)方法，看一下其他元素的解析过程：
public AbstractBeanDefinition parseBeanDefinitionElement(
Element ele, String beanName, BeanDefinition containingBean) {
this.parseState.push(new BeanEntry(beanName));
String className = null;
if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
}
try {
String parent = null;
if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
parent = ele.getAttribute(PARENT_ATTRIBUTE);
}
AbstractBeanDefinition bd = createBeanDefinition(className, parent);
parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
parseMetaElements(ele, bd);
parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
parseConstructorArgElements(ele, bd);
parsePropertyElements(ele, bd);
parseQualifierElements(ele, bd);
bd.setResource(this.readerContext.getResource());
bd.setSource(extractSource(ele));
return bd;
}
catch (ClassNotFoundException ex) {
error("Bean class [" + className + "] not found", ele, ex);
}
catch (NoClassDefFoundError err) {
error("Class that bean class [" + className + "] depends on not found", ele, err);
}
catch (Throwable ex) {
error("Unexpected failure during bean definition parsing", ele, ex);
}
finally {
this.parseState.pop();
}
return null;
}
回到主线上，解析封装成BeanDefinitionHolder对象之后，其实就可以进行注册了，这里先回到之前的processBeanDefinition(ele, delegate)：
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
//委托给delegate去进行各种标签的解析，parseBeanDefinitionElement方法中包含了各种标签元素的解析，
//并将解析好的内容封装成BeanDefinitionHolder对象
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// Send registration event.
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}
现在进入BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry())方法进行分析：
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
// Register aliases for bean name, if any.
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String alias : aliases) {
registry.registerAlias(beanName, alias);
}
}
}
这里的beanName就是之前封装好的bean的id。这个方法中分别以id和别名作为key来注册bean，其实就是存储在map中。
进入registerBeanDefinition(beanName, definitionHolder.getBeanDefinition())，在其子类DefaultListableBeanFactory中有实现：
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
if (existingDefinition != null) {
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + existingDefinition + "] bound.");
}
else if (existingDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (logger.isWarnEnabled()) {
logger.warn("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
existingDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(existingDefinition)) {
if (logger.isInfoEnabled()) {
logger.info("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
this.beanDefinitionMap.put(beanName, beanDefinition);
}
else {
if (hasBeanCreationStarted()) {
// Cannot modify startup-time collection elements anymore (for stable iteration)
synchronized (this.beanDefinitionMap) {
this.beanDefinitionMap.put(beanName, beanDefinition);
List<String> updatedDefinitions = new ArrayList<String>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set<String> updatedSingletons = new LinkedHashSet<String>(this.manualSingletonNames);
updatedSingletons.remove(beanName);
this.manualSingletonNames = updatedSingletons;
}
}
}
else {
// Still in startup registration phase
this.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
this.manualSingletonNames.remove(beanName);
}
this.frozenBeanDefinitionNames = null;
}
if (existingDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
我们可以看到：这个beanDefinitionMap就是用来存储解析好的bean的，以id作为key。至此，就将所有的bean标签解析好之后封装成BeanDefinition注册到了IOC容器中。但是，到目前为止，IOC容器并没有为我们将这些解析好的数据生成一个一个bean实例，我们仍然不能就这样直接使用。
到这里跟踪了IOC容器对bean标签进行解析之后存入Map中的过程，但是这些bean只是以BeanDefinition为载体单纯的存储起来了，并没有转换成一个个的对象
2.2.3、依赖注入
下面继续进行跟踪，看一看IOC容器是怎样实例化对象的。我们都使用过以下代码 (xml配置的方式)：
FileSystemXmlApplicationContext context=new FileSystemXmlApplicationContext("bean.xml");
User user=context.getBean("user",User.class);
这样我们就能获取到user对象了，所以，不难想象，这个getBean方法就是实例化对象的入口。接下来我们就以这个方法为切入点，来探究IOC容器中bean的实例化过程。getBean方法是在FileSystemXmlApplicationContext的基类AbstractApplicationContext中定义的，代码如下:
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
assertBeanFactoryActive();
return getBeanFactory().getBean(name, requiredType);
}
里面有很多重载方法，里面有调用了某个beanFactory的getBean方法。AbstractApplicationContext中并没有定义getBeanFactory这个方法，那一定是在FileSystemXmlApplicationtext的某个父类中定义的，我们再回过头看一下它的UML图：
经过查找之后，是在AbstractRefreshableApplicationContext中定义的，且这个beanFactory是DefaultListableBeanFactory类型的：
public abstract class AbstractRefreshableApplicationContext extends AbstractApplicationContext {
private Boolean allowBeanDefinitionOverriding;
private Boolean allowCircularReferences;
/** Bean factory for this context */
private DefaultListableBeanFactory beanFactory;
/** Synchronization monitor for the internal BeanFactory */
private final Object beanFactoryMonitor = new Object();
…………
直接进入DefaultListableBeanFactory中，查看它的getBean方法：
public <T> T getBean(Class<T> requiredType) throws BeansException {
return getBean(requiredType, (Object[]) null);
}
public <T> T getBean(Class<T> requiredType, Object... args) throws BeansException {
NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args);
if (namedBean != null) {
return namedBean.getBeanInstance();
}
BeanFactory parent = getParentBeanFactory();
if (parent != null) {
return parent.getBean(requiredType, args);
}
throw new NoSuchBeanDefinitionException(requiredType);
}
发现里面只有这两个重载方法，其他getBean方法，包括例子中使用的那个重载方法没有看到，我们看一下DefaultListableBeanFactory类的UML图：
发现在父类AbstractBeanFactory中定义了其他的getBean方法，如下：
public Object getBean(String name, Object... args) throws BeansException {
return doGetBean(name, null, args, false);
}
public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
return doGetBean(name, requiredType, args, false);
}
里面都调用了doGetBean方法，那么进入继续跟踪：
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
//将bean的name进行转换，比如将name前面的“&”符去掉，带“&”的name，获取的是beanFactory本身，而不是
//beanFactory生成出来的bean实例
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
//看缓存中是否已经有该bean
Object sharedInstance = getSingleton(beanName);
//如果缓存中有
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 + "'");
}
}
//如果sharedInstance是FactoryBean类型，则返回它生产的对象，否则，返回它本身
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
//如果缓存中没有，第一次创建的时候
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
//获取父容器
BeanFactory parentBeanFactory = getParentBeanFactory();
//如果父容器存在，且在当前容器中没有找到该名称的bean的数据
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
//则到父容器中进行处理
// Not found -> check parent.
//将name前加上“&”
String nameToLookup = originalBeanName(name);
//分别对是否有参数的情况进行处理
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);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
//将数据封装成RootBeanDefinition对象
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
//检查是否为抽象类
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//获取当前创建的bean的依赖的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 + "'");
}
//为当前bean和它的依赖对象建立映射关系
registerDependentBean(dep, beanName);
try {
//递归调用getBean方法，创建依赖对象，直到没有依赖对象为止
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
//回调方法
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, new ObjectFactory<Object>() {
@Override
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);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
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;
}
可以看到对不同scope域的bean的创建过程，其中会进行递归创建，现在进入createBean方法中，其实现是在AbstractAutowireCapableBeanFactory类中，代码如下：
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("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.
//判断需要创建的bean是否可以实例化，是否可以通过类装载其进行装载
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
//这里是对overrides属性和look-up属性的处理
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
//AOP就是这里操作的，如果配置了postProcessor，则生成一个proxy返回，即代理类
// 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);
}
//创建bean的方法调用
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
看见曙光了，进入doCreateBean方法：
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
//如果是单例，先把缓存中的同名Bean清除
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
//如果缓存中没有
if (instanceWrapper == null) {
//则创建一个实例
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
mbd.resolvedTargetType = beanType;
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//使用后置处理器进行处理
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
//这里是对单例的循环引用的处理
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<Object>() {
@Override
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
//这里是对bean的初始化，依赖注入往往是在这里进行的，这个exposedObject在初始化完成之后会作为依赖注入完成之后的Bean
// Initialize the bean instance.
Object exposedObject = bean;
try {
//属性的填充
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
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<String>(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.");
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
PS：这里要说一下，当在创建bean时，IOC会将该bean的名字存一份到singletonsCurrentlyInCreation这个map中，然后每次创建的时候都会到里面进行检查当前bean是否正在被创建。为了避免发生循环引用（A依赖B，B依赖C，C依赖A）引起是循环，在第一次创建bean时，IOC容器会把用于创建这个bean的工厂对象放入singletonFactories这个map中，key是这个正在被创建的bean的名字。这样发生循环依赖的时候，就不再调用getBean方法了，而是直接使用工厂创建一个bean给被依赖的对象。比如第一次创建A时，将A的名称存入了singletonsCurrentlyInCreation这个map中，并且调用addSingletonFactory方法，将创建A的工厂放到singletonFactories中了，然后递归调用getBean创建依赖对象B、C，创建C时，要先创建它的依赖对象A，此时，IOC容器检查到singletonsCurrentlyInCreation中已经有这个A了，说明它已经在创建的过程中，只是还没有完成创建，此时，IOC容器直接就使用这个工厂将A创建出来赋给C了，然后再往回完成B和A的创建。
这里可以再看一下addSingletonFactory方法的实现：
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//从缓存中获取该bean的实例，已经填充了属性值的实例
Object singletonObject = this.singletonObjects.get(beanName);
//创建bean时，IOC会在this.singletonsCurrentlyInCreation中存一个该bean的名称，表示正在创建这个bean
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
//也是从缓存中获取实例，但是这个缓存中的实例是没有经过填充的实例
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
//获取生成该bean的beanFactory
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//获取这个bean的实例
singletonObject = singletonFactory.getObject();
//将这个还未经填充属性的bean存入新生代缓存中（自己取的名字，类似于JVM）
this.earlySingletonObjects.put(beanName, singletonObject);
//将这个生产bean的工厂移除
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
在进行递归调用getBean方法创建依赖对象之前，getSignal方法是先调用前面的代码，在doGetBean方法中可以看到，可以多看几遍就能理解。
另外，对于原型实例，不允许循环引用。循环引用只针对单例。
2.2.4、bean属性的填充
先回到doCreateBean方法，代码如下：
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
//如果是单例，先把缓存中的同名Bean清除
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
//如果缓存中没有
if (instanceWrapper == null) {
//则创建一个实例
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
mbd.resolvedTargetType = beanType;
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//使用后置处理器进行处理
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
//这里是对单例的循环引用的处理，单例&&允许循环依赖&&正在被创建同时满足，才为true
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<Object>() {
@Override
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
//这里是对bean的初始化，依赖注入往往是在这里进行的，这个exposedObject在初始化完成之后会作为依赖注入完成之后的Bean
// Initialize the bean instance.
Object exposedObject = bean;
try {
//属性的填充
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
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<String>(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.");
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
对于bean的创建，IOC容器其实是采用反射和cglib技术来生成的，我们可以跟踪一下createBeanInstance方法：
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, 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());
}
//使用工厂方法对Bean进行实例化
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) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
//使用构造函数对Bean实例化
// Need to determine the constructor...
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
//使用默认的构造函数对Bean进行实例化
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
- 如果指定了工厂方法，则使用工厂进行创建；
- 如果没有指定工厂方法，则看有没指定某个构造器进行实例化；
- 都没有，则采用默认的构造器进行实例化。进入默认的构造器方法：
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
try {
Object beanInstance;
final BeanFactory parent = this;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
return getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
}, getAccessControlContext());
}
else {
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
虽然getInstantiationStrategy()方法获取的是CglibSubclassingInstantiationStrategy实例，但是CglibSubclassingInstantiationStrategy中只有一个instantiate带一个参数和一个可变参数的方法，和这里调用的并不是同一个方法，所以，这里调用的是它的父类SimpleInstantiationStrategy中的instantiate方法，进入：
public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (bd.getMethodOverrides().isEmpty()) {
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
//这里取得指定的构造器或者生产对象的工厂方法来对Bean进行实例化
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
final Class<?> clazz = bd.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() {
@Override
public Constructor<?> run() throws Exception {
return clazz.getDeclaredConstructor((Class[]) null);
}
});
}
else {
constructorToUse = clazz.getDeclaredConstructor((Class[]) null);
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
//使用反射进行实例化
return BeanUtils.instantiateClass(constructorToUse);
}
else {
//使用cglb进行实例化
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
这段代码可以看到，IOC是使用反射和cglib来进行实例化对象的。好了，我们回到之前的方法，到populateBean(beanName, mbd, instanceWrapper)方法进行跟踪：
protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
//在前面解析的过程中，XML中设置的属性已经填充到beanDefinition 的propertyValues属性中
PropertyValues pvs = mbd.getPropertyValues();
if (bw == null) {
if (!pvs.isEmpty()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
boolean continueWithPropertyPopulation = true;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
//开始进行依赖注入的操作
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
//按属性的名字来进行注入
// Add property values based on autowire by name if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
//按属性的类型来进行注入
// Add property values based on autowire by type if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
if (hasInstAwareBpps || needsDepCheck) {
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
//对属性进行注入
applyPropertyValues(beanName, mbd, bw, pvs);
}
可以看到：
1. 我们可以在XML中指定autowired的值，是按名称还是类型来进行注入；
2. 真正进行属性注入的是applyPropertyValues方法，进入：
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs == null || pvs.isEmpty()) {
return;
}
if (System.getSecurityManager() != null && bw instanceof BeanWrapperImpl) {
((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
}
MutablePropertyValues mpvs = null;
List<PropertyValue> original;
if (pvs instanceof MutablePropertyValues) {
mpvs = (MutablePropertyValues) pvs;
if (mpvs.isConverted()) {
// Shortcut: use the pre-converted values as-is.
try {
bw.setPropertyValues(mpvs);
return;
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
original = mpvs.getPropertyValueList();
}
else {
original = Arrays.asList(pvs.getPropertyValues());
}
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
//获取解析器
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
// Create a deep copy, resolving any references for values.
List<PropertyValue> deepCopy = new ArrayList<PropertyValue>(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
//如果已经转换过，就直接存入
if (pv.isConverted()) {
deepCopy.add(pv);
}
else {
//获取属性名称
String propertyName = pv.getName();
//获取属性值
Object originalValue = pv.getValue();
//将值进行转换，比如属性的值有可能是数组、集合，或者对象，并且该对象也需要进行属性的注入，
//那么还会进行递归调用getBean方法，将该对象生成好之后再注入给当前的属性
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}
// Possibly store converted value in merged bean definition,
// in order to avoid re-conversion for every created bean instance.
if (resolvedValue == originalValue) {
if (convertible) {
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
else if (convertible && originalValue instanceof TypedStringValue &&
!((TypedStringValue) originalValue).isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
if (mpvs != null && !resolveNecessary) {
mpvs.setConverted();
}
//设置依赖注入的地方
// Set our (possibly massaged) deep copy.
try {
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
至此，一个完整的bean就算创建完成了，要用的时候，直接找IOC容器拿就行了。
总结
源码分析的我一头雾水，后面要经常复习...这里主要跟踪的是xml配置文件方式创建bean的过程，后面在去看一下自己用到最多的注解方式的创建过程
后面的分析源码是抄来的笔记，在文章头部放了，估计也很少有人会看到这里吧...真的花了一天的时间去理解
在最后放一篇宝藏，spring mvc的运行流程： 点击前往