Spring是一个非常优秀的开源框架，对于Java开发人员来说，很多时候我们的开发工作都是围绕着它来进行的。因此我们不应该仅仅局限于会使用它，更应该去学习他的工作原理，这样当遇到问题时可以更好的定位与解决。
今天的文章主要结合源码来讲解SpringIOC的原理，也是自己这段时间的学习总结，分享出来，也希望能够对大家有那么些许的帮助。
使用的Spring源码版本为5.2.2.RELEASE，大家可自行去github下载。
1 BeanFactory和ApplicationContext
在Spring的IOC容器中主要有BeanFactory和ApplicationContext两条路径，这两者的区别如下：
- BeanFactory是IOC容器的顶层接口，其定义了一个容器需要满足的最基本的功能。
- ApplicationContext继承自BeanFactory，对容器进行了扩展（资源发现、事件支持、本地化），是高级的容器。
BeanFactory的类图如下：
ApplicationContext的类图如下：
通过上面的类图我们能够发现，BeanFactory路径下的容器实现类为DefaultListableBeanFactory。ApplicationContext路径的实现有如下：
- ClassPathXmlApplicationContext 从classPath下加载配置
- FileSystemXmlApplicationContext 从文件系统加载配置
- XmlWebApplicationContext 加载配置至web容器
2 使用ClassPathXmlApplicationContext
我们以ClassPathXmlApplicationContext类的源码查看Spring IOC的初始化过程，在看源码之前我们先写一个demo，目录结构如下：
代码逻辑很简单，在application.xml中配置了一个User对象，然后创建ClassPathXmlApplicationContext并从中获取该对象，application.xml的内容如下：
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:tx="http://www.springframework.org/schema/tx"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/tx
http://www.springframework.org/schema/tx/spring-tx.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-3.0.xsd"
default-autowire="byName" default-lazy-init="false">
<bean id="user" class="com.xh.spring.User">
<property name="age" value="18"/>
<property name="name" value="张三"/>
</bean>
</beans>
使用debug查看，我们可以看见创建后的容器对象如下：
通过上面的截图我们可以看到ClassPathXmlApplicationContext中的BeanFactory是一个DefaultListableBeanFactory对象，在该容器对象中有一个为beanDefinitionMap的ConcurrentHashMap实例，这个Map就是用来存储对象信息的，在BeanDefinition中存储了bean的描述，如下：
通过上面的例子我们能够看到我们指定一个xml的路径，即可在在创建好的容器容器中找到对该配置文件中配置的对象的描述信息，这便是Spring IOC需要完成的工作，概括如下：
- 资源的定位
- 将资源中的数据解析为一个个的BeanDefinition对象
- 将这些BeanDefinition对象注册到一个Map中
3 源码解析
了解了Spring IOC容器初始化所完成的事情后，我们正式开始查看源码，通过源码来详细的了解这三个过程都是怎样实现的。
创建容器时，最终调用的ClassPathXmlApplicationContext构造方法如下所示，容器的初始化逻辑在调用的refresh()方法中实现
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
// 设置资源路径 用于之后的加载BeanDefinitions
setConfigLocations(configLocations);
if (refresh) {
// IOC容器初始化逻辑
// 包括对Resource定位、载入和注册的过程
refresh();
}
}
refresh()方法的定义在AbstractApplicationContext类中，其源码如下：
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 容器初始化的准备工作，设置一些初始化数据 设置active标记
prepareRefresh();
// 创建IOC容器
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();
}
}
}
IOC容器的初始化逻辑是在obtainFreshBeanFactory()方法中定义的，我们一步步点击进去，会进入AbstractRefreshableApplicationContext类中的refreshBeanFactory()方法，该方法的处理逻辑如下：
@Override
protected final void refreshBeanFactory() throws BeansException {
// 关闭存在的容器
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
// 创建容器对象，使用的是DefaultListableBeanFactory
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
// 加载BeanDefinitions
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
上面代码的主要逻辑在loadBeanDefinitions(beanFactory)中，点进去会发现这个方法是调用的AbstractXmlApplicationContext类中的方法，源码如下
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// 创建一个XmlBeanDefinitionReader对象
// 用来解析xml文件生成的resource对象
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
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);
}
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
Resource[] configResources = getConfigResources();
if (configResources != null) {
reader.loadBeanDefinitions(configResources);
}
String[] configLocations = getConfigLocations();
if (configLocations != null) {
// 在我们的例子中最终会走该方法
reader.loadBeanDefinitions(configLocations);
}
}
不知大家是否还记得在我们创建ClassPathXmlApplicationContext对象的逻辑中，会调用一个setConfigLocations的方法。在上面的方法中会使用这个值进行资源的定位加载，我们点进去上面第二个if中调用的loadBeanDefinitions方法，会发现这是AbstractBeanDefinitionReader中的一个方法，其源码如下：
@Override
public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
Assert.notNull(locations, "Location array must not be null");
int count = 0;
for (String location : locations) {
count += loadBeanDefinitions(location);
}
return count;
}
@Override
public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
return loadBeanDefinitions(location, null);
}
public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
ResourceLoader resourceLoader = getResourceLoader();
if (resourceLoader == null) {
throw new BeanDefinitionStoreException(
"Cannot load bean definitions from location [" + location + "]: no ResourceLoader available");
}
if (resourceLoader instanceof ResourcePatternResolver) {
// Resource pattern matching available.
try {
Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
int count = loadBeanDefinitions(resources);
if (actualResources != null) {
Collections.addAll(actualResources, resources);
}
if (logger.isTraceEnabled()) {
logger.trace("Loaded " + count + " bean definitions from location pattern [" + location + "]");
}
return count;
}
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 count = loadBeanDefinitions(resource);
if (actualResources != null) {
actualResources.add(resource);
}
if (logger.isTraceEnabled()) {
logger.trace("Loaded " + count + " bean definitions from location [" + location + "]");
}
return count;
}
}
上面这段代码的逻辑是通过路径定位资源，并将资源转换为Resource对象，之后会调用BeanDefinitionReader中的loadBeanDefinitions(Resource resource)方法，我们使用的是XmlBeanDefinitionReader对象，继续点进去查看，其源码如下：
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (logger.isTraceEnabled()) {
logger.trace("Loading XML bean definitions from " + encodedResource);
}
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet<>(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());
}
// 真正开始加载BeanDefinitions的逻辑
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();
}
}
}
在上面的代码中我们会发现在这里会调用一个doLoadBeanDefinitions的方法，这是真正处理加载BeanDefinitions的逻辑，其源码如下
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
// 生成Document对象
Document doc = doLoadDocument(inputSource, resource);
// 注册BeanDefinition
int count = registerBeanDefinitions(doc, resource);
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + count + " bean definitions from " + resource);
}
return count;
}
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);
}
}
从上面的代码可以发现，在Spring中是使用Document对象来解析配置的内容并进行BeanDefinition的注册工作，registerBeanDefinitions方法的源码如下
@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
doRegisterBeanDefinitions(doc.getDocumentElement());
}
protected void doRegisterBeanDefinitions(Element root) {
BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createDelegate(getReaderContext(), root, parent);
if (this.delegate.isDefaultNamespace(root)) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
// We cannot use Profiles.of(...) since profile expressions are not supported
// in XML config. See SPR-12458 for details.
if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
if (logger.isDebugEnabled()) {
logger.debug("Skipped XML bean definition file due to specified profiles [" + profileSpec +
"] not matching: " + getReaderContext().getResource());
}
return;
}
}
}
preProcessXml(root);
// 解析文档中的标签
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;
}
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 {
// 解析用户自定义的标签
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}
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);
}
}
上面的代码是解析解析文档中的标签，这里我们查看下解析bean标签的逻辑，processBeanDefinition方法，源码如下
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.
// 注册BeanDefinition的逻辑
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));
}
}
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
// 注册BeanDefinition
// 调用的是DefaultListableBeanFactory中的方法
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);
}
}
}
最终会调用DefaultListableBeanFactory中的registerBeanDefinition方法，将BeanDefinition注册到容器中，使用容器中的beanDefinitionMap参数进行存储，这是一个ConcurrentHashMap对象，使用对象名称作为key，BeanDefinition作为值，可以跟我们先前例子的debug的结果是一致的。
@Override
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 BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
}
else if (existingDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (logger.isInfoEnabled()) {
logger.info("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.isDebugEnabled()) {
logger.debug("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("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<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
removeManualSingletonName(beanName);
}
}
else {
// Still in startup registration phase
this.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
removeManualSingletonName(beanName);
}
this.frozenBeanDefinitionNames = null;
}
if (existingDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
至此使用ClassPathXmlApplicationContext容器的初始化源码就分析结束了。写的比较粗糙，也就是简单的梳理了下容器的初始化流程，具体的内容需要大家去下载源码一步步的进行查看。
4 简单的总结
BeanFactory和ApplicationContext的关系
-
BeanFactory是容器的顶层接口，定义了容器应该具备的基本功能。
-
ApplicationContext继承自BeanFactory，对BeanFactory进行了扩展，是更高级的容器。
IOC的初始化过程
1、BeanDefinition资源的定位与加载（加载为Resource对象）
2、BeanDefinition的解析，使用的是（Document相关API）
3、BeanDefinition的注册，存入一个叫beanDefinitionMap的ConcurrentHashMap的对象。
Spring的IOC容器中是使用BeanDefinition来进行定义的，来自各个地方的资源，Spring都会将其转换为BeanDefinition。
