IoC:依赖倒置原理，个人觉得其核心思想就是减少代码的耦合度,很多复杂的应用都是依靠多个类之间的合作来完成的。如果类本身和合作类的引用需要靠自身来实现，那么代码的耦合度会很高。那么在Spring中，IoC是实现依赖反转的载体;Spring在对象初始化的时候将数据注入到对象中,或者将对象的引用注入到数据域中的方式来注入对方法调用的依赖。
Spring IoC提供了一个基本的javaBean容器，通过IoC模式管理依赖关系,并通过依赖注入和AOP切面增强了为javeBean这样的POJO对象赋予事务管理，生命周期管理等基本功能。而Bean都在Spring容器里，Spring容器负责它们的创建，装配，管理它们整个的生命周期。《深入理解Spring技术内幕》
SpringIoC容器系列(BeanFactory跟ApplicationContext)
BeanFactory跟ApplicationContext都属于Spring容器。
BeanFactory:定义了最基本的容器设计规范，如getBean(),containBean(),getType()等基本的方法。
ApplicationContext:由图可知ApplicationContext应用的上下文都是基于ConfigurableApplicationContext跟WebApplicationContext的实现。ApplicationContext继承了BeanFactory的接口同时又继承MessageSource（支持不同的信息源）,ResourceLoader（访问资源）,ApplicationEventPublisher（支持应用事件）。
IoC接口设计图图片引用《深入理解spring技术内幕》
容器是如何初始化的？
容器的初始化是通过refresh()方法来完成的，总共包括三个步骤：
- 定位:通过Resource定位BeanDefinition，BeanDefinition抽象了对bean的定义，比如bean的信息，依赖关系等。这个过程可以想象成寻找bean的过程。
下面我们通过代码来简要分析下容器是如何初始化的
public class ApplicationContextInit {
public static void main(String[] args) {
FileSystemXmlApplicationContext fileSystemXmlApplicationContext = new FileSystemXmlApplicationContext("bean.xml");
}
}
FileSystemXmlApplicationContext是通过文件来载入Resource的,运行上述代码
public class FileSystemXmlApplicationContext extends AbstractXmlApplicationContext {
//...省略部分源码
//通过文件的位置来定位到beanDefinition
public FileSystemXmlApplicationContext(String configLocation) throws BeansException {
this(new String[] {configLocation}, true, null);
}
//通过refresh()方法来完成BeanDefinition信息读取和载入
public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
@Override
protected Resource getResourceByPath(String path) {
if (path != null && path.startsWith("/")) {
path = path.substring(1);
}
return new FileSystemResource(path);
}
}
refresh()来启动IoC容器的初始化
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 容器初始化的一些准备工作.
prepareRefresh();
// 告知子类要初始化BeanFactory，BeanDefinition信息的读取是在子类的
// refreshBeanFactory()方法里完成的
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 准备bean工厂以在此上下文中使用。
prepareBeanFactory(beanFactory);
try {
// 设置beanFactory的后置处理
postProcessBeanFactory(beanFactory);
// 调用beanFactory的后置处理
invokeBeanFactoryPostProcessors(beanFactory);
// 注册beanFactory的后置处理
registerBeanPostProcessors(beanFactory);
// 初始化上下文的消息
initMessageSource();
// 初始化上下的事件
initApplicationEventMulticaster();
// 初始化一些特殊的bean
onRefresh();
// 检查一些监听的bean并注册到容器中
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// 发布容器事件，结束Refresh过程
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// 销毁已经生成的bean
destroyBeans();
// 重置激活状态.
cancelRefresh(ex);
throw ex;
}
finally {
resetCommonCaches();
}
}
}
beanDefinition信息是通过ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory()里的refreshBeanFactory()来完成的，而这个方法则是在AbstractRefreshableApplicationContext实现的。
@Override
protected final void refreshBeanFactory() throws BeansException {
//如果容器已经存在,那么销毁并且关闭该容器,保证每次产生的都是新的容器
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
//创建基础的BeanFactory
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
//载入BeanDefinition的信息
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
DefaultListableBeanFactory其实是一个最基础的容器,很多容器都是基于这个容器来作扩展,那么这个容器里自然也包含了很多基础重要的功能，那么通过loadBeanDefinitions()来完成BeanDefinition信息的载入的,这里是委托子类来完成这个工作的。
//抽象类,具体的resource定位跟BeanDefinition的载入是委托子类来完成的
protected abstract void loadBeanDefinitions(DefaultListableBeanFactory beanFactory)
throws BeansException, IOException;
//这是loadBeanDefinitions的具体实现
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
//创建XmlBeanDefinitionReader,并通过回调设置到beanFactory里面去
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// 使用此上下文的资源加载环境配置beanFactory
beanDefinitionReader.setEnvironment(this.getEnvironment());
beanDefinitionReader.setResourceLoader(this);
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
//启动bean信息的载入过程
initBeanDefinitionReader(beanDefinitionReader);
loadBeanDefinitions(beanDefinitionReader);
}
//这里应该算是容器初始化第一步resource定位,首先得到beanDefinition信息的Resource定位
//然后通过XmlBeanDefinitionReader来读取
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
//通过resource来定位
Resource[] configResources = getConfigResources();
if (configResources != null) {
reader.loadBeanDefinitions(configResources);
}
//通过文件路径来定位
String[] configLocations = getConfigLocations();
if (configLocations != null) {
reader.loadBeanDefinitions(configLocations);
}
}
public int loadBeanDefinitions(String location, Set actualResources) throws BeanDefinitionStoreException {
//获取ResourceLoader
ResourceLoader resourceLoader = getResourceLoader();
if (resourceLoader == null) {
throw new BeanDefinitionStoreException(
"Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
}
//判断ResourceLoader的路径模式，
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;
}
}
getResourceByPath被子类FileSystemXmlApplicationContext实现,最终返回FileSystemResource对象，通过这个对象,Spring进行相关的I/O操作，完成beanDefinition的定位。
总结下Resource定位BeanDefinition的流程
1.FileSystemXmlApplicationContext里调用refresh()方法初始化IoC容器。
2.在refresh()方法里调用obtainFreshBeanFactory()里面的refreshBeanFactory()来完成BeanDefinition的定位,而refreshBeanFactory()是由子类AbstractRefreshableApplicationContext来实现的。
3.refreshBeanFactory()中是通过loadBeanDefinitions()来完成BeanDefinition的定位,而loadBeanDefinitions()是一个抽象的方法,具体由AbstractBeanDefinitionReader里的loadBeanDefinitions()来实现。
4.在loadBeanDefinitions()通过DefaultResourceLoader的getResource方法里返回resource对象。
- 载入:BeanDefinition的信息已经定位到了，第二步就是把定义的BeanDefinition在Ioc容器中转化成一个Spring内部标示的数据结构的过程。
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (this.logger.isInfoEnabled()) {
this.logger.info("Loading XML bean definitions from " + encodedResource.getResource());
}
Set currentResources = (Set)this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet(4);
this.resourcesCurrentlyBeingLoaded.set(currentResources);
}
if (!((Set)currentResources).add(encodedResource)) {
throw new BeanDefinitionStoreException("Detected cyclic loading of " + encodedResource + " - check your import definitions!");
} else {
int var5;
try {
//准备读取xml文件
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
//具体的读取过程是在doLoadBeanDefinitions()这个方法里完成
var5 = this.doLoadBeanDefinitions(inputSource, encodedResource.getResource());
} finally {
inputStream.close();
}
} catch (IOException var15) {
throw new BeanDefinitionStoreException("IOException parsing XML document from " + encodedResource.getResource(), var15);
} finally {
((Set)currentResources).remove(encodedResource);
if (((Set)currentResources).isEmpty()) {
this.resourcesCurrentlyBeingLoaded.remove();
}
}
return var5;
}
}
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
int validationMode = getValidationModeForResource(resource);
//这里通过documentLoader来读取xml最终获取Document对象
Document doc = this.documentLoader.loadDocument(
inputSource, getEntityResolver(), this.errorHandler, validationMode, isNamespaceAware());
//这里通过registerBeanDefinitions()方法完成BeanDefinition的解析
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);
}
}
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
//获取BeanDefinitionDocumentReader对xml对BeanDefinition进行解析
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
documentReader.setEnvironment(this.getEnvironment());
int countBefore = getRegistry().getBeanDefinitionCount();
//具体解析是在registerBeanDefinitions()这个方法里进行对
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
logger.debug("Loading bean definitions");
Element root = doc.getDocumentElement();
//注册BeanDefinition
doRegisterBeanDefinitions(root);
}
protected void doRegisterBeanDefinitions(Element root) {
//委托父类解析
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);
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;
}
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
//对xml里根节点下所有对子节点进行解析
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);
}
}
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
//如果为bean的话 解析BeanDefinition
processBeanDefinition(ele, delegate);
}//beans
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// 递归解析
doRegisterBeanDefinitions(ele);
}
}
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}
}
首先通过调用XML的解析器得到Document对象，此时这些Document对象并没有按照Spring的Bean规则进行解析，在完成通用的XML解析以后，才是按照Spring Bean规则进行解析的地方，这个过程在documentReader中实现，使用的documentReader是默认设置好的DefaultBeanDefinitionDocumentReader。
- 注册:将抽象好的BeanDefinition统一注册到IoC容器中，IoC容器是通过hashMap来维护BeanDefinition信息的，key为beanName，value为BeanDefinition。
/** Map of bean definition objects, keyed by bean name */
private final Map beanDefinitionMap = new ConcurrentHashMap(256);
BeanDefinition的注册是发生在BeanDefinition信息载入之后的，
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 oldBeanDefinition;
//如果beanName已经在map里存在,那么抛出异常
oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (this.logger.isWarnEnabled()) {
this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(oldBeanDefinition)) {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + oldBeanDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + oldBeanDefinition +
"] with [" + beanDefinition + "]");
}
}
this.beanDefinitionMap.put(beanName, beanDefinition);
}
else {
if (hasBeanCreationStarted()) {
// 使用synchronize为了保证数据一致性，key为bean的名字,key为beanDefinition信息
synchronized (this.beanDefinitionMap) {
this.beanDefinitionMap.put(beanName, beanDefinition);
List updatedDefinitions = new ArrayList(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set updatedSingletons = new LinkedHashSet(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 (oldBeanDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
这样就完成了beanDefinition在IoC容器中的注册;
下一篇会大概分析下依赖注入的过程, 第一次写这种源码分析的文章,思路还是比较混乱,还请大家多多指教,共同进步！