Spring框架是Java最常用的框架之一，他可以以非侵入的方式，管理我们的程序，降低了我们程序的复杂性、耦合性，使用起来也特别的方便，但是对于Java基础较弱的同学来说，想要弄懂Spring的原理还是比较困难的，所以写这篇文章来一起学习Spring源码（源码读起来可能会相当枯燥，本文会尽量用比较通俗的话帮助大家理解，如果出现错误的话，欢迎指出，谢谢）
说到Spring离不开IOC/DI，至于什么是IOC/DI这里就不多说了。Spring要管理你程序的bean就一定需要一个 “容器” 来存放这些bean，而IOC容器就是Spring用来管理bean的“东西”。那么这个容器是怎么进行初始化，程序的bean又是怎么进到容器，然后又怎么自动注入到程序呢，别着急，慢慢来，先来看看容器是怎么进行初始化的。
在这之前，我们需要了解几个概念：
public interface BeanFactory {
String FACTORY_BEAN_PREFIX = "&";
Object getBean(String name) throws BeansException;
<T> T getBean(String name, @Nullable 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, @Nullable Class<?> typeToMatch) throws NoSuchBeanDefinitionException;
@Nullable
Class<?> getType(String name) throws NoSuchBeanDefinitionException;
String[] getAliases(String name);
}
为了省地方，我就直接把那些注释去掉了。如果有手动获取过Spring管理的bean的同学可以发觉，这些方法都见过，因为所有的IOC容器都是这个接口的实现类
到这可以看出来，BeanFactory只定义容器的基本功能规范（获取、判断），而并不关心bean是怎么产生，怎么保管，怎么依赖，而ListableBeanFactory则定义了bean的集合，HierarchicalBeanFactory定义了bean的关系，AutowireCapableBeanFactory定义了bean的行为（依赖），由这四个接口共同对程序的bean 进行保管和使用
以上，基本上可以知道IOC容器大概的结构组成，那么接下来就可以说一下IOC容器的初始化步骤，包括Resource的资源定位、载入、注册这三个基本步骤（下面会有大量源码）。
先从资源定位开始，以FileSystemXmlApplicationContext这个IOC容器为例，首先看一下他的部分源码：
public class FileSystemXmlApplicationContext extends AbstractXmlApplicationContext {
public FileSystemXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
...
}
这里是FileSystemXmlApplicationContext实际调用的构造方法，可以看到首先是调用了父类的构造方法，然后调用了setConfigLocations方法，这个方法也是父类的，所以我们一级一级的向上查找（因为这个继承关系很多，就只放几个我们需要的），首先是在父类AbstractRefreshableConfigApplicationContext找到了setConfigLocations方法，如下：
public abstract class AbstractRefreshableConfigApplicationContext extends AbstractRefreshableApplicationContext
implements BeanNameAware, InitializingBean {
@Nullable
private String[] configLocations;
public void setConfigLocations(@Nullable String... locations) {
if (locations != null) {
Assert.noNullElements(locations, "Config locations must not be null");
this.configLocations = new String[locations.length];
for (int i = 0; i < locations.length; i++) {
this.configLocations[i] = resolvePath(locations[i]).trim();
}
}
else {
this.configLocations = null;
}
}
...
}
到这，setConfigLocations方法是干什么的可以知道了，是设置bean的定义资源文件的定位路径，接着向上级查找，最后在AbstractApplicationContext类找到了实际调用的构造方法，如下：
public abstract class AbstractApplicationContext extends DefaultResourceLoader
implements ConfigurableApplicationContext {
public AbstractApplicationContext(@Nullable ApplicationContext parent) {
this();
setParent(parent);
}
public AbstractApplicationContext() {
this.resourcePatternResolver = getResourcePatternResolver();
}
public void setParent(@Nullable ApplicationContext parent) {
this.parent = parent;
if (parent != null) {
Environment parentEnvironment = parent.getEnvironment();
if (parentEnvironment instanceof ConfigurableEnvironment) {
getEnvironment().merge((ConfigurableEnvironment) parentEnvironment);
}
}
}
protected ResourcePatternResolver getResourcePatternResolver() {
return new PathMatchingResourcePatternResolver(this);
}
...
}
来看一下，构造方法主要做的事就是调用this()，然后获取了一个Spring Source的加载器，用于读入bean的定义资源文件
OK，来总结一下上面的源码，我们分析了FileSystemXmlApplicationContext这个IOC容器的创建过程，首先是调用了父类的构造方法，其目的是获取了一个Spring Source的加载器，用于读入bean的定义资源文件，然后再调用父类的setConfigLocations方法，来设置bean的定义资源文件的定位路径，这些做完，SpringIOC容器就有了一个可以读入资源文件的加载器，然后也知道了资源文件的定位路径，至此，定位步骤已经完成
还没有结束，回上去看FileSystemXmlApplicationContext的构造方法，在setConfigLocations之后还调用了refresh方法，该方法的实现也在AbstractApplicationContext中，我们先看一下源码：
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass 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();
}
}
}
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (logger.isDebugEnabled()) {
logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
}
return beanFactory;
}
这些注释是源码自带的，我就不翻译了，这个方法大概做了个什么事呢，就是载入bean定义资源文件，而整个载入都是从obtainFreshBeanFactory方法的执行开始，后面的都是在注册容器的信息源和生命周期事件，而obtainFreshBeanFactory方法可以看到主要是调用了一个refreshBeanFactory方法，该方法在AbstractApplicationContext中是抽象方法，其实现实际上是在子类AbstractRefreshableApplicationContext（类名绕晕了没）
public abstract class AbstractRefreshableApplicationContext extends AbstractApplicationContext {
protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
...
}
看一下代码的流程，首先是判断BeanFactory是否存在，存在就先销毁beans，然后关闭BeanFactory，然后在创建新的DefaultListableBeanFactory（这个类在最开头的地方我就介绍过），最后调用一个loadBeanDefinitions方法装载bean，loadBeanDefinitions方法在这里也是抽象的，具体的实现是在其子类AbstractXmlApplicationContext中：
public abstract class AbstractXmlApplicationContext extends AbstractRefreshableConfigApplicationContext {
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);
}
protected void initBeanDefinitionReader(XmlBeanDefinitionReader reader) {
reader.setValidating(this.validating);
}
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);
}
}
...
}
注释也是源码自带的，这一段是干了个啥呢，就是XmlBeanDefinitionReader调用loadBeanDefinitions读取bean的定义资源，而loadBeanDefinitions方法在其抽象父类AbstractBeanDefinitionReader定义了载入过程：
public abstract class AbstractBeanDefinitionReader implements EnvironmentCapable, BeanDefinitionReader {
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, @Nullable 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;
}
}
...
}
能坚持看到这的都是狼人，如果不是我自己写的我早就不读了
解释一下，loadBeanDefinitions这个方法干了什么呢，首先是调用resourceLoader.getResource(location)，获取到需要加载的资源，然后调用loadBeanDefinitions方法真正实行加载，先看这个getResource方法，这个方法是在ResourceLoader接口里面的，其实际的调用呢是DefaultResourceLoader（这一层一层的我看了都想打人）：
public class DefaultResourceLoader implements ResourceLoader {
public Resource getResource(String location) {
Assert.notNull(location, "Location must not be null");
Iterator var2 = this.getProtocolResolvers().iterator();
Resource resource;
do {
if (!var2.hasNext()) {
if (location.startsWith("/")) {
return this.getResourceByPath(location);
}
if (location.startsWith("classpath:")) {
return new ClassPathResource(location.substring("classpath:".length()), this.getClassLoader());
}
try {
URL url = new URL(location);
return (Resource)(ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
} catch (MalformedURLException var5) {
return this.getResourceByPath(location);
}
}
ProtocolResolver protocolResolver = (ProtocolResolver)var2.next();
resource = protocolResolver.resolve(location, this);
} while(resource == null);
return resource;
}
...
}
这一段又是干啥呢，就是获取配置文件的资源定义，简单来说就是找你程序中的配置文件，然后封装成Resource对象，这里面有个特殊的方法是getResourceByPath方法，因为咱们最开始就是用的FileSystemXmlApplicationContext的例子，而这个IOC容器对该方法进行了重写，如下：
protected Resource getResourceByPath(String path) {
if (path.startsWith("/")) {
path = path.substring(1);
}
return new FileSystemResource(path);
}
所以绕了一大圈，最后又回到了FileSystemXmlApplicationContext，他重写了这个方法，就可以从文件系统路径上对IOC配置文件进行加载
这只是分析了一个getResource方法，还有一个loadBeanDefinitions方法，这个方法是在XmlBeanDefinitionReader里面实现：
public class XmlBeanDefinitionReader extends AbstractBeanDefinitionReader {
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<>(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();
}
}
}
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);
}
}
...
}
可以看到，载入配置文件，最后就是转换成了一个Document对象（是不是感觉是xml配置的流程，是的，因为咱们用的是FileSystemXmlApplicationContext例子，注解配置的会放在后面）
总结一下，上面绕开了很大一圈，看了n多个父类子类的代码，到这，载入过程已经完成，SpringIOC容器已经可以获取到配置文件的内容了（注意区别，这里还没有对配置文件进行解析），那么接下来我们还要继续分析，SpringIOC容器在把bean的定义资源文件转换成Document对象之后，又是如何解析成BeanDefinition（这个东西我也在开头的地方就讲过了）并将其注册到容器中的
接着看上面这一段源码，XmlBeanDefinitionReader在doLoadDocument把文档转换成Document对象之后，还调用了registerBeanDefinitions方法，对bean的定义进行解析注册，来看一下这个方法
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
int countBefore = getRegistry().getBeanDefinitionCount();
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}
可以看到上面是把Document对象进行解析，使用的是BeanDefinitionDocumentReader的实现类DefaultBeanDefinitionDocumentReader（全篇最长源码来袭）
public class DefaultBeanDefinitionDocumentReader implements BeanDefinitionDocumentReader {
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
logger.debug("Loading bean definitions");
Element root = doc.getDocumentElement();
doRegisterBeanDefinitions(root);
}
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)) {
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 BeanDefinitionParserDelegate createDelegate(
XmlReaderContext readerContext, Element root, @Nullable BeanDefinitionParserDelegate parentDelegate) {
BeanDefinitionParserDelegate delegate = new BeanDefinitionParserDelegate(readerContext);
delegate.initDefaults(root, parentDelegate);
return delegate;
}
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);
}
}
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)) {
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}
protected void importBeanDefinitionResource(Element ele) {
String location = ele.getAttribute(RESOURCE_ATTRIBUTE);
if (!StringUtils.hasText(location)) {
getReaderContext().error("Resource location must not be empty", ele);
return;
}
// Resolve system properties: e.g. "${user.dir}"
location = getReaderContext().getEnvironment().resolveRequiredPlaceholders(location);
Set<Resource> actualResources = new LinkedHashSet<>(4);
// Discover whether the location is an absolute or relative URI
boolean absoluteLocation = false;
try {
absoluteLocation = ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();
}
catch (URISyntaxException ex) {
// cannot convert to an URI, considering the location relative
// unless it is the well-known Spring prefix "classpath*:"
}
// Absolute or relative?
if (absoluteLocation) {
try {
int importCount = getReaderContext().getReader().loadBeanDefinitions(location, actualResources);
if (logger.isDebugEnabled()) {
logger.debug("Imported " + importCount + " bean definitions from URL location [" + location + "]");
}
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error(
"Failed to import bean definitions from URL location [" + location + "]", ele, ex);
}
}
else {
// No URL -> considering resource location as relative to the current file.
try {
int importCount;
Resource relativeResource = getReaderContext().getResource().createRelative(location);
if (relativeResource.exists()) {
importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource);
actualResources.add(relativeResource);
}
else {
String baseLocation = getReaderContext().getResource().getURL().toString();
importCount = getReaderContext().getReader().loadBeanDefinitions(
StringUtils.applyRelativePath(baseLocation, location), actualResources);
}
if (logger.isDebugEnabled()) {
logger.debug("Imported " + importCount + " bean definitions from relative location [" + location + "]");
}
}
catch (IOException ex) {
getReaderContext().error("Failed to resolve current resource location", ele, ex);
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to import bean definitions from relative location [" + location + "]",
ele, ex);
}
}
Resource[] actResArray = actualResources.toArray(new Resource[0]);
getReaderContext().fireImportProcessed(location, actResArray, extractSource(ele));
}
protected void processAliasRegistration(Element ele) {
String name = ele.getAttribute(NAME_ATTRIBUTE);
String alias = ele.getAttribute(ALIAS_ATTRIBUTE);
boolean valid = true;
if (!StringUtils.hasText(name)) {
getReaderContext().error("Name must not be empty", ele);
valid = false;
}
if (!StringUtils.hasText(alias)) {
getReaderContext().error("Alias must not be empty", ele);
valid = false;
}
if (valid) {
try {
getReaderContext().getRegistry().registerAlias(name, alias);
}
catch (Exception ex) {
getReaderContext().error("Failed to register alias '" + alias +
"' for bean with name '" + name + "'", ele, ex);
}
getReaderContext().fireAliasRegistered(name, alias, extractSource(ele));
}
}
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.
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));
}
}
}
很长很长，从这上面可以看出，在Spring的配置文件中可以使用import标签元素来导入IOC容器所需的其他资源，SpringIOC容器在解析时会先将指定导入的资源加载进容器，使用ailas标签别名时，SpringIOC容器首先将别名元素所定义的别名注册到容器中，而对于既不是import标签，也不是ailas标签的其他标签，则认为是普通的bean标签，由BeanDefinitionParserDelegate类的parseBeanDefinitionElement方法解析：
public class BeanDefinitionParserDelegate {
@Nullable
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele) {
return parseBeanDefinitionElement(ele, null);
}
@Nullable
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, @Nullable BeanDefinition containingBean) {
String id = ele.getAttribute(ID_ATTRIBUTE);
String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
List<String> aliases = new ArrayList<>();
if (StringUtils.hasLength(nameAttr)) {
String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
aliases.addAll(Arrays.asList(nameArr));
}
String beanName = id;
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) {
checkNameUniqueness(beanName, aliases, ele);
}
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);
return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
}
return null;
}
protected void checkNameUniqueness(String beanName, List<String> aliases, Element beanElement) {
String foundName = null;
if (StringUtils.hasText(beanName) && this.usedNames.contains(beanName)) {
foundName = beanName;
}
if (foundName == null) {
foundName = CollectionUtils.findFirstMatch(this.usedNames, aliases);
}
if (foundName != null) {
error("Bean name '" + foundName + "' is already used in this <beans> element", beanElement);
}
this.usedNames.add(beanName);
this.usedNames.addAll(aliases);
}
@Nullable
public AbstractBeanDefinition parseBeanDefinitionElement(
Element ele, String beanName, @Nullable BeanDefinition containingBean) {
this.parseState.push(new BeanEntry(beanName));
String className = null;
if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
}
String parent = null;
if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
parent = ele.getAttribute(PARENT_ATTRIBUTE);
}
try {
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;
}
...
}
如果使用过xml配置的同学，可以看出来，我们配置的bean就是通过上面这段代码解析的，而我们最常使用的就是property标签，那下面再来看看这个标签是怎么解析的
public void parsePropertyElements(Element beanEle, BeanDefinition bd) {
NodeList nl = beanEle.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (isCandidateElement(node) && nodeNameEquals(node, PROPERTY_ELEMENT)) {
parsePropertyElement((Element) node, bd);
}
}
}
public void parsePropertyElement(Element ele, BeanDefinition bd) {
String propertyName = ele.getAttribute(NAME_ATTRIBUTE);
if (!StringUtils.hasLength(propertyName)) {
error("Tag 'property' must have a 'name' attribute", ele);
return;
}
this.parseState.push(new PropertyEntry(propertyName));
try {
if (bd.getPropertyValues().contains(propertyName)) {
error("Multiple 'property' definitions for property '" + propertyName + "'", ele);
return;
}
Object val = parsePropertyValue(ele, bd, propertyName);
PropertyValue pv = new PropertyValue(propertyName, val);
parseMetaElements(ele, pv);
pv.setSource(extractSource(ele));
bd.getPropertyValues().addPropertyValue(pv);
}
finally {
this.parseState.pop();
}
}
@Nullable
public Object parsePropertyValue(Element ele, BeanDefinition bd, @Nullable String propertyName) {
String elementName = (propertyName != null ?
"<property> element for property '" + propertyName + "'" :
"<constructor-arg> element");
// Should only have one child element: ref, value, list, etc.
NodeList nl = ele.getChildNodes();
Element subElement = null;
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (node instanceof Element && !nodeNameEquals(node, DESCRIPTION_ELEMENT) &&
!nodeNameEquals(node, META_ELEMENT)) {
// Child element is what we're looking for.
if (subElement != null) {
error(elementName + " must not contain more than one sub-element", ele);
}
else {
subElement = (Element) node;
}
}
}
boolean hasRefAttribute = ele.hasAttribute(REF_ATTRIBUTE);
boolean hasValueAttribute = ele.hasAttribute(VALUE_ATTRIBUTE);
if ((hasRefAttribute && hasValueAttribute) ||
((hasRefAttribute || hasValueAttribute) && subElement != null)) {
error(elementName +
" is only allowed to contain either 'ref' attribute OR 'value' attribute OR sub-element", ele);
}
if (hasRefAttribute) {
String refName = ele.getAttribute(REF_ATTRIBUTE);
if (!StringUtils.hasText(refName)) {
error(elementName + " contains empty 'ref' attribute", ele);
}
RuntimeBeanReference ref = new RuntimeBeanReference(refName);
ref.setSource(extractSource(ele));
return ref;
}
else if (hasValueAttribute) {
TypedStringValue valueHolder = new TypedStringValue(ele.getAttribute(VALUE_ATTRIBUTE));
valueHolder.setSource(extractSource(ele));
return valueHolder;
}
else if (subElement != null) {
return parsePropertySubElement(subElement, bd);
}
else {
// Neither child element nor "ref" or "value" attribute found.
error(elementName + " must specify a ref or value", ele);
return null;
}
}
通过对上面的源码分析，在Spring的配置文件中，property元素的ref会被封装成一个指向依赖对象的引用，而value配置对会被封装成字符串，ref和value都是通过.setSource(extractSource(ele))方法将属性值和引用与所引用的属性关联起来的，而最后对于子标签还会通过parsePropertySubElement方法解析
@Nullable
public Object parsePropertySubElement(Element ele, @Nullable BeanDefinition bd) {
return parsePropertySubElement(ele, bd, null);
}
@Nullable
public Object parsePropertySubElement(Element ele, @Nullable BeanDefinition bd, @Nullable String defaultValueType) {
if (!isDefaultNamespace(ele)) {
return parseNestedCustomElement(ele, bd);
}
else if (nodeNameEquals(ele, BEAN_ELEMENT)) {
BeanDefinitionHolder nestedBd = parseBeanDefinitionElement(ele, bd);
if (nestedBd != null) {
nestedBd = decorateBeanDefinitionIfRequired(ele, nestedBd, bd);
}
return nestedBd;
}
else if (nodeNameEquals(ele, REF_ELEMENT)) {
// A generic reference to any name of any bean.
String refName = ele.getAttribute(BEAN_REF_ATTRIBUTE);
boolean toParent = false;
if (!StringUtils.hasLength(refName)) {
// A reference to the id of another bean in a parent context.
refName = ele.getAttribute(PARENT_REF_ATTRIBUTE);
toParent = true;
if (!StringUtils.hasLength(refName)) {
error("'bean' or 'parent' is required for <ref> element", ele);
return null;
}
}
if (!StringUtils.hasText(refName)) {
error("<ref> element contains empty target attribute", ele);
return null;
}
RuntimeBeanReference ref = new RuntimeBeanReference(refName, toParent);
ref.setSource(extractSource(ele));
return ref;
}
else if (nodeNameEquals(ele, IDREF_ELEMENT)) {
return parseIdRefElement(ele);
}
else if (nodeNameEquals(ele, VALUE_ELEMENT)) {
return parseValueElement(ele, defaultValueType);
}
else if (nodeNameEquals(ele, NULL_ELEMENT)) {
// It's a distinguished null value. Let's wrap it in a TypedStringValue
// object in order to preserve the source location.
TypedStringValue nullHolder = new TypedStringValue(null);
nullHolder.setSource(extractSource(ele));
return nullHolder;
}
else if (nodeNameEquals(ele, ARRAY_ELEMENT)) {
return parseArrayElement(ele, bd);
}
else if (nodeNameEquals(ele, LIST_ELEMENT)) {
return parseListElement(ele, bd);
}
else if (nodeNameEquals(ele, SET_ELEMENT)) {
return parseSetElement(ele, bd);
}
else if (nodeNameEquals(ele, MAP_ELEMENT)) {
return parseMapElement(ele, bd);
}
else if (nodeNameEquals(ele, PROPS_ELEMENT)) {
return parsePropsElement(ele);
}
else {
error("Unknown property sub-element: [" + ele.getNodeName() + "]", ele);
return null;
}
}
我都不想写了
到这可以看出，在Spring的配置文件中，对property标签中配置的array、list、set、map等都是通过这个方法解析的，而生成的对象都是BeanDefiniton的数据封装
这里需要注意：到这都只是对Document对象解析成BeanDefiniton，而并没有创建实际的对象，BeanDefiniton只是bean的定义类，开头的时候对比Java的class，那么到这就相当于只是定义了这些class，而没有new对象，同理，现在也只是把bean标签中的配置信息设置到BeanDefiniton中，而只有在依赖注入的时候才会去实例化这些bean
最后，我们来看一下property中的配置是怎么解析的，以list为例
public List<Object> parseListElement(Element collectionEle, @Nullable BeanDefinition bd) {
String defaultElementType = collectionEle.getAttribute(VALUE_TYPE_ATTRIBUTE);
NodeList nl = collectionEle.getChildNodes();
ManagedList<Object> target = new ManagedList<>(nl.getLength());
target.setSource(extractSource(collectionEle));
target.setElementTypeName(defaultElementType);
target.setMergeEnabled(parseMergeAttribute(collectionEle));
parseCollectionElements(nl, target, bd, defaultElementType);
return target;
}
protected void parseCollectionElements(
NodeList elementNodes, Collection<Object> target, @Nullable BeanDefinition bd, String defaultElementType) {
for (int i = 0; i < elementNodes.getLength(); i++) {
Node node = elementNodes.item(i);
if (node instanceof Element && !nodeNameEquals(node, DESCRIPTION_ELEMENT)) {
target.add(parsePropertySubElement((Element) node, bd, defaultElementType));
}
}
}
这也没啥可以解读的，挺简单的代码
到目前为止，bean的定义资源文件经过这一层层的操作，SpringIOC已经将xml形式的文件，转换成了IOC容器能够识别的数据结构BeanDefinition，只剩下最后一步，把这些解析的BeanDefinition注册到容器（终于到最后一步了）
在上面的DefaultBeanDefinitionDocumentReader源码中可以看到，在parseDefaultElement方法中，对Document对象解析结束后，会返回封装BeanDefinition的BeanDefinitionHold对象，然后调用BeanDefinitionReaderUtils的registerBeanDefinition方法向IOC容器注册解析的bean
public class BeanDefinitionReaderUtils {
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);
}
}
}
...
}
可以看到真正注册调用的是registry.registerBeanDefinition方法，而这个方法的具体实现是在DefaultListableBeanFactory，我们来看一下这个家伙的源码
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
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<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set<String> 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 (existingDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
...
}
最后的bean的信息，实际上就是封装成BeanDefinition对象，然后保存在一个map中，到此为止，程序配置的bean已经真正的被注册到IOC容器，并且受到IOC容器的管理，注册步骤结束，IOC容器初始化过程结束，此时的IOC容器已经建立了整个bean的配置信息，并且可以使用，IOC容器的作用就是对这些注册的bean的配置信息进行处理和维护，这些注册的bean就是IOC容器控制反转的基础，容器后面才可以对程序进行依赖注入
还有最后一个总结，马上就结束了
上面说了这么多，最后来捋一下吧，总结一下IOC容器初始化的步骤（抽象总结，不以具体例子）
到这里为止，关于IOC容器初始化的过程就差不多了，上面的源码都是针对FileSystemXmlApplicationContext这个IOC容器的解析，可能别的容器会有一些不一样，但是初始化的步骤，公共的接口都是一样的，本来还想接着写基于xml的依赖注入和基于Annotation的依赖注入的，但是源码太占篇幅了，读起来又很枯燥，如果使用太通俗的话语又怕说错，所以还是另写一篇
因篇幅问题不能全部显示，请点此查看更多更全内容