【转自】http://geeekr.com/read-spring-source-1-how-to-load-bean/
最近几天跟同事聊起Spring的一些问题,对一些地方有些疑问,趁这两天有点空,看看Spring的源码,了解下具体的实现细节。本文基于Spring 4.0.5版本。
首先Web项目使用Spring是通过在web.xml里面配置
org.springframework.web.context.ContextLoaderListener初始化IOC容器的。
org.springframework.web.context.ContextLoaderListener
那就以此为切入点顺藤摸瓜。
public class ContextLoaderListener extends ContextLoader implements ServletContextListener
ContextLoaderListener继承了ContextLoader,并且实现ServletContextListener接口。当Server容器(一般指tomcat)启动时,会收到事件初始化。
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}
initWebApplicationContext方法是在org.springframework.web.context.ContextLoader类里面。方法太长,分段读一下。
if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
throw new IllegalStateException("Cannot initialize context because there is already a root application context present - " +"check whether you have multiple ContextLoader* definitions in your web.xml!");
}
Log logger = LogFactory.getLog(ContextLoader.class);
servletContext.log("Initializing Spring root WebApplicationContext");
if (logger.isInfoEnabled()) {
logger.info("Root WebApplicationContext: initialization started");
}
long startTime = System.currentTimeMillis();
首先是判断servletContext中是否已经注册了WebApplicationContext,如果有则抛出异常,避免重复注册。然后就是启用log,启动计时。本方法的关键就在于try代码块里的内容
try {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent ->
// determine parent for root web application context, if any.
ApplicationContext parent = loadParentContext(servletContext);
cwac.setParent(parent);
}
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
}
else if (ccl != null) {
currentContextPerThread.put(ccl, this.context);
}
if (logger.isDebugEnabled()) {
logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
}
if (logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
}
return this.context;
}
这里面有几个关键的方法。首先看一下createWebApplicationContext()
protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
Class> contextClass = determineContextClass(sc);
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
}
return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}
首先determineContextClass()方法查明具体的Context类,他会读取servletContext的初始化参数contextClass,此参数我们一半不配置,所以Spring就会读取跟org.springframework.web.context.WebApplicationContext同一个包下面的ContextLoader.properties文件读取默认设置,反射出org.springframework.web.context.support.XmlWebApplicationContext类来。接下来就是在configureAndRefreshWebApplicationContext()方法里将新创建的XmlWebApplicationContext进行初始化。首先会设置一个默认ID,即org.springframework.web.context.WebApplicationContext:+你项目的ContextPath。
if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
// The application context id is still set to its original default
// value
// -> assign a more useful id based on available information
String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
if (idParam != null) {
wac.setId(idParam);
} else {
// Generate default id...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(sc.getContextPath()));
}
}
紧接着就是将ServletContext设置成XmlWebApplicationContext的属性,这样Spring就能在上下文里轻松拿到ServletContext了。
wac.setServletContext(sc);
接下来就是读取web.xml文件中的contextConfigLocation参数。如果没有配置就会去读WEB-INF下的applicationContext.xml文件。
contextConfigLocation
classpath:beans.xml
并将值设置(就是我们的Spring配置文件的路径)进XmlWebApplicationContext中。然后就会在指定的路径加载配置文件。
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}
接下来就是customizeContext(sc, wac)方法,此方法会根据用户配置的globalInitializerClasses参数来初始化一些用户自定义的属性,一般我们不配置,所以这里什么也不做。
最后登场的就是最核心的方法了,
wac.refresh();
在这个方法里,会完成资源文件的加载、配置文件解析、Bean定义的注册、组件的初始化等核心工作,我们一探究竟。
@Override
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) {
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
}
}
方法是同步的,避免重复刷新,每个步骤都放在单独的方法内,流程清晰,是值得学习的地方。这里面有个重要的方法是finishBeanFactoryInitialization(beanFactory);,里面的内容是Spring如何实例化bean,并注入依赖的,这个内容下一节讲,本节只说明Spring是如何加载class文件的。
首先就是prepareRefresh()方法。
protected void prepareRefresh() {
this.startupDate = System.currentTimeMillis();
synchronized (this.activeMonitor) {
this.active = true;
}
if (logger.isInfoEnabled()) {
logger.info("Refreshing " + this);
}
// Initialize any placeholder property sources in the context environment
initPropertySources();
// Validate that all properties marked as required are resolvable
// see ConfigurablePropertyResolver#setRequiredProperties
getEnvironment().validateRequiredProperties();
}
此方法做一些准备工作,如记录开始时间,输出日志,initPropertySources();和getEnvironment().validateRequiredProperties();一般没干什么事。
接下来就是初始化BeanFactory,是整个refresh()方法的核心,其中完成了配置文件的加载、解析、注册
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
看看它里面都做了些什么?
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (logger.isDebugEnabled()) {
logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
}
return beanFactory;
}
首先refreshBeanFactory():
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);
}
}
我们看到会创建一个DefaultListableBeanFactory实例
DefaultListableBeanFactory beanFactory = createBeanFactory();
再设置一个ID
beanFactory.setSerializationId(getId());
然后设置一些自定义参数:
customizeBeanFactory(beanFactory);
这里面最重要的就是loadBeanDefinitions(beanFactory);方法了。
@Override
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);
}
此方法会通过XmlBeanDefinitionReader加载bean定义。具体的实现方法是在org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions方法中定义的。这里设计了层层调用,有好多重载方法,主要就是加载Spring所有的配置文件(可能会有多个),以备后面解析,注册之用。我一路追踪到org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions(Element root)
protected void doRegisterBeanDefinitions(Element root) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
Assert.state(this.environment != null, "Environment must be set for evaluating profiles");
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
if (!this.environment.acceptsProfiles(specifiedProfiles)) {
return;
}
}
BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createDelegate(this.readerContext, root, parent);
preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;
}
这里创建了一个BeanDefinitionParserDelegate示例,解析XML的过程就是委托它完成的,我们不关心它是怎样解析XML的,我们只关心是怎么加载类的,所以就要看parseBeanDefinitions(root, this.delegate)方法了。
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);
}
}
我们看到最终解析XML元素的是delegate.parseCustomElement(ele)方法,最终会走到一下方法.
public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
String namespaceUri = getNamespaceURI(ele);
NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
if (handler == null) {
error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
return null;
}
return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}
这里会根据不同的XML节点,会委托NamespaceHandlerSupport找出合适的BeanDefinitionParser,如果我们配置了
那么对应BeanDefinitionParser就是org.springframework.context.annotation.ComponentScanBeanDefinitionParser,来看看它的parse方法。
@Override
public BeanDefinition parse(Element element, ParserContext parserContext) {
String[] basePackages = StringUtils.tokenizeToStringArray(element.getAttribute(BASE_PACKAGE_ATTRIBUTE),
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
// Actually scan for bean definitions and register them.
ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);
Set beanDefinitions = scanner.doScan(basePackages);
registerComponents(parserContext.getReaderContext(), beanDefinitions, element);
return null;
}
不难看出这里定义了一个ClassPathBeanDefinitionScanner,通过它去扫描包中的类文件,注意:这里是类文件而不是类,因为现在这些类还没有被加载,只是ClassLoader能找到这些class的路径而已。到目前为止,感觉真想距离我们越来越近了。顺着继续往下摸。进入doSacn方法里,映入眼帘的又是一大坨代码,但是我们只关心观点的部分。
protected Set doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set beanDefinitions = new LinkedHashSet();
for (String basePackage : basePackages) {
Set candidates = findCandidateComponents(basePackage);
for (BeanDefinition candidate : candidates) {
ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
candidate.setScope(scopeMetadata.getScopeName());
String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
if (candidate instanceof AbstractBeanDefinition) {
postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
}
if (candidate instanceof AnnotatedBeanDefinition) {
AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
}
if (checkCandidate(beanName, candidate)) {
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
beanDefinitions.add(definitionHolder);
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}
一眼就能看出是通过
Set candidates = findCandidateComponents(basePackage);
有时候不得不佩服这些外国人起名字的功力,把扫描出来的类叫做candidates(候选人);真是不服不行啊,这种名字真的很容易理解有不有?哈哈,貌似扯远了。继续往下看。这里只列出方法的主题部分。
public Set findCandidateComponents(String basePackage) {
Set candidates = new LinkedHashSet();
try {
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
resolveBasePackage(basePackage) + "/" + this.resourcePattern;
Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);
boolean traceEnabled = logger.isTraceEnabled();
boolean debugEnabled = logger.isDebugEnabled();
for (Resource resource : resources) {
if (traceEnabled) {
logger.trace("Scanning " + resource);
}
if (resource.isReadable()) {
try {
MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource);
if (isCandidateComponent(metadataReader)) {
ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
sbd.setResource(resource);
sbd.setSource(resource);
先看这两句:
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX + resolveBasePackage(basePackage) + "/" + this.resourcePattern;
假设我们配置的需要扫描的包名为com.geeekr.service,那么packageSearchPath的值就是classpath:com.geeekr.service//.class,意思就是com.geeekr.service包(包括子包)下所有class文件;如果配置的是,那么packageSearchPath的值就是classpath://.class。这里的表达式是Spring自己定义的。Spring会根据这种表达式找出相关的class文件。
Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);
这一句就把相关class文件加载出来了,那我们就要看看,Spring究竟是如何把class文件找到的了。首先看看resourcePatternResolver的定义:
private ResourcePatternResolver resourcePatternResolver = new PathMatchingResourcePatternResolver();
进入getResources方法
@Override
public Resource[] getResources(String locationPattern) throws IOException {
Assert.notNull(locationPattern, "Location pattern must not be null");
if (locationPattern.startsWith(CLASSPATH_ALL_URL_PREFIX)) {
// a class path resource (multiple resources for same name possible)
if (getPathMatcher().isPattern(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()))) {
// a class path resource pattern
return findPathMatchingResources(locationPattern);
}
else {
// all class path resources with the given name
return findAllClassPathResources(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()));
}
}
else {
// Only look for a pattern after a prefix here
// (to not get fooled by a pattern symbol in a strange prefix).
int prefixEnd = locationPattern.indexOf(":") + 1;
if (getPathMatcher().isPattern(locationPattern.substring(prefixEnd))) {
// a file pattern
return findPathMatchingResources(locationPattern);
}
else {
// a single resource with the given name
return new Resource[] {getResourceLoader().getResource(locationPattern)};
}
}
}
这里会先判断表达式是否以classpath:开头。前面我们看到Spring已经给我们添加了这个头,这里当然符合条件了。接着会进入findPathMatchingResources方法。在这里又把/.class去掉了,然后在调用getResources方法,然后在进入findAllClassPathResources方法。这里的参数只剩下包名了例如com/geeekr/service/。
protected Resource[] findAllClassPathResources(String location) throws IOException {
String path = location;
if (path.startsWith("/")) {
path = path.substring(1);
}
ClassLoader cl = getClassLoader();
Enumeration resourceUrls = (cl != null ? cl.getResources(path) : ClassLoader.getSystemResources(path));
Set result = new LinkedHashSet(16);
while (resourceUrls.hasMoreElements()) {
URL url = resourceUrls.nextElement();
result.add(convertClassLoaderURL(url));
}
return result.toArray(new Resource[result.size()]);
}
真相大白了,Spring也是用的ClassLoader加载的class文件。一路追踪,原始的ClassLoader是Thread.currentThread().getContextClassLoader();。到此为止,就拿到class文件了。
Spring会将class信息封装成BeanDefinition,然后再放进DefaultListableBeanFactory的beanDefinitionMap中。
首先来看一段代码,看过上一节的朋友肯定对这段代码并不陌生。这一段代码诠释了Spring加载bean的完整过程,包括读取配置文件,扫描包,加载类,实例化bean,注入bean属性依赖。
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();
}
}
}
上一节介绍了Spring是如何加载class文件的,本节主要围绕finishBeanFactoryInitialization(beanFactory)方法,聊聊Spring是如何实例化bean的,从上面代码片段中的注解不难看出,此方法主要的任务就是实例化非懒加载的单例bean。闲话少叙,看代码。
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
beanFactory.preInstantiateSingletons();
}
上面代码主要看最后一句beanFactory.preInstantiateSingletons()。
public void preInstantiateSingletons() throws BeansException {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Pre-instantiating singletons in " + this);
}
List beanNames;
synchronized (this.beanDefinitionMap) {
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
beanNames = new ArrayList(this.beanDefinitionNames);
}
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
final FactoryBean> factory = (FactoryBean>) getBean(FACTORY_BEAN_PREFIX + beanName);
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(new PrivilegedAction() {
@Override
public Boolean run() {
return ((SmartFactoryBean>) factory).isEagerInit();
}
}, getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
else {
getBean(beanName);
}
}
}
}
此方法首先将加载进来的beanDefinitionNames循环分析,如果是我们自己配置的bean就会走else中的getBean(beanName),接着看。
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
doGetBean方法内容太多,一段一段看。
protected T doGetBean(
final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
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 + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
这里主要看Object sharedInstance = getSingleton(beanName)。
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
这里能看到,Spring会把实例化好的bean存入singletonObjects,这是一个ConcurrentHashMap,
private final Map singletonObjects = new ConcurrentHashMap(64);
当然这里我们bean并未实例化过,所以这里应该也不能get出什么东西来,也就是返回null了。if子句也就不会执行了。那么接着看else子句的内容。
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.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
这两条验证也都不会实现,接写来就是重点了。
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
if (isDependent(beanName, dependsOnBean)) {
throw new BeanCreationException("Circular depends-on relationship between '" +
beanName + "' and '" + dependsOnBean + "'");
}
registerDependentBean(dependsOnBean, beanName);
getBean(dependsOnBean);
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory
在这里拿到RootBeanDefinition并check,并获得bean的依赖,并循环迭代实例化bean。例如class A依赖于class B,就会先实例化B。下面的if ... else ...就是真正实例化bean的地方。其实真正实例化bean的方法是createBean(beanName, mbd, args),只是区分了isSingleton或isPrototype,两者的区别在于,单例的(Singleton)被缓存起来,而Prototype是不用缓存的。首先看一下createBean(beanName, mbd, args)。createBean方法中除了做了一些实例化bean前的检查准备工作外,最核心的方法就是
Object beanInstance = doCreateBean(beanName, mbd, args);
由于这个过程涉及到的代码都是一大坨,就不贴出所有代码了。
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
首先就是创建一个bean的实例且封装到BeanWrapper中,在这里bean已经实例化了。具体的实现方法是在org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner)中。
@Override
public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (beanDefinition.getMethodOverrides().isEmpty()) {
Constructor> constructorToUse;
synchronized (beanDefinition.constructorArgumentLock) {
constructorToUse = (Constructor>) beanDefinition.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
final Class> clazz = beanDefinition.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction>() {
@Override
public Constructor> run() throws Exception {
return clazz.getDeclaredConstructor((Class[]) null);
}
});
}
else {
constructorToUse = clazz.getDeclaredConstructor((Class[]) null);
}
beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Exception ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(beanDefinition, beanName, owner);
}
}
在这里不难看出实例化分两种情况,如果没有无参构造器是就生成CGLIB子类,否则就直接反射成实例。
public static T instantiateClass(Constructor ctor, Object... args) throws BeanInstantiationException {
Assert.notNull(ctor, "Constructor must not be null");
try {
ReflectionUtils.makeAccessible(ctor);
return ctor.newInstance(args);
}
既然已经有了实例对象了,那么,Spring是如何将bean的属性注入到bean的呢?返回到上面的doCreateBean方法中。往下看找到populateBean(beanName, mbd, instanceWrapper);,内幕就在这里。只贴部分代码:
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);
}
}
这里是调用InstantiationAwareBeanPostProcessor的具体子类的ibp.postProcessPropertyValues方法注入属性。当我们使用@Resource注解的时候,具体的子类是CommonAnnotationBeanPostProcessor;如果使用的是@Autowired注解,则具体的子类是AutowiredAnnotationBeanPostProcessor。此方法内是委托InjectionMetadata对象来完成属性注入。
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass());
try {
metadata.inject(bean, beanName, pvs);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}
findAutowiringMetadata方法能拿到使用了特定注解的属性(Field)、方法(Method)及依赖的关系保存到checkedElements集合
public void inject(Object target, String beanName, PropertyValues pvs) throws Throwable {
Collection elementsToIterate =
(this.checkedElements != null ? this.checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
boolean debug = logger.isDebugEnabled();
for (InjectedElement element : elementsToIterate) {
if (debug) {
logger.debug("Processing injected method of bean '" + beanName + "': " + element);
}
element.inject(target, beanName, pvs);
}
}
}
真正干事的还是InjectedElement的inject方法。
@Override
protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
Field field = (Field) this.member;
try {
Object value;
if (this.cached) {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
else {
DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
desc.setContainingClass(bean.getClass());
Set autowiredBeanNames = new LinkedHashSet(1);
TypeConverter typeConverter = beanFactory.getTypeConverter();
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
synchronized (this) {
if (!this.cached) {
if (value != null || this.required) {
this.cachedFieldValue = desc;
registerDependentBeans(beanName, autowiredBeanNames);
if (autowiredBeanNames.size() == 1) {
String autowiredBeanName = autowiredBeanNames.iterator().next();
if (beanFactory.containsBean(autowiredBeanName)) {
if (beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
this.cachedFieldValue = new RuntimeBeanReference(autowiredBeanName);
}
}
}
}
else {
this.cachedFieldValue = null;
}
this.cached = true;
}
}
}
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}
catch (Throwable ex) {
throw new BeanCreationException("Could not autowire field: " + field, ex);
}
}
}
其实别看代码这么多,最关键的部分就是:
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
在这里也就真相大白了,就是通过JDK反射特性,直接set值的。