一、简介
spring容器是整个spring框架的核心,通常我们说的spring容器就是bean工厂,bean工厂负责创建和初始化bean、装配bean并且管理应用程序中的bean.spring中提供了两个核心接口：BeanFactory和ApplicationContext,ApplicationContext是BeanFactory子接口,它提供了比BeanFactory更完善的功能.
二、ApplicationContext的工作原理
先建立一个新的java项目,搭建好spring的开发环境.然后启动spring的容器,如下面的代码:
public class Demo {
public static void main(String[] args) {
ApplicationContext cxt = new ClassPathXmlApplicationContext("beans.xml");
}
}
spring容器启动时,会完成两个重要的工作:加载bean的定义信息（BeanDefinition）以及初始化所有单例bean,在初始化bean的过程中注入bean的依赖.bean的定义信息是指：bean的基本属性,例如完整类名,是否单例等等,其实就是<bean id="" class="" scope="">元素的那些属性.在创建bean时需要用到这些属性,所以必须要先加载bean以及它的定义信息.
先说spring容器加载bean的定义信息的实现原理,spring中有两种配置bean的方法:
- 使用配置文件配置bean,需要在<bean>元素中声明bean的信息;spring容器启动时,会读取配置文件并进行解析,这种情况下,只要解析bean元素就可以获取bean的beanName和它的定义信息.
- 使用注解配置bean,需要在配置文件中配置bean的路径,例如:<context:component-scan base-package="cn.spring"/>,这样容器启动时就会扫描cn.spring包以及子包下面的所有类,如果类上有@Controller 或者 @Service 或者@Repository或者@Component注解,spring就会加载这些类的定义信息;这里就会有几个问题,第一个问题是如何获取base-package的子包以及包下的所有类?spring的做法是将包名转化成文件系统中的路径,然后traverse获取该目录下的所有.class文件,非常巧妙的一个解决方案!接下来的问题是如何从.class文件中获取bean的定义信息呢?有两种方式,第一种就是把通过.class文件的路径获取该类的包名,然后通过类加载器加载该类获取它的定义信息,第二种方式是用asm框架从class文件中直接读取类的定义信息。spring用的是第二种方式,个人觉得spring选择第二种方式是有以下几个原因,其一,可能需要对class文件进行增强处理,也就是在class文件中增加一些新的指令,在生成代理时可能会需要这样做;其二,反射无法获取类完完全全的信息(例如:方法的参数名称),其三,反射的性能问题;
接下来,就是容器初始化单例bean的过程:
spring容器在加载完所有bean的定义信息以后,会有一个refresh()操作,在refresh容器过程中完成两个重要的操作,第一个就是创建所有单例bean,第二个就是装配这些创建bean（注入它们所需要的依赖）;
因为前面的操作已经加载了所有bean的定义信息,并且维护了一个<beanName,BeanDefinition>对应关系的Map,遍历Map,就可以取得每个bean的定义信息,从bean的定义信息可以知道bean是否是单例,如果是单例的,下一步就会根据bean的定义信息来决定bean实例的创建策略,如果配置了bean的factory-method,就调用factory-method创建bean实例,如果没有配置factory-method,默认会调用bean的无参构造函数创建bean实例.
创建bean实例之后的工作就是装配bean,现在已经拿到了bean实例,如果bean是在配置文件中配置的,此时就会先把配置文件中配置的属性赋值给bean实例上对应的属性;而后由bean的后处理器(BeanPostProcessor)完成bean实例其他属性(通过注解配置的)的注入.如果bean是通过注解进行配置,这时直接就会由bean的后处理器完成bean的装配.完成bean装配的后处理器的工作原理:遍历bean对象的字段和方法,根据字段和方法上应的注解完成相对应的注入操作.
在装配bean的过程中会出现一个问题:A依赖B,装配A的时候B的实例还没有创建,spring解决这个问题的办法是:先创建B对象,装配好bean,然后把B注入A,继续完成A的装配.
三、容器初始化过程的源码分析
我们从ApplicationContext的构造函数开始,如下代码:
/**
* Create a new ClassPathXmlApplicationContext with the given parent,
* loading the definitions from the given XML files.
* @param configLocations array of resource locations
* @param refresh whether to automatically refresh the context,
* loading all bean definitions and creating all singletons. --->加载所有bean的定义信息,创建所有单例bean
* Alternatively, call refresh manually after further configuring the context.
* @param parent the parent context
* @throws BeansException if context creation failed
* @see #refresh()
*/
public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
throws BeansException {
super(parent); // 解析给定的配置文件,完成加载所有bean的定义信息的操作
setConfigLocations(configLocations);
if (refresh) { // refresh容器,完成创建单例bean的操作
refresh();
}
}
构造方法的注释上写的so nice.接下来,看加载bean的定义信息的过程,setConfigLocations()是在父类中实现的,接收到配置文件以后,容器开始解析配置文件.经过一系列的调用,会调用org.springframework.beans.factory.xml.XmlBeanDefinitionReader的doLoadBeanDefinitions(),到这里终于看到Document,下面是该方法的源码:
/**
* Actually load bean definitions from the specified XML file.
* @param inputSource the SAX InputSource to read from
* @param resource the resource descriptor for the XML file
* @return the number of bean definitions found
* @throws BeanDefinitionStoreException in case of loading or parsing errors
*/
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
int validationMode = getValidationModeForResource(resource);
// 取得Document对象
Document doc = this.documentLoader.loadDocument(
inputSource, getEntityResolver(), this.errorHandler, validationMode, isNamespaceAware());
// 从Document对象中解析bean的定义信息
return registerBeanDefinitions(doc, resource);
}
catch (BeanDefinitionStoreException ex) {
throw ex;
} // ...各种异常的处理
}
registerBeanDefinitions()又会经过一系列的检查和处理,然后调用
org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader的doRegisterBeanDefinitions(root)方法进行处理,这部分会从bean元素中解析出bean的定义信息,源代码如下:
/**
* Register each bean definition within the given root {@code <beans/>} element.
* @throws IllegalStateException if {@code <beans profile="..."} attribute is present
* and Environment property has not been set
* @see #setEnvironment
*/
protected void doRegisterBeanDefinitions(Element root) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
Assert.state(this.environment != null, "environment property must not be null");
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
if (!this.environment.acceptsProfiles(specifiedProfiles)) {
return;
}
}
// 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 = createHelper(readerContext, root, parent);
preProcessXml(root);
// 重点部分,解析bean的定义信息
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;
}
/**
* Parse the elements at the root level in the document:
* "import", "alias", "bean".
* @param root the DOM root element of the document
*/
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); // 重点:解析bean元素
}
else {
delegate.parseCustomElement(ele); // 重点:解析其他元素,例如:<context<context:component-scan> or <annotation:config/>
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}
到这里,终于到了关键的地方,如果bean是在配置文件中配置的,由parseDefaultElement(ele, delegate)处理bean元素的解析,如果是注解配置,parseCustomElement(ele)会扫描包下的class文件,并完成解析.我们先看配置文件中bean元素的解析方式。
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)) {
// 重点
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}
/**
* Process the given bean element, parsing the bean definition
* and registering it with the registry.
*/
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
// 获取bean的定义信息,用BeanDefinitionHodler对象封装
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.---》关键,将bean的定义信息保存到容器
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));
}
}
接下来就是调用org.springframework.beans.factory.support.BeanDefinitionReaderUtils的registerBeanDefinition()保存bean定义信息到容器的方法了.
/**
* Register the given bean definition with the given bean factory.
* @param definitionHolder the bean definition including name and aliases
* @param registry the bean factory to register with
* @throws BeanDefinitionStoreException if registration failed
*/
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.---》重点
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 aliase : aliases) {
registry.registerAlias(beanName, aliase);
}
}
}
ok,来看最终的保存代码:
org.springframework.beans.factory.support.DefaultListableBeanFactory中的registerBeanDefinition();先看DefaultListableBeanFactory保存bean定义信息的Map的声明：
/** Map of bean definition objects, keyed by bean name */
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>();
/** List of bean definition names, in registration order */
private final List<String> beanDefinitionNames = new ArrayList<String>();
保存bean定义信息的方法:
//---------------------------------------------------------------------
// Implementation of BeanDefinitionRegistry interface
//---------------------------------------------------------------------
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);
}
}
// 保存bean定义信息,线程同步
synchronized (this.beanDefinitionMap) { // 判断当前bean的定义信息是否已经保存
Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!this.allowBeanDefinitionOverriding) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean ‘" + beanName +
"‘: There is already [" + oldBeanDefinition + "] bound.");
}
else {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean ‘" + beanName +
"‘: replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
}
else { // 保存beanName
this.beanDefinitionNames.add(beanName);
this.frozenBeanDefinitionNames = null;
} // 保存beanName和bean的定义信息到Map
this.beanDefinitionMap.put(beanName, beanDefinition);
resetBeanDefinition(beanName);
}
}
上面就是spring解析配置文件中的bean定义信息,然后保存beanName和bean定义信息到Map中.这个过程主要就是xml的解析.接下来我们看spring是如何解析注解方式配置的bean.回到parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate)方法,现在重点关注:delegate.parseCustomElement(ele)方法.如果我们在配置文件用<context:component-scan base-package="">方式来指定自动扫描的包,之后就会调用org.springframework.context.annotation.ComponentScanBeanDefinitionParser的parse().下面是parse()方法的源代码:
public BeanDefinition parse(Element element, ParserContext parserContext) { // 解析<context:component-scan元素,获取base-package
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); //重点: 扫描basePackage下所有的class文件,读取bean的定义信息
Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages);
registerComponents(parserContext.getReaderContext(), beanDefinitions, element);
return null;
}
重点关注scanner.doScan(basePackges)方法,该方法完成整个核心操作--->根据包名获取包下所有的class的定义信息.直接看org.springframework.context.annotation.ClassPathBeanDefinitionScanner的scan():注意,看源码时一定要多关注注释,例如下面方法上的注释就非常有意义.
/**
* Perform a scan within the specified base packages,
* returning the registered bean definitions.
* <p>This method does <i>not</i> register an annotation config processor
* but rather leaves this up to the caller.
* @param basePackages the packages to check for annotated classes
* @return set of beans registered if any for tooling registration purposes (never {@code null})
*/
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
for (String basePackage : basePackages) { // 遍历每一个basepackages // 1.获取basePackage下bean的定义信息
Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
for (BeanDefinition candidate : candidates) { //2.根据扫描的信息,解析bean的一些定义信息
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); // 3.将bean的定义信息添加到容器中
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}
第1个步骤我们核心关注点,它完成从文件系统中读取class文件的操作,第3个步骤在之前已经说了,就是保存bean的定义信息到容器的DefaultListableBeanFactory的beanDefinitionMap 中.重点关注第1个步骤,看findCandidateComponents()的源代码:
/**
* Scan the class path for candidate components.
* @param basePackage the package to check for annotated classes
* @return a corresponding Set of autodetected bean definitions
*/
public Set<BeanDefinition> findCandidateComponents(String basePackage) {
Set<BeanDefinition> candidates = new LinkedHashSet<BeanDefinition>();
try {
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
resolveBasePackage(basePackage) + "/" + this.resourcePattern; // 1.获取包下的class文件路径,例如E:\Program Files (x86)\MyEclipse10\workplace2\spr\bin\cn\jack\domain\User.class, // 每一个class文件的路径封装成Resource对象.
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 { // 2.使用asm框架读取class文件,获取类的定义信息
MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource);
if (isCandidateComponent(metadataReader)) {
ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
sbd.setResource(resource);
sbd.setSource(resource);
if (isCandidateComponent(sbd)) {
if (debugEnabled) {
logger.debug("Identified candidate component class: " + resource);
}
candidates.add(sbd);
}
else {
if (debugEnabled) {
logger.debug("Ignored because not a concrete top-level class: " + resource);
}
}
}
else {
if (traceEnabled) {
logger.trace("Ignored because not matching any filter: " + resource);
}
}
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to read candidate component class: " + resource, ex);
}
}
else {
if (traceEnabled) {
logger.trace("Ignored because not readable: " + resource);
}
}
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
} //3.返回benadefinition集合
return candidates;
}
第1个步骤主要是解析文件路径,然后遍历文件夹获取每个class文件的地址;第2个步骤用asm框架来读取class文件获取类的信息封装成BeanDefinition对象.
第2个步骤最后调用的是org.springframework.core.type.classreading.SimpleMetadataReader的构造函数,下面是该类的部分源代码：
final class SimpleMetadataReader implements MetadataReader {
private final Resource resource;
private final ClassMetadata classMetadata;
private final AnnotationMetadata annotationMetadata;
SimpleMetadataReader(Resource resource, ClassLoader classLoader) throws IOException {
InputStream is = resource.getInputStream();
ClassReader classReader = null;
try { // asm框架读取class文件
classReader = new ClassReader(is);
} finally {
is.close();
}
// 采用访问者模式来获取class类信息
AnnotationMetadataReadingVisitor visitor = new AnnotationMetadataReadingVisitor(classLoader);
classReader.accept(visitor, true);
this.annotationMetadata = visitor;
// (since AnnotationMetadataReader extends ClassMetadataReadingVisitor)
this.classMetadata = visitor;
this.resource = resource;
}
ClassReader是asm框架中核心类,具体用法可以参考asm的官网.
上面说的过程就是spring容器加载bean定义信息的过程.过程很长,但实现原理却并不复杂.
2. 初始化单例bean的过程
上面分析了spring容器加载bean定义信息的过程,接下来分析bean的初始化以及创建bean的过程.回到ApplicationContext中的构造函数,入口为refresh().refresh方法在父类中实现的。下面是AbstractApplicationContext类refresh()方法的源代码:
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) { // 线程同步
// 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);// 容器的后处理器
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);//调用容器的后处理器
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);//注册bean的后处理器
// 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);// 重点,注释写的so nice，初始化所有单例bean
// 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;
}
}
}
后处理器是一种特殊bean,用于完成一些自身操作.
容器后处理器：对容器本身进行处理，在容器实例化其他任何Bean之前读取配置文件的元数据并可能修改这些数据.PropertyPlaceholderConfigurer就是一个容器后处理器,用于完成beans.xml中引入其他配置文件中内容操作.
Bean后处理器：即当Spring容器创建完Bean实例之后对bean进行一些处理，例如:完成bean的装配等操作。
回到refresh()方法,重点关注:finishBeanFactoryInitialization(beanFactory);这个方法会调用DefaultListableBeanFactory.preInstantiateSingletons方法.
public void preInstantiateSingletons() throws BeansException {
if (this.logger.isInfoEnabled()) {
this.logger.info("Pre-instantiating singletons in " + this);
}
synchronized (this.beanDefinitionMap) { // 线程同步
// 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.
List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames);
for (String beanName : beanNames) {// 遍历beanNames
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {//单例非延迟实例的bean
if (isFactoryBean(beanName)) { // 工厂bean.FactoryBean接口的子类
final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
public Boolean run() {
return ((SmartFactoryBean) factory).isEagerInit();
}
}, getAccessControlContext());
}
else {//普通bean
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
else {
getBean(beanName); // 从容器中获取bean,如果没有创建,并完成装配
}
}
}
}
}
getBean(beanName)方法会调用doGetBean方法.这是个很关键的地方,切记注释很重要
/**
* Return an instance, which may be shared or independent, of the specified bean.
* @param name the name of the bean to retrieve
* @param requiredType the required type of the bean to retrieve
* @param args arguments to use if creating a prototype using explicit arguments to a
* static factory method. It is invalid to use a non-null args value in any other case.
* @param typeCheckOnly whether the instance is obtained for a type check,
* not for actual use
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, final Class<T> 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) { //bean已创建,调用方法返回该bean
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean ‘" + beanName +
"‘ that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean ‘" + beanName + "‘");
}
} //如果是工厂bean,则返回beanFactory.getObject(),普通bean直接返回sharedInstance
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else { //bean未创建
// 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(); // 检查父容器是否已经创建该bean,有则从父容器获取bean返回
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);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
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) {
getBean(dependsOnBean);
registerDependentBean(dependsOnBean, beanName);
}
}
// Create bean instance.---》创建单例bean
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
try {
return createBean(beanName, mbd, args);// ---> 创建bean的方法
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
}
}); // 返回创建的单例bean
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) { // 创建原型bean,scope="prototype"
// It‘s a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope ‘" + scopeName + "‘");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope ‘" + scopeName + "‘ is not active for the current thread; " +
"consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean ‘" + name + "‘ to required type [" +
ClassUtils.getQualifiedName(requiredType) + "]", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
} // 返回bean
return (T) bean;
}
createBean(beanName, mbd, args)方法会调用doCreateBean()完成bean的创建工作,源代码如下：
/**
* Actually create the specified bean. Pre-creation processing has already happened
* at this point, e.g. checking <code>postProcessBeforeInstantiation</code> callbacks.
* <p>Differentiates between default bean instantiation, use of a
* factory method, and autowiring a constructor.
* @param beanName the name of the bean
* @param mbd the merged bean definition for the bean
* @param args arguments to use if creating a prototype using explicit arguments to a
* static factory method. This parameter must be <code>null</code> except in this case.
* @return a new instance of the bean
* @throws BeanCreationException if the bean could not be created
* @see #instantiateBean
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
*/
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) { // 1.创建bean的包装类,装饰设计模式
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean ‘" + beanName +
"‘ to allow for resolving potential circular references");
}
addSingletonFactory(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
Object exposedObject = bean;
try { //2.装配bean
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name ‘" + beanName + "‘ has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"‘getBeanNamesOfType‘ with the ‘allowEagerInit‘ flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
//3.返回
return exposedObject;
}
首先看第1个步骤,这个步骤中会创建bean实例和bean的包装类,这里使用了装饰设计模式.创建bean的实例过程比较简单,如果配置bean时指定了bean的创建方法 factory-method,就用factory-method创建bean实例,默认会使用无参构造函数创建bean实例.这部分重点关注装配bean的过程.
/**
* Create a new instance for the specified bean, using an appropriate instantiation strategy:
* factory method, constructor autowiring, or simple instantiation.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @param args arguments to use if creating a prototype using explicit arguments to a
* static factory method. It is invalid to use a non-null args value in any other case.
* @return BeanWrapper for the new instance
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
* @see #instantiateBean
*/
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// Make sure bean class is actually resolved at this point.
Class beanClass = resolveBeanClass(mbd, beanName);
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn‘t public, and non-public access not allowed: " + beanClass.getName());
}
if (mbd.getFactoryMethodName() != null) {// 使用工厂方法创建bean,<bean factory-method="">
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
// Need to determine the constructor...
Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// 用默认的构造函数创建bean,反射获取构造函数,constructor.newInstance()创建bean.
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
现在来看装配bean的过程,这个过程完成注入bean的依赖对象,如果bean是在配置文件配置的,则把从xml中解析出来的属性注入给bean实例,如果是用注解配置的依赖(@Resource 或者@AutoWired),则会解析bean的字段或者方法上的注解,根据这些注解找到对应的依赖,如果依赖对象已经创建,就直接注入依赖,否则,先创建依赖对象,在完成注入操作.
/**
* Populate the bean instance in the given BeanWrapper with the property values
* from the bean definition.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @param bw BeanWrapper with bean instance
*/
protected void populateBean(String beanName, AbstractBeanDefinition mbd, BeanWrapper bw) {
PropertyValues pvs = mbd.getPropertyValues();
if (bw == null) {
if (!pvs.isEmpty()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
boolean continueWithPropertyPopulation = true;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
// 根据beanName或者type完成自动装配
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
// 调用bean后处理器
if (hasInstAwareBpps || needsDepCheck) {
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; // 重点:获取bean要装配的属性和属性值
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
// 给bean的属性赋值
applyPropertyValues(beanName, mbd, bw, pvs);
}
InstantiationAwareBeanPostProcessor是BeanPostProcessor的子接口,能在bean初始化前后对bean进行处理.InstantiationAwareBeanPostProcessor有以下几个子类：
@Resource注解注入依赖的工作就是由CommonAnnotationBeanPostProcessor完成的.下面是该类postProcessPropertyValues()的源码:
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
//1.根据bean的字节码,遍历所有的字段和方法,获取需要注入的字段或者方法
InjectionMetadata metadata = findResourceMetadata(bean.getClass());
try { //2.从容器中查找依赖对象,并赋值给相应的字段,完成bean的装配
metadata.inject(bean, beanName, pvs);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of resource dependencies failed", ex);
}
return pvs;
}
第1个步骤主要就是根据字节码获取所有字段和方法，然后遍历查找有@Resource注解的字段或方法以及依赖bean的beanName,第2个步骤从容器中依赖对象的beanName(@Resource的name属性值),如果容器中没有该依赖对象就创建,有的话就直接获取,并赋值给bean的属性.这样,就通过bean的后处理器完成了bean的装配过程.
到这里,容器的启动过程就完成了,此时就可以对外提供服务了.上面就是本人对spring容器部分源码学习的一些总结.日后,了解更多会不定时更新上来!
时间： 2024-10-29 19:07:34