本文深入分析Spring的IOC容器初始化过程，讲解如何创建并管理单实例对象。从ClassPathXMLApplicationContext构造器开始，通过BeanFactory和ApplicationContext接口探讨bean的生命周期，重点在于refresh()方法及其实现，解释了BeanFactory如何通过反射创建和注入bean，并最终保存在DefaultSingletonBeanRegistry的singletonObjects映射中。
Spring（IOC）
IOC：（Inversion（反转） Of Control）：控制反转；
控制：资源的获取方式；
主动式：（要什么资源都自己创建即可）
BookServlet{
BookService bs = new BookService();
AirPlane ap = new AirPlane();//复杂对象的创建是比较庞大的工程；
}
被动式：资源的获取不是我们自己创建，而是交给一个容器来创建和设置；
BookServlet{
BookService bs;
public void test01(){
bs.checkout();//
}
}
容器：管理所有的组件（有功能的类）；假设，BookServlet受容器管理，BookService也受容器管理；容器可以自动的探查出那些组件（类）需要用到另一写组件（类）；容器帮我们创建BookService对象，并把BookService对象赋值过去；
容器：主动的new资源变为被动的接受资源；
DI：（Dependency Injection）依赖注入；
容器能知道哪个组件（类）运行的时候，需要另外一个类（组件）；容器通过反射的形式，将容器中准备好的BookService对象注入（利用反射给属性赋值）到BookServlet中；
只要IOC容器管理的组件，都能使用容器提供的强大功能；
IOC源码分析：
IOC：
1、IOC是一个容器
2、容器启动的时候创建所有单实例对象
3、我们可以直接从容器中获取到这个对象
SpringIOC：
1）、ioc容器的启动过程？启动期间都做了什么（什么时候创建所有单实例bean）
2）、ioc是如何创建这些单实例bean，并如何管理的；到底保存在了那里？
思路：
从HelloWorld开始，调试每个方法的作用；
1、ClassPathXMLApplicationContext构造器
ApplicationContext ioc =new ClassPathXmlApplicationContext("ioc.xml")
this(new String[] {configLocation}, true, null);
public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
//所有单实例bean创建完成
refresh();
}
}
BeanFactory：Bean工厂；
refresh();实现；Spring；BeanFactory的流程；也是ioc容器的创建流程
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {//同步锁，这里用来保证多线程情况下，ioc容器只被创建一次
// Prepare this context for refreshing.
prepareRefresh();//准备容器刷新；
// Tell the subclass to refresh the internal bean factory.
//Spring解析xml配置文件将要创建的所有bean的配置信息保存起来，观看Spring对xml的解析
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);//bean工厂准备一些其他东西，毕竟bean工厂很大
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);//bean工厂的后置处理器
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);//注册工厂的后置处理器
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);//注册spring自己里边的一些后置处理器
// Initialize message source for this context.
//支持国际化功能的；
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
//留给子类的方法，用来支持国际化功能的
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//初始化所有单实例bean的地方
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;
}
}
}
finishBeanFactoryInitialization(beanFactory);实现
AbstractApplicationContext：
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.初始化类型转化
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);//设置临时的类加载器
// Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();//冻结配置，让别人别修改他
// Instantiate all remaining (non-lazy-init) singletons.初始化所有单实例
beanFactory.preInstantiateSingletons();
}
DefaultListableBeanFactory：bean工厂；创建bean
@Override
public void preInstantiateSingletons() throws BeansException {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Pre-instantiating singletons in " + this);
}
//拿到所有要创建的bean的名字
List<String> 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<String>(this.beanDefinitionNames);//拿到所有要创建的bean名
}
//按配置顺序创建bean
for (String beanName : beanNames) {
//根据beandid获取到bean的定义信息（包括bean的详细信息）；
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//判断bean是单实例的，并且不是抽象的，并且不是懒加载的
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//是否是一个实现了FactoryBean接口的bean,比如之前实现过该接口的类，
//只要是这个接口的实现类，Spring都认为是一个工厂；1、ioc容器启动的时候不会创建实例2、FactoryBean；获取的时候的才创建对象
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<Boolean>() {
@Override
public Boolean run() {
return ((SmartFactoryBean<?>) factory).isEagerInit();
}
}, getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
else {
getBean(beanName);
}
}
}
}
getBean(beanName);创建bean的细节
getBean(){
//所有的getBean掉的是它
doGetBean(name, null, null, false);
}
AbstractBeanFactory：doGetBean(name, null, null, false);
@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);//获取要创建的bean的名字
Object bean;
// Eagerly check singleton cache for manually registered singletons.
//先从已经注册的所有单实例bean中看有没有个bean。第一次创建bean是没有的；
Object sharedInstance = getSingleton(beanName);//第一次创建等于null
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);
}
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);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);//标志bean被创建，防止多线程别人一进来，看到这个bean已经被创建，就不创建了
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);//获取合并了的本地的bean的定义信息
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//拿到创建当前bean之前需要提前创建的bean。depends-on属性；如果有就循环创建
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.创建bean实例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
return createBean(beanName, mbd, args);
}
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;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// 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<Object>() {
@Override
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);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw 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());
}
}
return (T) bean;
}
DefaultSingletonBeanRegistry：
/** Cache of singleton objects: bean name --> bean instance */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);
getSingleton方法；
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "'beanName' must not be null");
synchronized (this.singletonObjects) {
//先从一个地方将这个bean get出来
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while the singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
beforeSingletonCreation(beanName);
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<Exception>();
}
try {
//创建bean
singletonObject = singletonFactory.getObject();
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
//添加创建的bean
addSingleton(beanName, singletonObject);
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
创建好的对象最终会保存在一个map中；
ioc容器之一：保存单实例bean的地方；
ioc就是一个容器，单实例bean保存在一个map中；
DefaultSingletonBeanRegistry-singletonObjects
BeanFactory和ApplicationContext的区别；
ApplicationContext是BeanFactory的子接口；
BeanFactory：bean工厂接口；负责创建bean实例；容器里面保存的所有单例bean其实是一个map；Spring最底层的接口；
ApplicationContext：是容器接口；更多的负责容器功能的实现；（可以基于beanFactory创建好的对象之上完成强大的容器）
容器可以从map获取这个bean，并且aop。di。在ApplicationContext接口的下的这些类里面；
BeanFactory最底层的接口，ApplicationContext留给程序员使用的ioc容器接口；ApplicationContext是BeanFactory的子接口；
ApplicationContext
Spring里面最大的模式就是工厂模式；
BeanFactory：bean工厂；工厂模式；帮用户创建bean
IOC容器初始化总结：
ApplicationContext ioc =new ClassPathXmlApplicationContext(“ioc.xml”)
首先调用ClassPathXMLApplicationContext构造器，进入构造器中，
public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
//所有单实例bean创建完成
refresh();
}
}
调用refresh()方法，完成所有单实例bean的创建。
refresh()的实现过程如下，首先该方法中调用了一个初始化所有单实例bean的方法finishBeanFactoryInitialization(beanFactory）
1、finishBeanFactoryInitialization(beanFactory);该方法在AbstractApplicationContext类中，然后冻结了配置，让别人别修改他，然后调用beanFactory.preInstantiateSingletons();初始化所有单实例
2、beanFactory.preInstantiateSingletons()；该方法在DefaultListableBeanFactory类中，在该方法中，
- ①首先是先拿到所有要创建的bean的名字List beanNames;
- ②然后调用增强 for (String beanName : beanNames) {}去根据beandid获取到bean的定义信息（包括bean的详细信息）-> RootBeanDefinition bd =
getMergedLocalBeanDefinition(beanName); - ③判断bean是单实例的，并且不是抽象的，并且不是懒加载的
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
//是否是一个实现了FactoryBean接口的bean,比如之前实现过该接口的类
//只要是这个接口的实现类，Spring都认为是一个工厂；
//1、ioc容器启动的时候不会创建实例
//2、FactoryBean；获取的时候的才创建对象
}
}else{
getBean(beanName);//调用getBean方法，该方法是创建bean的细节
}
}
3、getBean(){ doGetBean(name, null, null, false); //所有的getBean掉的是它 }
4、在doGetBean方法中，
- ① 先获取要创建的bean的名字 final String beanName = transformedBeanName(name);
- ②从已经注册的所有单实例bean中看有没有个bean。第一次创建bean是没有的；bject sharedInstance = getSingleton(beanName);//第一次创建等于null，若有就返回获取到的bean–>return (T) bean;即第⑦步
- ③ markBeanAsCreated(beanName);//标志bean被创建，防止多线程别人一进来，看到这个bean已经被创建，就不创建了
- ④根据id获取合并了的本地的bean的定义信息final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
- ⑤判断拿到创建当前bean之前需要提前创建的bean。depends-on属性；如果有就循环创建String[] dependsOn = mbd.getDependsOn();若存在，则调用增强for (String dependsOnBean : dependsOn) {}对每一个都进行getBean()操作。
- ⑥开始创建bean实例 ，调用getSingleton方法，进行单实例的创建
if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override//重写了getObject方法
public Object getObject() throws BeansException {
try {
return createBean(beanName, mbd, args);
}
} 然后通过bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
- ⑦最后return (T) bean;
5、在getSingleton(String beanName, ObjectFactory<?> singletonFactory)方法中，
- ①先从一个保存单实例的map根据id将这个bean get出来 Object singletonObject = this.singletonObjects.get(beanName);
- ②若bean为空，就开始创建bean singletonObject = singletonFactory.getObject();
- ③然后添加创建的bean到Map中，Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>addSingleton(beanName, singletonObject);
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