一个Spring Bean从无到有的过程
有开头没结尾
经历了java开发变迁史，从早期编写原生的servlet、自研mvc和jdbc工具、和使用开源框架struts、hibernate、jsp、spring、springmvc、freemarker、springboot，到最后前后端分离式开发，一开始开发工具用的是editplus。相对来说，现在开发很好了，框架生态(只有spring生态传承下来了，其他都成了历史)。到spring生态圈终结了，它的体系太过庞大了，席卷了各国，项目产品技术换型的几率很小了。
一开始做Spring相关开发，写了大量xml配置，到后来换成了注解式开发，虽然能干活，可并不知道它的内在原理和设计理念是什么，随着时间的累计，需要做sping 扩展或集成，就要研究源码级是如何实现的了。我就以一个Spring Bean如何创建开始，不要小看一个Bean，很多码农都说不清它的创建过程。
简单介绍Spring框架是一个开放源代码的J2EE应用程序框架，由Rod Johnson发起，是针对bean的生命周期进行管理的轻量级容器（lightweight container）。 Spring解决了开发者在J2EE开发中遇到的许多常见的问题，提供了功能强大IOC、AOP及Web MVC等功能。Spring可以单独应用于构筑应用程序，也可以和Struts、Webwork、Tapestry等众多Web框架组合使用，并且可以与 Swing等桌面应用程序AP组合。因此， Spring不仅仅能应用于JEE应用程序之中，也可以应用于桌面应用程序以及小应用程序之中。Spring框架主要由七部分组成，分别是 Spring Core、 Spring AOP、 Spring ORM、 Spring DAO、Spring Context、 Spring Web和 Spring Web MVC。
就是图片里的Beans，下面开始进入正题，看仔细了，这很重要
介绍bean之前，说下ioc和context上下文（可以理解为一个宿主环境）
ioc是Inversion of Control的简称，行内话叫控制反转，早期开发都是需要对象，自己new出来一个，可有了ioc后，我们不需要
自己new对象了，让spring ioc容器负责对象的创建和管理。一句话概括ioc的作用颠倒了对象的依赖关系，ioc容器管理对象。
尽量搞明白bean，ioc，context，对以后学spring生态很重要。
Spring bean 分两种，一种普通bean，一种是工厂型的bean，但处理类型很多
一， 普通bean（以xml配置为例，现在改用注解的人越来越多了）
准备好原材料
spring-bean.xml文件（以演示为例）
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="user" class="spring.model.User">
<property name="id" value="1"/>
<property name="name" value="dongguangming"/>
<property name="age" value="99"/>
</bean>
</beans>User.java
public class User {
private int id;
private String name;
private int age;
//set,get略
}测试类
/**
*
* @author dgm
* @describe "xml bean"
* @date 2020年4月16日
*/
public class XMLConfigurationBeanApp {
public static void main(String[] args) {
/*DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);
reader.loadBeanDefinitions("conf/spring-bean.xml");*/
ApplicationContext applicationContext =
new ClassPathXmlApplicationContext("conf/spring-bean.xml");
User user1 = applicationContext.getBean("user", User.class);
User user2 = (User) applicationContext.getBean("user");
System.out.println(user1);
System.out.println(user2);
System.out.println(user1==user2);
//由于是个普通的bean，故出现异常，生成环境就不需要这样写了，我是测试两种不同类型的bean
System.out.println(applicationContext.getBean("&user"));
}
}输出结果
解析初始化过程，就从这行代码
ApplicationContext applicationContext =
new ClassPathXmlApplicationContext("spring-bean.xml");说起，看看spring做了多少事
三大阶段：bean解析，bean实例化，bean初始化，销毁
1. bean解析定义注册阶段
早期开发人员都知道，配置文件以xml文件（现在人都喜欢注解解析了）居多，要把xml文件内容解析成java对应的类，简称dom解析，如
spring也是如此，一开始进行大量的xml文件解析工作，和java对应的类映射好
文件路径org.springframework.beans.factory.xml.XmlBeanDefinitionReader.doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException
/**
* 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
* @see #doLoadDocument
* @see #registerBeanDefinitions
*/
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
//xml文档解析
Document doc = doLoadDocument(inputSource, resource);
//调用注册bean定义
return registerBeanDefinitions(doc, resource);
}
}
文件路径org.springframework.beans.factory.xml.XmlBeanDefinitionReader.registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
int countBefore = getRegistry().getBeanDefinitionCount();
//调用bean注册
documentReader.registerBeanDefinitions(doc,
createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}
。。。。。。嵌套很多
文件路径org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate)
/**
* Process the given bean element, parsing the bean definition
* and registering it with the registry.
*/
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.
//调用注册工具注册
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));
}
}
文件路径org.springframework.beans.factory.support.DefaultListableBeanFactory.registerBeanDefinition(String beanName, BeanDefinition beanDefinition) throws BeanDefinitionStoreException
@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
BeanDefinition oldBeanDefinition;
oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (this.logger.isWarnEnabled()) {
this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(oldBeanDefinition)) {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + oldBeanDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + oldBeanDefinition +
"] with [" + beanDefinition + "]");
}
}
//终于测测成功了，一种数据结构
this.beanDefinitionMap.put(beanName, beanDefinition);
}
else {
if (hasBeanCreationStarted()) {
// Cannot modify startup-time collection elements anymore (for stable iteration)
synchronized (this.beanDefinitionMap) {
//同上
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 (oldBeanDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
这只是一个bean定义（开发过程时有很多很多个Bean定义注册），很重要做那么多工作就是为了组装成这种数据结构：this.beanDefinitionMap.put(beanName, beanDefinition);它是这么定义的：
/** Map of bean definition objects, keyed by bean name */
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);beanDefinition的部分属性，把它当然一种组装数据的结构就行
2. Bean实例化和初始化阶段
调用时
ClassPathXmlApplicationContext(AbstractApplicationContext).finishBeanFactoryInitialization(ConfigurableListableBeanFactory) line: 861
{
//部分代码
// Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(new StringValueResolver() {
@Override
public String resolveStringValue(String strVal) {
return getEnvironment().resolvePlaceholders(strVal);
}
});
}
// Instantiate all remaining (non-lazy-init) singletons.
//这里面东西可多了去了
beanFactory.preInstantiateSingletons();
}preInstantiateSingletons核心代码：
@Override
public void preInstantiateSingletons() throws BeansException {
// 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<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
//工厂型bean（系统自带很多，当然也可以自己写）,实现FactoryBean接口或继承AbstractFactoryBean
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 {
//普通bean,人为实现，没有实现FactoryBean接口或没有继承AbstractFactoryBean
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
smartSingleton.afterSingletonsInstantiated();
return null;
}
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
0：DefaultListableBeanFactory(AbstractBeanFactory).getBean(String)
1 DefaultListableBeanFactory(AbstractBeanFactory).doGetBean(String, Class<T>, Object[], boolean) line: 303
2 DefaultListableBeanFactory(DefaultSingletonBeanRegistry).getSingleton(String) line: 231 ，第一次getSingleton
3 Object org.springframework.beans.factory.support.AbstractBeanFactory.createBean(String beanName, RootBeanDefinition mbd, Object[] args)
要分叉了：
4. org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd)
判断是否有InstantiationAwareBeanPostProcessor，applyBeanPostProcessorsBeforeInstantiation()>postProcessBeforeInstantiation(),
applyBeanPostProcessorsAfterInitialization()>postProcessAfterInitialization().
返回代理对象的机会，一旦返回，就没有下面的事了，aop代理由此可生
如果return ，直接跳到10阶段
5 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(String beanName, 4 RootBeanDefinition mbd, Object[] args) 很重要
6 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args), 实例化完成，详细请继续看源码，注意实例化策略方式不止一种：构造器，工厂方法，也有机会通过cglib
cglid代理
7 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(beanName, mbd, instanceWrapper)，,详细请继续看源码，下面是部分
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;
}此阶段也有机会产生分叉，特别是实现了InstantiationAwareBeanPostProcessor，提前返回，不再进行后续的autowireByName，autowireByType和属性赋值操作，但并不影响初始化操作
8 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(String beanName, Object bean, RootBeanDefinition mbd)，初始化完成
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
invokeAwareMethods(beanName, bean);
return null;
}
}, getAccessControlContext());
}
else {
//实现了后缀名Aware接口的调用在这个阶段，通知感应
//BeanNameAware
//BeanClassLoaderAware
//BeanFactoryAware
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
//实现BeanPostProcessor接口
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
//调用初始化方法，判断是否实现了InitializingBean，方法名afterPropertiesSet()，自定义初始化方法也在这里
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
//实现BeanPostProcessor接口
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
10 org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(String beanName, ObjectFactory<?> singletonFactory)，第二次getSingleton，但实现方法不一样
org.springframework.beans.factory.support.DefaultSingletonBeanRegistry
/** Cache of singleton objects: bean name --> bean instance */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name --> ObjectFactory */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name --> bean instance */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
/** Set of registered singletons, containing the bean names in registration order */
private final Set<String> registeredSingletons = new LinkedHashSet<>(256);注意篇幅有限: 列了一些核心方法，其实还有很多，由于代码量超超多，就不贴了
工厂型bean
该Bean实现了FactoryBean接口
public class UserFactoryBean<User> implements FactoryBean<User> {
private User user;
/**
* @return the user
*/
public User getUser() {
return user;
}
/**
* @param user the user to set
*/
public void setUser(User user) {
this.user = user;
}
/* (non-Javadoc)
* @see org.springframework.beans.factory.FactoryBean#getObjectType()
*/
@Override
public Class<?> getObjectType() {
return user.getClass();
}
/* (non-Javadoc)
* @see org.springframework.beans.factory.FactoryBean#getObject()
*/
@Override
public User getObject() throws Exception {
// TODO Auto-generated method stub
return user;
}
}它和上面没有实现FactoryBean接口的处理方式不太一样，大体上相同不再赘述
/**
* Obtain an object to expose from the given FactoryBean.
* @param factory the FactoryBean instance
* @param beanName the name of the bean
* @param shouldPostProcess whether the bean is subject to post-processing
* @return the object obtained from the FactoryBean
* @throws BeanCreationException if FactoryBean object creation failed
* @see org.springframework.beans.factory.FactoryBean#getObject()
*/
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
object = doGetObjectFromFactoryBean(factory, beanName);
// Only post-process and store if not put there already during getObject() call above
// (e.g. because of circular reference processing triggered by custom getBean calls)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (object != null && shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
}
this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
}
}
return (object != NULL_OBJECT ? object : null);
}
}
else {
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (object != null && shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}顺序图：
3. bean销毁
三种体现：
3.1 和初始化InitializingBean对应的DisposableBean接口，
3.2 自定义方法，destroy-method="destroyXML"
3.3 和@PostConstruct对应的 @PreDestroy
销毁顺序如图
代码就不举例了。
总结：其实你可以new User(6,"dongguangmming",99)，但和spring没关系，Spring很强大，各个阶段都有机会改造参与bean的过程，组合度也很高，****BeanDefinitionRegistryPostProcessor，***BeanPostProcessor，***BeanFactory，****BeanFactoryPostProcessor
记住Spring bean：解析注册，实例化，初始化，销毁，才能做扩展性开发或集成第三方组件（比如mybatis，dubbo，email，zk，redis等）到spring的生态圈里
参考：
0 Spring 框架简介 https://www.ibm.com/developerworks/cn/java/wa-spring1/
1. spring bean是什么 https://www.awaimai.com/2596.html
2 what-in-the-world-are-spring-beans https://stackoverflow.com/questions/17193365/what-in-the-world-are-spring-beans
3. Spring Bean Lifecycle https://www.benchresources.net/spring-bean-lifecycle/
4. Detailed tutorial on Redis caching in the SpringBoot series https://laptrinhx.com/detailed-tutorial-on-redis-caching-in-the-springboot-series-3352915639/
5. Spring Bean creation process http://www.programmersought.com/article/55942589567/
6. Inversion of Control Containers and the Dependency Injection pattern https://martinfowler.com/articles/injection.html
