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<!– TOC –>
- 前言
- BeanDefinition
- BeanFactory 简介
- Web 容器启动过程
- bean 的加载
- FactoryBean
- 循环依赖
- bean 生命周期
- 公众号
<!– /TOC –>
前言
许多文章都是剖析的 xml 配置，然而当初 Spring Boot 开发多基于注解。本文从 注解 的角度剖析 Spring IOC 容器源码。
版本：
- Spring Boot：2.1.6.RELEASE
- Spring FrameWork：5.1.8.RELEASE
- Java 8
文章局部内容参考自：https://www.javadoop.com/post…
BeanDefinition
BeanDefinition 接口定义了一个蕴含属性、结构器参数、其余具体信息的 bean 实例。
public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
// ConfigurableBeanFactory 中只有 2 种：singleton 和 prototype。// request, session 等是基于 Web 的扩大
String SCOPE_SINGLETON = ConfigurableBeanFactory.SCOPE_SINGLETON;
String SCOPE_PROTOTYPE = ConfigurableBeanFactory.SCOPE_PROTOTYPE;
// 不重要
int ROLE_APPLICATION = 0;
int ROLE_SUPPORT = 1;
int ROLE_INFRASTRUCTURE = 2;
// 设置父 Bean 的信息（继承父 Bean 的配置信息）void setParentName(@Nullable String parentName);
@Nullable
String getParentName();
// 设置 Bean 的类名称，要通过反射来生成实例
void setBeanClassName(@Nullable String beanClassName);
// 返回以后 Bean 的 class name
String getBeanClassName();
void setScope(@Nullable String scope);
@Nullable
String getScope();
// 是否提早初始化
void setLazyInit(boolean lazyInit);
boolean isLazyInit();
// 设置该 Bean 依赖的所有的 Bean，并非 @Autowire 标记的
void setDependsOn(@Nullable String... dependsOn);
@Nullable
String[] getDependsOn();
// 设置该 Bean 是否能够注入到其余 Bean 中，只对依据类型注入无效，// 如果依据名称注入，即便这边设置了 false，也是能够的
void setAutowireCandidate(boolean autowireCandidate);
boolean isAutowireCandidate();
// 同一接口的多个实现，如果不指定名字，Spring 会优先选择设置 primary 为 true 的 bean
void setPrimary(boolean primary);
boolean isPrimary();
// 如果该 Bean 采纳工厂办法生成，指定工厂名称；否则用反射生成
void setFactoryBeanName(@Nullable String factoryBeanName);
@Nullable
String getFactoryBeanName();
// 指定工厂类中的 工厂办法名称
void setFactoryMethodName(@Nullable String factoryMethodName);
@Nullable
String getFactoryMethodName();
// 返回该 bean 的结构器参数
ConstructorArgumentValues getConstructorArgumentValues();
default boolean hasConstructorArgumentValues() {return !getConstructorArgumentValues().isEmpty();}
// Bean 中的属性值，返回的实例在 bean factory post-processing 期间会被更改
MutablePropertyValues getPropertyValues();
default boolean hasPropertyValues() {return !getPropertyValues().isEmpty();}
void setInitMethodName(@Nullable String initMethodName);
@Nullable
String getInitMethodName();
void setDestroyMethodName(@Nullable String destroyMethodName);
@Nullable
String getDestroyMethodName();
void setRole(int role);
int getRole();
void setDescription(@Nullable String description);
@Nullable
String getDescription();
// Read-only attributes
boolean isSingleton();
boolean isPrototype();
boolean isAbstract();
@Nullable
String getResourceDescription();
@Nullable
BeanDefinition getOriginatingBeanDefinition();}AnnotationConfigUtils#processCommonDefinitionAnnotations(…)
static void processCommonDefinitionAnnotations(AnnotatedBeanDefinition abd, AnnotatedTypeMetadata metadata) {AnnotationAttributes lazy = attributesFor(metadata, Lazy.class);
if (lazy != null) {abd.setLazyInit(lazy.getBoolean("value"));
}
else if (abd.getMetadata() != metadata) {lazy = attributesFor(abd.getMetadata(), Lazy.class);
if (lazy != null) {abd.setLazyInit(lazy.getBoolean("value"));
}
}
if (metadata.isAnnotated(Primary.class.getName())) {abd.setPrimary(true);
}
AnnotationAttributes dependsOn = attributesFor(metadata, DependsOn.class);
if (dependsOn != null) {abd.setDependsOn(dependsOn.getStringArray("value"));
}
AnnotationAttributes role = attributesFor(metadata, Role.class);
if (role != null) {abd.setRole(role.getNumber("value").intValue());
}
AnnotationAttributes description = attributesFor(metadata, Description.class);
if (description != null) {abd.setDescription(description.getString("value"));
}
}能够看到，processCommonDefinitionAnnotations 办法会依据注解来填充 AnnotatedBeanDefinition，这些注解有：
- Lazy
- Primary
- DependsOn
- Role
- Description
向上查看调用，发现会在 ConfigurationClassBeanDefinitionReader#registerBeanDefinitionForImportedConfigurationClass 将其注册为一个 bean definition。
private void registerBeanDefinitionForImportedConfigurationClass(ConfigurationClass configClass) {AnnotationMetadata metadata = configClass.getMetadata();
AnnotatedGenericBeanDefinition configBeanDef = new AnnotatedGenericBeanDefinition(metadata);
ScopeMetadata scopeMetadata = scopeMetadataResolver.resolveScopeMetadata(configBeanDef);
configBeanDef.setScope(scopeMetadata.getScopeName());
String configBeanName = this.importBeanNameGenerator.generateBeanName(configBeanDef, this.registry);
// 1. 通过注解填充 configBeanDef
AnnotationConfigUtils.processCommonDefinitionAnnotations(configBeanDef, metadata);
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(configBeanDef, configBeanName);
definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
// 2. 将 bean definition 注册到 registry 中
this.registry.registerBeanDefinition(definitionHolder.getBeanName(), definitionHolder.getBeanDefinition());
configClass.setBeanName(configBeanName);
if (logger.isTraceEnabled()) {logger.trace("Registered bean definition for imported class'" + configBeanName + "'");
}
}最终会被 AbstractApplicationContext#refresh 的 invokeBeanFactoryPostProcessors(beanFactory) 办法调用。
@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();}
...
}
}BeanFactory 简介
BeanFactory 是生产 bean 的工厂，它负责生产和治理各个 bean 实例。从下图能够看到，ApplicationContext 也是一个 BeanFactory。如果说 BeanFactory 是 Spring 的心脏，那么 ApplicationContext 就是残缺的身躯。
ApplicationContext 是利用程序运行时提供配置信息的通用接口。ApplicationContext 在程序运行时是不可更改的，然而实现类能够从新再入配置信息。
ApplicationContext 的实现类有很多，比方 AnnotationConfigApplicationContext, AnnotationConfigWebApplicationContext, ClassPathXmlApplicationContext, FileSystemXmlApplicationContext, XmlWebApplicationContext 等。咱们下面剖析的就是 AnnotationConfigApplicationContext，其采纳注解的形式提供配置信息，这样咱们就不必写 XML 配置文件了，十分简洁。
Web 容器启动过程
本文应用 Spring Boot 开发，其启动的代码是：
@SpringBootApplication
@EnableScheduling
@EnableAspectJAutoProxy
public class AppApplication {public static void main(String[] args) {SpringApplication.run(AppApplication.class, args);
}
}外围的点是这一句：
SpringApplication.run(AppApplication.class, args);SpringApplication 的代码就不剖析了，明确本次看源码的目标是剖析 容器源码，Spring Boot 的启动过程和其余信息都疏忽了，因为 Spring 代码切实是庞杂。剖析下面的 run 办法，最终会追踪到 SpringApplication#run(…) 办法。
public ConfigurableApplicationContext run(String... args) {StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
configureHeadlessProperty();
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
configureIgnoreBeanInfo(environment);
Banner printedBanner = printBanner(environment);
context = createApplicationContext();
exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class}, context);
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
refreshContext(context);
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
}
listeners.started(context);
callRunners(context, applicationArguments);
}
catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
}
try {listeners.running(context);
}
catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
}
return context;
}跟 context 相干的，是上面这 3 句代码：
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
refreshContext(context);
afterRefresh(context, applicationArguments);refreshContext 办法就是刷新给定的 context：
private void refreshContext(ConfigurableApplicationContext context) {refresh(context);
if (this.registerShutdownHook) {
try {context.registerShutdownHook();
}
catch (AccessControlException ex) {// Not allowed in some environments.}
}
}protected void refresh(ApplicationContext applicationContext) {Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
((AbstractApplicationContext) applicationContext).refresh();}会发现最终调用到了 AbstractApplicationContext#refresh 办法。正文参考自：https://www.javadoop.com/post…
@Override
public void refresh() throws BeansException, IllegalStateException {// 来个锁，不然 refresh() 还没完结，你又来个启动或销毁容器的操作，那不就乱套了嘛
synchronized (this.startupShutdownMonitor) {
// 筹备工作，记录下容器的启动工夫、标记“已启动”状态、解决配置文件中的占位符
prepareRefresh();
// 这步比拟要害，这步实现后，配置文件就会解析成一个个 Bean 定义，注册到 BeanFactory 中，// 当然，这里说的 Bean 还没有初始化，只是配置信息都提取进去了，// 注册也只是将这些信息都保留到了注册核心(说到底外围是一个 beanName-> beanDefinition 的 map)
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 设置 BeanFactory 的类加载器，增加几个 BeanPostProcessor，手动注册几个非凡的 bean
prepareBeanFactory(beanFactory);
try {
//【这里须要晓得 BeanFactoryPostProcessor 这个知识点，Bean 如果实现了此接口，// 那么在容器初始化当前，Spring 会负责调用外面的 postProcessBeanFactory 办法。】// 这里是提供给子类的扩大点，到这里的时候，所有的 Bean 都加载、注册实现了，然而都还没有初始化
// 具体的子类能够在这步的时候增加一些非凡的 BeanFactoryPostProcessor 的实现类或做点什么事
postProcessBeanFactory(beanFactory);
// 调用 BeanFactoryPostProcessor 各个实现类的 postProcessBeanFactory(factory) 办法
invokeBeanFactoryPostProcessors(beanFactory);
// 注册 BeanPostProcessor 的实现类，留神看和 BeanFactoryPostProcessor 的区别
// 此接口两个办法: postProcessBeforeInitialization 和 postProcessAfterInitialization
// 两个办法别离在 Bean 初始化之前和初始化之后失去执行。留神，到这里 Bean 还没初始化
registerBeanPostProcessors(beanFactory);
// 初始化以后 ApplicationContext 的 MessageSource，国际化这里就不开展说了，不然没完没了了
initMessageSource();
// 初始化以后 ApplicationContext 的事件播送器，这里也不开展了
initApplicationEventMulticaster();
// 从办法名就能够晓得，典型的模板办法(钩子办法)，// 具体的子类能够在这里初始化一些非凡的 Bean（在初始化 singleton beans 之前）onRefresh();
// 注册事件监听器，监听器须要实现 ApplicationListener 接口。这也不是咱们的重点，过
registerListeners();
// 重点，重点，重点
// 初始化所有的 singleton beans
//（lazy-init 的除外）finishBeanFactoryInitialization(beanFactory);
// 最初，播送事件，ApplicationContext 初始化实现
finishRefresh();}
catch (BeansException ex) {if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization -" +
"cancelling refresh attempt:" + ex);
}
// Destroy already created singletons to avoid dangling resources.
// 销毁曾经初始化的 singleton 的 Beans，免得有些 bean 会始终占用资源
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();}
}
}外围流程就是 try 代码块里的内容，咱们应该理解整体原理，本篇文章并不能逐行逐句剖析。如果那样做，齐全就变成一部字典了……
bean 的加载
bean 加载的调用函数：org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// 提取对应 bean 的名字
final String beanName = transformedBeanName(name);
Object bean;
// 1. 重要，重要，重要！// 创立单例 bean 防止循环依赖，尝试从缓存中获取
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {if (logger.isTraceEnabled()) {if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean'" + beanName +
"'that is not fully initialized yet - a consequence of a circular reference");
}
else {logger.trace("Returning cached instance of singleton bean'" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// 存在循环依赖
if (isPrototypeCurrentlyInCreation(beanName)) {
// 原型模式间接抛出异样（循环依赖仅能在单例模式下解决）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 (parentBeanFactory instanceof AbstractBeanFactory) {return ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {return (T) parentBeanFactory.getBean(nameToLookup);
}
}
// 如果不是仅仅做类型查看，则是创立 bean，须要做记录
if (!typeCheckOnly) {markBeanAsCreated(beanName);
}
try {
// 获取 RootBeanDefinition，如果指定 beanName 是子 bean 的话，须要合并父类属性
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 若存在依赖，须要递归实例化依赖的 bean
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {for (String dep : dependsOn) {if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between'" + beanName + "'and'" + dep + "'");
}
registerDependentBean(dep, beanName);
try {getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'"+ beanName +"' depends on missing bean '"+ dep +"'", ex);
}
}
}
// 创立 bean 实例
// Singleton 模式的创立
if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, () -> {
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);
}
// Prototype 模式的创立
else if (mbd.isPrototype()) {
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 name'" + scopeName + "'");
}
try {Object scopedInstance = scope.get(beanName, () -> {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;
}
}
// 检测 requiredType 是否为 bean 的理论类型，不是则转换，不胜利则抛出异样
if (requiredType != null && !requiredType.isInstance(bean)) {
try {T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {if (logger.isTraceEnabled()) {
logger.trace("Failed to convert bean'" + name + "'to required type'" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}能够看到 bean 的加载是相当简单的。加载的步骤大抵如下：
- 转换对应 beanName
- 尝试从缓存中加载单例
- bean 的实例化
- 原型模式的依赖查看
- 检测 parentBeanFactory
- 将配置文件转换为 RootBeanDefinition
- 寻找依赖
- 针对不同的 scope 进行 bean 的创立
- 类型转换
FactoryBean
后面提到了 BeanFactory，这里又来了个 FactoryBean …… 据说 Spring 提供了 70 多个 FactoryBean 的实现，可见其在 Spring 框架中的位置。它们暗藏了实例化简单 bean 的细节，给下层利用带来便捷。
public interface FactoryBean<T> {// 返回 FactoryBean 创立的 bean 实例，如果 isSingleton() 返回 true，则该实例会放到 Spring 容器的单例缓存池中
@Nullable
T getObject() throws Exception;
// 返回 FactoryBean 创立的 bean 类型
@Nullable
Class<?> getObjectType();
default boolean isSingleton() {return true;}
}循环依赖
循环依赖就是 循环援用，两个或多个 bean 相互之间持有对方。那么 Spring 是如何解决循环依赖的？
在 Spring 中循环依赖一共有 3 种状况：
- 结构器循环依赖
- setter 循环依赖
- prototype 范畴的依赖解决
其中结构器循环依赖是无奈解决的，因为一个 bean 创立时首先要通过结构器，然而结构器相互依赖，就相当于 Java 中多线程死锁。
setter 注入造成的依赖是通过 Spring 容器提前裸露刚实现结构器注入但未实现其余步骤（如 setter 注入）的 bean 来实现的，而且只能解决 单例作用域的 bean 循环依赖。通过提前裸露一个单例工厂办法，从而使其余 bean 能援用到该 bean，代码如下：
@Nullable
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;
}其中 earlySingletonObjects 的定义为：
/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);对于 prototype 作用域的 bean，Spring 容器无奈实现依赖注入，因为 Spring 容器不缓存 prototype 作用域的 bean。
bean 生命周期
面试的话，Spring 的外围就在这里了，不过只有记住大体流程就行。
公众号
coding 笔记、点滴记录，当前的文章也会同步到公众号（Coding Insight）中，心愿大家关注 ^_^
代码和思维导图在 GitHub 我的项目中，欢送大家 star！
正文完