【Spring源码解析】—— 依赖注入结合SpringMVC Demo-xml配置理解
2024-10-02 09:23:01
在IOC容器初始化的梳理之后，对依赖注入做一个总结，就是bean实例化的过程，bean的定义有两种方式，一种是xml文件配置，一种是注解，这里是对xml配置文件的依赖注入的介绍，后续对bean与该部分的不同会再做介绍。
先对整个过程做一个整体说明：从refresh()的入口进入之后，因为是注入依赖实例化bean的过程，所以在这个过程中，会不断判断是否有可用的快捷方式，是什么类型的，是singleton的，还是protype类型的，进行getBean获取bean的操作时，针对已经创建的可以直接返回的直接结束，若无则进行创建，创建中会要判断是否有parentFactory，是否有依赖bean，set创建过程中可能需要的条件，例如method overrides的验证和准备等，获取可用的constructor，有可用的构造器，有参数的情况等，都要走自主选择的逻辑，否则全部最后走兜底策略，就是默认构造器，注意，在对constructor选择的过程中，可能会出现无可用，或者是Ambiguous的情况，会抛出异常；具体最终创建bean要用什么策略，提供两种选择
一、Demo部分的修改：
首先是xml配置相关，首先在cn.lx.controller包中新建一个TestController2.java，将之前配置的注解的形式全部注释掉，在xml中将bean配置清楚，TestController2的类内容如下：
//@Controller
public class TestController2 {
// @RequestMapping("/test.form")
public void execute(){
return ;
}
}
修改applicationContext.xml文件，为如下：
<?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="simpleUrlHandlerMapping" class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
<property name="mappings">
<props>
<prop key="/test.form">startControl</prop>
</props>
</property>
</bean>
<bean id="startControl" class="cn.lx.controller.TestController2"></bean>
</beans>
二、依赖注入过程介绍：
依赖注入会发生两种场景，一种是：IOC容器初始化到最后阶段，会将none-Lazy-init的bean全部都实例化，其他的会在具体用到的时候实例化；先从第一种情况IOC容器初始化最后阶段来看实例化的整个过程，实例化的过程后续阶段都是相同的，只是入口不同而已
现在，正文开始，共分为三个步骤：入口-xml配置方式的实例化入口、获取bean、new-bean（实例化bean）
入口部分-xml配置方式的实例化入口：
从AbstractApplicationContext类的refresh()方法中，之前在IOC初始化中主要从obtainFreshBeanFactory()方法调用后得到了ConfigurableListableBeanFactory类型的beanFactory，在beanFactry中已经对xml文件做了解析并且将需要的信息都存储到了beanDefinitionMap中，之后将beanFactory作为参数传递给finishBeanFactoryInitialization方法，进入到其具体实现中：
//完成beanfactory的初始化，初始化所有保留的单例bean
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
//类型转换服务，用于提供线程安全的类型转换
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));
}
// 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(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// 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.
//预实例化，对未设置lazy-init=true的进行预实例化处理
beanFactory.preInstantiateSingletons();
}
之后，之后看最后一步，通过beanFactory.preInstantiateSingletons()进入到：DefaultListableBeanFactory类中，该方法会进行判断，会判断是否不是抽象且为singleton且是non lazy-init的，就会进入到是否为FactoryBean的判断，最终发现不是FactoryBean,就进入了else的逻辑分支，即：getBean(beanName)了，从这个部分就进入到下一个关键阶段：获取Bean阶段
@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
// 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...
//触发对于所有非懒加载的singleton beans的实例化
//通过getBean()方法重点执行，其中getBean会找到doGetBean
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//判断是否不为abstract、为singleton、lazyinit属性为false，则进入到下一步
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//判断是否是FactoryBean
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
//判断不是FactoryBean，直接进入getBean逻辑
else {
getBean(beanName);
}
}
}
获取Bean阶段：
从亮色的：getBean方法进入到类：AbstractBeanFactory中，然后就看到do真正做事情的来啦：
@Override
public Object getBean(String name) throws BeansException {
//实际执行
return doGetBean(name, null, null, false);
}
兴奋地点击doGetBean，去看看它究竟干了啥？进入到类：AbstractBeanFactory类中，doGetBean所完成的内容主要包含：判断是否已经有存在singleton的beanName，若有可以直接返回，若无则获取此bean所依赖的beans-dependsOn，然后通过一个for循环判断，是否这个中的所有dependsOn中的某个bean是否存在环形依赖，若存在的话，直接抛出异常，无异常则为给定的bean注册依赖bean（依赖bean也会在给定bean销毁之前被销毁），最后走到createBean逻辑，至此进入到下一个阶段：new-bean真正创建的过程了
doGetBean部分的重要代码如下：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException { //规范化beanName
final String beanName = transformedBeanName(name);
//类型:Object
Object bean; // Eagerly check singleton cache for manually registered singletons.
//获取已经创建的singleton的bean的实例对象，将之赋给Object的sharedInstance
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 + "'");
}
} //对于已经存在的直接返回
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
} //没有已经存在的singleton的bean的实例对象
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 (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);
}
} if (!typeCheckOnly) {
markBeanAsCreated(beanName);
} try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args); // Guarantee initialization of beans that the current bean depends on.
//确保被依赖的bean要初始化之后，其他的才能继续初始化
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 + "'");
}
//给给定bean注册依赖bean
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
} // Create bean instance.
//实例化 if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, ()-> {
try {
//实例化
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 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;
}
} // Check if required type matches the type of the actual bean instance.
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;
}
createBean过程：
进入到AbstractAutowireCapableBeanFactory类的createBean方法中，该方法主要做了三件事情：首先将RootBeanDefinition（注：一个RootBeanDefinition类型的mbdToUse，RootBeanDefinition就是根bean定义，表示合并的bean定义，在运行时支持Spring Factory中定义的特定的bean）类的一个实例对其设置beanClass，并验证和准备method override，其次在真正进行create之前先判断是会否有short-cut快捷方式可以直接返回，最后，进入到doCreateBean()中，其中doCreateBean()方法的参数有三个，一个是：一个是beanName，一个是mbdToUse，一个是参数args；关键部分的代码如下：
//依赖注入，实例化一个bean
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException { if (logger.isTraceEnabled()) {
logger.trace("Creating instance of bean '" + beanName + "'");
}
//一个RootBeanDefinition类型的mbdToUse
//RootBeanDefinition就是根bean定义，表示合并的bean定义，在运行时支持Spring Factory中定义的特定的bean
RootBeanDefinition mbdToUse = mbd; // Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition. //resolveBeanClass的作用是：根据特定的beanDefinition解析bean class，并得到一个class的引用；将已解析的类存储到bean定义中以供后续使用
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
//将解析后得到的resolvedCalss设置给RootBeanDefinition 根bean定义的 mbdToUse
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
} //在RootBeanDefinition中可以去准备Bean所需要的各种属性，例如：beanClass解析后的类信息的set，例如method override
//这些信息在后续去进行实际的创建bean实例化的过程都是需要用到的 // Prepare method overrides.
try {
//要回头去看解析bean的存入到BeanDefinition中的时候，就已经设置过overrides的相关属性了
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
} try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
//在实例化之前判断，是否有short-cut快捷方式可以应用
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
} try {
//doCreateBean是真正做实例化操作的部分
//这里的三个参数，一个是beanName，一个是mbdToUse，一个是参数args
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
继续：doCreateBean，会做三件事情：首先，判断是否是单例，如果单例的话，需要将其FactoryBeanInstanceCache中的该beanName取出，若为null，则需要继续创建，否则直接用cache中取出的接口；后面会重点说一下：创建createBeanInstance的过程，之后initializeBean，最后registerDisposableBeanIfNecessary，将bean注册为一次性的
主要部分代码代码见下方：
//方法说明：真正执行CreatBean的操作
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException { // Instantiate the bean.
BeanWrapper instanceWrapper = null;
//判断是单例，则从factoryBeanInstanceCache中将其取出，若存在，则基于当前已经缓存的进行后续处理
//否则需要继续创建
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//创建一个特定bean的new instance新的实例
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
} // Initialize the bean instance.
//填充过程
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
//initializeBean
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);
}
} // Register bean as disposable.
//将bean注册为一次性的
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
} return exposedObject;
}
下面重点说明：createBeanInstance过程，会根据有参和无参，分别进入到：autowireConstructor和instantiateBean中，重点代码见：
//创建一个特定bean的new instance新的实例
//会使用恰当的实例化策略：工厂方法、构造函数自动装配或简单实例化
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable 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());
} Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
} if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
} // Shortcut when re-creating the same bean...
//快捷方式
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) {
//有参数的情况走到的
return autowireConstructor(beanName, mbd, null, null);
}
else {
//无参的情况走到的
return instantiateBean(beanName, mbd);
}
} // Candidate constructors for autowiring?
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
} // Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
} // No special handling: simply use no-arg constructor.
//调用无参构造器
return instantiateBean(beanName, mbd);
}
如果是有参所用到的autowireConstructor,主要是：解析参数、获取constructorToUse，如果一直没有找到可用的，直接抛出异常，有可用的继续bw.setBeanInstance(instantiate(beanName, mbd, constructorToUse, argsToUse))；进入instantiate中，该方法是在：ContructorReSolver类中，该方法中可以看到：strategy的字眼，这里针对不同的情况进行了不同的策略选择，具体见下方：
/类内部的私有方法
//实例化，通过对应的Constructor和参数进行某个类的对象的实例化
//autowire的走到了这里
private Object instantiate(
String beanName, RootBeanDefinition mbd, Constructor constructorToUse, Object[] argsToUse) { try {
//获取初始化策略
InstantiationStrategy strategy = this.beanFactory.getInstantiationStrategy();
if (System.getSecurityManager() != null) {
return AccessController.doPrivileged((PrivilegedAction<Object>) () ->
strategy.instantiate(mbd, beanName, this.beanFactory, constructorToUse, argsToUse),
this.beanFactory.getAccessControlContext());
}
else {
return strategy.instantiate(mbd, beanName, this.beanFactory, constructorToUse, argsToUse);
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean instantiation via constructor failed", ex);
}
}
那么具体所给出的不同策略的支持是什么呢？进入到了：通过jdk反射和cglib来进行具体的实例化的选择
SimpleInstantiationStrategy类中，
//最终的很多实现的调到的都是：Simplexxx、或者是Defalutxxx之类的
//这是有参数的contructor的走的路径
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
final Constructor<?> ctor, Object... args) { //没有method是override的情况，就是说不能通过继承来实现的，那么必须不能用cglib了，
//否则可以用cglib动态代理
if (!bd.hasMethodOverrides()) {
if (System.getSecurityManager() != null) {
// use own privileged to change accessibility (when security is on)
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
ReflectionUtils.makeAccessible(ctor);
return null;
});
}
return BeanUtils.instantiateClass(ctor, args);
}
else {
return instantiateWithMethodInjection(bd, beanName, owner, ctor, args);
}
}
三、依赖注入过程时序图：（备注：原图若出现看不清的情况，可下载后查看，请保留版权）
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