依赖注入发生的时间
当Spring IOC容器完成了Bean资源的定位、载入和解析注册之后,IOC容器中已经管理了Bean定义的相关数据,但是此时IOC容器还没有对所管理的Bean进行依赖注入。
依赖注入发生的场景
1、用户第一次使用getBean()方法时,IOC容器触发依赖注入。
2、当用户在配置文件将<bean>元素配置了lazy-init=false属性的时候,即让容器在解析注册Bean定义时进行预实例化,触发依赖注入。
依赖注入的关键类:
1、IOC容器
BeanFactory getBean()
AbstractBeanFactory
BeanFactory接口定义了Spring IOC容器的基本功能规范,是Spring IOC容器所应该遵守的最底层和最基本的编程规范,BeanFactory接口中定义了几个getBean()方法,就是用户向IOC容器索取管理的Bean的方法,
2、实例化策略:SimpleInstantiationStrategy
3、存储实例化所有相关的信息scope、proxy、instance、BeanWrapper
依赖注入的执行细节
寻找入口------>开始实例化-------->选择实例化策略-------->执行实例化-------->准备依赖注入-------->解析注入规则-------->注入赋值
寻找入口:
AbstractBeanFactory的getBean()相关方法源码如下:
//获取IOC容器中指定名称的Bean
@Override
public Object getBean(String name) throws BeansException {
//doGetBean才是真正向IoC容器获取被管理Bean的过程
return doGetBean(name, null, null, false);
}
//获取IOC容器中指定名称和类型的Bean
@Override
public <T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException {
//doGetBean才是真正向IoC容器获取被管理Bean的过程
return doGetBean(name, requiredType, null, false);
}
//获取IOC容器中指定名称和参数的Bean
@Override
public Object getBean(String name, Object... args) throws BeansException {
//doGetBean才是真正向IoC容器获取被管理Bean的过程
return doGetBean(name, null, args, false);
}
/**
* 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 when creating a bean instance using explicit arguments
* (only applied when creating a new instance as opposed to retrieving an existing one)
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
//获取IOC容器中指定名称、类型和参数的Bean
public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
throws BeansException {
//doGetBean才是真正向IoC容器获取被管理Bean的过程
return doGetBean(name, requiredType, args, false);
}
在spring中,如果Bean定义的单例模式(Singleton),则容器在创建之前现在缓存中查找,以确保只有一个实例对象存在;如果定义的是原型模式(Prototype),则容器每次都会创建一个新的实例对象。除此之外,Bean定义还可以扩展为指定其生命周期范围。
根据Bean定义的模式,采取不同的创建Bean实例对象的策略,具体的Bean实例对象的创建过程由实现了ObjectFactory接口的匿名内部类的createBean()方法完成。
ObjectFactory使用了委派模式,具体的Bean实例创建过程交给其实现类AbstractAutowireCapableBeanFactory完成,
//真正实现向IOC容器获取Bean的功能,也是触发依赖注入功能的地方
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//根据指定的名称获取被管理Bean的名称,剥离指定名称中对容器的相关依赖
//如果指定的是别名,将别名转换为规范的Bean名称
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
//先从缓存中取是否已经有被创建过的单态类型的Bean
//对于单例模式的Bean整个IOC容器中只创建一次,不需要重复创建
Object sharedInstance = getSingleton(beanName);
//IOC容器创建单例模式Bean实例对象
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
//如果指定名称的Bean在容器中已有单例模式的Bean被创建
//直接返回已经创建的Bean
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 + "'");
}
}
//获取给定Bean的实例对象,主要是完成FactoryBean的相关处理
//注意:BeanFactory是管理容器中Bean的工厂,而FactoryBean是
//创建创建对象的工厂Bean,两者之间有区别
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
//缓存没有正在创建的单例模式Bean
//缓存中已经有已经创建的原型模式Bean
//但是由于循环引用的问题导致实例化对象失败
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
//对IOC容器中是否存在指定名称的BeanDefinition进行检查,首先检查是否
//能在当前的BeanFactory中获取的所需要的Bean,如果不能则委托当前容器
//的父级容器去查找,如果还是找不到则沿着容器的继承体系向父级容器查找
BeanFactory parentBeanFactory = getParentBeanFactory();
//当前容器的父级容器存在,且当前容器中不存在指定名称的Bean
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
//解析指定Bean名称的原始名称
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.
//委派父级容器根据指定名称和显式的参数查找
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
//委派父级容器根据指定名称和类型查找
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
//创建的Bean是否需要进行类型验证,一般不需要
if (!typeCheckOnly) {
//向容器标记指定的Bean已经被创建
markBeanAsCreated(beanName);
}
try {
//根据指定Bean名称获取其父级的Bean定义
//主要解决Bean继承时子类合并父类公共属性问题
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//获取当前Bean所有依赖Bean的名称
String[] dependsOn = mbd.getDependsOn();
//如果当前Bean有依赖Bean
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 + "'");
}
//递归调用getBean方法,获取当前Bean的依赖Bean
registerDependentBean(dep, beanName);
//把被依赖Bean注册给当前依赖的Bean
getBean(dep);
}
}
// Create bean instance.
//创建单例模式Bean的实例对象
if (mbd.isSingleton()) {
//这里使用了一个匿名内部类,创建Bean实例对象,并且注册给所依赖的对象
sharedInstance = getSingleton(beanName, () -> {
try {
//创建一个指定Bean实例对象,如果有父级继承,则合并子类和父类的定义
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.
//显式地从容器单例模式Bean缓存中清除实例对象
destroySingleton(beanName);
throw ex;
}
});
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//IOC容器创建原型模式Bean实例对象
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
//原型模式(Prototype)是每次都会创建一个新的对象
Object prototypeInstance = null;
try {
//回调beforePrototypeCreation方法,默认的功能是注册当前创建的原型对象
beforePrototypeCreation(beanName);
//创建指定Bean对象实例
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
//回调afterPrototypeCreation方法,默认的功能告诉IOC容器指定Bean的原型对象不再创建
afterPrototypeCreation(beanName);
}
//获取给定Bean的实例对象
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
//要创建的Bean既不是单例模式,也不是原型模式,则根据Bean定义资源中
//配置的生命周期范围,选择实例化Bean的合适方法,这种在Web应用程序中
//比较常用,如:request、session、application等生命周期
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
//Bean定义资源中没有配置生命周期范围,则Bean定义不合法
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
//这里又使用了一个匿名内部类,获取一个指定生命周期范围的实例
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
//获取给定Bean的实例对象
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.
//对创建的Bean实例对象进行类型检查
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.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;
}
BeanFactory和FactoryBean
FactoryBean Spring内部实现一种规范&开头作为beanName Spring中的所有容器都是FactoryBean,因为容器本身也是由容器管理,root来创建,都是单例存放在IOC容器中
BeanFactory:Bean工厂的顶层规范,接口中只是定义了getBean()方法,就是用户向IOC容器索取管理的Bean的方法,在用户索取Bean的过程中完成了依赖注入。