经过前面的分析我们结束了对XML配置文件的解析,接下来就是对bean加载的探索,bean加载的功能实现远比bean的解析要复杂得多。
下面的示例演示了bean加载的功能:
MyTestBean bean = (MyTestBean) bf.getBean("myTestBean");
这里调用getBean方法时执行了哪些逻辑呢,我们可以先快速的体验一下Spring中的代码是如何实现的
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
//提取对应的beanName
final String beanName = transformedBeanName(name);
Object bean;
/**
* 检查缓存中或者实例工厂中是否有对应的实例
* 为什么首先会使用这段代码呢,
* 因为在创建单例bean的时候会存在依赖注入的情况,而在创建依赖的时候为了避免循环依赖,Spring创建bean的原则是不等bean创建完成就会将创建的bean的ObjectFactory提早曝光
* 也就是将ObjectFactory加入到缓存中,一旦下一个bean创建时候需要依赖上一个bean则直接使用ObjectFactory
*/
//直接尝试从缓存中获取或者singletonFactories中的ObjectFactory中获取
Object sharedInstance = getSingleton(beanName);
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 + "'");
}
}
//返回对应的实例,有时候存在诸如BeanFactory的情况并不是直接返回实例本身而是返回指定方法返回的实例
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
//只有在单例的情况下才会尝试解决循环依赖,原型模式情况下,如果存在A中有B的属性,B中有A的属性,那么当依赖注入的时候,就会产生当A还未创建完成的时候因为对于B的创建在返回创建A,造成循环依赖,也就是下面的情况
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
//如果beanDefinitionMap中也就是在所有已经加载的类中不包括beanName则尝试从parentBeanFactory中检测
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
//递归到BeanFactory中寻找
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);
}
}
//如果不是仅仅做类型检查则是创建bean,这里要进行记录
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
//将存储XMl配置文件的GernericBeanDefinition转换为RootBeanDefinition,如果指定BeanName是子Bean的话同时会合并父类的相关属性
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
//若存在依赖则需要递归实例化依赖的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 + "'");
}
//缓存依赖调用
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.
//实例化依赖的bean后便可以实例化mbd本身了
//singleton模式的创建
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.
//prototype模式的创建(new)
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
//指定的scope上实例化bean
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, 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.
//检查需要的类型是否符合bean的实际类型
if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
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;
}
从代码量上来看,bean的加载经历了一个很复杂的过程,粗略的了解Spring加载bean的过程,大致步骤如下:
1、转换对应beanName
这里的beanName传入的参数可能是别名,也可能是FactoryBean,所有要进行一系列的解析,解析的内容包括下面这些:
(1)、去除FactoryBean的修饰符,也就是如果name=“&aa”,那么会首先去除&,
(2)、取指定alias所表示的最终beanName,如别名A指向的名称为B则返回B,如果别名A指向B,别名B又指向C,则返回C。
2、尝试从缓存中加载单例
单例在Spring的同一个容器内只会被创建一次,后续获取Bean时就直接从缓存中获取。这里只是尝试加载,首先尝试从缓存中加载,如果加载不成功则再次从singletonFactories中加载,因为在创建单例bean的时候会存在依赖注入的情况,而在创建依赖的时候为了避免循环依赖,在Spring中创建bean的原则是不等bean创建完成就会将创建bean的ObjectFactory提早曝光加入到缓存中,一旦下一个bean创建的时候需要依赖上一个bean则直接使用ObjectFactory
3、bean的实例化
如果从缓存中得到了bean的原始状态,则需要对bean进行实例化,缓存中记录的只是最原始的bean的状态,并不一定是我们最终想要的bean。如果要对工厂bean进行处理,那么这里得到的其实不是工厂bean的初始状态,但是我们真正需要的是工厂bena中定义的factory-method方法中返回的bean,而getObjectForBeanInstance就是完成这个工作的。
4、原型模式的依赖检查
只有在单例情况下才会尝试解决循环依赖,如果存在A中有B的属性,B中有A的属性,那么当依赖注入的时候,就会产生当A还未创建完成的时候因为对于B的创建再次返回创建A,造成循环依赖,也就是情况:isPrototypeCurrentlyInCreation(beanName)判断为true。
5、检测parentBeanFactory
如果缓存没有数据的话直接转到父类工厂上去加载。这里有一个很重要的判断条件,parentBeanFactory!=null&&!containsBeanDefinition(beanName),parentBeanFactory!=null。parentBeanFactory如果为空,则其他一切都是浮云,但是!containsBeanDefinition(beanBane)比较重要,它是检测如果当前加载的XML配置文件中不包含beanName所对应的配置,就只能到parentBeanFactory中去尝试,然后递归的调用getBean方法。
6、将存储XML配置文件的GernericBeanDefinition转换为RootBeanDefinition
因为从XML配置文件中读取到的Bean信息时存储在GernericBeanDefinition中的,但是所有的bean后续处理都是针对于RootBeanDefinition的,所以这里需要进行一个转换,转换的同时如果父类bean不为空的话,则会一并合并父类的属性。
7、寻找依赖
因为bean的初始化过程中很可能会用到某些属性,而某些属性很可能是动态配置的,并且配置成依赖于其他的bean,那么这个时候就有必要加载依赖的bean,所以,在Spring的加载顺寻中,在初始化某一个bean的时候首先会初始化这个bean对应的依赖
8、针对不同的scope进行bean的创建
在Spring总存在着不同的scope,其中默认的是singleton,但是还有些其他的配置诸如prototype、request之类的,在这个步骤中Spring会根据不同的配置进行不同的初始化策略
9、类型的转换
程序到这里返回bean后,已经基本结束了,通常对该方法的调用参数requiredType是为空的。可能会存在这样的情况,返回的bean其实是一个String,但是requiredType却传入Integer类型,那么这个时候本步骤就会起作用了,它的功能是将返回的bean转换为requiredType所指定的类型。
经过上面的步骤后bean的加载就结束了,这个时候就可以返回我们需要的bean。
下面的图直观的反映了整个过程,其中最重要的是步骤8,针对不同的scope进行bean的创建。

1、FactoryBean的使用
Spring通过反射机制利用bean的class属性指定实现类来实例化bean。在一些情况下,实例化bean的过程比较复杂,如果按照传统的方式需要在中提供大量的配置信息,配置的方式的灵活性是受限的,这个时候采用编码的方式可能会得到一个简单的方案,Spring提供BeanFactory的工厂类接口,我们可以通过实现该接口定制实例化逻辑。
FactoryBean接口对于Spring框架来说占有重要地位,Spring自身就提供70多个FactoryBean的实现,从Spring 3.0开始FactoryBean支持泛型:
package org.springframework.beans.factory;
public interface FactoryBean<T> {
//返回由FactoryBean创建的bean实例,如果isSingleton()返回true,则该实例会放到Spring容器的单实例缓冲池中
T getObject() throws Exception;
//返回FactoryBean创建的bean类型
Class<?> getObjectType();
//返回由FactoryBean创建的bean实例的作用域是singleton还是prototype
boolean isSingleton();
}
当配置文件中的class属性配置的实现类是FactoryBean时,通过getBean()方法返回的不是FactoryBean本身,而是FactoryBean#getObject()方法所返回的对象,相当于FactoryBean#getObject()代理了getBean()方法。
package test.factorybean;
public class Car {
private int maxSpeed;
private String brand;
private double price;
//省略get,set方法
}
package test.factorybean;
import org.springframework.beans.factory.FactoryBean;
public class CarFactoryBean implements FactoryBean<Car> {
private String carInfo;
@Override
public Car getObject() throws Exception {
Car car = new Car();
String[] infos = carInfo.split(",");
car.setBrand(infos[0]);
car.setMaxSpeed(Integer.parseInt(infos[1]));
car.setPrice(Double.parseDouble(infos[2]));
return car;
}
@Override
public Class<?> getObjectType() {
return Car.class;
}
@Override
public boolean isSingleton() {
return false;
}
public String getCarInfo() {
return carInfo;
}
public void setCarInfo(String carInfo) {
this.carInfo = carInfo;
}
}
有了这个CarFactoryBean之后,就可以在配置文件中使用这种自定义的配置方式配置Car这个类
<?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="car" class="test.factorybean.CarFactoryBean">
<property name="carInfo" value="超级跑车,400,2000000"/>
</bean>
</beans>
当调用getBean(“car”)时,Spring通过反射机制发现CarFactoryBean实现了FactoryBean的接口,这时Spring容器就调用接口方法CarFactoryBean#getObject()方法返回,如果希望获取的是CarFactoryBean的实例,则需要在使用geiBean(beanName)方法时在beanName前显式的加上”&”前缀。
package test.factorybean;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Main {
public static void main(String[] args) throws Exception {
ApplicationContext applicationContext = new ClassPathXmlApplicationContext(
"test/factorybean/FactoryBeanTest.xml");
// 如果要获取CarFactoryBean的实例,加上&前缀.否则报错java.lang.ClassCastException:
// test.factorybean.Car cannot be cast to test.factorybean.CarFactoryBean
CarFactoryBean cfb = (CarFactoryBean) applicationContext.getBean("&car");
Car car = (Car) applicationContext.getBean("car");
System.out.println(car.getBrand());
String carInfo = cfb.getCarInfo();
System.out.println(carInfo);
}
}
2、缓存中获取单例bean
单例在Spring的同一个容器内只会被创建一次,后续再获取bean直接从单例缓存中获取,首先是尝试从缓存中加载,然后再次尝试从singletonFactories中加载,因为在创建单例bean的时候会存在依赖注入的情况,而在创建依赖的时候为了避免循环依赖,Spring创建bean的原则是不等bean创建完成就会将创建bean的ObjectFactory提早曝光加入到缓存中,一旦下一个bean创建时需要依赖上个bean,则直接使用ObjectFactory。
public Object getSingleton(String beanName) {
//参数true设置标识允许早期依赖
return getSingleton(beanName, true);
}
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//检查缓存中是否存在实例
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
//如果为空,则锁定全局变量并进行处理
synchronized (this.singletonObjects) {
//如果此bean正在加载则不处理
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
//当某些方法需要提前初始化的时候则会调用addSingletonFactory方法将对应的ObjectFactory初始化策略存储在singletonFactories
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//调用预先设定的getObject()方法
singletonObject = singletonFactory.getObject();
//记录在缓存中,earlySingletonObjects和singletonFactories互斥
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
首先尝试从singletonObjects中获取实例,如果获取不到再从earlySingletonObjects中获取,如果还获取不到,再尝试从singletonFactories里面获取beanName对应的ObjectFactory,然后调用这个ObjectFactory的getObject来创建bean,并放到earlySingletonObjects里面去,并且从singletonFactories里面remove掉这个ObjectFactory,而对于后续的所有内存操作都只是为了循环依赖检测的时候使用,也就是在allowEarlyReference为true的情况下才会使用
这里用到的存储bean的map,它们的作用如下:
//用于保存BeanName和创建bena实例之间的关系,bean name --> bean instance
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(256);
// 用于保存BeanName和创建bean的工厂之间的关系,bean name --> ObjectFactory
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<String, ObjectFactory<?>>(16);
//也是用于保存BeanName和创建bean实例之间的关系,与singletonObjects的不同之处在于,当一个单例bean被放到这里面后,那么当bean还在创建过程中,就可以通过getBean方法获取到了,其目的是用来检测循环引用。
private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16);
//用来保存当前所有已注册的bean
private final Set<String> registeredSingletons = new LinkedHashSet<String>(256);
3、从bean的实例中获取对象
在getBean方法中,getObjectForBeanInstance是个高频率使用的方法,无论是从缓存中获得bean还是根据不同的scope策略加载bean。得到bean的实例要做的第一件事就是调用这个方法来检测它的正确性,其实就是检测当前bean是否是FactoryBean类型的bean,如果是的话,那么需要调用该bean对应的FactoryBean实例中的getObject()作为返回值。
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// 如果bean不是工厂,不要让调用代码试图解除对工厂的引用
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
// 现在有了bean的实例,这个实例可能是正常的bean或者是FactoryBean,如果是FactoryBean我们使用它创建实例,但是如果用户想要获取的是工厂实例而不是工厂的getObject方法对应的实例,那么传入的name应该加前缀&
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
// 加载FactoryBean
Object object = null;
if (mbd == null) {
// 尝试从缓存中加载bean
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// 这里已经明确beanInstance一定是FactoryBean类型
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// containsBeanDefinition检测beanDefinitionMap中也就是在所有已经加载的bean中检测是否定义beanName
if (mbd == null && containsBeanDefinition(beanName)) {
// 将存储XML配置文件的GernericBeanDefinition转换为RootBeanDefinition,如果指定BeanName是子Bean的话同时会合并父类的相关属性
mbd = getMergedLocalBeanDefinition(beanName);
}
// 是否是用户定义的而不是程序本身定义的
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
上面代码大多数是一些辅助代码以及一些功能性的判断,而真正的核心代码却委托给了getObjectFromFactoryBean,下面看看getObjectForBeanInstance中所作的工作。
- 1、对FactoryBean正确性的验证
- 2、对非FactoryBean不做任何处理
- 3、对bean进行转换
- 4、将从Factory中解析bean的工作委托给getObjectFromFactoryBean
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
// 如果是单例模式
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) {
if (isSingletonCurrentlyInCreation(beanName)) {
// Temporarily return non-post-processed object, not storing it yet..
return object;
}
beforeSingletonCreation(beanName);
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
finally {
afterSingletonCreation(beanName);
}
}
if (containsSingleton(beanName)) {
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;
}
}
上面这段代码中做了一件事情就是返回的bean如果是单例,那就要必须保证全局唯一。同时不需重复创建,使用缓存来提高性能。
在doGetObjectFromFactoryBean方法中看到object = factory.getObject(),这句代码是最内部的代码实现。
private Object doGetObjectFromFactoryBean(
final FactoryBean<?> factory, final String beanName, final boolean shouldPostProcess)
throws BeanCreationException {
Object object;
try {
// // 需要权限验证
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
@Override
public Object run() throws Exception {
return factory.getObject();
}
}, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
// 直接调用getObject()方法
object = factory.getObject();
}
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
if (object == null && isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
if (object != null && shouldPostProcess) {
try {
// 调用ObjectFactory的后处理器
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of the FactoryBean's object failed", ex);
}
}
return object;
}
上面讲述了FactoryBean的调用方法,如果bean声明为FactoryBean类型,则当前提取的bean并不是FactoryBean,而是FactoryBean中对应的getObject方法返回的bean类型,doGetObjectForFactoryBean正是实现这个功能的。