一、前言
本文是 Spring源码分析:bean的加载过程 的衍生文章。主要是因为本人菜鸡,在分析源码的过程中还有一些其他的内容不理解,故开设衍生篇来完善内容以学习。
关于 FactoryBean
在源码中的设计,可以参考上面提及的正文bean的加载过程内容。在getObjectForBeanInstance
方法中有详细的分析。
二、简介
一般情况下,Spring通过反射机制利用bean的class属性指定实现类来实例化bean。但是在某些情况下,实例化bean 的过程比较复杂,在XML配置下如果按照传统的编码方式,则需要进行大量配置,灵活性受限,这时候Spring 提供了一个 org.springframework.beans.factory.FactoryBean 的工厂类接口。可以通过该接口定制实例化bean 的逻辑。
除此之外,FactoryBean在Spring框架中都占有很重要的地位,Spring自身就提供了非常多的FactoryBean的实现。
首先我们来看看 FactoryBean 接口的定义。方法比较简单,见名知意。
public interface FactoryBean<T> {
// 属性名
String OBJECT_TYPE_ATTRIBUTE = "factoryBeanObjectType";
// 获取bean实例,实例化bean 的逻辑就在这里实现
@Nullable
T getObject() throws Exception;
// 获取bean类型
@Nullable
Class<?> getObjectType();
// 是否是单例
default boolean isSingleton() {
return true;
}
}
T getObject()
: 返回由FactoryBean创建的bean实例,如果isSingleton() 返回true,则会将该实例方法Spring容器的单例缓冲池中Class<?> getObjectType()
: 返回由FactoryBean 创建的bean 的类型boolean isSingleton()
: 返回由FactoryBean 创建的bean实例的作用域是singleton还是Prototype。
三、正文
下面举个简单的例子进一步说明FactoryBean 的用法。
@Component
public class DemoFactoryBean implements FactoryBean<DemoBean> {
@Override
public DemoBean getObject() throws Exception {
System.out.println("DemoFactoryBean.getObject");
return new DemoBean();
}
@Override
public Class<?> getObjectType() {
return DemoBean.class;
}
}
@SpringBootApplication
public class BeanInitDemoApplication {
public static void main(String[] args) {
ConfigurableApplicationContext run = SpringApplication.run(BeanInitDemoApplication.class, args);
Object demoFactoryBean = run.getBean("demoFactoryBean");
Object bean = run.getBean("&demoFactoryBean");
System.out.println("BeanInitDemoApplication.main");
}
}
如上一个简单的例子,调用结果如下,可以看到:我们这里直接调用 run.getBean("demoFactoryBean");
返回的并不是 DemoFactoryBean
,而是DemoBean
。而我们使用 run.getBean("&demoFactoryBean");
返回的结果却是DemoFactoryBean
。
实际上,当调用 run.getBean("demoFactoryBean");
时,Spring通过反射会发现 DemoFactoryBean
实现了FactoryBean
接口,则会直接调用 其getObject()
方法,并将方法的返回值注入到Spring容器中。而如果想要获得DemoFactoryBean
实例,则需要在 beanName
前加上 &
,即 run.getBean("&demoFactoryBean");
四、源码解读
比较完成的源码分析可以看 bean的加载过程。
这里仅仅介绍一下 FactoryBean 的处理过程,所以内容覆盖可能不够全面。
对于 FactoryBean的处理在 AbstractBeanFactory#getObjectForBeanInstance 方法中完成
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
代码逻辑大体如下:
- 判断是否需要获取
FactoryBean
实例。 通过BeanFactoryUtils.isFactoryDereference(name)
来判断
判断name 是否以&
开头。如果是则说明是想获取 FactoryBean 的实例,所以经过简单校验后就直接返回beanInstance。 - 经过第一步,现在可以确定,当前bean不是为了获取FactoryBean实现类,那么这个bean还可能是普通bean或者FactoryBean。
if (!(beanInstance instanceof FactoryBean))
判断如果不是FactoryBean则没必要进行下一步处理,直接返回即可。 - 这一步可以确定,这里的bean是 FactoryBean类型,并且name 是“beanName”,表明是想获取bean实例,而不是beanFactory实例,则将解析bean的工作委托给
getObjectFromFactoryBean
完成。getObjectFromFactoryBean
方法对FactoryBean的解析还是在 中完成。(getObjectFromFactoryBean 这一步调用了ObjectFactory的后置处理器)
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
// 1. 检测name 是否是想获取 工厂类(name 以 & 开头)
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
// 以&开头又不是FactoryBean实现类,则抛出异常
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
}
if (mbd != null) {
mbd.isFactoryBean = true;
}
return beanInstance;
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
// 2. 判断如果beanInstance 不是 FactoryBean,就直接返回
if (!(beanInstance instanceof FactoryBean)) {
return beanInstance;
}
// 3. 到这一步就可以确定,当前beanInstance 是FactoryBean,并且需要获取getObject() 的结果
Object object = null;
if (mbd != null) {
mbd.isFactoryBean = true;
}
else {
// 尝试从缓存中加载bean
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
// containsBeanDefinition 检测 beanDefinitionMap中也就是所有已经加载的类中检测是否定义beanName
if (mbd == null && containsBeanDefinition(beanName)) {
// 合并父类bean 定义的属性
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
// 4. 这一步中对FactoryBean进行了解析。
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
getObjectFromFactoryBean
方法中 调用 doGetObjectFromFactoryBean
方法进行了解析。
上面已经说了,因为这里的bean是FactoryBean,且并不是为了获取FactoryBean实例,所以会调用factory.getObject() 方法将得到的bean返回。
getObjectFromFactoryBean
代码如下
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
// 判断是否是单例模式
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
// 尝试从缓存中获取
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
// 在这个方法中进行解析
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)
// 因为是单例模式,所以要保证变量的全局唯一。所以这里如果缓存中已经创建好了bean则替换为已经创建好的bean
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (shouldPostProcess) {
if (isSingletonCurrentlyInCreation(beanName)) {
// Temporarily return non-post-processed object, not storing it yet..
return object;
}
beforeSingletonCreation(beanName);
try {
// 调用了ObjectFactory的后置处理器。
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);
}
}
}
return object;
}
}
else {
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}
....
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
// 需要权限验证
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) 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) {
if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();
}
return object;
}
以上:内容部分参考
《Spring源码深度解析》
如有侵扰,联系删除。 内容仅用于自我记录学习使用。如有错误,欢迎指正