一. FactoryBean简介
在前面分析SpringIoc的时候，我们肯定见过这个FactoryBean这个类，这个类和BeanFactory经常放在一块区分，这里就再说一下这个两个分别是什么：
- FactoryBean ： 是一个Java Bean，但是它是一个能生产出当前对象的工厂Bean,它的实现和工厂模式及修饰器模式很像。
- BeanFactory：这就是一个工厂，是整个Spring IOC容器的核心内容,生产并存储很多的bean。
二. FactoryBean源码
public interface FactoryBean<T> {
String OBJECT_TYPE_ATTRIBUTE = "factoryBeanObjectType";
T getObject() throws Exception;
Class<?> getObjectType();
default boolean isSingleton() {
return true;
}
}
T getObject()获取泛型T的实例。用来创建Bean。当IoC容器通过getBean方法来FactoryBean创建的实例时实际获取的不是FactoryBean 本身而是具体创建的T泛型实例。Class<?> getObjectType()获取 T getObject()中的返回值 T 的具体类型。这里强烈建议如果T是一个接口，返回其具体实现类的类型。default boolean isSingleton()用来规定 Factory创建的的bean是否是单例。这里通过默认方法定义为单例。
三. 通过例子分析
applicationIoc.xml 配置
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.springframework.org/schema/beans"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.2.xsd">
<bean id="factoryBeanTest" class="com.jingchuan.model.FactoryBeanTest"></bean>
</beans>
Fruits 类
public class Fruits {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
FactoryBeanTest 类
import org.springframework.beans.factory.FactoryBean;
public class FactoryBeanTest implements FactoryBean{
@Override
public Object getObject() throws Exception {
Fruits fruits = new Fruits();
fruits.setName("FactoryBeanTest");
return fruits;
}
@Override
public Class<?> getObjectType() {
return FactoryBeanTest.class;
}
}
测试TestIoc 类
import com.jingchuan.model.Fruits;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class TestIoc {
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("classpath:applicationIoc.xml");
Fruits fruits = (Fruits) context.getBean("factoryBeanTest");
System.out.println(fruits.getName());
}
}
在上面这个例子中，可以看到我们通过context.getBean("factoryBeanTest")获取到的实际是个Fruits 对象实例。接下来通过源码分析
四. 通过例子分析源码
在上面的例子中，对于IOC加载对象的相关这里就不细说了，我们直接来到org.springframework.beans.factory.support.DefaultListableBeanFactory#preInstantiateSingletons方法，这个方法是在bean定义加载完成，实例化bean的这一步，也就是org.springframework.context.support.AbstractApplicationContext#refresh的finishBeanFactoryInitialization(beanFactory)方法过程中，接下来我们细看这个方法：
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...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//判断是不是一个FactoryBean 对于我们上面的例子factoryBeanTest类 很明显它是实现了FactoryBean的
if (isFactoryBean(beanName)) {
// ioc获取bean实例 但是注意这里给bean名字前面加上了FACTORY_BEAN_PREFIX
// 所以这里获取的到的是FactoryBeanTest类的实例，因为它是一个实现了FactoryBean的对象
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
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);
}
}
}
else {
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
.tag("beanName", beanName);
SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
smartInitialize.end();
}
}
}
在上面这段代码中，我们可以看到在获取factoryBeanTest的时候发现才实现了FactoryBean的，所以在getBean的时候给名字前面加上了FACTORY_BEAN_PREFIX，这个其实就是org.springframework.beans.factory.BeanFactory#FACTORY_BEAN_PREFIX前缀，如果加了这个前缀那么获取到的实例就会是创建这个对象的工厂的实例。
那么进入getBean(FACTORY_BEAN_PREFIX + beanName)中，其实就是IOC获取对象实例的步骤，这里就不再一一复述了，不理解的同学可以先去看看我的这篇IOC源码详解，不想看的同学也可以自己断点跟进，其实这里获取的就是FactoryBeanTest的实例对象了。
那么其实到这里，我们应该明白，在我们的测试类ApplicationContext context = new ClassPathXmlApplicationContext("classpath:applicationIoc.xml");这一步已经获取到了实现了FactoryBean接口的factoryBeanTest类的实例了，factoryBeanTest这个类的实例已经被加载到SpringIOC容器中了。
那么接下来我们再看例子中测试类的Fruits fruits = (Fruits) context.getBean("factoryBeanTest");
在getBean方法中，我们会来到：
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
String beanName = transformedBeanName(name);
Object beanInstance;
// Eagerly check singleton cache for manually registered singletons.
// 去缓存中获取bean实例 这个时候我们是能获取到前面已经加载到的bean实例的
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 + "'");
}
}
// 然后重点在这里
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
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);
}
StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
.tag("beanName", name);
try {
if (requiredType != null) {
beanCreation.tag("beanType", requiredType::toString);
}
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
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);
}
}
}
// Create bean instance.
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;
}
});
beanInstance = 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);
}
beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
if (!StringUtils.hasLength(scopeName)) {
throw new IllegalStateException("No scope name defined for bean ′" + beanName + "'");
}
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);
}
});
beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new ScopeNotActiveException(beanName, scopeName, ex);
}
}
}
catch (BeansException ex) {
beanCreation.tag("exception", ex.getClass().toString());
beanCreation.tag("message", String.valueOf(ex.getMessage()));
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
finally {
beanCreation.end();
}
}
return adaptBeanInstance(name, beanInstance, requiredType);
}
在上面这段代码中，Object sharedInstance = getSingleton(beanName);是可以从缓存中获取到已经创建好的实例的，然后进入if中来到beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);，这一步是FactoryBean自定义加载类的重点。我们跟进去看实际会来到org.springframework.beans.factory.support.AbstractBeanFactory#getObjectForBeanInstance：
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.
// 如果是&开头 那么说明是个工厂bean 直接返回 不进行后续处理
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
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.
// 如果不是实现自FactoryBean 那么也直接返回
if (!(beanInstance instanceof FactoryBean)) {
return beanInstance;
}
Object object = null;
if (mbd != null) {
mbd.isFactoryBean = true;
}
else {
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.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
// 这里是真正调用FactoryBean的getObject获取自定义类实例创建的地方
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
根据注释，我们看object = getObjectFromFactoryBean(factory, beanName, !synthetic);
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
//首先判断是不是单例 其实是不是单例区别只是在创建的对象要不要加入缓存等一些步骤上
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
// 调用FactoryBean的getObject获取自定义实例
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 (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);
}
}
}
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;
}
}
根据上面方法的注释分析，我们来到：doGetObjectFromFactoryBean(factory, beanName);
private Object doGetObjectFromFactoryBean(FactoryBean<?> factory, String beanName) throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
// 调用FactoryBean的getObject方法获取自定义实例
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;
}
可以看到在这段代码中真正的调用了FactoryBean的getObject方法获取自定义实例然后进行返回，这个factory.getObject()自然就是调用到了我们的FactoryBeanTest类实现的getObject()方法中了，所以返回的实例是我们自定义的Fruits对象。
返回到org.springframework.beans.factory.support.FactoryBeanRegistrySupport#getObjectFromFactoryBean方法中，自定义实例已经创建完成，然后做后续的bean实例创建成的处理，并加入org.springframework.beans.factory.support.FactoryBeanRegistrySupport#factoryBeanObjectCache缓存，下次再获取的时候就可以直接从缓存中获取了。
至此实现了FactoryBean的类的加载和使用就分析完成了！
