FactoryBean
public interface FactoryBean {
// 返回真实的对象
T getObject() throws Exception;
// 返回对象类型
Class getObjectType();
// 是否单例；如果是单例会将其创建的对象缓存到缓存池中。
boolean isSingleton();
} 该接口就是一个工厂Bean，在获取对象时，先判断当前对象是否是FactoryBean，如果是再根据getObjectType的返回类型判断是否需要的类型，如果匹配则会调用getObject方法返回真实的对象。该接口用来自定义对象的创建。
注意：如果A.class 实现了FactoryBean，如果想获取A本身这个对象则bean的名称必须添加前缀 '&'，也就是获取Bean则需要ctx.getBean("&a")
当注入属性是ObjectFactory或者ObjectProvider类型时，系统会直接创建DependencyObjectProvider对象然后进行注入，只有在真正调用getObject方法的时候系统才会根据字段上的泛型类型进行查找注入。
2 实际应用
ObjectFactory在Spring源码中应用的比较多
2.1 创建Bean实例
public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
protected T doGetBean(String name, @Nullable Class requiredType, @Nullable Object[] args, boolean typeCheckOnly) throws BeansException {
// other code
if (mbd.isSingleton()) {
//getSingleton方法的第二个参数就是ObjectFactory对象（这里应用了lamda表达式）
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);
}
// other code
}
} 这里的getSingleton方法就是通过不同的Scope（singleton，prototype，request，session）创建Bean；具体的创建细节都是交个ObjectFactory来完成。
2.2 Servlet API注入
在Controller中注入Request，Response相关对象时也是通过ObjectFactory接口。
容器启动时实例化的上下文对象是AnnotationConfigServletWebServerApplicationContext；
调用在AbstractApplicationContext#refresh.postProcessBeanFactory
public class AnnotationConfigServletWebServerApplicationContext {
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
super.postProcessBeanFactory(beanFactory);
if (this.basePackages != null && this.basePackages.length > 0) {
this.scanner.scan(this.basePackages);
}
if (!this.annotatedClasses.isEmpty()) {
this.reader.register(ClassUtils.toClassArray(this.annotatedClasses));
}
}
}super.postProcessBeanFactory(beanFactory)方法进入到ServletWebServerApplicationContext中
public class ServletWebServerApplicationContext extends GenericWebApplicationContext implements ConfigurableWebServerApplicationContext {
@Override
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
beanFactory.addBeanPostProcessor(new WebApplicationContextServletContextAwareProcessor(this));
beanFactory.ignoreDependencyInterface(ServletContextAware.class);
registerWebApplicationScopes();
}
private void registerWebApplicationScopes() {
ExistingWebApplicationScopes existingScopes = new ExistingWebApplicationScopes(getBeanFactory());
WebApplicationContextUtils.registerWebApplicationScopes(getBeanFactory());
existingScopes.restore();
}
}WebApplicationContextUtils工具类
public abstract class WebApplicationContextUtils {
public static void registerWebApplicationScopes(ConfigurableListableBeanFactory beanFactory) {
registerWebApplicationScopes(beanFactory, null);
}
public static void registerWebApplicationScopes(ConfigurableListableBeanFactory beanFactory, @Nullable ServletContext sc) {
beanFactory.registerScope(WebApplicationContext.SCOPE_REQUEST, new RequestScope());
beanFactory.registerScope(WebApplicationContext.SCOPE_SESSION, new SessionScope());
if (sc != null) {
ServletContextScope appScope = new ServletContextScope(sc);
beanFactory.registerScope(WebApplicationContext.SCOPE_APPLICATION, appScope);
// Register as ServletContext attribute, for ContextCleanupListener to detect it.
sc.setAttribute(ServletContextScope.class.getName(), appScope);
}
beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory());
beanFactory.registerResolvableDependency(ServletResponse.class, new ResponseObjectFactory());
beanFactory.registerResolvableDependency(HttpSession.class, new SessionObjectFactory());
beanFactory.registerResolvableDependency(WebRequest.class, new WebRequestObjectFactory());
if (jsfPresent) {
FacesDependencyRegistrar.registerFacesDependencies(beanFactory);
}
}
}这里的RequestObjectFactory，ResponseObjectFactory，SessionObjectFactory，WebRequestObjectFactory都是ObjectFactory接口。
这几个接口实际获取的对象都是从当前线程的上下文中获取的（通过ThreadLocal），所以在Controller中直接属性注入相应的对象是线程安全的。
注意：这里registerResolvableDependency方法意图就是当有Bean需要注入相应的Request，Response对象时直接注入第二个参数的值即可。
2.3 自定义定义ObjectFactory
在IOC容器，如果有两个相同类型的Bean，这时候在注入的时候肯定是会报错的，示例如下：
public interface AccountDAO {
}
@Component
public class AccountADAO implements AccountDAO {
}
@Component
public class AccountBDAO implements AccountDAO {
}
@RestController
@RequestMapping("/accounts")
public class AccountController {
@Resource
private AccountDAO dao ;
}当我们有如上的Bean后，启动容器会报错如下：
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.pack.objectfactory.AccountDAO' available: expected single matching bean but found 2: accountADAO,accountBDAO
at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:220) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]期望一个AccountDAO类型的Bean，但当前环境却有两个。
解决这个办法可以通过@Primary和@Qualifier来解决，这两个方法这里不做介绍；接下来我们通过BeanFactory#registerResolvableDependency的方式来解决；
自定义BeanFactoryPostProcessor
// 方式1：直接通过beanFactory获取指定的bean注入。
@Component
public class CustomBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
// beanFactory的实例是DefaultListableBeanFactory，该实例内部维护了一个ConcurrentMap resolvableDependencies 的集合，Class作为key。
beanFactory.registerResolvableDependency(AccountDAO.class, beanFactory.getBean("accountBDAO"));
}
}自定义ObjectFactory
public class AccountObjectFactory implements ObjectFactory {
@Override
public AccountDAO getObject() throws BeansException {
return new AccountBDAO() ;
}
}
// 对应的BeanFactoryPostProcessor
beanFactory.registerResolvableDependency(AccountDAO.class, new AccountObjectFactory()); 当一个Bean的属性在填充（注入）时调用AbstractAutowireCapableBeanFactory.populateBean方法时，会在当前的IOC容器中查找符合的Bean，最终执行如下方法：
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
protected Map findAutowireCandidates(@Nullable String beanName, Class requiredType, DependencyDescriptor descriptor) {
// 从当前IOC容器中查找所有指定类型的Bean
String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this, requiredType, true, descriptor.isEager());
Map result = new LinkedHashMap<>(candidateNames.length);
// 遍历DefaultListableBeanFactory对象中通过registerResolvableDependency方法注册的
for (Map.Entry, Object> classObjectEntry : this.resolvableDependencies.entrySet()) {
Class autowiringType = classObjectEntry.getKey();
if (autowiringType.isAssignableFrom(requiredType)) {
Object autowiringValue = classObjectEntry.getValue();
// 解析自动装配的类型值（主要就是判断当前的值对象是否是ObjectFactory对象）
autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
if (requiredType.isInstance(autowiringValue)) {
// 要注意这里，如果通过registerResolvableDependency添加的对象是个ObjectFactory，那么最终会调用factory.getObject方法返回真实的对象并且加入到result集合中。这时候相当于当前类型还是找到了多个Bean还是会报错。
result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
break;
}
}
}
for (String candidate : candidateNames) {
if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {
addCandidateEntry(result, candidate, descriptor, requiredType);
}
}
return result;
}
}
// AutowireUtils.resolveAutowiringValue方法
// 该方法中会判断对象是否是ObjectFactory
public static Object resolveAutowiringValue(Object autowiringValue, Class requiredType) {
if (autowiringValue instanceof ObjectFactory && !requiredType.isInstance(autowiringValue)) {
ObjectFactory factory = (ObjectFactory) autowiringValue;
if (autowiringValue instanceof Serializable && requiredType.isInterface()) {
autowiringValue = Proxy.newProxyInstance(requiredType.getClassLoader(), new Class[] {requiredType}, new ObjectFactoryDelegatingInvocationHandler(factory));
} else {
// 进入这里之间调用getObject返回对象
return factory.getObject();
}
}
return autowiringValue;
} 完毕！！！
