【spring】@Lockup注解源码分析

@Lockup简单介绍

本文源码基于spring-framework-5.3.10。
用于解决单例依赖原型Bean，原型Bean不生效的情况。
核心思路是生成生成代理对象，执行代理对象的方法。
生成的代理对象是单例Bean去生成代理对象，不是原型Bean！！！
本文源码位置之寻找含有@Lockup注解的方法：org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.determineCandidateConstructors(Class<?>, String)
本文源码位置之生成代理对象源码：org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(String, RootBeanDefinition)
本文源码位置之执行代理对象源码：- 本文源码位置之生成代理对象源码：org.springframework.beans.factory.support.CglibSubclassingInstantiationStrategy.LookupOverrideMethodInterceptor(内部类).intercept(Object obj, Method method, Object[] args, MethodProxy mp)

使用方式

@Component
public class Tset1Service {
    public void printClass() {
        System.out.println(a());
    }

    // 配置具体的原型Bean
    @Lookup("tset2Service")
    public ClassB a() {
        return null;
    }
}

注入点源码分析，找到含有@Lockup注解的方法

public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName)
		throws BeanCreationException {

	// Let's check for lookup methods here...
	if (!this.lookupMethodsChecked.contains(beanName)) {
		// 判断beanClass是不是java.开头的类，比如String
		if (AnnotationUtils.isCandidateClass(beanClass, Lookup.class)) {
			try {
				Class<?> targetClass = beanClass;
				do {
					// 遍历targetClass中的method，查看是否写了@Lookup方法
					ReflectionUtils.doWithLocalMethods(targetClass, method -> {
						Lookup lookup = method.getAnnotation(Lookup.class);
						if (lookup != null) {
							Assert.state(this.beanFactory != null, "No BeanFactory available");

							// 将当前method封装成LookupOverride并设置到BeanDefinition的methodOverrides中
							LookupOverride override = new LookupOverride(method, lookup.value());
							try {
								RootBeanDefinition mbd = (RootBeanDefinition)
										this.beanFactory.getMergedBeanDefinition(beanName);
								mbd.getMethodOverrides().addOverride(override);
							}
							catch (NoSuchBeanDefinitionException ex) {
								throw new BeanCreationException(beanName,
										"Cannot apply @Lookup to beans without corresponding bean definition");
							}
						}
					});
					targetClass = targetClass.getSuperclass();
				}
				while (targetClass != null && targetClass != Object.class);

			}
			catch (IllegalStateException ex) {
				throw new BeanCreationException(beanName, "Lookup method resolution failed", ex);
			}
		}
		this.lookupMethodsChecked.add(beanName);
	}
	......
}

生成代理对象的方法

protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
	try {
		Object beanInstance;
		if (System.getSecurityManager() != null) {
			beanInstance = AccessController.doPrivileged(
					(PrivilegedAction<Object>) () -> getInstantiationStrategy().instantiate(mbd, beanName, this),
					getAccessControlContext());
		}
		else {
			// 策略模式，默认是CglibSubclassingInstantiationStrategy
			beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
		}
		BeanWrapper bw = new BeanWrapperImpl(beanInstance);
		initBeanWrapper(bw);
		return bw;
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
	}
}

/**
 * 实例化
 */
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
	// Don't override the class with CGLIB if no overrides.
	// 判断当前BeanDefinition对应的beanClass中是否存在@Lookup的方法
	if (!bd.hasMethodOverrides()) {
		......
	}
	else {
		// Must generate CGLIB subclass.
		// 如果存在@Lookup，则会生成一个代理对象
		return instantiateWithMethodInjection(bd, beanName, owner);
	}
}

/**
 * 生成代理对象的重载方法
 */
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
	return instantiateWithMethodInjection(bd, beanName, owner, null);
}

/**
 * 生成代理对象的重载方法
 */
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
		@Nullable Constructor<?> ctor, Object... args) {

	// Must generate CGLIB subclass...
	// 调用Cglib去生成一个代理对象
	return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
}

/**
 * 生成代理对象
 */
public Object instantiate(@Nullable Constructor<?> ctor, Object... args) {
	Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
	Object instance;
	// 生成代理对象
	if (ctor == null) {
		instance = BeanUtils.instantiateClass(subclass);
	}
	else {
		try {
			Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
			instance = enhancedSubclassConstructor.newInstance(args);
		}
		catch (Exception ex) {
			throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
					"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
		}
	}
	// 代理对象具体的执行逻辑
	// SPR-10785: set callbacks directly on the instance instead of in the
	// enhanced class (via the Enhancer) in order to avoid memory leaks.
	Factory factory = (Factory) instance;
	factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
			new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
			new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
	return instance;
}

代理对象具体的执行逻辑：直接调用getBean

public Object intercept(Object obj, Method method, Object[] args, MethodProxy mp) throws Throwable {
	// Cast is safe, as CallbackFilter filters are used selectively.
	// 当前执行的方法上有没有@Lockup注解
	LookupOverride lo = (LookupOverride) getBeanDefinition().getMethodOverrides().getOverride(method);
	Assert.state(lo != null, "LookupOverride not found");
	// 得到所有参数
	Object[] argsToUse = (args.length > 0 ? args : null);  // if no-arg, don't insist on args at all
	if (StringUtils.hasText(lo.getBeanName())) {
		// 调用getBean
		Object bean = (argsToUse != null ? this.owner.getBean(lo.getBeanName(), argsToUse) :
				this.owner.getBean(lo.getBeanName()));
		// Detect package-protected NullBean instance through equals(null) check
		// 直接返回Bean，不去执行具体的方法
		return (bean.equals(null) ? null : bean);
	}
	else {
		// Find target bean matching the (potentially generic) method return type
		ResolvableType genericReturnType = ResolvableType.forMethodReturnType(method);
		return (argsToUse != null ? this.owner.getBeanProvider(genericReturnType).getObject(argsToUse) :
				this.owner.getBeanProvider(genericReturnType).getObject());
	}
}