【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Stringif (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CglibSubclassingInstantiationStrategybeanInstance = 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 allif (StringUtils.hasText(lo.getBeanName())) {// 调用getBeanObject 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 typeResolvableType genericReturnType = ResolvableType.forMethodReturnType(method);return (argsToUse != null ? this.owner.getBeanProvider(genericReturnType).getObject(argsToUse) :this.owner.getBeanProvider(genericReturnType).getObject());}
}

结束语

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