Spring @Lookup深入分析实现原理

@Component
public class Person {
    @Autowired
    Car car;
    public Car getCar() {
        return car;
    }
}
@Component
@Scope("prototype")
public class Car {
}

@Component
public class Person implements ApplicationContextAware {
    private ApplicationContext applicationContext;
    @Lookup
    public Car getCar() {
        return applicationContext.getBean(Car.class);
    }
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
}

@Component
@Scope(value = "prototype", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class Car {
}

@Component
public class Person {
    @Lookup
    public Car getCar() {
        return null;
    }
}

public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    /**
     * 如果bean没有要重写的方法，直接反射调用构造函数创建对象
     * 反之，需要通过CGLIB创建增强子类代理对象
     */
    if (!bd.hasMethodOverrides()) {
        Constructor<?> constructorToUse;
        synchronized (bd.constructorArgumentLock) {
            constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
            if (constructorToUse == null) {
                final Class<?> clazz = bd.getBeanClass();
                if (clazz.isInterface()) {
                    throw new BeanInstantiationException(clazz, "Specified class is an interface");
                }
                try {
                    if (System.getSecurityManager() != null) {
                        constructorToUse = AccessController.doPrivileged(
                                (PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
                    } else {
                        constructorToUse = clazz.getDeclaredConstructor();
                    }
                    bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                } catch (Throwable ex) {
                    throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                }
            }
        }
        return BeanUtils.instantiateClass(constructorToUse);
    } else {
        /**
         * 存在 lookup-method 和 replaced-method
         * 通过CGLIB生成代理类
         */
        return instantiateWithMethodInjection(bd, beanName, owner);
    }
}

@Override
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
        @Nullable Constructor<?> ctor, Object... args) {
    // Must generate CGLIB subclass...
    return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
}

public Object instantiate(@Nullable Constructor<?> ctor, Object... args) {
    Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
    Object instance;
    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);
        }
    }
    Factory factory = (Factory) instance;
    factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
            new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
            new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
    return instance;
}

@Override
public Object intercept(Object obj, Method method, Object[] args, MethodProxy mp) throws Throwable {
    // Cast is safe, as CallbackFilter filters are used selectively.
    LookupOverride lo = (LookupOverride) getBeanDefinition().getMethodOverrides().getOverride(method);
    Assert.state(lo != null, "LookupOverride not found");
    Object[] argsToUse = (args.length > 0 ? args : null);  // if no-arg, don't insist on args at all
    if (StringUtils.hasText(lo.getBeanName())) {
        return (argsToUse != null ? this.owner.getBean(lo.getBeanName(), argsToUse) :
                this.owner.getBean(lo.getBeanName()));
    }
    else {
        return (argsToUse != null ? this.owner.getBean(method.getReturnType(), argsToUse) :
                this.owner.getBean(method.getReturnType()));
    }
}