@Lockup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。
@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());
}
}
结束语
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