环境：Spring5.3.23
1. 简介
Lookup方法注入能够根据@Lookup注解的value属性值或被注解该方法的返回值，从容器中查找bean作为方法的返回值对象使用。Spring容器会通过CGLIB生成当前类的代理，然后重写被@Lookup注解的方法。
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Lookup {
/**
* 根据你设置的value值，从容器中查找对应的bean，如果没有指定value值
* 那么会根据被注解方法的返回值类型从容器中查找相应的bean
*/
String value() default "";
}2. 应用案例
准备基础类
static interface HttpSecurity {
void http() ;
}
static class HtmlHttpSecurity implements HttpSecurity {
@Override
public void http() {
System.out.println("Html HttpSecurity...") ;
}
}定义一个抽象类
该类中有一个抽象方法被@Lookup注解标注
static abstract class SecurityManager {
public void execute() {
HttpSecurity httpSecurity = httpSecurity() ;
System.out.println(httpSecurity.getClass()) ;
httpSecurity.http() ;
}
@Lookup("html")
protected abstract HttpSecurity httpSecurity() ;
}注册Bean
将上面的类注册到容器中
public static void main(String[] args) {
try (AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext()) {
context.registerBean("html", HtmlHttpSecurity.class) ;
context.register(SecurityManager.class) ;
context.refresh() ;
SecurityManager sm = context.getBean(SecurityManager.class);
sm.execute() ;
System.out.println(sm.getClass()) ;
}
}执行结果
class com.pack.main.lookup.MethodLookupInjectMain2$HtmlHttpSecurity
Html HttpSecurity...
class com.pack.main.lookup.MethodLookupInjectMain2$SecurityManager$$EnhancerBySpringCGLIB$$ae697832SecurityManager通过CGLIB被创建为了代理类。同时execute方法中HttpSecurity对象就是HtmlHttpSecurity类，也就是容器通过查找注入的。
去掉@Lookup注解的value属性
@Lookup
protected abstract HttpSecurity httpSecurity() ;继续执行上面的测试代码，程序也能正常的输出。
在添加一个HttpSecurity的实现
static class CssHttpSecurity implements HttpSecurity {
@Override
public void http() {
System.out.println("Css HttpSecurity...") ;
}
}将上面的类也注册到容器中，如果这时候你的@Lookup注解没有value属性将会报错
Exception in thread "main" org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.pack.main.lookup.MethodLookupInjectMain2$HttpSecurity' available: expected single matching bean but found 2: html,css
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveNamedBean(DefaultListableBeanFactory.java:1273)expected single matching bean but found 2: html,css；错误提示你有2个bean，期望的是一个，所以这里我们必须添加value属性，指明要查找的bean对象。
继续测试，被@Lookup注解的方法是不是必须是抽象的？
修改代码如下：
@Lookup("html")
protected HttpSecurity httpSecurity() {
return null ;
};方法返回null，执行结果
class com.pack.main.lookup.MethodLookupInjectMain2$HtmlHttpSecurity
Html HttpSecurity...
class com.pack.main.lookup.MethodLookupInjectMain2$SecurityManager$$EnhancerBySpringCGLIB$$ae697832程序正常，没有问题。这也说明了，这里和具体的返回值是没有关系的。
3. 实现原理
容器在创建一个Bean对象时会执行如下步骤
public abstract class AbstractAutowireCapableBeanFactory {
protected Object doCreateBean() {
BeanWrapper instanceWrapper = null;
// ...
if (instanceWrapper == null) {
// 创建实例
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
// ...
}
protected BeanWrapper createBeanInstance() {
// 查找构造方法
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
// 实例化bean对象；上面一步对@Lookup注解的方法进行了初始化查找
return instantiateBean(beanName, mbd);
}
protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(@Nullable Class<?> beanClass, String beanName)
throws BeansException {
if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) {
// 调用BeanPostProcessor
for (SmartInstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().smartInstantiationAware) {
// 这里通过了AutowiredAnnotationBeanPostProcessor处理器
// 这里其实最终返回了null，关键就是执行了下面的动作，查找了@Lookup注解的方法进行解析
// 保存到BeanDefinition中
Constructor<?>[] ctors = bp.determineCandidateConstructors(beanClass, beanName);
if (ctors != null) {
return ctors;
}
}
}
return null;
}
}AutowiredAnnotationBeanPostProcessor处理器
public class AutowiredAnnotationBeanPostProcessor {
public Constructor<?>[] determineCandidateConstructors() {
do {
// 遍历当前类的所有方法
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
// 获取方法是的@Lookup注解
Lookup lookup = method.getAnnotation(Lookup.class);
if (lookup != null) {
// 如果存在则构造LookupOverride对象，将当前的Method及注解的value值传入
LookupOverride override = new LookupOverride(method, lookup.value());
try {
RootBeanDefinition mbd = (RootBeanDefinition)this.beanFactory.getMergedBeanDefinition(beanName);
// 最后将其保存到BeanDefinition中
mbd.getMethodOverrides().addOverride(override);
}
}
});
targetClass = targetClass.getSuperclass();
}
while (targetClass != null && targetClass != Object.class);
}
}继续上面执行到instantiateBean方法
public abstract class AbstractAutowireCapableBeanFactory {
protected BeanWrapper instantiateBean() {
// getInstancetiationStrategy方法返回了CglibSubclassingInstantiationStrategy
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
}
}进入SimpleInstantiationStrategy#instanceiate方法
public class SimpleInstantiationStrategy implements InstantiationStrategy {
public Object instantiate(RootBeanDefinition bd) {
// 判断当前的BeanDefinition中是否有LookupOverride
// 上面查找到后已经将其保存到了BeanDefinition中。
if (!bd.hasMethodOverrides()) {
// ...
}
else {
// 存在则通过CGLIB进行创建代理
// 进入CglibSubclassingInstantiationStrategy
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
}CglibSubclassingInstantiationStrategy
public class CglibSubclassingInstantiationStrategy extends SimpleInstantiationStrategy {
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
return instantiateWithMethodInjection(bd, beanName, owner, null);
}
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
@Nullable Constructor<?> ctor, Object... args)
return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
}
}CglibSubclassCreator
private static class CglibSubclassCreator {
private static final Class<?>[] CALLBACK_TYPES = new Class<?>[]
{NoOp.class, LookupOverrideMethodInterceptor.class, ReplaceOverrideMethodInterceptor.class};
public Object instantiate(@Nullable Constructor<?> ctor, Object... args) {
// 创建代理，当前类的子类
Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
// ...
}
private Class<?> createEnhancedSubclass(RootBeanDefinition beanDefinition) {
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(beanDefinition.getBeanClass());
enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
if (this.owner instanceof ConfigurableBeanFactory) {
ClassLoader cl = ((ConfigurableBeanFactory) this.owner).getBeanClassLoader();
enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(cl));
}
enhancer.setCallbackFilter(new MethodOverrideCallbackFilter(beanDefinition));
// 我们主要关注这里，这些都是CGLIB相关的知识了。
// 关注LookupOverrideMethodInterceptor拦截器
enhancer.setCallbackTypes(CALLBACK_TYPES);
return enhancer.createClass();
}
}LookupOverrideMethodInterceptor拦截器
private static class LookupOverrideMethodInterceptor extends CglibIdentitySupport implements MethodInterceptor {
public Object intercept(Object obj, Method method, Object[] args, MethodProxy mp) throws Throwable {
LookupOverride lo = (LookupOverride) getBeanDefinition().getMethodOverrides().getOverride(method);
Object[] argsToUse = (args.length > 0 ? args : null); // if no-arg, don't insist on args at all
// 判断是否设置了@Lookup value值
if (StringUtils.hasText(lo.getBeanName())) {
// 根据value指定的值查找bean对象
Object bean = (argsToUse != null ? this.owner.getBean(lo.getBeanName(), argsToUse) :
this.owner.getBean(lo.getBeanName()));
return (bean.equals(null) ? null : bean);
}
// 如果没有设置value，那么会根据方法的返回值类型在容器中查找bean
else {
ResolvableType genericReturnType = ResolvableType.forMethodReturnType(method);
return (argsToUse != null ? this.owner.getBeanProvider(genericReturnType).getObject(argsToUse) :
this.owner.getBeanProvider(genericReturnType).getObject());
}
}
}以上就是@Lookup注解的原理
总结：Spring的@Lookup注解提供了一种灵活的机制，用于在运行时动态地创建和初始化beans。通过@Lookup，开发者可以在配置中指定一个方法，该方法会在运行时被调用以获取相应的bean实例。这使得在某些特定条件下或在运行时配置变更时，能够动态地选择和创建不同的bean实现。
完毕！！！
