关于java:Spring-Bean的实例化

Bean的实例化

结构器的抉择

SimpleInstantiationStrategy

Bean实例化有两种策略，一种是CGLIB，另一种是通过BeanUtil，即用JVM反射生成对象。
如果Bean中有@LookUp注解，或者xml中指定了lookup-method，那么就会采纳CGLIB来实例化对象，因为须要重写@LookUp标记的办法，这种状况下很容易想到应用代理，应用代理，被代理类又不需实现接口，无疑CGLB形式适合。

public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
   // Don't override the class with CGLIB if no overrides.
 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");
 }
 //获取无参结构器去实例化Bean
            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);
 }
         }
      }
      // 通过JVM反射性能生成实例化对象
      return BeanUtils.instantiateClass(constructorToUse);
 }
   else {
      // 通过CGLIB来生成实例化对象
 return instantiateWithMethodInjection(bd, beanName, owner);
 }
}