java - Use of proxies in Spring AOP

Question

I am reading a book, which talks about enabling AspectJ support in Spring AOP.

Given below is a paragraph taken from the book:

To enable AspectJ annotation support in the Spring IoC container, you only have to define an empty XML element aop:aspectj-autoproxy in your bean configuration file. Then, Spring will automatically create proxies for any of your beans that are matched by your AspectJ aspects.

For cases in which interfaces are not available or not used in an application’s design, it’s possible to create proxies by relying on CGLIB. To enable CGLIB, you need to set the attribute proxy-target-class=true in <aop:aspectj-autoproxy />.

I am not able to get the second paragraph. What is meant by 'interfaces are not available'. Can anyone illustrate this with an example ?

Answer 1

Spring AOP uses either JDK dynamic proxies or CGLIB to create the proxies for your target objects.

According to Spring documentation, in case your target implements at least one interface, a JDK dynamic proxy will be used. However if your target object does not implement any interfaces then a CGLIB proxy will be created.

This is how you can force creation of the CGLIB proxies (set proxy-target-class="true"):

 <aop:config proxy-target-class="true">
    <!-- other beans defined here... -->
 </aop:config>

When using AspectJ and its autopoxy support you can also force CGLIB proxies. This is where the <aop:aspectj-autoproxy> is used and also here the "proxy-target-class" must be set to true:

<aop:aspectj-autoproxy proxy-target-class="true"/>

Please refer to Proxying mechanisms section of Aspect Oriented Programming with Spring documentation for more details.