There are more advantages to interfaces – As in proxying . If your class implements an interface , JDK dynamic proxies will be used by default for AOP . If you use the implementations directly, you’ll be forced to use CGLIB proxies by making proxy-target-class=true . These require byte code manipulation unlike JDK proxies .
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Read another discussion at what reasons are there to use interfaces (Java EE or Spring and JPA) for more info .