Core Spring interfaces declared with Java 8 default methods and @FunctionalInterface markers [SPR-14432]

[spring-projects-issues]

Juergen Hoeller opened SPR-14432 and commented

We have many multi-method interfaces in the core framework codebase, a few of which are obvious candidates for Java 8 default methods. We should also consistently mark user-level single-method interfaces as @FunctionalInterface. As a special consideration, even a multi-method interface can be a @FunctionalInterface if only one method is not declared as default..

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Issue Links:

• Upgrade framework codebase to JDK 8+ [SPR-13188] #17780 Upgrade framework codebase to JDK 8+

[spring-projects-issues]

Juergen Hoeller commented

Our FactoryBean, BeanPostProcessor and HandlerInterceptor variants are selectively declared with default methods now.

Also, all applicable interfaces with a functional characteristic to the user are marked with @FunctionalInterface. We could have marked all technically compatible interfaces but decided to avoid SPI bean contracts and other accidentally-single-method interfaces in favor of a more user-focused marker with stronger guidance towards actual lambda-friendly interfaces by design.