Java：任意对象的方法引用

Question

Given the following code (copied out of context to keep things simple) It works fine, and enables OAuth2 JWT token validation.

...
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.annotation.web.configurers.oauth2.server.resource.OAuth2ResourceServerConfigurer;
...
protected void configure(HttpSecurity http) throws Exception {
    http.oauth2ResourceServer(OAuth2ResourceServerConfigurer::jwt);
}
...

I have a hard time understanding the OAuth2ResourceServerConfigurer::jwt method reference

httpConfig.oauth2ResourceServer expects an Customizer as input. The Customizer is a functionnal interface that defines one instance method which is void customize(T t) .

The jwt instance method of OAuth2ResourceServerConfigurer returns an object of type OAuth2ResourceServerConfigurer.JwtConfigurer , which does not define the void customize(T t) method.

How can it comply with the Customizer interface?

Also, I don't understand which instance of OAuth2ResourceServerConfigurer will be used to call the 'jwt()' method.

The basic Java article does not help me. ( https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html )

I am familiar with Lambdas and method references, but this one I don't get.

Answer 1

How can it comply with the Customizer interface?

When a method reference implements a functional interface, the parameters and the return type have to comply (be compatible), not the method name.

Consistent with how a lambda expression has to have parameters and a return type that is compatible with the functional method, but the method name isn't given anywhere.

Also, I don't understand which instance of OAuth2ResourceServerConfigurer will be used to call the jwt() method.

In the link you provided, see the section named " Reference to an Instance Method of an Arbitrary Object of a Particular Type ".

Since your method reference is OAuth2ResourceServerConfigurer::jwt , which is a no-arg instance method, the t parameter of the customize(T t) method becomes the this value of the jwt() call.

It is basically the same as this lambda expression:

http.oauth2ResourceServer(t -> t.jwt());

Which is equivalent to this anonymous class:

http.oauth2ResourceServer(new Customizer<OAuth2ResourceServerConfigurer<HttpSecurity>>() {
    @Override
    public void customize(OAuth2ResourceServerConfigurer<HttpSecurity> t) {
        t.jwt();
    }
});