gradle

// retrofit2
    implementation 'com.squareup.retrofit2:retrofit:2.9.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.1.0'

// OkHttp
    implementation 'com.squareup.okhttp3:okhttp:4.9.0'
    implementation 'com.squareup.okhttp3:logging-interceptor:4.9.0'

public class HeaderInterceptor implements Interceptor {

    private static final String TAG = "HeaderInterceptor";
    private String token;

    public HeaderInterceptor(String token) {
        this.token = token;
    }

    @NotNull
    @Override
    public Response intercept(@NotNull Chain chain) throws IOException {
        Log.d(TAG, "intercept: 실행됨");
        token = SessionUser.token;
        Request request = chain.request().newBuilder()
                .addHeader("Authorization", token).build();
        return chain.proceed(request);
    }
}

public interface PostService {

    OkHttpClient client = new OkHttpClient.Builder().
            addInterceptor(new HeaderInterceptor(SessionUser.token)).build();

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("<http://localhost:8080>")
            .addConverterFactory(GsonConverterFactory.create())
            .client(client)
            .build();

    PostService service = retrofit.create(PostService.class);

}

헤더에 따로 추가하지 않고 요청해도 정상적으로 응답이 오는 것을 확인할 수 있음

@GET("/post/{id}")
    Call<CMRespDTO<Post>> findById(@Path("id")int postId);

참고

[Android] Retrofit을 이용한 토큰 인증 방식 구현](https://kimch3617.tistory.com/entry/Retrofit을-이용한-토큰-인증-방식-구현)

Retrofit OAuth2 Bearer Token Authentication OkHttp Android