How to allow a User only access their own data in Spring Boot / Spring Security?

In any @Controller, @RestController annotated bean you can use Principal directly as a method argument.

    @RequestMapping("/users/{user_id}")
    public String getUserInfo(@PathVariable("user_id") Long userId, Principal principal){
        // test if userId is current principal or principal is an ADMIN
        ....
    }

If you don’t want the security checks in your Controllers you could use Spring EL expressions.
You probably already use some build-in expressions like hasRole([role]).

And you can write your own expressions.

1. Create a bean

    @Component("userSecurity")
    public class UserSecurity {
         public boolean hasUserId(Authentication authentication, Long userId) {
            // do your check(s) here
        }
    }

2. Use your expression

    http
     .authorizeRequests()
     .antMatchers("/user/{userId}/**")
          .access("@userSecurity.hasUserId(authentication,#userId)")
        ...

The nice thing is that you can also combine expressions like:

    hasRole('admin') or @userSecurity.hasUserId(authentication,#userId)