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To utilize IDs instead of complete entities when making a JSON POST request through a Spring Boot application, you can follow the steps below:

1. Create a DTO (Data Transfer Object) that contains only the necessary fields for the request. The DTO should have an ID field that represents the ID of the entity that the request is related to.

2. In the controller method that handles the POST request, parse the DTO and retrieve the entity by its ID using a service method.

3. Validate the retrieved entity and update the necessary fields based on the DTO.

4. Save the updated entity using the service method.

Example:

Suppose you have two entities: User and Role, with a Many-to-Many relationship between them. You want to add a new Role for a User by sending a JSON POST request. Instead of sending the complete User and Role entities in the request, you can send only the IDs of the User and Role.

DTO:

public class UserRoleDTO {
  private Long userId;
  private Long roleId;

  // getters and setters
}

Controller:

@PostMapping("/users/{userId}/roles")
public ResponseEntity addRoleToUser(@PathVariable Long userId,
                                    @RequestBody UserRoleDTO userRoleDTO) {
  // retrieve the User entity by its ID
  User user = userService.findById(userId);

  // retrieve the Role entity by its ID
  Role role = roleService.findById(userRoleDTO.getRoleId());

  // validate the entities
  if (user == null || role == null) {
    return new ResponseEntity(HttpStatus.BAD_REQUEST);
  }

  // add the Role to the User's roles list
  user.getRoles().add(role);

  // save the updated User entity
  userService.save(user);

  return new ResponseEntity(HttpStatus.OK);
}

In this example, we only need the IDs of the User and Role to add a new Role for the User. By following this approach, we minimize the amount of data that is sent over the network and improve the performance of our application.