Employee.java

package com.example.demo;

public class Employee {
}

EmployeeController.java

package com.example.demo;

import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping(path = "/employees")
public class EmployeeController {
    @Autowired
    private EmployeeService employeeService;

    @GetMapping
    public ResponseEntity<List<Employee>> findEmployees(@RequestParam("firstName") String firstName) {
        List<Employee> employees = employeeService.findAllCustomersByFirstName(firstName);
        return new ResponseEntity<>(employees, HttpStatus.OK);
    }
}

EmployeeRepository.java

package com.example.demo;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.JpaSpecificationExecutor;

public interface EmployeeRepository extends JpaRepository<Employee, Long>, JpaSpecificationExecutor<Employee> {
}

EmployeeService.java

package com.example.demo;

import java.util.List;

public interface EmployeeService {
    List<Employee> findAllCustomersByFirstName(String firstName);
}

EmployeeServiceImpl.java

package com.example.demo;

import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class EmployeeServiceImpl implements EmployeeService {
    @Autowired
    private EmployeeRepository employeeRepository;

    @Override
    public List<Employee> findAllCustomersByFirstName(String firstName) {
        return employeeRepository.findAll(EmployeeSpecification.textInAllColumns(firstName));
    }
}

EmployeeSpecification.java

package com.example.demo;

import javax.persistence.criteria.Predicate;

import org.springframework.data.jpa.domain.Specification;

public class EmployeeSpecification {

    public static Specification<Employee> textInAllColumns(String text) {
        if (!text.contains("%")) {
            text = "%" + text + "%";
        }
        final String finalText = text;

        return (root, query, builder) -> builder
                .or(root.getModel().getDeclaredSingularAttributes().stream().filter(a -> {
                    return a.getJavaType().getSimpleName().equalsIgnoreCase("string") ? true : false;
                }).map(a -> builder.like(root.get(a.getName()), finalText)).toArray(Predicate[]::new));
    }
}