I am new to spring boot and I am wondering how to allow a response JSON to include null values for database columns that are nullable. This is my controller, service, repository and model:

package com.findersgame.questtracker.controller;

import java.util.List;

import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

import com.findersgame.questtracker.model.MapArea;
import com.findersgame.questtracker.service.MapAreaService;

@RestController
public class MapAreaController {

    private MapAreaService mapAreaService;

    public MapAreaController(MapAreaService mapAreaService) {
        super();
        this.mapAreaService = mapAreaService;
    }
    
    @PostMapping("selected_map_areas")
    public List<MapArea> selectedMapAreas(@RequestBody List<Integer> selectedMapAreas) {
        return mapAreaService.selectedMapAreas(selectedMapAreas);
    }
}


package com.findersgame.questtracker.service;

import java.util.ArrayList;
import java.util.List;

import org.springframework.stereotype.Service;

import com.findersgame.questtracker.model.MapArea;
import com.findersgame.questtracker.repository.MapAreaRepository;

@Service
public class MapAreaService {
    
    private MapAreaRepository mapAreaRepository;
    
    public MapAreaService(MapAreaRepository mapAreaRepository) {
        super();
        this.mapAreaRepository = mapAreaRepository;
    }
    
    public List<MapArea> selectedMapAreas(List<Integer> selectedIds) {
        return mapAreaRepository.findAllById(selectedIds);
    }
}

package com.findersgame.questtracker.repository;

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

import com.findersgame.questtracker.model.MapArea;

public interface MapAreaRepository extends JpaRepository<MapArea, Integer> {

}

package com.findersgame.questtracker.model;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.Table;

@Entity
@Table(name = "map_areas")
public class MapArea {
    
    @Id
    private Integer id;
    
    @Column
    private Integer mapTabId;

    @Column
    private String name;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }
    
    public Integer getMapTabId() {
        return mapTabId;
    }

    public void setMapTabId(Integer mapTabId) {
        this.mapTabId = mapTabId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

My problem is that when I call http://localhost:8080/selected_map_areas with body [14, 15, 16] I get the following error.

Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Cannot invoke "java.lang.Integer.intValue()" because "this.mapTabId" is null]

I guess it makes sense because in the database, the map_tab_id value is null for both id 15 and 16. I'm guessing that I'm missing an annotation in one of the classes/interfaces above but I have not been able to find which one.

Note that it works fine and I get the correct result in Postman when `map_tab_id` has a value.

Please help.

java.lang.NullPointerException: Cannot invoke "com.example.demo.student.StudentRepository.findAll()" because "this.studentRepository" is null

I do have a @Repository annotated above my StudentRepository class. I also used @Autorwired above the constructor in my StudentService class. I think the problem is with dependency injection but I don't see why.

Related code:

//StudentRepository.java
package com.example.demo.student;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface StudentRepository extends JpaRepository<Student, Long> {
}

//StudentService.java
package com.example.demo.student;

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

import java.time.LocalDate;
import java.time.Month;
import java.util.List;

@Service
public class StudentService {

    private final StudentRepository studentRepository;

    @Autowired
    public StudentService(StudentRepository studentRepository) {
        this.studentRepository = studentRepository;
    }

    public List<Student> getStudents() {
        return studentRepository.findAll();
    }
}

//StudentController.java
package com.example.demo.student;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.List;

@RestController
@RequestMapping(path = "api/v1/student")
public class StudentController {

    private final StudentService studentService;

    @Autowired
    public StudentController(StudentService studentService) {
        this.studentService = studentService;
    }

    @GetMapping
    public List<Student> getStudents(StudentService studentService) {
        return studentService.getStudents();
    }
}

I'm pretty new at Java and was trying to do a CRUD, the process was going pretty smoothly until i had to make the create function to the table with a composite key.

At first i thought the problem was with the Dependecy Injection, but it's working just fine in the methods getByStateAndModelId() and getAll().I also tried to set the names for the EquipmentModel and EquipmentState in the createEmshe (In the POST i just insert the Id of both State and Models, and the Value, so the names from Model and State coming from the DTO are null and i thought that maybe that was the cause). But then, both the equipmentModelRepository and the equipmentStateRepository returned the NullPointerException too, what i'm missing?here's the relevant code:

The service:

package com.api.forestoperation.emshe;

import java.util.List;
import java.util.UUID;

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

import com.api.forestoperation.equipmentmodel.EquipmentModelModel;
import com.api.forestoperation.equipmentmodel.EquipmentModelRepository;
import com.api.forestoperation.equipmentstate.EquipmentStateModel;
import com.api.forestoperation.equipmentstate.EquipmentStateRepository;

@Service
public class EquipmentModelStateHourlyEarningsService {
	@Autowired
	EquipmentModelStateHourlyEarningsRepository emsheRepository;
	@Autowired
	EquipmentModelRepository equipmentModelRepository;
	@Autowired
	EquipmentStateRepository equipmentStateRepository;

	public List<EquipmentModelStateHourlyEarningsModel> getAllEmshe() {
		return emsheRepository.findAll();
	}

	public EquipmentModelStateHourlyEarningsModel createEmshe(EquipmentModelStateHourlyEarningsDTO emsheDTO) {
		var emsheModel = new EquipmentModelStateHourlyEarningsModel();

		BeanUtils.copyProperties(emsheDTO, emsheModel);

		EquipmentModelModel emsheModelInfo = emsheModel.getId().getEquipmentModel();
		EquipmentStateModel emsheStateInfo = emsheModel.getId().getEquipmentState();
		EquipmentModelStateHourlyEarningsPK emshePk = new EquipmentModelStateHourlyEarningsPK(emsheModelInfo,
				emsheStateInfo);

		emsheModel.setId(emshePk);

		return emsheRepository.save(emsheModel);
	}

	public EquipmentModelStateHourlyEarningsModel getEmsheByStateAndModelId(UUID modelId, UUID stateId) {
		var modelExists = equipmentModelRepository.findById(modelId).orElse(null);
		var stateExists = equipmentStateRepository.findById(stateId).orElse(null);
		if (modelExists != null && stateExists != null) {
			EquipmentModelStateHourlyEarningsPK emshePk = new EquipmentModelStateHourlyEarningsPK(modelExists,
					stateExists);
			EquipmentModelStateHourlyEarningsModel emsheModel = emsheRepository.findById(emshePk).orElse(null);
			return emsheModel;
		}
		return null;
	}
}

The Controller:

package com.api.forestoperation.emshe;

import java.util.List;
import java.util.UUID;

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.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class EquipmentModelStateHourlyEarningsController {
	@Autowired
	EquipmentModelStateHourlyEarningsService emsheService;

	@PostMapping("/equipment-model-state-hourly-earnings")
	public ResponseEntity<Object> saveEmshe(@RequestBody EquipmentModelStateHourlyEarningsDTO emsheDTO) {
		var savedEmshe = new EquipmentModelStateHourlyEarningsService().createEmshe(emsheDTO);
		return savedEmshe != null
				? ResponseEntity.status(HttpStatus.CREATED)
						.body("EquipmentModelStateHourlyEarnings created with Sucess")
				: ResponseEntity.status(HttpStatus.BAD_REQUEST).build();
	}

	@GetMapping("/equipment-model-state-hourly-earnings")
	public ResponseEntity<List<EquipmentModelStateHourlyEarningsModel>> getAllEmshe() {
		List<EquipmentModelStateHourlyEarningsModel> equipments = emsheService.getAllEmshe();
		return ResponseEntity.status(HttpStatus.OK).body(equipments);
	}

	@GetMapping("/equipment-model-state-hourly-earnings/{modelId}/{stateId}")
	public ResponseEntity<Object> getEmsheByModelAndStateId(@PathVariable(value = "modelId") UUID modelId,
			@PathVariable(value = "stateId") UUID stateId, EquipmentModelStateHourlyEarningsPK emshePk) {
		EquipmentModelStateHourlyEarningsModel emsheModel = emsheService.getEmsheByStateAndModelId(modelId, stateId);
		return emsheModel == null ? ResponseEntity.status(HttpStatus.BAD_REQUEST).body("EMSHE nulo")
				: ResponseEntity.status(HttpStatus.OK).body(emsheModel);

	}
}

The Repository:

	package com.api.forestoperation.equipment;

import java.util.UUID;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface EquipmentRepository extends JpaRepository<EquipmentModel, UUID> {

}

The Model:

	package com.api.forestoperation.emshe;

import java.io.Serializable;

import com.api.forestoperation.equipmentmodel.EquipmentModelModel;
import com.api.forestoperation.equipmentstate.EquipmentStateModel;

import jakarta.persistence.Column;
import jakarta.persistence.EmbeddedId;
import jakarta.persistence.Entity;
import jakarta.persistence.Table;

@Entity
@Table(name="equipment_model_state_hourly_earnings", schema="operation")
public class EquipmentModelStateHourlyEarningsModel implements Serializable {
	@EmbeddedId
	private EquipmentModelStateHourlyEarningsPK id;
	private static final long serialVersionUID = 1L;
	
	@Column(name="value")
	private double value;
	
	public EquipmentModelStateHourlyEarningsPK getId() {
		return id;
	}

	public void setId(EquipmentModelStateHourlyEarningsPK id) {
		this.id = id;
	}

	public double getValue() {
		return value;
	}

	public void setValue(double value) {
		this.value = value;
	}
	
	
	public EquipmentModelModel getEquipmentModel() {
		return id.getEquipmentModel();
	}
	
	public EquipmentStateModel getEquipmentState() {
		return id.getEquipmentState();
	}

}

The Pk:

package com.api.forestoperation.emshe;

import java.io.Serializable;
import java.util.Objects;

import com.api.forestoperation.equipmentmodel.EquipmentModelModel;
import com.api.forestoperation.equipmentstate.EquipmentStateModel;

import jakarta.persistence.Embeddable;
import jakarta.persistence.JoinColumn;
import jakarta.persistence.ManyToOne;

@Embeddable
public class EquipmentModelStateHourlyEarningsPK implements Serializable {

	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;

	@ManyToOne
	@JoinColumn(name = "equipment_model_id")
	private EquipmentModelModel equipmentModel;

	@ManyToOne
	@JoinColumn(name = "equipment_state_id")
	private EquipmentStateModel equipmentState;

	public EquipmentModelStateHourlyEarningsPK() {

	}

	public EquipmentModelStateHourlyEarningsPK(EquipmentModelModel equipmentModelModel,
			EquipmentStateModel equipmentStateModel) {
		this.equipmentModel = equipmentModelModel;
		this.equipmentState = equipmentStateModel;
	}

	@Override
	public int hashCode() {
		return Objects.hash(equipmentModel, equipmentState);
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		EquipmentModelStateHourlyEarningsPK other = (EquipmentModelStateHourlyEarningsPK) obj;
		return Objects.equals(equipmentModel, other.equipmentModel)
				&& Objects.equals(equipmentState, other.equipmentState);
	}

	public EquipmentModelModel getEquipmentModel() {
		return equipmentModel;
	}

	public void setEquipmentModel(EquipmentModelModel equipmentModel) {
		this.equipmentModel = equipmentModel;
	}

	public EquipmentStateModel getEquipmentState() {
		return equipmentState;
	}

	public void setEquipmentState(EquipmentStateModel equipmentState) {
		this.equipmentState = equipmentState;
	}
}

Here's the error i get:

java.lang.NullPointerException: Cannot invoke     "com.api.forestoperation.emshe.EquipmentModelStateHourlyEarningsRepository.save(Object)" because "this.emsheRepository" is null
at com.api.forestoperation.emshe.EquipmentModelStateHourlyEarningsService.createEmshe(EquipmentModelStateHourlyEarningsService.java:39) ~[classes/:na]
at com.api.forestoperation.emshe.EquipmentModelStateHourlyEarningsController.saveEmshe(EquipmentModelStateHourlyEarningsController.java:22) ~[classes/:na]
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[na:na]
at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[na:na]
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) ~[spring-web-6.0.13.jar:6.0.13]
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:150) ~[spring-web-6.0.13.jar:6.0.13]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:118) ~[spring-webmvc-6.0.13.jar:6.0.13]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:884) ~[spring-webmvc-6.0.13.jar:6.0.13]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797) ~[spring-webmvc-6.0.13.jar:6.0.13]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-6.0.13.jar:6.0.13]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1081) ~[spring-webmvc-6.0.13.jar:6.0.13]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:974) ~[spring-webmvc-6.0.13.jar:6.0.13]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1011) ~[spring-webmvc-6.0.13.jar:6.0.13]
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:914) ~[spring-webmvc-6.0.13.jar:6.0.13]
at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:590) ~[tomcat-embed-core-10.1.15.jar:6.0]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885) ~[spring-webmvc-6.0.13.jar:6.0.13]
at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:658) ~[tomcat-embed-core-10.1.15.jar:6.0]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:205) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51) ~[tomcat-embed-websocket-10.1.15.jar:10.1.15]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-6.0.13.jar:6.0.13]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.13.jar:6.0.13]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-6.0.13.jar:6.0.13]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.13.jar:6.0.13]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-6.0.13.jar:6.0.13]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.13.jar:6.0.13]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:167) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:90) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:482) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:115) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:340) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:391) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:896) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1744) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]