Getting status 500 error "Internal Server Error" while running SpringBoot & POSTMAN

Question

I am new to SpringBoot and Postman and just started learning Java I was following a tutorial, but I keep getting the following error when I try to POST using POSTMAN after running my files on SpringBoot:

I have double checked the database and that is running fine, I am also not getting any errors on SpringBoot.

LOG from SpringBoot:

2023-01-20T17:57:07.313Z[0;39m [31mERROR[0;39m [35m8208[0;39m [2m---[0;39m [2m[nio-8080-exec-5][0;39m [36mo.a.c.c.C.[.[.[/].[dispatcherServlet]   [0;39m [2m:[0;39m Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed: org.springframework.dao.DataIntegrityViolationException: not-null property references a null or transient value : com.advanced.app1.model.Employee.firstName] with root cause

org.hibernate.PropertyValueException: not-null property references a null or transient value : com.advanced.app1.model.Employee.firstName
    at org.hibernate.engine.internal.Nullability.checkNullability(Nullability.java:111) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.engine.internal.Nullability.checkNullability(Nullability.java:55) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.action.internal.AbstractEntityInsertAction.nullifyTransientReferencesIfNotAlready(AbstractEntityInsertAction.java:119) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.action.internal.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:68) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.engine.spi.ActionQueue.execute(ActionQueue.java:653) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.engine.spi.ActionQueue.addResolvedEntityInsertAction(ActionQueue.java:283) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.engine.spi.ActionQueue.addInsertAction(ActionQueue.java:264) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.engine.spi.ActionQueue.addAction(ActionQueue.java:322) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.event.internal.AbstractSaveEventListener.addInsertAction(AbstractSaveEventListener.java:340) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:286) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:192) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:122) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:184) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:129) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:53) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:107) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:737) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:721) ~[hibernate-core-6.1.6.Final.jar:6.1.6.Final]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
    at org.springframework.orm.jpa.ExtendedEntityManagerCreator$ExtendedEntityManagerInvocationHandler.invoke(ExtendedEntityManagerCreator.java:360) ~[spring-orm-6.0.4.jar:6.0.4]
    at jdk.proxy2/jdk.proxy2.$Proxy96.persist(Unknown Source) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
    at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:307) ~[spring-orm-6.0.4.jar:6.0.4]
    at jdk.proxy2/jdk.proxy2.$Proxy96.persist(Unknown Source) ~[na:na]
    at org.springframework.data.jpa.repository.support.SimpleJpaRepository.save(SimpleJpaRepository.java:610) ~[spring-data-jpa-3.0.1.jar:3.0.1]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
    at org.springframework.data.repository.core.support.RepositoryMethodInvoker$RepositoryFragmentMethodInvoker.lambda$new$0(RepositoryMethodInvoker.java:288) ~[spring-data-commons-3.0.1.jar:3.0.1]
    at org.springframework.data.repository.core.support.RepositoryMethodInvoker.doInvoke(RepositoryMethodInvoker.java:136) ~[spring-data-commons-3.0.1.jar:3.0.1]
    at org.springframework.data.repository.core.support.RepositoryMethodInvoker.invoke(RepositoryMethodInvoker.java:120) ~[spring-data-commons-3.0.1.jar:3.0.1]
    at org.springframework.data.repository.core.support.RepositoryComposition$RepositoryFragments.invoke(RepositoryComposition.java:516) ~[spring-data-commons-3.0.1.jar:3.0.1]
    at org.springframework.data.repository.core.support.RepositoryComposition.invoke(RepositoryComposition.java:285) ~[spring-data-commons-3.0.1.jar:3.0.1]
    at org.springframework.data.repository.core.support.RepositoryFactorySupport$ImplementationMethodExecutionInterceptor.invoke(RepositoryFactorySupport.java:628) ~[spring-data-commons-3.0.1.jar:3.0.1]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.4.jar:6.0.4]
    at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.doInvoke(QueryExecutorMethodInterceptor.java:168) ~[spring-data-commons-3.0.1.jar:3.0.1]
    at org.springframework.data.repository.core.support.QueryExecutorMethodInterceptor.invoke(QueryExecutorMethodInterceptor.java:143) ~[spring-data-commons-3.0.1.jar:3.0.1]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.4.jar:6.0.4]
    at org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor.invoke(DefaultMethodInvokingMethodInterceptor.java:77) ~[spring-data-commons-3.0.1.jar:3.0.1]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.4.jar:6.0.4]
    at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:123) ~[spring-tx-6.0.4.jar:6.0.4]
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:388) ~[spring-tx-6.0.4.jar:6.0.4]
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:119) ~[spring-tx-6.0.4.jar:6.0.4]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.4.jar:6.0.4]
    at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:137) ~[spring-tx-6.0.4.jar:6.0.4]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.4.jar:6.0.4]
    at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:163) ~[spring-data-jpa-3.0.1.jar:3.0.1]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.4.jar:6.0.4]
    at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:97) ~[spring-aop-6.0.4.jar:6.0.4]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.4.jar:6.0.4]
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:218) ~[spring-aop-6.0.4.jar:6.0.4]
    at jdk.proxy2/jdk.proxy2.$Proxy100.save(Unknown Source) ~[na:na]
    at com.advanced.app1.service.impl.EmployeeServiceImpl.saveEmployee(EmployeeServiceImpl.java:21) ~[classes/:na]
    at com.advanced.app1.controller.EmployeeController.saveEmployee(EmployeeController.java:27) ~[classes/:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:207) ~[spring-web-6.0.4.jar:6.0.4]
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:152) ~[spring-web-6.0.4.jar:6.0.4]
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117) ~[spring-webmvc-6.0.4.jar:6.0.4]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:884) ~[spring-webmvc-6.0.4.jar:6.0.4]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797) ~[spring-webmvc-6.0.4.jar:6.0.4]
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-6.0.4.jar:6.0.4]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1080) ~[spring-webmvc-6.0.4.jar:6.0.4]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:973) ~[spring-webmvc-6.0.4.jar:6.0.4]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1011) ~[spring-webmvc-6.0.4.jar:6.0.4]
    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:914) ~[spring-webmvc-6.0.4.jar:6.0.4]
    at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:731) ~[tomcat-embed-core-10.1.5.jar:6.0]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885) ~[spring-webmvc-6.0.4.jar:6.0.4]
    at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:814) ~[tomcat-embed-core-10.1.5.jar:6.0]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:223) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:158) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:185) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:158) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-6.0.4.jar:6.0.4]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.4.jar:6.0.4]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:185) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:158) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-6.0.4.jar:6.0.4]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.4.jar:6.0.4]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:185) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:158) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-6.0.4.jar:6.0.4]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.4.jar:6.0.4]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:185) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:158) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:177) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:119) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:400) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:859) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1734) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-10.1.5.jar:10.1.5]
    at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]

 {
     "timestamp": "2023-01-20T12:47:38.113+00:00", 
     "status": 500, 
     "error": "Internal Server Error", 
     "path": "/api/employees" 
 }

My pom.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.0.1</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.advanced</groupId>
    <artifactId>sprintboot-advanced</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>war</packaging>
    <name>sprintboot-advanced</name>
    <description>Sprinboot Advanced</description>
    <properties>
        <java.version>17</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>com.mysql</groupId>
            <artifactId>mysql-connector-j</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <excludes>
                        <exclude>
                            <groupId>org.projectlombok</groupId>
                            <artifactId>lombok</artifactId>
                        </exclude>
                    </excludes>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

Main:

package com.advanced.app1;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class SprintbootAdvanced1Application {

    public static void main(String[] args) {
        SpringApplication.run(SprintbootAdvanced1Application.class, args);
    }

}

Controller:

package com.advanced.app1.controller;

import org.springframework.http.HttpStatus; 
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import com.advanced.app1.model.Employee;
import com.advanced.app1.service.EmployeeService;

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

    public EmployeeController(EmployeeService employeeService) {
        super();
        this.employeeService = employeeService;
    }
    
    // build REST API
    @PostMapping()
    public ResponseEntity<Employee> saveEmployee(@RequestBody Employee employee){
        return new ResponseEntity<Employee>(employeeService.saveEmployee(employee), HttpStatus.CREATED);
    }
}

Exception:

package com.advanced.app1.exception;

import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.ResponseStatus;

@ResponseStatus(value = HttpStatus.NOT_FOUND)
public class ResourceNotFoundException extends RuntimeException{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    private String resourceName;
    private String fieldName;
    private Object fieldValue;
    
    public ResourceNotFoundException(String resourceName, String fieldName, Object fieldValue) {
        super(String.format("%s not found with %s : '%s'", resourceName, fieldName, fieldValue));
        this.resourceName = resourceName;
        this.fieldName = fieldName;
        this.fieldValue = fieldValue;
    }
    
    public String getResourceName() {
        return resourceName;
    }
    
    public String getFieldName() {
        return fieldName;
    }
    
    public Object getFieldValue() {
        return fieldValue;
    }   
}

Model:

package com.advanced.app1.model;

import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.Table;
import lombok.Data;

@Data
@Entity
@Table(name="employees")
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    
    @Column(name = "first_name", nullable = false)
    private String firstName;
    
    @Column(name = "last_name")
    private String lastName;
    
    @Column(name = "email")
    private String email;
}

Repository:

package com.advanced.app1.repository;

import org.springframework.data.jpa.repository.JpaRepository;   
import com.advanced.app1.model.Employee;

public interface EmployeeRepository extends JpaRepository<Employee, Long> {

}

Service:

package com.advanced.app1.service;

import com.advanced.app1.model.Employee;

public interface EmployeeService {

    Employee saveEmployee(Employee employee);
} 

ServiceImpl:

package com.advanced.app1.service.impl;

import org.springframework.stereotype.Service;  
import com.advanced.app1.model.Employee;
import com.advanced.app1.repository.EmployeeRepository;
import com.advanced.app1.service.EmployeeService;

@Service
public class EmployeeServiceImpl implements EmployeeService{

    private EmployeeRepository employeeRepository;
    
    public EmployeeServiceImpl(EmployeeRepository employeeRepository) {
        super();
        this.employeeRepository = employeeRepository;
    }

    @Override
    public Employee saveEmployee(Employee employee) {
        return employeeRepository.save(employee);
    }

}

application.properties:

spring.datasource.url=jdbc:mysql://localhost:3306/test?useSSL=false
spring.datasource.username=root
spring.datasource.password=root
spring.jpa.properties.hibernate.dialect= org.hibernate.dialect.MySQLDialect
spring.jpa.hibernate.ddl-auto = update

Headers for request and return POSTMAN:

Answer 1

You say you're not getting and errors in your Spring Boot log, but you've posted a stack trace, which is the best type of error you could hope for. In fact, it tells you exactly what the problem is:

org.hibernate.PropertyValueException: not-null property references a null or transient value : com.advanced.app1.model.Employee.firstName

Your entity does not have "firstName" marked as nullable

@Column(name = "first_name", nullable = false)

Somehow, your Employee entity is not being populated with the data from your POST request. This can happen if the deserialization can't "automatically" map from your JSON fields to your Employee object fields.

One possible reason is that you do not have any getters or setters defined on your Entity. I see you're using lombok for this, but I would suggest at least for now ruling out any issue with this not working with Spring's framework and manually generating these methods directly.

You're also writing directly from the REST API into a database Entity. While this isn't strictly a problem, it's not a best practice and can sometimes lead to weird behaviour. I'd recommend creating a model object (POJO) specifically intended for use in your REST interface, and then map to an entity in your Repository.

Answer 2

The cause of the issue:

{
    "timestamp": "2023-01-22T14:43:44.841+00:00",
    "status": 500,
    "error": "Internal Server Error",
    "path": "/api/employees"
}

as correctly, mentioned, Hypino, is inside your provided entity:

@Data
@Entity
@Table(name="employees")
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    
    @Column(name = "first_name", nullable = false)
    private String firstName;
    
    @Column(name = "last_name")
    private String lastName;
    
    @Column(name = "email")
    private String email;
}

This implies that you labeled first_name to not being able to have null and so Spring isn't able to map the json request to an object, so first_name is always null.

In your case:

nullable = false means this value can't be null.

@Column(name = "first_name", nullable = false)

this means when creating the schema for the table, it will use the name "first_name" as the name and name = "firstName", nullable = false means that while saving to the database, if the value is null, it will not save and throw an exception:

Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed: org.springframework.dao.DataIntegrityViolationException: not-null property references a null or transient value

if you were to look at the actual sql that is generated.

In the same time, @Column only cares about the database, so when you are calling it via a Rest API, Spring is only caring about the variable name and not the column name.

As result, you have several options how to handle it and it really depends on you and your project requirements:

---

Option #1

(To use explicitly nullable = true as the option)

@Data
@Entity
@Table(name = "employees")
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    @Column(name = "first_name", nullable = true)
    private String first_name;

    @Column(name = "last_name", nullable = true)
    private String last_name;

    @Column(name = "email", nullable = true)
    private String email;
}    

in accordance with the body in Postman as:

{
    "first_name": "testFirstName",
    "last_name": "testLastName",
    "email": "testEmail@gmail.com"
}

Pros

• you can see right away that it is nullable. This could be useful when you are analyzing the code directly from github for example or you don't know the default value.

• if default value ever changes, you have it set and don't have to worry about it.

Cons:

• there is very very small chance that default value ever changes

• it makes code more verbose and that is generaly frowned upon.

true is the default option for nullable, so can be omitted. In the absence of a @Column annotation with a name, Hibernate will create a column using the field name, replacing camel-case with snake-case.

If you like keeping things explicit - fine.

---

Option #2

(To avoid the usage of annotations: @Column(name = "first_name", nullable = false for specific fields, as attached below)

Controller class:

@RestController
@RequestMapping("/api/employees")
public class EmployeeController {

    private EmployeeService employeeService;

    public EmployeeController(EmployeeService employeeService) {
        this.employeeService = employeeService;
    }

    @PostMapping
    public ResponseEntity<Employee> saveEmployee(@RequestBody Employee employee){
        return new ResponseEntity<>
                (employeeService.saveEmployee(employee), HttpStatus.CREATED);
    }
}

ResourceNotFoundException class:

@ResponseStatus(value = HttpStatus.NOT_FOUND)
public class ResourceNotFoundException extends RuntimeException {

    private static final long serialVersionUID = 1L;
    private String resourceName;
    private String fieldName;
    private Object fieldValue;

    public ResourceNotFoundException(String resourceName, String fieldName, Object fieldValue) {
        super(String.format("%s not found with %s : '%s'", resourceName, fieldName, fieldValue));
        this.resourceName = resourceName;
        this.fieldName = fieldName;
        this.fieldValue = fieldValue;
    }

    public String getResourceName() {
        return resourceName;
    }

    public String getFieldName() {
        return fieldName;
    }

    public Object getFieldValue() {
        return fieldValue;
    }
}

Model class:

@Data
@Entity
@Table(name = "employees")
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    private String firstName;

    private String lastName;

    private String email;
}

Repository:

public interface EmployeeRepository extends JpaRepository<Employee, Long> {

}

Service implementation (business-logic):

@Service
public class EmployeeServiceImpl implements EmployeeService {

    private final EmployeeRepository employeeRepository;

    public EmployeeServiceImpl(EmployeeRepository employeeRepository) {
        super();
        this.employeeRepository = employeeRepository;
    }

    @Override
    public Employee saveEmployee(Employee employee) {
        return employeeRepository.save(employee);
    }
}

Service:

public interface EmployeeService {

    Employee saveEmployee(Employee employee);
}

Main class:

@SpringBootApplication
public class SpringBootAdvancedApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringBootAdvancedApplication.class, args);
    }
}

and accordingly database configuration:

spring.datasource.url=jdbc:mysql://localhost:3306/testdb?useSSL=false&allowPublicKeyRetrieval=true
spring.datasource.username=root
spring.datasource.password=...
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQLDialect
spring.jpa.hibernate.ddl-auto=update

As result, you can test via Postman using POST request as the following:

http://localhost:8080/api/employees

with body as in the example:

{
    "firstName": "testFirstName",
    "lastName": "testLastName",
    "email": "testEmail@gmail.com"
}

choosing raw & JSON as your options.

Headers for request:

The content of POM file:

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
        <version>3.0.1</version>
    </dependency>

    <dependency>
        <groupId>com.mysql</groupId>
        <artifactId>mysql-connector-j</artifactId>
        <scope>runtime</scope>
    </dependency>

    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <optional>true</optional>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-tomcat</artifactId>
        <scope>provided</scope>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
                <source>10</source>
                <target>10</target>
                <encoding>UTF-8</encoding>
            </configuration>
        </plugin>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <excludes>
                    <exclude>
                        <groupId>org.projectlombok</groupId>
                        <artifactId>lombok</artifactId>
                    </exclude>
                </excludes>
            </configuration>
        </plugin>
    </plugins>
</build>

If you prefer avoiding boilerplate when you are aware of the defaults - also fine. The key here is keeping the rules consistent throughout the entire project.

---

Additional useful & informative links:

1. Hibernate Tips: What’s the difference between @Column(nullable = false) and @NotNull
2. Hibernate @NotNull vs @Column(nullable = false)
3. Hibernate handling of nullable=true in @Column
4. Does it make sense to write @Column(nullable = true) for entity while using Hibernate?

Answer 3

A status 500 "Internal Server Error" is a general error message that indicates that something went wrong on the server side of your application. There could be multiple reasons for this error, but some common causes include:

Configuration issues: Check your application's configuration, such as the properties files, to make sure everything is set up correctly.

Coding errors: This could be a bug in your code that is causing the server to crash or return the error. Check for syntax errors, null pointer exceptions, and other common coding issues.

Database issues: Make sure your database is running and configured correctly. Check your database connection settings and ensure that the correct user has access to the database.

Dependency issues: If you're using any dependencies or libraries, make sure they are up-to-date and compatible with your application.

Permission issues: check if the user running the application has the correct permissions to access the resources it needs.

It's recommended to check the server logs to see more detailed information about the error, which could help you identify the root cause of the problem and fix it.

It's worth mentioning that testing your application using Postman could not reveal all the possible issues, so it could be worth testing your application by running it directly on a browser or other clients as well.