When I issue a Postman Get request, it returns duplicate result. Even when I had only three records in my database, Postman returns hundreds of same record duplicated.

pom.xml:

<?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>2.5.5</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example.resource</groupId>
    <artifactId>akademiks</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>akademiks</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>1.8</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-data-rest</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
            <scope>runtime</scope>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</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>
            </plugin>
        </plugins>
    </build>

</project>

Entity class:

@Entity
@Table(name = "Chemistry")
public class Chemistry {
    
    @Id
    @Column(name = "id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    
    @Column(name = "date")
    private Date date;
    
    @Column(name = "question_no")
    private Integer questionNo;
    
    @Column(name = "question")
    private String question;
    
    @OneToOne(mappedBy = "question",cascade = CascadeType.ALL)
    private ChemistryAnswer answer = new ChemistryAnswer();
    
    public Chemistry() {}

    public Chemistry(Date date, Integer questionNo, String question) {
        this.date = date;
        this.questionNo = questionNo;
        this.question = question;
        
        this.answer.setDate(date);
        this.answer.setQuestionNo(questionNo);
        this.answer.setQuestion(this);
    }//accessors

Entity class:

@Entity
@Table(name = "Chemistryanswer")
public class ChemistryAnswer {

    @Id
    @Column(name = "id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    @Column(name = "date")
    private Date date;
    
    @Column(name = "question_no")
    private Integer questionNo;
    
    @Column(name = "solution")
    private String solution;
    
    @Column(name = "solution_url")
    private String solutionUrl;
    
    @OneToOne
    @JoinColumn(name = "question_id")
    private Chemistry question;

    public ChemistryAnswer() {}

    public ChemistryAnswer(Integer questionNo, String solution, String solutionUrl) {
        
        this.questionNo = questionNo;
        this.solution = solution;
        this.solutionUrl = solutionUrl;
    }

    public ChemistryAnswer(Date date, Integer questionNo) {
        this.date = date;
        this.questionNo = questionNo;
    }

    public Date getDate() {
        return date;
    }//accessors

jpaRepository:

@RepositoryRestResource(collectionResourceRel = "chemistry", path = "chemistry")
public interface ChemistryRepo extends JpaRepository<Chemistry, Integer> {

}

Service class:

public interface ChemistryService {
    
    public List<Chemistry>findAll();
    
    public void save(Chemistry chemistry);

}

Service implementation:

@Service
public class ChemistryServiceImpl implements ChemistryService {
 
    private ChemistryRepo repo;
    
    public ChemistryServiceImpl() {}
    
    @Autowired
    public ChemistryServiceImpl(ChemistryRepo repo) {
        this.repo = repo;
    }
    
    @Override
    public List<Chemistry> findAll() {
        
        return repo.findAll();
    }

    @Override
    public void save(Chemistry chemistry) {
        Chemistry tempChemistry = new Chemistry(chemistry.getDate(), 
                chemistry.getQuestionNo(), chemistry.getQuestion());
        ChemistryAnswer answer = tempChemistry.getAnswer();
        tempChemistry.setAnswer(answer);
        repo.save(tempChemistry);

    }

}

RestController class:

@RestController
public class ChemistryController {
    
    private ChemistryService service;
    
    @Autowired
    public ChemistryController(ChemistryService service) {
        this.service = service;
    }

    @GetMapping("/chemistries")
    public ResponseEntity<Object>findAll(){
        
        return new ResponseEntity<Object>(service.findAll(), HttpStatus.OK);
    }
    
    @PostMapping("/chemistry")
    public void save(@RequestBody Chemistry chemistry, 
            HttpServletResponse response) throws IOException {
        service.save(chemistry);
        
        response.sendRedirect("/chemistries");
    }
}

Stacktrace Get request:

at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.12.5.jar:2.12.5]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.12.5.jar:2.12.5]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:770) ~[jackson-databind-2.12.5.jar:2.12.5]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.12.5.jar:2.12.5]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.12.5.jar:2.12.5]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:770) ~[jackson-databind-2.12.5.jar:2.12.5]

CodePudding user response：

The issue you are facing is quite abstract. "Postman returns hundreds of same record duplicated." does not tell us much. Still, I guess that the issue is your bi-directional relationship. Try adding @JsonManagedReference and @JsonBackReference to your bi-directional relationship in your model as follows:

@JsonManagedReference
@OneToOne(mappedBy = "question",cascade = CascadeType.ALL)
private ChemistryAnswer answer = new ChemistryAnswer();

@OneToOne
@JsonBackReference
@JoinColumn(name = "question_id")
private Chemistry question;