JPA 一对多双向堆栈溢出问题

Question

我正在使用 Java 1.8、Spring Boot、REST、JPA 创建一个 Spring Boot REST 微服务 API，它的实体关系具有以下基数：

Owner can have many Cars.
Cars only have one Owner.

能够通过我的 REST Web 服务创建和查看所有者。

每次我尝试创建具有关联所有者的 Car 时，它都会正确填充数据库的行，但似乎无限递归会导致堆栈溢出错误（见下文）。

---

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>2.2.5.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.myapi</groupId>
    <artifactId>car-api</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>car-api</name>
    <description>Car REST API</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>

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

        <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>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>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

---

src/main/resources/applications.properties:

server.servlet.context-path=/car-api
server.port=8080
server.error.whitelabel.enabled=false

# Database specific
spring.jpa.hibernate.ddl-auto=create
spring.datasource.url=jdbc:mysql://localhost:3306/car_db?useSSL=false
spring.datasource.ownername=root
spring.datasource.password=

---

所有者实体：

@Entity
@Table(name = "owner")
public class Owner {

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

    @NotNull
    private String name;


    @OneToMany(cascade = CascadeType.ALL,
                fetch = FetchType.LAZY,
                mappedBy = "owner")
    private List<Car> cars = new ArrayList<>();

    public Owner() {
    }

    // Getter & Setters omitted for brevity.
}

汽车实体：

@Entity
@Table(name="car")
public class Car {

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

    String make;
    String model;
    String year;

    @JsonIgnore
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "owner_id", nullable = false)
    private Owner owner;

    // Getter & Setters omitted for brevity.
}

---

所有者存储库：

@Repository
public interface OwnerRepository extends JpaRepository<Owner, Long> {
}

---

汽车存储库：

@Repository
public interface CarRepository extends JpaRepository<Car, Long> {
}

---

业主服务：

public interface OwnerService {

    boolean createOwner(Owner owner);

    Owner getOwnerByOwnerId(Long ownerId);

    List<Owner> getAllOwners();

}

---

OwnerServiceImpl:

@Service
public class OwnerServiceImpl implements OwnerService {


    @Autowired
    OwnerRepository ownerRepository;

    @Autowired
    CarRepository carRepository;

    @Override
    public List<Owner> getAllOwners() {
        return ownerRepository.findAll();
    }

    @Override
    public boolean createOwner(Owner owner) {
        boolean created = false;
        if (owner != null) {
            ownerRepository.save(owner);
            created = true;
        }
        return created;
    }

    @Override
    public Owner getOwnerByOwnerId(Long ownerId) {
        Optional<Owner> owner = null;
        if (ownerRepository.existsById(ownerId)) {
            owner = ownerRepository.findById(ownerId);
        }
        return owner.get();
    }
}

---

汽车服务：

public interface CarService {

    boolean createCar(Long ownerId, Car car);
}

---

CarServiceImpl:

@Service
public class CarServiceImpl implements CarService {

    @Autowired
    OwnerRepository ownerRepository;

    @Autowired
    CarRepository carRepository;

    @Override
    public boolean createCar(Long ownerId, Car car) {
        boolean created = false;
        if (ownerRepository.existsById(ownerId)) {
            Optional<Owner> owner = ownerRepository.findById(ownerId);
            if (owner != null) {
                List<Car> cars = owner.get().getCars();
                cars.add(car);
                owner.get().setCars(cars);
                car.setOwner(owner.get());
                carRepository.save(car);
                created = true;
            }
        }
        return created;
    }

}

---

所有者控制器：

@RestController
public class OwnerController {


    private HttpHeaders headers = null;

    @Autowired
    OwnerService ownerService;

    public OwnerController() {
        headers = new HttpHeaders();
        headers.add("Content-Type", "application/json");
    }

    @RequestMapping(value = { "/owners" }, method = RequestMethod.POST, produces = "APPLICATION/JSON")
    public ResponseEntity<Object> createOwner(@Valid @RequestBody Owner owner) {
        boolean isCreated = ownerService.createOwner(owner);
        if (isCreated) {
            return new ResponseEntity<Object>(headers, HttpStatus.OK);
        }
        else {
            return new ResponseEntity<Object>(HttpStatus.NOT_FOUND);
        }
    }


    @RequestMapping(value = { "/owners" }, method = RequestMethod.GET, produces = "APPLICATION/JSON")
    public ResponseEntity<Object> getAllOwners() {
        List<Owner> owners = ownerService.getAllOwners();

        if (owners.isEmpty()) {
            return new ResponseEntity<Object>(HttpStatus.NOT_FOUND);
        }
        return new ResponseEntity<Object>(owners, headers, HttpStatus.OK);
    }


    @RequestMapping(value = { "/owners/{ownerId}" }, method = RequestMethod.GET, produces = "APPLICATION/JSON")
    public ResponseEntity<Object> getOwnerByOwnerId(@PathVariable Long ownerId) {
        if (null == ownerId || "".equals(ownerId)) {
            return new ResponseEntity<Object>(HttpStatus.NOT_FOUND);
        }
        Owner owner = ownerService.getOwnerByOwnerId(ownerId);
        return new ResponseEntity<Object>(owner, headers, HttpStatus.OK);
    }

}

---

汽车控制器：

@RestController
public class CarController {

    private HttpHeaders headers = null;

    @Autowired
    CarService carService;

    public VehicleController() {
        headers = new HttpHeaders();
        headers.add("Content-Type", "application/json");
    }

    @RequestMapping(value = { "/cars/{ownerId}" }, method = RequestMethod.POST, produces = "APPLICATION/JSON")
    public ResponseEntity<Object> createVehicleBasedOnOwnerId(@Valid @RequestBody Car car, Long ownerId) {
        boolean isCreated = carService.createCar(ownerId, vehicle);
        if (isCreated) {
            return new ResponseEntity<Object>(headers, HttpStatus.OK);
        }
        else {
            return new ResponseEntity<Object>(HttpStatus.NOT_FOUND);
        }
    }

---

而我可以通过将其作为请求正文传递来创建新的所有者（并在数据库中查看它们并通过 curl / Postman 调用 getAllOwners 来查看它们）：

{
    "owner": "John Doe"
}

---

在数据库car_db.owner ：

-------------------------------------
|id | name                          |
-------------------------------------  
|1  | John Doe                      |
-------------------------------------  

---

当我尝试通过使用此 REST 调用/cars/{ownerId}为所有者创建全新汽车时出现问题：

POST http://localhost:8080/car-api/cars/1

具有以下请求正文：

{
    "make": "Honda",
    "model": "Accord"
    "year": 2020
}

它将它正确插入 MySQL 数据库的car_db.car表中，如下所示：

---------------------------------------
|id | make  | model  | year | owner_id|
---------------------------------------  
|1  | Honda | Accord | 2020 |     1   |
---------------------------------------  

我在CarServiceImpl.createCar() method做了什么导致双向关系中断的事情吗？

创建一个 Stack Over Flow : null execption:

-03-08 01:43:20,106 ERROR org.apache.juli.logging.DirectJDKLog [http-nio-8080-exec-1] Servlet.service() for servlet [dispatcherServlet] in context with path [/car-api] threw exception [Handler dispatch failed; nested exception is java.lang.StackOverflowError] with root cause
java.lang.StackOverflowError: null
    at java.base/java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:512)
    at java.base/java.lang.StringBuilder.append(StringBuilder.java:141)
    at com.myapi.model.Car.toString(Car.java:87)
    at java.base/java.lang.String.valueOf(String.java:2788)
    at java.base/java.lang.StringBuilder.append(StringBuilder.java:135)
    at java.base/java.util.AbstractCollection.toString(AbstractCollection.java:473)
    at org.hibernate.collection.internal.PersistentBag.toString(PersistentBag.java:622)
    at java.base/java.lang.String.valueOf(String.java:2788)
    at java.base/java.lang.StringBuilder.append(StringBuilder.java:135)
    at com.myapi.model.Owner.toString(Owner.java:105)
    at java.base/java.lang.String.valueOf(String.java:2788)
    at java.base/java.lang.StringBuilder.append(StringBuilder.java:135)
    at com.myapi.model.Car.toString(Car.java:87)
    at java.base/java.lang.String.valueOf(String.java:2788)
    at java.base/java.lang.StringBuilder.append(StringBuilder.java:135)
    at java.base/java.util.AbstractCollection.toString(AbstractCollection.java:473)
    at org.hibernate.collection.internal.PersistentBag.toString(PersistentBag.java:622)
    at java.base/java.lang.String.valueOf(String.java:2788)
    at java.base/java.lang.StringBuilder.append(StringBuilder.java:135)
    at com.myapi.model.Owner.toString(Owner.java:105)
    at java.base/java.lang.String.valueOf(String.java:2788)
    at java.base/java.lang.StringBuilder.append(StringBuilder.java:135)
    at com.myapi.model.Car.toString(Car.java:87)
    at java.base/java.lang.String.valueOf(String.java:2788)
    at java.base/java.lang.StringBuilder.append(StringBuilder.java:135)
    at java.base/java.util.AbstractCollection.toString(AbstractCollection.java:473)
    at org.hibernate.collection.internal.PersistentBag.toString(PersistentBag.java:622)

---

奇怪的是，尽管这是每次我创建一辆新车时都会出现的堆栈跟踪，但数据库中的一切都很好（插入在汽车表中，行中的 ownerId 正确），我能够当我执行以下任一 GET 请求时，查看 JSON 响应负载：

GET http://localhost:8080/owners/1

产量：

{
    "name": "John Doe",
    "cars": [
       {
           "make": "Honda",
           "model": "Accord",
           "year": 2020
       }
    ]
}

---

GET http://localhost:8080/owners

产量：

[
   {
      "name": "John Doe",
      "cars": [
         {
             "make": "Honda",
             "model": "Accord",
             "year": 2020
         }
      ]
   }
]

---

尽管所有 GET 和数据库插入都在工作，但为什么我会收到此堆栈溢出错误？

Answer 1

JPA 与此错误无关。 查看堆栈跟踪 - 您的Car#toString()打印其Owner 。 而Owner#toString()打印它的汽车集合。

因此，当您的代码中的某些内容对这些对象之一调用toString()时 - 它会导致无限的调用链，只有在达到线程堆栈的最大深度时才会结束，从而导致 StackOverflowError。

通常在toString()我们只想打印当前类中的原语/ValueObjects。 如果我们也开始打印关联的实体 - 这将导致惰性字段被初始化。

Answer 2

在我的 Owner 类中，这是 IntelliJ IDEA 为我的toString()方法生成的内容：

public String toString() {
  return "Owner{" +
              "id=" + id +
              "name=" + name + 
              ", cars=" + cars +
        '}';
}

当我取出包含", cars=" + cars +的行时，它变成了：

public String toString() {
  return "Owner{" +
              "id=" + id +
              "name=" + name +
        '}';
}

---

有效！