在spring boot中将Entity映射到DTO时的循环依赖

Question

我有两个实体类：

@Data
@Entity
@Table(name="Foo", schema="REF_DATA")
public class Foo {
 private Long fooId;
 private String fooName;
  @OneToMany(mappedBy="foo")
  @ToString.Exclude
 private List<Bar> bars;
}

@Data
@Entity
@Table(name="Bar", schema="REF_DATA")
public class Bar  {
    private Long barId;
    private String barName;
    @ManyToOne
    @JoinColumn(name="fooId")
    private Foo foo;
}

他们各自的 DTO 看起来像：

public class FooDto {
 private Long fooId;
 private String fooName;
 @JsonIgnoreProperties("foo")
 private List<BarDto> bars;
}

public class BarDto  {
    private Long barId;
    private String barName;
    @JsonIgnoreProperties("bars") //added to avoid the cyclic dependency
    private FooDto foo;
}

当用户调用 /getAllData 时，我们将返回以下 DTO：

public class FooBar {
    List<FooDto> foo;
    List<BarDto> bars;
}

我们能够使用 JPA 从后端获取数据，但在将实体映射到 Dto 时出错。映射器代码如下：

public interface DomainToDtoMapper<E extends BaseEntity, D extends BaseDto> {

   D mapDomainToDto(E domain);
}

public class FooDomainToDtoMapper implements DomainToDtoMapper<Foo, FooDto> {
    @Override
    public FooDto mapDomainToDto(Foo domain) {
        FooDto fooDto = new FooDto();
        FooDto.setfooId(domain.getfooId());
        FooDto.setfooName(domain.getfooName());
        if (domain.getBars() != null) {
            BarDomainToDtoMapper barDomainToDtoMapper= new BarDomainToDtoMapper();
            fooDto.setBars(domain.getBars().stream()
                    .map(barDomainToDtoMapper::mapDomainToDto)
                    .collect(Collectors.toList()));
        }
        return fooDto;
    }

}


public class BarDomainToDtoMapper implements DomainToDtoMapper<Bar, BarDto> {
    @Override
    public BarDto mapDomainToDto(Bar domain) {
        BarDto barDto = new BarDto();
        barDto.setBarId(domain.getBarId());
        barDto.setBarName(domain.getBarName());
        //works fine if we remove this If condition but Bar won't have Foo in final output
        if (domain.getFoo() != null) {
            FooDomainToDtoMapper fooDomainToDtoMapper= new FooDomainToDtoMapper();
            barDto.setFoo(domain.getFoo());
        }
        return barDto ;
    }

}

在将 Bar 映射到 BarDto 时，出现 StackOverflow 错误。

org.springframework.web.util.NestedServletException: Handler dispatch failed; nested exception is java.lang.StackOverflowError
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1055) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) [spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) [spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:626) [tomcat-embed-core-9.0.37.jar:4.0.FR]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) [spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:733) [tomcat-embed-core-9.0.37.jar:4.0.FR]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [tomcat-embed-core-9.0.37.jar:9.0.37]

Caused by: java.lang.StackOverflowError: null
    at com.jpmorgan.markets.dto.mapper.FooDomainToDtoMapper .mapDomainToDto(FooDomainToDtoMapper .java:17) ~[classes/:na]
    at com.jpmorgan.markets.dto.mapper.BarDomainToDtoMapper .mapDomainToDto(BarDomainToDtoMapper .java:23) ~[classes/:na]
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[na:1.8.0_231]
    at java.util.Iterator.forEachRemaining(Iterator.java:116) ~[na:1.8.0_231]
    at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801) ~[na:1.8.0_231]
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482) ~[na:1.8.0_231]
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472) ~[na:1.8.0_231]
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[na:1.8.0_231]
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_231]
    at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) ~[na:1.8.0_231]

同样，如果用户通过 ID 请求它们，我们希望返回个人 BarDto 和 FooDto。但问题是在最终输出中必须包含另一个对象。我尝试在实体中添加@JsonBackReference 和@JsonManagedReference，仍然得到相同的错误。是因为我们将实体显式映射到 DTO 而不是直接返回实体吗？在不使用Mapstruct 之类的东西的情况下，有没有更好的方法来做到这一点？

Answer 1

您需要在下面的代码中使用@JsonBackReference 和@JsonManagedReference。

Foo实体

@Data
@Entity
@Table(name="Foo", schema="REF_DATA")
public class Foo {
   private Long fooId;
   private String fooName;
   @OneToMany(mappedBy="foo")
   @ToString.Exclude
   @JsonManagedReference
   private List<Bar> bars;
}

条形实体

@Data
@Entity
@Table(name="Bar", schema="REF_DATA")
public class Bar  {
    private Long barId;
    private String barName;
    @ManyToOne
    @JoinColumn(name="fooId")
    @JsonBackReference
    private Foo foo;
}

*注意：如果仍然遇到任何问题，请尝试从 Foo 实体中删除 @JsonManagedReference。