Spring Data JPA和Projection获得TupleConverter的ConverterNotFoundException

Question

Is there a way to write and register a TupleConverter converter in Spring Data? I'm getting this exception when I have an @Query annotation in the Repository interface and asking for Projection.

The Interface:

public interface ProjectRepository extends JpaRepository<Project, Integer> {
    @Query("select p.projectId, p.projectName, p.techstack from Project p")
    public List<ProjectItem> findAllForTest();
}

The DTO:

public class ProjectItem {
    private final Integer projectId;
    private final String projectName;
    private final String techstack;
    @JsonCreator
    public ProjectItem(
        @JsonProperty("projectId") Integer projectId, 
        @JsonProperty("projectName") String projectName, 
        @JsonProperty("techstack") String techstack
    ) {
        this.projectId = projectId;
        this.projectName = projectName;
        this.techstack = techstack;
    }
    public Integer getProjectId() {
        return projectId;
    }
    public String getProjectName() {
        return projectName;
    }
    public String getTechstack() {
        return techstack;
    }
}

The exception

No converter found capable of converting from type [org.springframework.data.jpa.repository.query.AbstractJpaQuery$TupleConverter$TupleBackedMap] to type [project.item.ProjectItem]] with root cause

Answer 1

使用JPQL构造函数表达式 ：

@Query("select new com.company.path.to.ProjectItem(p.projectId, p.projectName, p.techstack) from Project p")

Answer 2

You're close. If you just want a DTO with a few of the items from the original item, just use the interface projection technique with methods having the same signatures as the Project class method items you want:

public interface ProjectTestSummary {
    public Integer getProjectId();
    public String getProjectName();
    public String getTechstack();
}

And in your DAO:

public interface ProjectRepository extends JpaRepository<Project, Integer> {
    public List<ProjectTestSummary> findAllProjectedBy();
}