继承实体一对一关联配置问题：关联对象为空的解决

你的问题出在OneToOne关系的维护端设置错误。在ShapeDetailsEntity中使用mappedBy = "shapeDetailsEntity"，说明ShapeEntity是这个一对一关系的拥有端（Owning Side），数据库中的外键字段会存储在ShapeEntity对应的表中，指向ShapeDetailsEntity的ID。

目前你只在ShapeDetailsEntity中设置了关联的shapeEntity，但没有更新ShapeEntity中的shapeDetailsEntity字段。由于拥有端负责维护外键关系，这就导致ShapeEntity表中的外键字段始终为null，进而调用shapeEntity.getShapeDetailsEntity()时出现空指针异常。

1. 双向绑定关联关系并更新拥有端 #

修改createShapeDetails方法，在创建ShapeDetailsEntity后，同时将其绑定到ShapeEntity上，并保存ShapeEntity以更新外键：

public ShapeDetailsEntity createShapeDetails(ShapeEntity shapeEntity) {
    ShapeDetailsEntity shapeDetailsEntity = new ShapeDetailsEntity();
    // 绑定反向关联
    shapeDetailsEntity.setShapeEntity(shapeEntity);
    // 绑定拥有端关联（关键步骤）
    shapeEntity.setShapeDetailsEntity(shapeDetailsEntity);
    
    // 先保存详情实体（生成ID）
    shapeDetailsEntity = shapeDetailsEntityRepository.saveAndFlush(shapeDetailsEntity);
    // 保存形状实体，更新外键字段
    switch (shapeEntity.getClass().getSimpleName()) {
        case "CircleEntity":
            circleEntityRepository.saveAndFlush((CircleEntity) shapeEntity);
            break;
        case "SquareEntity":
            squareEntityRepository.saveAndFlush((SquareEntity) shapeEntity);
            break;
        case "RectangleEntity":
            rectangleEntityRepository.saveAndFlush((RectangleEntity) shapeEntity);
            break;
        default:
            throw new IllegalArgumentException();
    }
    
    return shapeDetailsEntity;
}

2. 优化：为拥有端添加级联操作（可选） #

如果希望通过保存ShapeEntity自动完成ShapeDetailsEntity的持久化，可以在ShapeEntity的shapeDetailsEntity字段上添加cascade = CascadeType.ALL：

@Entity
@Inheritance(strategy = TABLE_PER_CLASS)
@Getter
@Setter
@NoArgsConstructor
public abstract class ShapeEntity {
    // ... 其他字段
    
    @OneToOne(cascade = CascadeType.ALL) // 添加级联配置
    private ShapeDetailsEntity shapeDetailsEntity;
    
    // ... 抽象方法
}

此时createShapeDetails方法可以简化为：

public ShapeDetailsEntity createShapeDetails(ShapeEntity shapeEntity) {
    ShapeDetailsEntity shapeDetailsEntity = new ShapeDetailsEntity();
    shapeDetailsEntity.setShapeEntity(shapeEntity);
    shapeEntity.setShapeDetailsEntity(shapeDetailsEntity);
    
    // 只需要保存ShapeEntity，级联操作会自动保存ShapeDetailsEntity
    switch (shapeEntity.getClass().getSimpleName()) {
        case "CircleEntity":
            circleEntityRepository.saveAndFlush((CircleEntity) shapeEntity);
            break;
        case "SquareEntity":
            squareEntityRepository.saveAndFlush((SquareEntity) shapeEntity);
            break;
        case "RectangleEntity":
            rectangleEntityRepository.saveAndFlush((RectangleEntity) shapeEntity);
            break;
        default:
            throw new IllegalArgumentException();
    }
    
    return shapeDetailsEntity;
}

3. 验证结果 #

修改后，再次调用shapeEntity.getShapeDetailsEntity()时就不会出现空指针，同时数据库中ShapeEntity表对应的外键字段也会正确存储ShapeDetailsEntity的ID。

内容的提问来源于stack exchange，提问作者Lulex97