The cycle reference processing principle and mechanism in the MapStruct Core framework

MapStruct is a Java annotation processor that automatically generates a safe Bean mapping code.When using MapStructs for mapping, we often encounter cyclic references, that is, a class contains references to its own type.This article will introduce the principles and mechanisms of processing cycle references in the MapStruct Core framework in detail. Circular references usually occur between the two objects and form a closed loop.During the Bean mapping process, circular references may cause infinite recursive and eventually cause errors in stack overflow.To solve this problem, the MapStruct Core framework uses a mechanism called "MAPPING Context".This context provides some status information in the mapping operation and a cache of a mapping relationship. When processing cycle references, the Mapstruct Core framework is identified that the unique identifier (usually the memory address) is maintained on the mapping source and target objects to identify the object that has been accessed.When the mapping operation is performed, if the source object has been accessed, the MAPSTRUCT CORE framework will directly return to the reference of the target object to avoid infinite recursion.This process is performed recursively until the entire mapping process is completed. In order to enable the circular reference processing mechanism of the MapStruct Core framework, we need to set a attribute value in the @mapper annotation of the Mapper interface: `(UN) MappedTargetPolicy = ReportingPolicy.ignore`.This setting tells the MapStruct Core framework that ignores the attributes of the unable to mapping target object, and regards the cycle reference as an unspoiled attribute.In this way, the MAPSTRUCT CORE framework can properly handle the cycle reference, avoiding possible stack overflow errors. The following is a sample code that shows how to use the MapStruct Core framework to process the cycle reference: @Mapper(unmappedTargetPolicy = ReportingPolicy.IGNORE) public interface UserMapper { UserMapper INSTANCE = Mappers.getMapper(UserMapper.class); @Mapping (target = "friend", iGnore = TRUE) // ignore the mapping of the friend attribute UserDTO userToUserDTO(User user); User userDTOToUser(UserDTO userDTO); } In the above example, the UserMapper interface uses the @EnableMapStruct annotation to enable the MapStruct Core framework.By setting the `UnmappedTargetPolicy = reportingPolicy.ignore`, we told the MapStruct Core framework to ignore the unspoiled target attribute.In addition, we use the @Mapping annotation to set the mapping of the "Friends" property of the user object to ignore it to avoid the problem of cycle reference. In summary, the MapStruct Core framework is processed by maintaining the context and cache of the mapping operation, and setting a specific annotation attribute.It can automatically identify and avoid unlimited recursive, and ensure the smooth progress of the bean mapping operation of the type safe.By using the relevant configuration and annotation of the MapStruct Core framework, we can elegantly handle the cycle reference, thereby improving the code readability and maintainability.