如何通过MapStruct将实体的多个字段映射到DTO的List集合？以Bank与BankDto为例

Absolutely! MapStruct makes this kind of custom field mapping straightforward—you have a few clean options to convert those individual employee1/employee2 fields into the employeeList collection in BankDto. Let's walk through the most common approaches:

First, let's recap your classes for clarity:

// Your entity class
class Bank {
    private String name;
    private String employee1;
    private String employee2;

    // Getters and setters omitted for brevity
}

// Your DTO class
class BankDto {
    private String name;
    private List<String> employeeList;

    // Getters and setters omitted for brevity
}

Approach 1: Use @Mapping with an expression #

For simple cases where you just want to wrap the two fields into a list (even if one is null), you can use MapStruct's expression attribute directly in the @Mapping annotation:

import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import java.util.Arrays;

@Mapper(componentModel = "spring") // Use "default" if you don't need Spring integration
public interface BankMapper {

    @Mapping(target = "employeeList", expression = "java(Arrays.asList(bank.getEmployee1(), bank.getEmployee2()))")
    BankDto bankToBankDto(Bank bank);
}

Note: This will include null values in the list if either employee1 or employee2 is null. If you want to exclude nulls, see the next approaches.

Approach 2: Use a custom default method (for extra logic) #

If you need to add filtering (like removing null values) or other custom logic, a default method in your mapper gives you full control:

import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import java.util.ArrayList;
import java.util.List;

@Mapper(componentModel = "spring")
public interface BankMapper {

    // Map all fields except employeeList first
    @Mapping(target = "employeeList", ignore = true)
    BankDto bankToBankDto(Bank bank);

    // Custom method to handle the employee list
    default BankDto mapBankToDtoWithEmployees(Bank bank) {
        BankDto dto = bankToBankDto(bank);
        List<String> employees = new ArrayList<>();
        
        if (bank.getEmployee1() != null) {
            employees.add(bank.getEmployee1());
        }
        if (bank.getEmployee2() != null) {
            employees.add(bank.getEmployee2());
        }
        
        dto.setEmployeeList(employees);
        return dto;
    }
}

Approach 3: Use @AfterMapping (cleaner post-processing) #

Another clean option is to use the @AfterMapping annotation to handle the employee list after the basic mapping is done:

import org.mapstruct.AfterMapping;
import org.mapstruct.Mapper;
import org.mapstruct.MappingTarget;
import java.util.ArrayList;
import java.util.List;

@Mapper(componentModel = "spring")
public interface BankMapper {

    // Basic mapping (MapStruct handles the 'name' field automatically)
    BankDto bankToBankDto(Bank bank);

    // Post-process the DTO to populate employeeList
    @AfterMapping
    default void populateEmployeeList(Bank bank, @MappingTarget BankDto dto) {
        List<String> employees = new ArrayList<>();
        
        if (bank.getEmployee1() != null) {
            employees.add(bank.getEmployee1());
        }
        if (bank.getEmployee2() != null) {
            employees.add(bank.getEmployee2());
        }
        
        dto.setEmployeeList(employees);
    }
}

How to use the mapper #

MapStruct automatically generates an implementation of your mapper interface at compile time. You can inject it (if using Spring) or instantiate it manually:

// With Spring
@Autowired
private BankMapper bankMapper;

// Or manual instantiation (for non-Spring projects)
BankMapper bankMapper = Mappers.getMapper(BankMapper.class);

// Convert a Bank to BankDto
Bank bank = new Bank();
bank.setName("First National Bank");
bank.setEmployee1("Alice");
bank.setEmployee2("Bob");

BankDto dto = bankMapper.bankToBankDto(bank);
// dto.getEmployeeList() will contain ["Alice", "Bob"]

All these approaches work well—pick the one that best fits your needs (simple expression for basic cases, default method/@AfterMapping for more control).

内容的提问来源于stack exchange，提问作者Andrey