1. 简介
MapStruct是一个高效、类型安全的库，它简化了Java对象之间的数据映射，无需手动转换逻辑。
在本教程中，我们将探讨使用MapStruct将枚举映射到字符串。
2. 将枚举映射到字符串
使用Java枚举作为字符串而不是序数简化了与外部API的数据交换，使数据检索更容易并增强了UI的可读性。
假设我们要将DayOfWeek枚举转换为字符串。
DayOfWeek是Java Date Time API中的一个枚举，代表一周的七天，从星期一到星期日。
让我们实现MapStruct映射器：
@Mapper
public interface DayOfWeekMapper {
DayOfWeekMapper INSTANCE = Mappers.getMapper(DayOfWeekMapper.class);
String toString(DayOfWeek dayOfWeek);
// additional mapping methods as needed
}
DayOfWeekMapper接口是一个MapStruct映射器，由@Mapper指定。我们定义toString()方法，该方法接收DayOfWeek枚举并将其转换为字符串表示形式。默认情况下，MapStruct使用name()方法获取枚举的字符串值：
class DayOfWeekMapperUnitTest {
private DayOfWeekMapper dayOfWeekMapper = DayOfWeekMapper.INSTANCE;
@ParameterizedTest
@CsvSource({"MONDAY,MONDAY", "TUESDAY,TUESDAY", "WEDNESDAY,WEDNESDAY", "THURSDAY,THURSDAY",
"FRIDAY,FRIDAY", "SATURDAY,SATURDAY", "SUNDAY,SUNDAY"})
void whenDayOfWeekMapped_thenGetsNameString(DayOfWeek source, String expected) {
String target = dayOfWeekMapper.toString(source);
assertEquals(expected, target);
}
}
这验证了toString()将DayOfWeek枚举值映射到其预期的字符串名称，这种类型的参数化测试还允许我们通过单个测试测试所有可能的变异。
2.1 处理Null
现在，让我们看看MapStruct如何处理null。默认情况下，MapStruct将null源映射到null目标。但是，此行为可以修改。
让我们验证映射器是否为null输入返回null结果：
@Test
void whenNullDayOfWeekMapped_thenGetsNullResult() {
String target = dayOfWeekMapper.toString(null);
assertNull(target);
}
MapStruct提供MappingConstants.NULL来管理null值：
@Mapper
public interface DayOfWeekMapper {
@ValueMapping(target = "MONDAY", source = MappingConstants.NULL)
String toStringWithDefault(DayOfWeek dayOfWeek);
}
对于null值，此映射返回默认值MONDAY：
@Test
void whenNullDayOfWeekMappedWithDefaults_thenReturnsDefault() {
String target = dayOfWeekMapper.toStringWithDefault(null);
assertEquals("MONDAY", target);
}
3. 将字符串映射到枚举
现在，让我们看一下将字符串转换回枚举的映射器方法：
@Mapper
public interface DayOfWeekMapper {
DayOfWeek nameStringToDayOfWeek(String day);
}
该映射器将代表星期几的字符串转换为相应的DayOfWeek枚举值：
@ParameterizedTest
@CsvSource(
{"MONDAY,MONDAY", "TUESDAY,TUESDAY", "WEDNESDAY,WEDNESDAY", "THURSDAY,THURSDAY",
"FRIDAY,FRIDAY", "SATURDAY,SATURDAY", "SUNDAY,SUNDAY"})
void whenNameStringMapped_thenGetsDayOfWeek(String source, DayOfWeek expected) {
DayOfWeek target = dayOfWeekMapper.nameStringToDayOfWeek(source);
assertEquals(expected, target);
}
我们验证nameStringToDayOfWeek()将某天的字符串表示形式映射到其对应的枚举。
3.1 处理未映射的值
如果字符串与枚举名称或通过@ValueMapping的另一个常量不匹配，MapStruct将抛出错误。通常，这样做是为了确保所有值都安全且可预测地映射。如果出现无法识别的源值，MapStruct创建的映射方法将抛出IllegalStateException：
@Test
void whenInvalidNameStringMapped_thenThrowsIllegalArgumentException() {
String source = "Mon";
IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> {
dayOfWeekMapper.nameStringToDayOfWeek(source);
});
assertTrue(exception.getMessage().equals("Unexpected enum constant: " + source));
}
为了改变这种行为，MapStruct还提供了MappingConstants.ANY_UNMAPPED，这指示MapStruct将任何未映射的源值映射到目标常量值：
@Mapper
public interface DayOfWeekMapper {
@ValueMapping(target = "MONDAY", source = MappingConstants.ANY_UNMAPPED)
DayOfWeek nameStringToDayOfWeekWithDefaults(String day);
}
最后，此@ValueMapping注解设置未映射源的默认行为。因此，任何未映射的输入都默认为MONDAY：
@ParameterizedTest
@CsvSource({"Mon,MONDAY"})
void whenInvalidNameStringMappedWithDefaults_thenReturnsDefault(String source, DayOfWeek expected) {
DayOfWeek target = dayOfWeekMapper.nameStringToDayOfWeekWithDefaults(source);
assertEquals(expected, target);
}
4. 将枚举映射到自定义字符串
现在，我们将枚举转换为DayOfWeek的自定义简短表示，如“Mon”、“Tue”等：
@Mapper
public interface DayOfWeekMapper {
@ValueMapping(target = "Mon", source = "MONDAY")
@ValueMapping(target = "Tue", source = "TUESDAY")
@ValueMapping(target = "Wed", source = "WEDNESDAY")
@ValueMapping(target = "Thu", source = "THURSDAY")
@ValueMapping(target = "Fri", source = "FRIDAY")
@ValueMapping(target = "Sat", source = "SATURDAY")
@ValueMapping(target = "Sun", source = "SUNDAY")
String toShortString(DayOfWeek dayOfWeek);
}
相反，这个toShortString()映射配置使用@ValueMapping将DayOfWeek枚举转换为简短字符串：
@ParameterizedTest
@CsvSource(
{"MONDAY,Mon", "TUESDAY,Tue", "WEDNESDAY,Wed", "THURSDAY,Thu",
"FRIDAY,Fri", "SATURDAY,Sat", "SUNDAY,Sun"})
void whenDayOfWeekMapped_thenGetsShortString(DayOfWeek source, String expected) {
String target = dayOfWeekMapper.toShortString(source);
assertEquals(expected, target);
}
5. 将自定义字符串映射到枚举
最后，我们看看如何将缩写字符串转换为DayOfWeek枚举：
@Mapper
public interface DayOfWeekMapper {
@InheritInverseConfiguration(name = "toShortString")
DayOfWeek shortStringToDayOfWeek(String day);
}
此外，@InheritInverseConfiguration注解定义了一个反向映射，它允许shortStringToDayOfWeek()从toShortString()方法继承其配置，将缩写的星期名称转换为相应的DayOfWeek枚举：
@ParameterizedTest
@CsvSource(
{"Mon,MONDAY", "Tue,TUESDAY", "Wed,WEDNESDAY", "Thu,THURSDAY",
"Fri,FRIDAY", "Sat,SATURDAY", "Sun,SUNDAY"})
void whenShortStringMapped_thenGetsDayOfWeek(String source, DayOfWeek expected) {
DayOfWeek target = dayOfWeekMapper.shortStringToDayOfWeek(source);
assertEquals(expected, target);
}
6. 总结
在本文中，我们学习了如何使用MapStruct的@ValueMapping注解将枚举映射到字符串，我们还使用了@InheritInverseConfiguration来在来回映射时保持一致性。使用这些技巧，我们可以顺利地处理枚举到字符串的转换，并使我们的代码清晰易维护。
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