BAEL-8307 Map empty string to null in MapStruct (#18467)

* BAEL-8307 Map empty string to null in MapStruct

* BAEL-8307 Add example using targetPropertyName and sourcePropertyName

* BAEL-8307 use source instead of target property

* BAEL-8307 retrigger build

Author: sam-gardner

mapstruct-2/src/main/java/com/baeldung/mapstruct/emptystringtonull/EmptyStringToNullCondition.java

@@ -0,0 +1,18 @@
+package com.baeldung.mapstruct.emptystringtonull;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+@Mapper
+public interface EmptyStringToNullCondition {
+
+    EmptyStringToNullCondition INSTANCE = Mappers.getMapper(EmptyStringToNullCondition.class);
+
+    Teacher toTeacher(Student student);
+
+    @Condition
+    default boolean isNotEmpty(String value) {
+        return value != null && !value.isEmpty();
+    }
+}

mapstruct-2/src/main/java/com/baeldung/mapstruct/emptystringtonull/EmptyStringToNullConditionSourceProperty.java

@@ -0,0 +1,23 @@
+package com.baeldung.mapstruct.emptystringtonull;
+
+import org.mapstruct.Condition;
+import org.mapstruct.Mapper;
+import org.mapstruct.TargetPropertyName;
+import org.mapstruct.SourcePropertyName;
+import org.mapstruct.factory.Mappers;
+
+@Mapper
+public interface EmptyStringToNullConditionSourceProperty {
+
+    EmptyStringToNullConditionSourceProperty INSTANCE = Mappers.getMapper(EmptyStringToNullConditionSourceProperty.class);
+
+    Teacher toTeacher(Student student);
+
+    @Condition
+    default boolean isNotEmpty(String value, @TargetPropertyName String targetPropertyName, @SourcePropertyName String sourcePropertyName) {
+        if( sourcePropertyName.equals("lastName")) {
+            return value != null && !value.isEmpty();
+        }
+        return true;
+    }
+}

mapstruct-2/src/main/java/com/baeldung/mapstruct/emptystringtonull/EmptyStringToNullExpression.java

@@ -0,0 +1,13 @@
+package com.baeldung.mapstruct.emptystringtonull;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.factory.Mappers;
+
+@Mapper
+public interface EmptyStringToNullExpression {
+    EmptyStringToNullExpression INSTANCE = Mappers.getMapper(EmptyStringToNullExpression.class);
+
+    @Mapping(target = "lastName", expression = "java(student.lastName.isEmpty() ? null : student.lastName)")
+    Teacher toTeacher(Student student);
+}

mapstruct-2/src/main/java/com/baeldung/mapstruct/emptystringtonull/EmptyStringToNullGlobal.java

@@ -0,0 +1,16 @@
+package com.baeldung.mapstruct.emptystringtonull;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+@Mapper
+public interface EmptyStringToNullGlobal {
+
+    EmptyStringToNullGlobal INSTANCE = Mappers.getMapper(EmptyStringToNullGlobal.class);
+
+    Teacher toTeacher(Student student);
+
+    default String mapEmptyString(String string) {
+        return string != null && !string.isEmpty() ? string : null;
+    }
+}

mapstruct-2/src/main/java/com/baeldung/mapstruct/emptystringtonull/Student.java

@@ -0,0 +1,11 @@
+package com.baeldung.mapstruct.emptystringtonull;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Data
+@AllArgsConstructor
+public class Student {
+    String firstName;
+    String lastName;
+}

mapstruct-2/src/main/java/com/baeldung/mapstruct/emptystringtonull/Teacher.java

@@ -0,0 +1,11 @@
+package com.baeldung.mapstruct.emptystringtonull;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Data
+@AllArgsConstructor
+public class Teacher {
+    String firstName;
+    String lastName;
+}

mapstruct-2/src/test/java/com/baeldung/mapstruct/emptystringtonull/MapEmptyStringToNullUnitTest.java

@@ -0,0 +1,46 @@
+package com.baeldung.mapstruct.emptystringtonull;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import org.junit.jupiter.api.Test;
+
+class MapEmptyStringToNullUnitTest {
+
+    @Test
+    void givenAMapperWithGlobalNullHandling_whenConvertingEmptyString_thenOutputNull() {
+        EmptyStringToNullGlobal globalMapper = EmptyStringToNullGlobal.INSTANCE;
+        Student student = new Student("Steve", "");
+        Teacher teacher = globalMapper.toTeacher(student);
+        assertEquals("Steve", teacher.firstName);
+        assertNull(teacher.lastName);
+    }
+
+    @Test
+    void givenAMapperWithConditionAnnotationNullHandling_whenConvertingEmptyString_thenOutputNull() {
+        EmptyStringToNullCondition conditionMapper = EmptyStringToNullCondition.INSTANCE;
+        Student student = new Student("Steve", "");
+        Teacher teacher = conditionMapper.toTeacher(student);
+        assertEquals("Steve", teacher.firstName);
+        assertNull(teacher.lastName);
+    }
+
+    @Test
+    void givenAMapperUsingExpressionBasedNullHandling_whenConvertingEmptyString_thenOutputNull() {
+        EmptyStringToNullExpression expressionMapper = EmptyStringToNullExpression.INSTANCE;
+        Student student = new Student("Steve", "");
+        Teacher teacher = expressionMapper.toTeacher(student);
+        assertEquals("Steve", teacher.firstName);
+        assertNull(teacher.lastName);
+    }
+
+    @Test
+    void givenAMapperUsingConditionBasedNullHandlingWithPropertyNames_whenConvertingEmptyString_thenOutputNull() {
+        EmptyStringToNullConditionSourceProperty expressionMapper = EmptyStringToNullConditionSourceProperty.INSTANCE;
+        Student student = new Student("Steve", "");
+        Teacher teacher = expressionMapper.toTeacher(student);
+        assertEquals("Steve", teacher.firstName);
+        assertNull(teacher.lastName);
+    }
+
+}