#479 Add method for unwrapping RETURN_NULL to LeafValueHandler

Author: ljacqu

src/main/java/ch/jalu/configme/beanmapper/MapperImpl.java

@@ -122,13 +122,13 @@ public MapperImpl(@NotNull BeanDefinitionService beanDefinitionService,
         // Step 1: attempt simple value transformation
         Object exportValue = leafValueHandler.toExportValue(value, exportContext);
         if (exportValue != null || value == null) {
-            return unwrapReturnNull(exportValue);
+            return LeafValueHandler.unwrapReturnNull(exportValue);
         }
 
         // Step 2: handle special cases like Collection
         exportValue = createExportValueForSpecialTypes(value, exportContext);
         if (exportValue != null) {
-            return unwrapReturnNull(exportValue);
+            return LeafValueHandler.unwrapReturnNull(exportValue);
         }
 
         // Step 3: treat as bean
@@ -195,10 +195,6 @@ public MapperImpl(@NotNull BeanDefinitionService beanDefinitionService,
         return null;
     }
 
-    protected static @Nullable Object unwrapReturnNull(@Nullable Object o) {
-        return o == LeafValueHandler.RETURN_NULL ? null : o;
-    }
-
     // ---------
     // Bean mapping
     // ---------

src/main/java/ch/jalu/configme/beanmapper/leafvaluehandler/LeafValueHandler.java

@@ -45,4 +45,16 @@ public interface LeafValueHandler {
      */
     @Nullable Object toExportValue(@Nullable Object value, @NotNull ExportContext exportContext);
 
+    /**
+     * Returns null if the object is {@link #RETURN_NULL}, otherwise the given object. Used to process return values
+     * from methods like {@link #toExportValue}, where {@code null} means the instance doesn't support the value,
+     * while {@link #RETURN_NULL} means null should be used as export value.
+     *
+     * @param object the object to potentially unwrap
+     * @param <T> the object type
+     * @return null, or the provided object
+     */
+    static <T> @Nullable T unwrapReturnNull(@Nullable T object) {
+        return object == RETURN_NULL ? null : object;
+    }
 }

src/test/java/ch/jalu/configme/beanmapper/leafvaluehandler/LeafValueHandlerTest.java

@@ -0,0 +1,26 @@
+package ch.jalu.configme.beanmapper.leafvaluehandler;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.TimeUnit;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.nullValue;
+
+/**
+ * Test for {@link LeafValueHandler}.
+ */
+class LeafValueHandlerTest {
+
+    @Test
+    void shouldUnwrapReturnNull() {
+        // given / when / then
+        assertThat(LeafValueHandler.unwrapReturnNull(null), nullValue());
+        assertThat(LeafValueHandler.unwrapReturnNull(LeafValueHandler.RETURN_NULL), nullValue());
+
+        assertThat(LeafValueHandler.unwrapReturnNull("null"), equalTo("null"));
+        assertThat(LeafValueHandler.unwrapReturnNull(TimeUnit.SECONDS), equalTo(TimeUnit.SECONDS));
+        assertThat(LeafValueHandler.unwrapReturnNull(12), equalTo(12));
+    }
+}