Tighten nullability contract of PersistentPropertyPath.

We should change the definition of `PersistentPropertyPath` to — in its public API — not allow empty instances anymore. Those violate the concept and bleed into the concept's API by having to make all methods nullable (returning null in exactly that "empty" case). An empty property path doesn't make any actual sense as you cannot reasonably answer the methods declared on the interface except by returning null, which then causes client code having to verify the returned values all the time.

This is now changed into only making `PersistentPropertyPath.getParentPath()` nullable and letting it return null for single segment paths. Adapted client code accordingly. `….getRequiredLeadProperty()` is now deprecated in favour of `….getLeafProperty()` not returning null anymore.

Fixes #2813.

src/test/java/org/springframework/data/mapping/PersistentPropertyAccessorUnitTests.java

@@ -81,15 +81,6 @@ public class PersistentPropertyAccessorUnitTests {
 		assertThat(customer.firstname).isEqualTo("Oliver August");
 	}
 
-	@Test // DATACMNS-1275
-	public void rejectsEmptyPathToSetValues() {
-
-		setUp(new Order(null), "");
-
-		assertThatIllegalArgumentException() //
-				.isThrownBy(() -> accessor.setProperty(path, "Oliver August"));
-	}
-
 	@Test // DATACMNS-1275
 	public void rejectsIntermediateNullValuesForRead() {
 

src/test/java/org/springframework/data/mapping/context/DefaultPersistentPropertyPathUnitTests.java

@@ -27,7 +27,6 @@ import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
-
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.data.mapping.PersistentProperty;
 import org.springframework.data.mapping.PersistentPropertyPath;
@@ -110,18 +109,10 @@ class DefaultPersistentPropertyPathUnitTests<P extends PersistentProperty<P>> {
 	}
 
 	@Test
-	void returnsEmptyPathForRootLevelProperty() {
-		assertThat(oneLeg.getParentPath()).isEmpty();
-	}
+	void returnsNullForRootLevelProperty() {
 
-	@Test
-	void returnItselfForEmptyPath() {
-
-		var parent = oneLeg.getParentPath();
-		var parentsParent = parent.getParentPath();
-
-		assertThat(parentsParent).isEmpty();
-		assertThat(parentsParent).isSameAs(parent);
+		assertThat(oneLeg.isRootPath()).isTrue();
+		assertThat(oneLeg.getParentPath()).isNull();
 	}
 
 	@Test
@@ -132,27 +123,27 @@ class DefaultPersistentPropertyPathUnitTests<P extends PersistentProperty<P>> {
 
 	@Test // DATACMNS-444
 	void skipsMappedPropertyNameIfConverterReturnsNull() {
-		assertThat(twoLegs.toDotPath(source -> null)).isNull();
+		assertThat(twoLegs.toDotPath(source -> null)).isEmpty();
 	}
 
 	@Test // DATACMNS-444
 	void skipsMappedPropertyNameIfConverterReturnsEmptyStrings() {
-		assertThat(twoLegs.toDotPath(source -> "")).isNull();
+		assertThat(twoLegs.toDotPath(source -> "")).isEmpty();
 	}
 
 	@Test // DATACMNS-1466
-	void returnsNullForLeafPropertyOnEmptyPath() {
+	void throwsExceptionForLeafPropertyOnEmptyPath() {
 
 		PersistentPropertyPath<P> path = new DefaultPersistentPropertyPath<P>(Collections.emptyList());
 
-		assertThat(path.getLeafProperty()).isNull();
+		assertThatIllegalStateException().isThrownBy(() -> path.getLeafProperty());
 	}
 
 	@Test // DATACMNS-1466
-	void returnsNullForBasePropertyOnEmptyPath() {
+	void throwsExceptionForBasePropertyOnEmptyPath() {
 
 		PersistentPropertyPath<P> path = new DefaultPersistentPropertyPath<P>(Collections.emptyList());
 
-		assertThat(path.getBaseProperty()).isNull();
+		assertThatIllegalStateException().isThrownBy(() -> path.getBaseProperty());
 	}
 }

src/test/java/org/springframework/data/mapping/context/PersistentPropertyPathFactoryUnitTests.java

@@ -22,7 +22,6 @@ import jakarta.inject.Inject;
 import java.util.List;
 
 import org.junit.jupiter.api.Test;
-
 import org.springframework.data.annotation.Reference;
 import org.springframework.data.mapping.MappingException;
 import org.springframework.data.mapping.PersistentPropertyPath;
@@ -106,8 +105,8 @@ class PersistentPropertyPathFactoryUnitTests {
 	}
 
 	@Test // DATACMNS-1275
-	void createsEmptyPropertyPathCorrectly() {
-		assertThat(factory.from(Wrapper.class, "")).isEmpty();
+	void rejectsEmptyPropertyPathCreation() {
+		assertThatIllegalArgumentException().isThrownBy(() -> factory.from(Wrapper.class, ""));
 	}
 
 	@Test // DATACMNS-1275