DATACMNS-1114 - Introduced usage of nullable annotations for API validation.

Marked all packages with Spring Frameworks @NonNullApi. Added Spring's @Nullable to methods, parameters and fields that take or produce null values. Adapted using code to make sure the IDE can evaluate the null flow properly. Fixed Javadoc in places where an invalid null handling policy was advertised. Strengthened null requirements for types that expose null-instances.

Removed null handling from converters for JodaTime and ThreeTenBP. Introduced factory methods Page.empty() and Page.empty(Pageable). Introduced default methods getRequiredGetter(), …Setter() and …Field() on PersistentProperty to allow non-nullable lookups of members. The same for TypeInformation.getrequiredActualType(), …SuperTypeInformation().

Tweaked PersistentPropertyCreator.addPropertiesForRemainingDescriptors() to filter unsuitable PropertyDescriptors before actually trying to create a Property instance from them as the new stronger nullability requirements would cause exceptions downstream.

Lazy.get() now expects a non-null return value. Clients being able to cope with null need to call ….orElse(…).

Original pull request: #232.

src/test/java/org/springframework/data/domain/SortUnitTests.java

@@ -47,6 +47,7 @@ public class SortUnitTests {
 	 * 
 	 * @throws Exception
 	 */
+	@SuppressWarnings("null")
 	@Test(expected = IllegalArgumentException.class)
 	public void preventsNullProperties() throws Exception {
 
@@ -90,14 +91,15 @@ public class SortUnitTests {
 	public void allowsCombiningSorts() {
 
 		Sort sort = Sort.by("foo").and(Sort.by("bar"));
-		assertThat(sort).containsExactly(new Sort.Order("foo"), new Sort.Order("bar"));
+		assertThat(sort).containsExactly(Order.by("foo"), Order.by("bar"));
 	}
 
 	@Test
 	public void handlesAdditionalNullSort() {
 
-		Sort sort = Sort.by("foo").and(null);
-		assertThat(sort).containsExactly(new Sort.Order("foo"));
+		Sort sort = Sort.by("foo").and(Sort.unsorted());
+
+		assertThat(sort).containsExactly(Order.by("foo"));
 	}
 
 	@Test // DATACMNS-281, DATACMNS-1021
@@ -108,7 +110,7 @@ public class SortUnitTests {
 	@Test // DATACMNS-281, DATACMNS-1021
 	public void orderDoesNotIgnoreCaseByDefault() {
 
-		assertThat(new Order(Direction.ASC, "foo").isIgnoreCase()).isFalse();
+		assertThat(Order.by("foo").isIgnoreCase()).isFalse();
 		assertThat(Order.asc("foo").isIgnoreCase()).isFalse();
 		assertThat(Order.desc("foo").isIgnoreCase()).isFalse();
 	}
@@ -123,8 +125,8 @@ public class SortUnitTests {
 	@Test // DATACMNS-436
 	public void ordersWithDifferentIgnoreCaseDoNotEqual() {
 
-		Order foo = new Order("foo");
-		Order fooIgnoreCase = new Order("foo").ignoreCase();
+		Order foo = Order.by("foo");
+		Order fooIgnoreCase = Order.by("foo").ignoreCase();
 
 		assertThat(foo).isNotEqualTo(fooIgnoreCase);
 		assertThat(foo.hashCode()).isNotEqualTo(fooIgnoreCase.hashCode());
@@ -132,28 +134,28 @@ public class SortUnitTests {
 
 	@Test // DATACMNS-491
 	public void orderWithNullHandlingHintNullsFirst() {
-		assertThat(new Order("foo").nullsFirst().getNullHandling()).isEqualTo(NULLS_FIRST);
+		assertThat(Order.by("foo").nullsFirst().getNullHandling()).isEqualTo(NULLS_FIRST);
 	}
 
 	@Test // DATACMNS-491
 	public void orderWithNullHandlingHintNullsLast() {
-		assertThat(new Order("foo").nullsLast().getNullHandling()).isEqualTo(NULLS_LAST);
+		assertThat(Order.by("foo").nullsLast().getNullHandling()).isEqualTo(NULLS_LAST);
 	}
 
 	@Test // DATACMNS-491
 	public void orderWithNullHandlingHintNullsNative() {
-		assertThat(new Order("foo").nullsNative().getNullHandling()).isEqualTo(NATIVE);
+		assertThat(Order.by("foo").nullsNative().getNullHandling()).isEqualTo(NATIVE);
 	}
 
 	@Test // DATACMNS-491
 	public void orderWithDefaultNullHandlingHint() {
-		assertThat(new Order("foo").getNullHandling()).isEqualTo(NATIVE);
+		assertThat(Order.by("foo").getNullHandling()).isEqualTo(NATIVE);
 	}
 
 	@Test // DATACMNS-908
 	public void createsNewOrderForDifferentProperty() {
 
-		Order source = new Order(Direction.DESC, "foo").nullsFirst().ignoreCase();
+		Order source = Order.desc("foo").nullsFirst().ignoreCase();
 		Order result = source.withProperty("bar");
 
 		assertThat(result.getProperty()).isEqualTo("bar");
@@ -163,6 +165,7 @@ public class SortUnitTests {
 	}
 
 	@Test
+	@SuppressWarnings("null")
 	public void preventsNullDirection() {
 
 		assertThatExceptionOfType(IllegalArgumentException.class)//