Consistent use of @Nullable across the codebase (even for internals)

Beyond just formally declaring the current behavior, this revision actually enforces non-null behavior in selected signatures now, not tolerating null values anymore when not explicitly documented. It also changes some utility methods with historic null-in/null-out tolerance towards enforced non-null return values, making them a proper citizen in non-null assignments.

Some issues are left as to-do: in particular a thorough revision of spring-test, and a few tests with unclear failures (ignored as "TODO: NULLABLE") to be sorted out in a follow-up commit.

Issue: SPR-15540

spring-core/src/test/java/org/springframework/core/annotation/AnnotatedElementUtilsTests.java

@@ -70,8 +70,8 @@ public class AnnotatedElementUtilsTests {
 
 	@Test
 	public void getMetaAnnotationTypesOnNonAnnotatedClass() {
-		assertNull(getMetaAnnotationTypes(NonAnnotatedClass.class, TransactionalComponent.class));
-		assertNull(getMetaAnnotationTypes(NonAnnotatedClass.class, TransactionalComponent.class.getName()));
+		assertTrue(getMetaAnnotationTypes(NonAnnotatedClass.class, TransactionalComponent.class).isEmpty());
+		assertTrue(getMetaAnnotationTypes(NonAnnotatedClass.class, TransactionalComponent.class.getName()).isEmpty());
 	}
 
 	@Test

spring-core/src/test/java/org/springframework/core/annotation/AnnotationUtilsTests.java

@@ -860,12 +860,6 @@ public class AnnotationUtilsTests {
 		assertEquals("value attribute: ", "webController", synthesizedComponent.value());
 	}
 
-	@Test
-	public void synthesizeAnnotationsFromNullSources() throws Exception {
-		assertNull("null annotation", synthesizeAnnotation(null, null));
-		assertNull("null map", synthesizeAnnotation(null, WebMapping.class, null));
-	}
-
 	@Test
 	public void synthesizeAlreadySynthesizedAnnotation() throws Exception {
 		Method method = WebController.class.getMethod("handleMappedWithValueAttribute");

spring-core/src/test/java/org/springframework/core/annotation/OrderUtilsTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -45,7 +45,7 @@ public class OrderUtilsTests {
 
 	@Test
 	public void getDefaultOrder() {
-		assertEquals(Integer.valueOf(33), OrderUtils.getOrder(NoOrder.class, 33));
+		assertEquals(33, OrderUtils.getOrder(NoOrder.class, 33));
 	}
 
 	@Test

spring-core/src/test/java/org/springframework/core/io/ResourceTests.java

@@ -246,21 +246,6 @@ public class ResourceTests {
 		assertThat(resource.contentLength(), is(3L));
 	}
 
-	@Test(expected = IllegalStateException.class)
-	public void testContentLength_withNullInputStream() throws IOException {
-		AbstractResource resource = new AbstractResource() {
-			@Override
-			public InputStream getInputStream() throws IOException {
-				return null;
-			}
-			@Override
-			public String getDescription() {
-				return null;
-			}
-		};
-		resource.contentLength();
-	}
-
 	@Test
 	public void testGetReadableByteChannel() throws IOException {
 		Resource resource = new FileSystemResource(getClass().getResource("Resource.class").getFile());

spring-core/src/test/java/org/springframework/mock/env/MockPropertySource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -48,6 +48,7 @@ public class MockPropertySource extends PropertiesPropertySource {
 	 */
 	public static final String MOCK_PROPERTIES_PROPERTY_SOURCE_NAME = "mockProperties";
 
+
 	/**
 	 * Create a new {@code MockPropertySource} named {@value #MOCK_PROPERTIES_PROPERTY_SOURCE_NAME}
 	 * that will maintain its own internal {@link Properties} instance.
@@ -84,6 +85,7 @@ public class MockPropertySource extends PropertiesPropertySource {
 		super(name, properties);
 	}
 
+
 	/**
 	 * Set the given property on the underlying {@link Properties} object.
 	 */

spring-core/src/test/java/org/springframework/util/StringUtilsTests.java

@@ -301,7 +301,6 @@ public class StringUtilsTests {
 
 	@Test
 	public void testStripFilenameExtension() {
-		assertEquals(null, StringUtils.stripFilenameExtension(null));
 		assertEquals("", StringUtils.stripFilenameExtension(""));
 		assertEquals("myfile", StringUtils.stripFilenameExtension("myfile"));
 		assertEquals("myfile", StringUtils.stripFilenameExtension("myfile."));