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-oxm/src/main/java/org/springframework/oxm/castor/CastorMarshaller.java

@@ -429,7 +429,7 @@ public class CastorMarshaller extends AbstractMarshaller implements Initializing
 	}
 
 	@Override
-	public void setBeanClassLoader(@Nullable ClassLoader classLoader) {
+	public void setBeanClassLoader(ClassLoader classLoader) {
 		this.beanClassLoader = classLoader;
 	}
 

spring-oxm/src/main/java/org/springframework/oxm/jaxb/ClassPathJaxb2TypeScanner.java

@@ -87,7 +87,8 @@ class ClassPathJaxb2TypeScanner {
 					MetadataReader metadataReader = metadataReaderFactory.getMetadataReader(resource);
 					if (isJaxb2Class(metadataReader, metadataReaderFactory)) {
 						String className = metadataReader.getClassMetadata().getClassName();
-						Class<?> jaxb2AnnotatedClass = this.resourcePatternResolver.getClassLoader().loadClass(className);
+						Class<?> jaxb2AnnotatedClass =
+								ClassUtils.forName(className, this.resourcePatternResolver.getClassLoader());
 						jaxb2Classes.add(jaxb2AnnotatedClass);
 					}
 				}

spring-oxm/src/main/java/org/springframework/oxm/jaxb/Jaxb2Marshaller.java

@@ -432,7 +432,7 @@ public class Jaxb2Marshaller implements MimeMarshaller, MimeUnmarshaller, Generi
 
 
 	@Override
-	public void setBeanClassLoader(@Nullable ClassLoader classLoader) {
+	public void setBeanClassLoader(ClassLoader classLoader) {
 		this.beanClassLoader = classLoader;
 	}
 
@@ -1001,7 +1001,11 @@ public class Jaxb2Marshaller implements MimeMarshaller, MimeUnmarshaller, Generi
 				}
 				contentId = '<' + contentId + '>';
 			}
-			return this.mimeContainer.getAttachment(contentId);
+			DataHandler dataHandler = this.mimeContainer.getAttachment(contentId);
+			if (dataHandler == null) {
+				throw new IllegalArgumentException("No attachment found for " + contentId);
+			}
+			return dataHandler;
 		}
 
 		@Override

spring-oxm/src/main/java/org/springframework/oxm/jibx/JibxMarshaller.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 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.
@@ -419,7 +419,7 @@ public class JibxMarshaller extends AbstractMarshaller implements InitializingBe
 		return transformAndUnmarshal(new SAXSource(xmlReader, inputSource), inputSource.getEncoding());
 	}
 
-	private Object transformAndUnmarshal(Source source, String encoding) throws IOException {
+	private Object transformAndUnmarshal(Source source, @Nullable String encoding) throws IOException {
 		try {
 			Transformer transformer = this.transformerFactory.newTransformer();
 			if (encoding != null) {

spring-oxm/src/main/java/org/springframework/oxm/support/MarshallingSource.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.
@@ -32,6 +32,7 @@ import org.xml.sax.SAXParseException;
 import org.xml.sax.XMLReader;
 import org.xml.sax.ext.LexicalHandler;
 
+import org.springframework.lang.Nullable;
 import org.springframework.oxm.Marshaller;
 import org.springframework.util.Assert;
 
@@ -130,6 +131,7 @@ public class MarshallingSource extends SAXSource {
 		}
 
 		@Override
+		@Nullable
 		public ContentHandler getContentHandler() {
 			return this.contentHandler;
 		}
@@ -140,6 +142,7 @@ public class MarshallingSource extends SAXSource {
 		}
 
 		@Override
+		@Nullable
 		public DTDHandler getDTDHandler() {
 			return this.dtdHandler;
 		}
@@ -150,6 +153,7 @@ public class MarshallingSource extends SAXSource {
 		}
 
 		@Override
+		@Nullable
 		public EntityResolver getEntityResolver() {
 			return this.entityResolver;
 		}
@@ -160,6 +164,7 @@ public class MarshallingSource extends SAXSource {
 		}
 
 		@Override
+		@Nullable
 		public ErrorHandler getErrorHandler() {
 			return this.errorHandler;
 		}

spring-oxm/src/main/java/org/springframework/oxm/xstream/XStreamMarshaller.java

@@ -380,7 +380,7 @@ public class XStreamMarshaller extends AbstractMarshaller implements BeanClassLo
 	}
 
 	@Override
-	public void setBeanClassLoader(@Nullable ClassLoader classLoader) {
+	public void setBeanClassLoader(ClassLoader classLoader) {
 		this.beanClassLoader = classLoader;
 	}