GH-10083 Add Nullability to the mapping package

Related to: #10083

* `AbstractHeaderMapper` - Enhanced null safety by adding `@Nullable` annotations to fields and methods.
* Updated method signatures to match those in `AbstractHeaderMapper` for the following classes:
   - `DefaultAmqpHeaderMapper`
   - `DefaultSoapHeaderMapper`
   - `DefaultXmppHeaderMapper`

* `buildResolvableType` should accept a null `ClassLoader`
* Move `@Contract` to `getClassForValue`
*Remove redundant `Assert.state()`.

Author: cppwfs

spring-integration-amqp/src/main/java/org/springframework/integration/amqp/support/DefaultAmqpHeaderMapper.java

@@ -56,6 +56,7 @@
  * @author Artem Bilan
  * @author Stephane Nicoll
  * @author Steve Singer
+ * @author Glenn Renfro
  *
  * @since 2.1
  */
@@ -107,8 +108,8 @@ protected DefaultAmqpHeaderMapper(String @Nullable [] requestHeaderNames, String
 	 * Extract "standard" headers from an AMQP MessageProperties instance.
 	 */
 	@Override
-	protected Map<String, Object> extractStandardHeaders(MessageProperties amqpMessageProperties) {
-		Map<String, Object> headers = new HashMap<>();
+	protected Map<String, @Nullable Object> extractStandardHeaders(MessageProperties amqpMessageProperties) {
+		Map<String, @Nullable Object> headers = new HashMap<>();
 		try {
 			JavaUtils.INSTANCE
 					.acceptIfNotNull(AmqpHeaders.APP_ID, amqpMessageProperties.getAppId(), headers::put)
@@ -326,13 +327,13 @@ else if (contentType instanceof String) {
 	}
 
 	@Override
-	public Map<String, Object> toHeadersFromRequest(MessageProperties source) {
-		Map<String, Object> headersFromRequest = super.toHeadersFromRequest(source);
+	public Map<String, @Nullable Object> toHeadersFromRequest(MessageProperties source) {
+		Map<String, @Nullable Object> headersFromRequest = super.toHeadersFromRequest(source);
 		addConsumerMetadata(source, headersFromRequest);
 		return headersFromRequest;
 	}
 
-	private void addConsumerMetadata(MessageProperties messageProperties, Map<String, Object> headers) {
+	private void addConsumerMetadata(MessageProperties messageProperties, Map<String, @Nullable Object> headers) {
 		String consumerTag = messageProperties.getConsumerTag();
 		if (consumerTag != null) {
 			headers.put(AmqpHeaders.CONSUMER_TAG, consumerTag);

spring-integration-core/src/main/java/org/springframework/integration/mapping/AbstractHeaderMapper.java

@@ -51,6 +51,7 @@
  * @author Stephane Nicoll
  * @author Gary Russell
  * @author Artem Bilan
+ * @author Glenn Renfro
  *
  * @since 2.1
  */
@@ -87,7 +88,7 @@ public abstract class AbstractHeaderMapper<T> implements RequestReplyHeaderMappe
 
 	private HeaderMatcher replyHeaderMatcher;
 
-	private ClassLoader classLoader = ClassUtils.getDefaultClassLoader();
+	private @Nullable ClassLoader classLoader = ClassUtils.getDefaultClassLoader();
 
 	/**
 	 * Create a new instance.
@@ -113,7 +114,7 @@ public void setBeanClassLoader(ClassLoader classLoader) {
 		this.classLoader = classLoader;
 	}
 
-	protected ClassLoader getClassLoader() {
+	protected @Nullable ClassLoader getClassLoader() {
 		return this.classLoader;
 	}
 
@@ -204,12 +205,12 @@ public void fromHeadersToReply(MessageHeaders headers, T target) {
 	}
 
 	@Override
-	public Map<String, Object> toHeadersFromRequest(T source) {
+	public Map<String, @Nullable Object> toHeadersFromRequest(T source) {
 		return toHeaders(source, this.requestHeaderMatcher);
 	}
 
 	@Override
-	public Map<String, Object> toHeadersFromReply(T source) {
+	public Map<String, @Nullable Object> toHeadersFromReply(T source) {
 		return toHeaders(source, this.replyHeaderMatcher);
 	}
 
@@ -264,18 +265,19 @@ private boolean isMessageChannel(String headerName, Object headerValue) {
 	 * Map headers from a source instance to the {@link MessageHeaders} of
 	 * a {@link org.springframework.messaging.Message}.
 	 */
-	private Map<String, Object> toHeaders(T source, HeaderMatcher headerMatcher) {
-		Map<String, Object> headers = new HashMap<>();
-		Map<String, Object> standardHeaders = extractStandardHeaders(source);
+	private Map<String, @Nullable Object> toHeaders(T source, HeaderMatcher headerMatcher) {
+		Map<String, @Nullable Object> headers = new HashMap<>();
+		Map<String, @Nullable Object> standardHeaders = extractStandardHeaders(source);
 		copyHeaders(standardHeaders, headers, headerMatcher);
-		Map<String, Object> userDefinedHeaders = extractUserDefinedHeaders(source);
+		Map<String, @Nullable Object> userDefinedHeaders = extractUserDefinedHeaders(source);
 		copyHeaders(userDefinedHeaders, headers, headerMatcher);
 		return headers;
 	}
 
-	private void copyHeaders(Map<String, Object> source, Map<String, Object> target, HeaderMatcher headerMatcher) {
+	private void copyHeaders(Map<String, @Nullable Object> source, Map<String, @Nullable Object> target,
+			HeaderMatcher headerMatcher) {
 		if (!CollectionUtils.isEmpty(source)) {
-			for (Map.Entry<String, Object> entry : source.entrySet()) {
+			for (Map.Entry<String, @Nullable Object> entry : source.entrySet()) {
 				try {
 					String headerName = createTargetPropertyName(entry.getKey(), false);
 					if (shouldMapHeader(headerName, headerMatcher)) {
@@ -322,8 +324,7 @@ private boolean shouldMapHeader(String headerName, HeaderMatcher headerMatcher)
 	}
 
 	@SuppressWarnings("unchecked")
-	@Nullable
-	protected <V> V getHeaderIfAvailable(Map<String, Object> headers, String name, Class<V> type) {
+	protected @Nullable <V> V getHeaderIfAvailable(Map<String, Object> headers, String name, Class<V> type) {
 		Object value = headers.get(name);
 		if (value == null) {
 			return null;
@@ -365,7 +366,7 @@ protected Collection<String> getTransientHeaderNames() {
 	 * @param source the source object to extract standard headers.
 	 * @return the map of headers to be mapped.
 	 */
-	protected abstract Map<String, Object> extractStandardHeaders(T source);
+	protected abstract Map<String, @Nullable Object> extractStandardHeaders(T source);
 
 	/**
 	 * Extract the user-defined headers from the specified source.

spring-integration-core/src/main/java/org/springframework/integration/mapping/package-info.java

@@ -1,5 +1,5 @@
 /**
  * Provides classes related to mapping to/from message headers.
  */
-@org.springframework.lang.NonNullApi
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.mapping;

spring-integration-core/src/main/java/org/springframework/integration/mapping/support/JsonHeaders.java

@@ -24,6 +24,7 @@
 
 import org.springframework.core.ResolvableType;
 import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.lang.Contract;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -32,6 +33,7 @@
  *
  * @author Artem Bilan
  * @author Gary Russell
+ * @author Glenn Renfro
  *
  * @since 3.0
  */
@@ -67,18 +69,17 @@ private JsonHeaders() {
 	 * @return the {@link ResolvableType} based on provided class components
 	 * @since 5.2.4
 	 */
-	public static ResolvableType buildResolvableType(ClassLoader classLoader, Object targetClassValue,
+	public static ResolvableType buildResolvableType(@Nullable ClassLoader classLoader, Object targetClassValue,
 			@Nullable Object contentClassValue, @Nullable Object keyClassValue) {
 
 		Class<?> targetClass = getClassForValue(classLoader, targetClassValue);
 		Class<?> keyClass = getClassForValue(classLoader, keyClassValue);
 		Class<?> contentClass = getClassForValue(classLoader, contentClassValue);
-
 		return buildResolvableType(targetClass, contentClass, keyClass);
 	}
 
-	@Nullable
-	private static Class<?> getClassForValue(ClassLoader classLoader, @Nullable Object classValue) {
+	@Contract("_, null -> null; _, !null -> !null")
+	private static @Nullable Class<?> getClassForValue(@Nullable ClassLoader classLoader, @Nullable Object classValue) {
 		if (classValue instanceof Class<?>) {
 			return (Class<?>) classValue;
 		}
@@ -105,7 +106,6 @@ else if (classValue != null) {
 	 */
 	public static ResolvableType buildResolvableType(Class<?> targetClass, @Nullable Class<?> contentClass,
 			@Nullable Class<?> keyClass) {
-
 		if (keyClass != null) {
 			return TypeDescriptor
 					.map(targetClass,

spring-integration-core/src/main/java/org/springframework/integration/mapping/support/package-info.java

@@ -1,4 +1,5 @@
 /**
  * Support classes for mapping.
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.mapping.support;

spring-integration-ws/src/main/java/org/springframework/integration/ws/DefaultSoapHeaderMapper.java

@@ -56,6 +56,7 @@
  * @author Artem Bilan
  * @author Gary Russell
  * @author Jooyoung Pyoung
+ * @author Glenn Renfro
  *
  * @since 2.0
  */
@@ -74,10 +75,10 @@ public DefaultSoapHeaderMapper() {
 	}
 
 	@Override
-	protected Map<String, Object> extractStandardHeaders(SoapMessage source) {
+	protected Map<String, @Nullable Object> extractStandardHeaders(SoapMessage source) {
 		final String soapAction = source.getSoapAction();
 		if (StringUtils.hasText(soapAction)) {
-			Map<String, Object> headers = new HashMap<>(1);
+			Map<String, @Nullable Object> headers = new HashMap<>(1);
 			headers.put(WebServiceHeaders.SOAP_ACTION, soapAction);
 			return headers;
 		}

spring-integration-xmpp/src/main/java/org/springframework/integration/xmpp/support/DefaultXmppHeaderMapper.java

@@ -41,6 +41,7 @@
  * @author Florian Schmaus
  * @author Stephane Nicoll
  * @author Artem Bilan
+ * @author Glenn Renfro
  *
  * @since 2.1
  */
@@ -61,8 +62,8 @@ public DefaultXmppHeaderMapper() {
 	}
 
 	@Override
-	protected Map<String, Object> extractStandardHeaders(MessageBuilder source) {
-		Map<String, Object> headers = new HashMap<>();
+	protected Map<String, @Nullable Object> extractStandardHeaders(MessageBuilder source) {
+		Map<String, @Nullable Object> headers = new HashMap<>();
 		Jid from = source.getFrom();
 		if (from != null) {
 			headers.put(XmppHeaders.FROM, from.toString());