Update code based on second review.

* Added style to IDE to resolve style issues
* Resolved the missed remove from IntegrationContextUtils.
* Made sure that private test methods were marked static
* Updated Assert.NotNulls to Assert.state and updated their messages
* Rebased and updated nullification for changes

Author: cppwfs

spring-integration-core/src/main/java/org/springframework/integration/aggregator/AbstractCorrelatingMessageHandler.java

@@ -937,7 +937,7 @@ protected Collection<Message<?>> completeGroup(Message<?> message, Object correl
 			this.logger.debug(() -> "Completing group with correlationKey [" + correlationKey + "]");
 
 			result = this.outputProcessor.processMessageGroup(group);
-			Assert.notNull(result, "the group returned a null result");
+			Assert.state(result != null, "The processorMessageGroup returned a null result. Null result is not expected.");
 			if (isResultCollectionOfMessages(result)) {
 				partialSequence = (Collection<Message<?>>) result;
 			}

spring-integration-core/src/main/java/org/springframework/integration/aggregator/ExpressionEvaluatingMessageGroupProcessor.java

@@ -61,7 +61,7 @@ public void setExpectedType(Class<?> expectedType) {
 	@Override
 	protected Object aggregatePayloads(MessageGroup group, Map<String, Object> headers) {
 		Object object = this.processor.process(group.getMessages());
-		Assert.notNull(object, "Result from processor must not be null");
+		Assert.state(object != null, "The process returned a null result. Null result is not expected.");
 		return object;
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/aggregator/ExpressionEvaluatingMessageListProcessor.java

@@ -106,7 +106,7 @@ public void setExpectedType(Class<?> expectedType) {
 	@Override
 	public Object process(Collection<? extends Message<?>> messages) {
 		Object object = this.evaluateExpression(this.expression, messages, this.expectedType);
-		Assert.state(object != null, "Failed to evaluate expression: " + this.expression);
+		Assert.state(object != null, "The evaluation of the expression returned a null. Null result is not expected." + this.expression);
 		return object;
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/aggregator/MethodInvokingMessageGroupProcessor.java

@@ -89,7 +89,7 @@ public void setBeanFactory(BeanFactory beanFactory) {
 	protected final Object aggregatePayloads(MessageGroup group, Map<String, Object> headers) {
 		final Collection<Message<?>> messagesUpForProcessing = group.getMessages();
 		Object object = this.processor.process(messagesUpForProcessing, headers);
-		Assert.notNull(object, "Result from processor must not be null");
+		Assert.state(object != null, "The process returned a null result. Null result is not expected.");
 		return object;
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/aggregator/MethodInvokingMessageListProcessor.java

@@ -22,6 +22,7 @@
 import java.util.Map;
 
 import org.jspecify.annotations.NonNull;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.integration.handler.support.MessagingMethodInvokerHelper;
@@ -89,7 +90,7 @@ public String toString() {
 	}
 
 	@SuppressWarnings("unchecked")
-	public T process(Collection<Message<?>> messages, Map<String, Object> aggregateHeaders) {
+	public @Nullable T process(Collection<Message<?>> messages, @Nullable Map<String, Object> aggregateHeaders) {
 		return (T) this.delegate.process(messages, aggregateHeaders);
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/context/IntegrationContextUtils.java

@@ -131,9 +131,10 @@ public static MetadataStore getMetadataStore(BeanFactory beanFactory) {
 	 * @param beanFactory BeanFactory for lookup, must not be null.
 	 * @return The {@link MessageChannel} bean whose name is "errorChannel".
 	 */
-	@Nullable
 	public static MessageChannel getErrorChannel(BeanFactory beanFactory) {
-		return getBeanOfType(beanFactory, ERROR_CHANNEL_BEAN_NAME, MessageChannel.class);
+		MessageChannel channel = getBeanOfType(beanFactory, ERROR_CHANNEL_BEAN_NAME, MessageChannel.class);
+		Assert.state(channel != null, "Error Channel was not found");
+		return channel;
 	}
 
 	/**

spring-integration-ip/src/test/java/org/springframework/integration/ip/tcp/TcpReceivingChannelAdapterTests.java

@@ -451,12 +451,6 @@ public void testNetInterceptors() throws Exception {
 		scf.stop();
 	}
 
-	private AbstractServerConnectionFactory getDefaultServerConnectionFactory() {
-		AbstractServerConnectionFactory scf = new TcpNioServerConnectionFactory(0);
-		scf.setTaskScheduler(new SimpleAsyncTaskScheduler());
-		return scf;
-	}
-
 	@Test
 	public void testNetSingleNoOutboundInterceptors() throws Exception {
 		AbstractServerConnectionFactory scf = getDefaultServerConnectionFactory();
@@ -660,6 +654,12 @@ public void testException() throws Exception {
 		scf.stop();
 	}
 
+	private static AbstractServerConnectionFactory getDefaultServerConnectionFactory() {
+		AbstractServerConnectionFactory scf = new TcpNioServerConnectionFactory(0);
+		scf.setTaskScheduler(new SimpleAsyncTaskScheduler());
+		return scf;
+	}
+
 	private class FailingService {
 
 		@SuppressWarnings("unused")

spring-integration-ip/src/test/java/org/springframework/integration/ip/tcp/connection/FailoverClientConnectionFactoryTests.java

@@ -706,11 +706,12 @@ private TcpNetServerConnectionFactory getTcpNetServerConnectionFactory(int port)
 		return result;
 	}
 
-	private TcpNetClientConnectionFactory getTcpNetServerConnectionFactory(String host, int port) {
+	private static TcpNetClientConnectionFactory getTcpNetServerConnectionFactory(String host, int port) {
 		TcpNetClientConnectionFactory result = new TcpNetClientConnectionFactory(host, port);
 		result.setTaskScheduler(new SimpleAsyncTaskScheduler());
 
 		return result;
 	}
+
 }