Add JSR-305 annotations to public APIs

(Partially) Resolves BATCH-2688

spring-batch-integration/src/main/java/org/springframework/batch/integration/async/package-info.java

@@ -2,5 +2,9 @@
  * Components for executing item processing asynchronously and writing the results when processing is complete.
  *
  * @author Michael Minella
+ * @author Mahmoud Ben Hassine
  */
-package org.springframework.batch.integration.async;
+@NonNullApi
+package org.springframework.batch.integration.async;
+
+import org.springframework.lang.NonNullApi;

spring-batch-integration/src/main/java/org/springframework/batch/integration/chunk/ChunkResponse.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2007 the original author or authors.
+ * Copyright 2006-2018 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.
@@ -19,11 +19,13 @@ package org.springframework.batch.integration.chunk;
 import java.io.Serializable;
 
 import org.springframework.batch.core.StepContribution;
+import org.springframework.lang.Nullable;
 
 /**
  * Encapsulates a response to processing a chunk of items, summarising the result as a {@link StepContribution}.
  * 
  * @author Dave Syer
+ * @author Mahmoud Ben Hassine
  * 
  */
 public class ChunkResponse implements Serializable {
@@ -50,7 +52,7 @@ public class ChunkResponse implements Serializable {
 		this(status, sequence, jobId, stepContribution, null);
 	}
 
-	public ChunkResponse(boolean status, int sequence, Long jobId, StepContribution stepContribution, String message) {
+	public ChunkResponse(boolean status, int sequence, Long jobId, StepContribution stepContribution, @Nullable String message) {
 		this(status, sequence, jobId, stepContribution, message, false);
 	}
 
@@ -58,7 +60,7 @@ public class ChunkResponse implements Serializable {
 		this(input.status, input.sequence, input.jobId, input.stepContribution, input.message, redelivered);
 	}
 
-	public ChunkResponse(boolean status, int sequence, Long jobId, StepContribution stepContribution, String message, boolean redelivered) {
+	public ChunkResponse(boolean status, int sequence, Long jobId, StepContribution stepContribution, @Nullable String message, boolean redelivered) {
 		this.status = status;
 		this.sequence = sequence;
 		this.jobId = jobId;

spring-batch-integration/src/main/java/org/springframework/batch/integration/chunk/package-info.java

@@ -2,5 +2,9 @@
  * Components for remote chunking.
  *
  * @author Michael Minella
+ * @author Mahmoud Ben Hassine
  */
-package org.springframework.batch.integration.chunk;
+@NonNullApi
+package org.springframework.batch.integration.chunk;
+
+import org.springframework.lang.NonNullApi;

spring-batch-integration/src/main/java/org/springframework/batch/integration/config/annotation/package-info.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2018 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.
+ * You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * APIs for the configuration of Spring Integration components through annotations.
+ *
+ * @author Mahmoud Ben Hassine
+ */
+@NonNullApi
+package org.springframework.batch.integration.config.annotation;
+
+import org.springframework.lang.NonNullApi;

spring-batch-integration/src/main/java/org/springframework/batch/integration/config/xml/package-info.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2018 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.
+ * You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * APIs for the configuration of Spring Integration components through XML.
+ *
+ * @author Mahmoud Ben Hassine
+ */
+@NonNullApi
+package org.springframework.batch.integration.config.xml;
+
+import org.springframework.lang.NonNullApi;

spring-batch-integration/src/main/java/org/springframework/batch/integration/launch/package-info.java

@@ -2,5 +2,9 @@
  * Message based job launching components.
  *
  * @author Michael Minella
+ * @author Mahmoud Ben Hassine
  */
-package org.springframework.batch.integration.launch;
+@NonNullApi
+package org.springframework.batch.integration.launch;
+
+import org.springframework.lang.NonNullApi;

spring-batch-integration/src/main/java/org/springframework/batch/integration/partition/MessageChannelPartitionHandler.java

@@ -213,7 +213,7 @@ public class MessageChannelPartitionHandler implements PartitionHandler, Initial
 		final Set<StepExecution> split = stepExecutionSplitter.split(masterStepExecution, gridSize);
 
 		if(CollectionUtils.isEmpty(split)) {
-			return null;
+			return split;
 		}
 
 		int count = 0;

spring-batch-integration/src/main/java/org/springframework/batch/integration/partition/package-info.java

@@ -2,5 +2,9 @@
  * Remote partitioning components.
  *
  * @author Michael Minella
+ * @author Mahmoud Ben Hassine
  */
-package org.springframework.batch.integration.partition;
+@NonNullApi
+package org.springframework.batch.integration.partition;
+
+import org.springframework.lang.NonNullApi;

spring-batch-integration/src/main/java/org/springframework/batch/integration/step/package-info.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2018 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.
+ * You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Support classes related to steps when used with Spring Integration.
+ *
+ * @author Mahmoud Ben Hassine
+ */
+@NonNullApi
+package org.springframework.batch.integration.step;
+
+import org.springframework.lang.NonNullApi;