GH-10082: Add io.spring.nullability Gradle plugin

Fixes: https://github.com/spring-projects/spring-integration/issues/10082

* Experiment with `NullAway` in some HTTP module packages

build.gradle

@@ -25,6 +25,7 @@ plugins {
     id 'com.github.spotbugs' version '6.1.13'
     id 'com.google.protobuf' version '0.9.5' apply false
     id 'io.freefair.aggregate-javadoc' version '8.13.1'
+    id 'io.spring.nullability' version '0.0.1' apply false
 }
 
 if (isCI) {
@@ -202,6 +203,7 @@ configure(javaProjects) { subproject ->
     apply plugin: 'checkstyle'
     apply plugin: 'kotlin'
     apply plugin: 'kotlin-spring'
+    apply plugin: 'io.spring.nullability'
 
     apply from: "${rootDir}/gradle/publish-maven.gradle"
 

spring-integration-http/src/main/java/org/springframework/integration/http/outbound/AbstractHttpRequestExecutingMessageHandler.java

@@ -22,6 +22,7 @@ import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -91,8 +92,10 @@ public abstract class AbstractHttpRequestExecutingMessageHandler extends Abstrac
 
 	private final Expression uriExpression;
 
+	@SuppressWarnings("NullAway.Init")
 	private StandardEvaluationContext evaluationContext;
 
+	@SuppressWarnings("NullAway.Init")
 	private SimpleEvaluationContext simpleEvaluationContext;
 
 	private boolean trustedSpel;
@@ -314,7 +317,7 @@ public abstract class AbstractHttpRequestExecutingMessageHandler extends Abstrac
 				() -> "'uriExpression' evaluation must result in a 'String' or 'URI' instance, not: "
 						+ (uri == null ? "null" : uri.getClass()));
 
-		Map<String, ?> uriVariables = null;
+		Map<String, ?> uriVariables = Collections.emptyMap();
 
 		if (uri instanceof String) {
 			uriVariables = determineUriVariables(requestMessage);
@@ -325,7 +328,7 @@ public abstract class AbstractHttpRequestExecutingMessageHandler extends Abstrac
 
 	@Nullable
 	protected abstract Object exchange(Object uri, HttpMethod httpMethod, HttpEntity<?> httpRequest,
-			@Nullable Object expectedResponseType, Message<?> requestMessage, @Nullable Map<String, ?> uriVariables);
+			Object expectedResponseType, Message<?> requestMessage, Map<String, ?> uriVariables);
 
 	protected Object getReply(ResponseEntity<?> httpResponse) {
 		HttpHeaders httpHeaders = httpResponse.getHeaders();
@@ -518,12 +521,10 @@ public abstract class AbstractHttpRequestExecutingMessageHandler extends Abstrac
 		}
 	}
 
-	@Nullable
 	private Object determineExpectedResponseType(Message<?> requestMessage) {
 		return evaluateTypeFromExpression(requestMessage, this.expectedResponseTypeExpression, "expectedResponseType");
 	}
 
-	@Nullable
 	protected Object evaluateTypeFromExpression(Message<?> requestMessage, @Nullable Expression expression,
 			String property) {
 
@@ -548,6 +549,9 @@ public abstract class AbstractHttpRequestExecutingMessageHandler extends Abstrac
 				}
 			}
 		}
+		else {
+			type = Void.class;
+		}
 		return type;
 	}
 

spring-integration-http/src/main/java/org/springframework/integration/http/outbound/HttpRequestExecutingMessageHandler.java

@@ -116,11 +116,19 @@ public class HttpRequestExecutingMessageHandler extends AbstractHttpRequestExecu
 	 */
 	public HttpRequestExecutingMessageHandler(Expression uriExpression, @Nullable RestTemplate restTemplate) {
 		super(uriExpression);
-		this.restTemplateExplicitlySet = restTemplate != null;
-		this.restTemplate = (this.restTemplateExplicitlySet ? restTemplate : new RestTemplate());
-		if (!this.restTemplateExplicitlySet) {
-			this.restTemplate.setUriTemplateHandler(this.uriFactory);
+		RestTemplate restTemplateToSet;
+
+		if (restTemplate != null) {
+			restTemplateToSet = restTemplate;
+			this.restTemplateExplicitlySet = true;
 		}
+		else {
+			restTemplateToSet = new RestTemplate();
+			restTemplateToSet.setUriTemplateHandler(this.uriFactory);
+			this.restTemplateExplicitlySet = false;
+		}
+
+		this.restTemplate = restTemplateToSet;
 	}
 
 	@Override
@@ -174,7 +182,7 @@ public class HttpRequestExecutingMessageHandler extends AbstractHttpRequestExecu
 	@Override
 	@Nullable
 	protected Object exchange(Object uri, HttpMethod httpMethod, HttpEntity<?> httpRequest,
-			@Nullable Object expectedResponseType, Message<?> requestMessage, @Nullable Map<String, ?> uriVariables) {
+			Object expectedResponseType, Message<?> requestMessage, Map<String, ?> uriVariables) {
 
 		ResponseEntity<?> httpResponse;
 		try {

spring-integration-http/src/test/java/org/springframework/integration/http/HttpProxyScenarioTests.java

@@ -54,7 +54,6 @@ import org.springframework.web.servlet.HandlerMapping;
 import org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.mockito.ArgumentMatchers.isNull;
 
 /**
  * @author Artem Bilan
@@ -134,7 +133,7 @@ public class HttpProxyScenarioTests {
 					return new ResponseEntity<>(responseHeaders, HttpStatus.OK);
 				}).when(template)
 				.exchange(Mockito.anyString(), Mockito.any(HttpMethod.class),
-						Mockito.any(HttpEntity.class), (Class<?>) isNull(), Mockito.anyMap());
+						Mockito.any(HttpEntity.class), Mockito.<Class<?>>any(), Mockito.anyMap());
 
 		PropertyAccessor dfa = new DirectFieldAccessor(this.handler);
 		dfa.setPropertyValue("restTemplate", template);
@@ -195,7 +194,7 @@ public class HttpProxyScenarioTests {
 					return new ResponseEntity<>(responseHeaders, HttpStatus.OK);
 				}).when(template)
 				.exchange(Mockito.anyString(), Mockito.any(HttpMethod.class),
-						Mockito.any(HttpEntity.class), (Class<?>) isNull(), Mockito.anyMap());
+						Mockito.any(HttpEntity.class), Mockito.<Class<?>>any(), Mockito.anyMap());
 
 		PropertyAccessor dfa = new DirectFieldAccessor(this.handlermp);
 		dfa.setPropertyValue("restTemplate", template);