GH-3079: Use getMostSpecificMethod for SpEL calls

Fixes #3079

It turns out that we need to use a *most specific* method for the target
to be called reflectively from the SpEL

NOTE: We drop a *Method not found* error in case of wrong method source
since now we find a right source via `ClassUtils.getMostSpecificMethod()`

**Cherry-pick to 5.1.x**

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/handler/support/MessagingMethodInvokerHelper.java

@@ -417,7 +417,8 @@ private void prepareEvaluationContext() {
 		StandardEvaluationContext context = getEvaluationContext();
 		Class<?> targetType = AopUtils.getTargetClass(this.targetObject);
 		if (this.method != null) {
-			context.registerMethodFilter(targetType, new FixedMethodFilter(this.method));
+			context.registerMethodFilter(targetType,
+					new FixedMethodFilter(ClassUtils.getMostSpecificMethod(this.method, targetType)));
 			if (this.expectedType != null) {
 				Assert.state(context.getTypeConverter()
 								.canConvert(TypeDescriptor.valueOf((this.method).getReturnType()), this.expectedType),

spring-integration-core/src/test/java/org/springframework/integration/dsl/flows/IntegrationFlowTests.java

@@ -74,6 +74,7 @@
 import org.springframework.integration.store.SimpleMessageStore;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.support.MutableMessageBuilder;
+import org.springframework.integration.transformer.GenericTransformer;
 import org.springframework.integration.transformer.PayloadSerializingTransformer;
 import org.springframework.integration.util.NoBeansOverrideAnnotationConfigContextLoader;
 import org.springframework.messaging.Message;
@@ -362,10 +363,10 @@ public void testWireTap() {
 		assertThat(out).isNotNull();
 		assertThat(out.getPayload()).isEqualTo("bar");
 
-		this.tappedChannel5.send(new GenericMessage<>("foo"));
+		this.tappedChannel5.send(new GenericMessage<>(""));
 		out = this.wireTapSubflowResult.receive(10000);
 		assertThat(out).isNotNull();
-		assertThat(out.getPayload()).isEqualTo("FOO");
+		assertThat(out.getPayload()).isEqualTo("");
 	}
 
 	@Autowired
@@ -700,7 +701,15 @@ public IntegrationFlow wireTapFlow4() {
 		public IntegrationFlow wireTapFlow5() {
 			return f -> f
 					.wireTap(sf -> sf
-							.<String, String>transform(String::toUpperCase)
+							.transform(// Must not be lambda for SpEL fallback behavior on empty payload
+									new GenericTransformer<String, String>() {
+
+										@Override
+										public String transform(String source) {
+											return source.toUpperCase();
+										}
+
+									})
 							.channel(MessageChannels.queue("wireTapSubflowResult")))
 					.channel("nullChannel");
 		}

spring-integration-core/src/test/java/org/springframework/integration/handler/MethodInvokingMessageProcessorTests.java

@@ -55,7 +55,6 @@
 import org.springframework.core.MethodParameter;
 import org.springframework.expression.Expression;
 import org.springframework.expression.spel.SpelCompilerMode;
-import org.springframework.expression.spel.SpelEvaluationException;
 import org.springframework.expression.spel.SpelParserConfiguration;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
 import org.springframework.expression.spel.support.StandardEvaluationContext;
@@ -449,18 +448,6 @@ public void testProcessMessageCheckedException() throws Exception {
 				.withRootCauseInstanceOf(CheckedException.class);
 	}
 
-	@Test
-	public void testProcessMessageMethodNotFound() throws Exception {
-		TestDifferentErrorService service = new TestDifferentErrorService();
-		Method method = TestErrorService.class.getMethod("checked", String.class);
-		MethodInvokingMessageProcessor processor = new MethodInvokingMessageProcessor(service, method);
-		processor.setUseSpelInvoker(true);
-		processor.setBeanFactory(mock(BeanFactory.class));
-		assertThatExceptionOfType(MessageHandlingException.class)
-				.isThrownBy(() -> processor.processMessage(new GenericMessage<>("foo")))
-				.withCauseInstanceOf(SpelEvaluationException.class);
-	}
-
 	@Test
 	public void messageAndHeaderWithAnnotatedMethod() throws Exception {
 		AnnotatedTestService service = new AnnotatedTestService();