INT-4551: Fail if old Java DSL jar is present

JIRA: https://jira.spring.io/browse/INT-4551

**Cherry-pick to 5.0.x**

spring-integration-core/src/main/java/org/springframework/integration/config/IntegrationRegistrar.java

@@ -38,6 +38,7 @@ import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.beans.factory.support.ManagedList;
 import org.springframework.beans.factory.support.ManagedSet;
 import org.springframework.beans.factory.support.RootBeanDefinition;
+import org.springframework.context.ApplicationContextException;
 import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
 import org.springframework.core.io.Resource;
 import org.springframework.core.io.support.PathMatchingResourcePatternResolver;
@@ -85,6 +86,17 @@ public class IntegrationRegistrar implements ImportBeanDefinitionRegistrar, Bean
 		this.classLoader = classLoader;
 	}
 
+	static {
+		if (ClassUtils.isPresent("org.springframework.integration.dsl.support.Function",
+				IntegrationRegistrar.class.getClassLoader())) {
+
+			throw new ApplicationContextException("Starting with Spring Integration 5.0, "
+					+  "the 'spring-integration-java-dsl' dependency is no longer needed; "
+					+ "the Java DSL has been merged into the core project. "
+					+ "Having it present it on the classpath causes class loading conflicts.");
+		}
+	}
+
 	/**
 	 * Invoked by the framework when an @EnableIntegration annotation is encountered.
 	 * Also called with {@code null} {@code importingClassMetadata} from {@code AbstractIntegrationNamespaceHandler}