INT-4332 updated DSL with Supplier support

- add support for Supplier as sources of messages to the DSL
- add tests and javadocs

spring-integration-core/src/main/java/org/springframework/integration/dsl/IntegrationFlows.java

@@ -17,6 +17,7 @@
 package org.springframework.integration.dsl;
 
 import java.util.function.Consumer;
+import java.util.function.Supplier;
 
 import org.reactivestreams.Publisher;
 
@@ -28,6 +29,7 @@ import org.springframework.integration.dsl.support.FixedSubscriberChannelPrototy
 import org.springframework.integration.dsl.support.MessageChannelReference;
 import org.springframework.integration.endpoint.MessageProducerSupport;
 import org.springframework.integration.endpoint.MethodInvokingMessageSource;
+import org.springframework.integration.endpoint.SourcePollingChannelAdapter;
 import org.springframework.integration.gateway.AnnotationGatewayProxyFactoryBean;
 import org.springframework.integration.gateway.GatewayProxyFactoryBean;
 import org.springframework.integration.gateway.MessagingGatewaySupport;
@@ -40,6 +42,7 @@ import org.springframework.util.Assert;
  *
  * @author Artem Bilan
  * @author Gary Russell
+ * @author Oleg Zhurakousky
  *
  * @since 5.0
  *
@@ -146,6 +149,30 @@ public final class IntegrationFlows {
 		return from(service, methodName, null);
 	}
 
+	/**
+	 * Provides {@link Supplier} as source of messages to the integration flow which will
+	 * be triggered by the application context's default poller (which must be declared).
+	 * @param messageSource the {@link Supplier} to populate.
+	 * @return new {@link IntegrationFlowBuilder}.
+	 * @see Supplier
+	 */
+	public static <T> IntegrationFlowBuilder from(Supplier<T> messageSource) {
+		return from(messageSource, "get", null);
+	}
+
+	/**
+	 * Provides {@link Supplier} as source of messages to the integration flow.
+	 * which will be triggered by a <b>provided</b> {@link SourcePollingChannelAdapter}.
+	 * @param messageSource the {@link Supplier} to populate.
+	 * @param endpointConfigurer the {@link Consumer} to provide more options for the
+	 * {@link org.springframework.integration.config.SourcePollingChannelAdapterFactoryBean}.
+	 * @return new {@link IntegrationFlowBuilder}.
+	 * @see Supplier
+	 */
+	public static <T> IntegrationFlowBuilder from(Supplier<T> messageSource, Consumer<SourcePollingChannelAdapterSpec> endpointConfigurer) {
+		return from(messageSource, "get", endpointConfigurer);
+	}
+
 	/**
 	 * Populate the provided {@link MethodInvokingMessageSource} for the method of the provided service.
 	 * The {@link org.springframework.integration.dsl.IntegrationFlow} {@code startMessageSource}.