INT-4095: Support Limiting (S)FTP Files Fetched

JIRA: https://jira.spring.io/browse/INT-4095

Limit the number of remote files fetched on each poll (when it is necessary to fetch files).

Polishing - PR Comments

Polishing - Decouple MaxFetchSize from Poller

Polishing - PR Comments

Schemas and Docs

More Polishing

* Polishing according PR comments

spring-integration-jmx/src/test/java/org/springframework/integration/monitor/MessageSourceTests.java

@@ -0,0 +1,128 @@
+/*
+ * Copyright 2016 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.
+ */
+
+package org.springframework.integration.monitor;
+
+import static org.hamcrest.Matchers.equalTo;
+import static org.junit.Assert.assertThat;
+
+import java.util.Set;
+
+import javax.management.Attribute;
+import javax.management.MBeanServer;
+import javax.management.ObjectName;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.integration.annotation.InboundChannelAdapter;
+import org.springframework.integration.channel.QueueChannel;
+import org.springframework.integration.config.EnableIntegration;
+import org.springframework.integration.endpoint.AbstractFetchLimitingMessageSource;
+import org.springframework.integration.jmx.config.EnableIntegrationMBeanExport;
+import org.springframework.integration.scheduling.PollerMetadata;
+import org.springframework.jmx.support.MBeanServerFactoryBean;
+import org.springframework.messaging.PollableChannel;
+import org.springframework.test.context.junit4.SpringRunner;
+
+/**
+ * @author Gary Russell
+ * @since 5.0
+ *
+ */
+@RunWith(SpringRunner.class)
+public class MessageSourceTests {
+
+	@Autowired
+	private MBeanServer server;
+
+	@Autowired
+	private MaxFetchSource source1;
+
+	@Autowired
+	private MaxFetchSource source2;
+
+	@Test
+	public void testMaxFetch() throws Exception {
+		Set<ObjectName> query = this.server.queryNames(new ObjectName("foo:type=MessageSource,*"), null);
+		for (ObjectName instance : query) {
+			assertThat(this.server.getAttribute(instance, "MaxFetchSize"), equalTo(123));
+			this.server.setAttribute(instance, new Attribute("MaxFetchSize", 456));
+			assertThat(this.server.getAttribute(instance, "MaxFetchSize"), equalTo(456));
+		}
+		assertThat(this.source1.getMaxFetchSize(), equalTo(456));
+		assertThat(this.source2.getMaxFetchSize(), equalTo(456));
+	}
+
+	@Configuration
+	@EnableIntegrationMBeanExport(defaultDomain = "foo")
+	@EnableIntegration
+	public static class Config {
+
+		@Bean
+		public MBeanServerFactoryBean fb() {
+			MBeanServerFactoryBean fb = new MBeanServerFactoryBean();
+			fb.setLocateExistingServerIfPossible(true);
+			return fb;
+		}
+
+		@Bean
+		@InboundChannelAdapter(channel = "out")
+		public MaxFetchSource source1() {
+			return new MaxFetchSource();
+		}
+
+		@Bean
+		public MaxFetchSource source2() { // raw source, no consumer
+			return new MaxFetchSource();
+		}
+
+		@Bean
+		public PollableChannel out() {
+			return new QueueChannel();
+		}
+
+		@Bean(name = PollerMetadata.DEFAULT_POLLER)
+		public PollerMetadata defaultPoller() {
+			PollerMetadata pollerMetadata = new PollerMetadata();
+			pollerMetadata.setTrigger(t -> null);
+			return pollerMetadata;
+		}
+
+	}
+
+	public static class MaxFetchSource extends AbstractFetchLimitingMessageSource<String> {
+
+		{
+			setMaxFetchSize(123);
+		}
+
+		@Override
+		public String getComponentType() {
+			return null;
+		}
+
+		@Override
+		protected Object doReceive(int maxFetchSize) {
+			return null;
+		}
+
+	}
+
+}