GH-3410: Add UDP SocketCustomizer

Resolves https://github.com/spring-projects/spring-integration/issues/3410

**cherry-pick to 5.3.x**

* Doc polishing.

spring-integration-ip/src/test/java/org/springframework/integration/ip/config/ParserTests-context.xml

@@ -19,6 +19,8 @@
 
 	<task:executor id="externalTE" pool-size="10"/>
 
+	<bean id="udpCust" class = "org.springframework.integration.ip.config.ParserUnitTests$SocketCust"/>
+
 	<ip:udp-inbound-channel-adapter id="testInUdp"
 		channel="udpChannel"
 		check-length="true"
@@ -35,6 +37,7 @@
 		lookup-host="false"
 		auto-startup="false"
 		phase="1234"
+		socket-customizer="udpCust"
 	/>
 
 	<ip:udp-inbound-channel-adapter id="testInUdpMulticast"
@@ -141,6 +144,7 @@
 		local-address="127.0.0.1"
 		task-executor="externalTE"
 		order="23"
+		socket-customizer="udpCust"
 	/>
 
 	<ip:udp-outbound-channel-adapter id="testOutUdpiMulticast"

spring-integration-ip/src/test/java/org/springframework/integration/ip/config/ParserUnitTests.java

@@ -20,6 +20,8 @@ import static org.assertj.core.api.Assertions.assertThat;
 import static org.mockito.BDDMockito.given;
 import static org.mockito.Mockito.mock;
 
+import java.net.DatagramSocket;
+import java.net.SocketException;
 import java.time.Duration;
 import java.util.Iterator;
 import java.util.Set;
@@ -66,6 +68,7 @@ import org.springframework.integration.ip.tcp.connection.TcpSocketSupport;
 import org.springframework.integration.ip.udp.DatagramPacketMessageMapper;
 import org.springframework.integration.ip.udp.MulticastReceivingChannelAdapter;
 import org.springframework.integration.ip.udp.MulticastSendingMessageHandler;
+import org.springframework.integration.ip.udp.SocketCustomizer;
 import org.springframework.integration.ip.udp.UnicastReceivingChannelAdapter;
 import org.springframework.integration.ip.udp.UnicastSendingMessageHandler;
 import org.springframework.integration.test.util.TestUtils;
@@ -273,6 +276,9 @@ public class ParserUnitTests {
 	@Autowired
 	TcpMessageMapper mapper;
 
+	@Autowired
+	SocketCust socketCustomizer;
+
 	private static CountDownLatch adviceCalled = new CountDownLatch(1);
 
 	@Test
@@ -293,6 +299,7 @@ public class ParserUnitTests {
 		assertThat((Boolean) mapperAccessor.getPropertyValue("lookupHost")).isFalse();
 		assertThat(TestUtils.getPropertyValue(udpIn, "autoStartup", Boolean.class)).isFalse();
 		assertThat(dfa.getPropertyValue("phase")).isEqualTo(1234);
+		assertThat(dfa.getPropertyValue("socketCustomizer")).isSameAs(this.socketCustomizer);
 	}
 
 	@Test
@@ -365,6 +372,7 @@ public class ParserUnitTests {
 		assertThat(dfa.getPropertyValue("order")).isEqualTo(23);
 		assertThat(udpOut.getComponentName()).isEqualTo("testOutUdp");
 		assertThat(udpOut.getComponentType()).isEqualTo("ip:udp-outbound-channel-adapter");
+		assertThat(dfa.getPropertyValue("socketCustomizer")).isSameAs(this.socketCustomizer);
 	}
 
 	@Test
@@ -697,4 +705,12 @@ public class ParserUnitTests {
 
 	}
 
+	public static class SocketCust implements SocketCustomizer {
+
+		@Override
+		public void configure(DatagramSocket socket) throws SocketException {
+		}
+
+	}
+
 }

spring-integration-ip/src/test/java/org/springframework/integration/ip/dsl/IpIntegrationTests.java

@@ -18,6 +18,7 @@ package org.springframework.integration.ip.dsl;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
+import java.net.DatagramSocket;
 import java.util.Collections;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
@@ -56,6 +57,7 @@ import org.springframework.integration.ip.tcp.serializer.TcpCodecs;
 import org.springframework.integration.ip.udp.MulticastSendingMessageHandler;
 import org.springframework.integration.ip.udp.UdpServerListeningEvent;
 import org.springframework.integration.ip.udp.UnicastReceivingChannelAdapter;
+import org.springframework.integration.ip.udp.UnicastSendingMessageHandler;
 import org.springframework.integration.ip.util.TestingUtilities;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.test.util.TestUtils;
@@ -100,6 +102,9 @@ public class IpIntegrationTests {
 	@Autowired
 	private UnicastReceivingChannelAdapter udpInbound;
 
+	@Autowired
+	private UnicastSendingMessageHandler udpOutbound;
+
 	@Autowired
 	private QueueChannel udpIn;
 
@@ -176,6 +181,8 @@ public class IpIntegrationTests {
 		Message<?> received = this.udpIn.receive(10000);
 		assertThat(received).isNotNull();
 		assertThat(Transformers.objectToString().transform(received).getPayload()).isEqualTo("foo");
+		assertThat(TestUtils.getPropertyValue(this.udpOutbound, "socket", DatagramSocket.class).getTrafficClass())
+				.isEqualTo(0x10);
 	}
 
 	@Test
@@ -303,7 +310,8 @@ public class IpIntegrationTests {
 
 		@Bean
 		public IntegrationFlow outUdpAdapter() {
-			return f -> f.handle(Udp.outboundAdapter(m -> m.getHeaders().get("udp_dest")));
+			return f -> f.handle(Udp.outboundAdapter(m -> m.getHeaders().get("udp_dest"))
+					.configureSocket(socket -> socket.setTrafficClass(0x10)));
 		}
 
 		@Bean