GH-3062: Fix Mail Tests Hang

Resolves https://github.com/spring-projects/spring-integration/issues/3062

Reliably stop the test mail servers before the test context is destroyed.

spring-integration-mail/src/test/java/org/springframework/integration/mail/dsl/MailTests.java

@@ -33,13 +33,13 @@ import javax.mail.search.FlagTerm;
 import javax.mail.search.FromTerm;
 import javax.mail.search.SearchTerm;
 
-import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Qualifier;
+import org.springframework.context.SmartLifecycle;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.integration.IntegrationMessageHeaderAccessor;
@@ -92,14 +92,6 @@ public class MailTests {
 		assertThat(n < 100).isTrue();
 	}
 
-	@AfterClass
-	public static void tearDown() {
-		smtpServer.stop();
-		pop3Server.stop();
-		imapServer.stop();
-		imapIdleServer.stop();
-	}
-
 	@Autowired
 	private MessageChannel sendMailChannel;
 
@@ -189,6 +181,35 @@ public class MailTests {
 	@EnableIntegration
 	public static class ContextConfiguration {
 
+		@Bean
+		public SmartLifecycle serverStopper() {
+			return new SmartLifecycle() {
+
+				@Override
+				public int getPhase() {
+					return Integer.MAX_VALUE;
+				}
+
+				@Override
+				public void stop() {
+					smtpServer.stop();
+					pop3Server.stop();
+					imapServer.stop();
+					imapIdleServer.stop();
+				}
+
+				@Override
+				public void start() {
+				}
+
+				@Override
+				public boolean isRunning() {
+					return true;
+				}
+
+			};
+		}
+
 		@Bean
 		public IntegrationFlow sendMailFlow() {
 			return IntegrationFlows.from("sendMailChannel")

spring-integration-test-support/src/main/java/org/springframework/integration/test/mail/TestMailServer.java

@@ -412,6 +412,8 @@ public final class TestMailServer {
 
 		protected final List<String> messages = new ArrayList<>(); // NOSONAR protected
 
+		private final List<Socket> clientSockets = new ArrayList<>();
+
 		private volatile boolean listening;
 
 		MailServer(int port) throws IOException {
@@ -445,6 +447,7 @@ public final class TestMailServer {
 			try {
 				while (!serverSocket.isClosed()) {
 					Socket socket = this.serverSocket.accept();
+					this.clientSockets.add(socket);
 					exec.execute(mailHandler(socket));
 				}
 			}
@@ -457,6 +460,9 @@ public final class TestMailServer {
 
 		public void stop() {
 			try {
+				for (Socket socket : this.clientSockets) {
+					socket.close();
+				}
 				this.serverSocket.close();
 			}
 			catch (IOException e) {