Default Tomcat to not save SESSIONS.ser file

Update TomcatEmbeddedServletContainerFactory so that session data isn't
serialized by default. Prior to this commit the SESSIONS.ser file would
either be written to `/tmp` or into `baseDir` (if one was set).

By not saving session data we align Tomcat with the other embedded
servlet containers and reduce the risk of sensitive information being
left in `/tmp`.

Fixes gh-4156

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatEmbeddedServletContainerFactory.java

@@ -394,12 +394,20 @@ public class TomcatEmbeddedServletContainerFactory
 	private void configureSession(Context context) {
 		long sessionTimeout = getSessionTimeoutInMinutes();
 		context.setSessionTimeout((int) sessionTimeout);
-		if (isPersistSession()) {
 		Manager manager = context.getManager();
 		if (manager == null) {
 			manager = new StandardManager();
 			context.setManager(manager);
 		}
+		if (isPersistSession()) {
+			configurePersistSession(manager);
+		}
+		else {
+			disablePersistSession(manager);
+		}
+	}
+
+	private void configurePersistSession(Manager manager) {
 		Assert.state(manager instanceof StandardManager,
 				"Unable to persist HTTP session state using manager type "
 						+ manager.getClass().getName());
@@ -407,6 +415,11 @@ public class TomcatEmbeddedServletContainerFactory
 		File file = new File(folder, "SESSIONS.ser");
 		((StandardManager) manager).setPathname(file.getAbsolutePath());
 	}
+
+	private void disablePersistSession(Manager manager) {
+		if (manager instanceof StandardManager) {
+			((StandardManager) manager).setPathname(null);
+		}
 	}
 
 	private long getSessionTimeoutInMinutes() {

spring-boot/src/test/java/org/springframework/boot/context/embedded/AbstractEmbeddedServletContainerFactoryTests.java

@@ -723,7 +723,7 @@ public abstract class AbstractEmbeddedServletContainerFactoryTests {
 		return bean;
 	}
 
-	private ServletContextInitializer sessionServletRegistration() {
+	protected final ServletContextInitializer sessionServletRegistration() {
 		ServletRegistrationBean bean = new ServletRegistrationBean(new ExampleServlet() {
 
 			@Override

spring-boot/src/test/java/org/springframework/boot/context/embedded/tomcat/TomcatEmbeddedServletContainerFactoryTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.context.embedded.tomcat;
 
+import java.io.File;
 import java.io.IOException;
 import java.net.InetSocketAddress;
 import java.net.ServerSocket;
@@ -45,6 +46,7 @@ import org.springframework.util.SocketUtils;
 
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.not;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertThat;
@@ -333,6 +335,31 @@ public class TomcatEmbeddedServletContainerFactoryTests
 		assertForwardHeaderIsUsed(factory);
 	}
 
+	@Test
+	public void disableDoesNotSaveSessionFiles() throws Exception {
+		File baseDir = this.temporaryFolder.newFolder();
+		TomcatEmbeddedServletContainerFactory factory = getFactory();
+		// If baseDir is not set SESSIONS.ser is written to a different temp folder
+		// each time. By setting it we can really ensure that data isn't saved
+		factory.setBaseDirectory(baseDir);
+		this.container = factory
+				.getEmbeddedServletContainer(sessionServletRegistration());
+		this.container.start();
+		String s1 = getResponse(getLocalUrl("/session"));
+		String s2 = getResponse(getLocalUrl("/session"));
+		this.container.stop();
+		this.container = factory
+				.getEmbeddedServletContainer(sessionServletRegistration());
+		this.container.start();
+		String s3 = getResponse(getLocalUrl("/session"));
+		System.out.println(s1);
+		System.out.println(s2);
+		System.out.println(s3);
+		String message = "Session error s1=" + s1 + " s2=" + s2 + " s3=" + s3;
+		assertThat(message, s2.split(":")[0], equalTo(s1.split(":")[1]));
+		assertThat(message, s3.split(":")[0], not(equalTo(s2.split(":")[1])));
+	}
+
 	@Override
 	protected Wrapper getJspServlet() {
 		Container context = ((TomcatEmbeddedServletContainer) this.container).getTomcat()