Polish "Configure a temporary directory with Undertow"

See gh-17778

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/MultipartAutoConfigurationTests.java

@@ -148,6 +148,16 @@ public class MultipartAutoConfigurationTests {
 				.isSameAs(this.context.getBean(DispatcherServlet.class).getMultipartResolver());
 	}
 
+	@Test
+	public void webServerWithNonAbsoluteMultipartLocationUndertowConfiguration() {
+		this.context = new AnnotationConfigServletWebServerApplicationContext(
+				WebServerWithNonAbsolutePathUndertow.class, BaseConfiguration.class);
+		this.context.getBean(MultipartConfigElement.class);
+		verifyServletWorks();
+		assertThat(this.context.getBean(StandardServletMultipartResolver.class))
+				.isSameAs(this.context.getBean(DispatcherServlet.class).getMultipartResolver());
+	}
+
 	@Test
 	public void webServerWithMultipartConfigDisabled() {
 		testWebServerWithCustomMultipartConfigEnabledSetting("false", 0);
@@ -365,6 +375,27 @@ public class MultipartAutoConfigurationTests {
 
 	}
 
+	@Configuration
+	@EnableWebMvc
+	public static class WebServerWithNonAbsolutePathUndertow {
+
+		@Bean
+		MultipartConfigElement multipartConfigElement() {
+			return new MultipartConfigElement("test/not-absolute");
+		}
+
+		@Bean
+		UndertowServletWebServerFactory webServerFactory() {
+			return new UndertowServletWebServerFactory();
+		}
+
+		@Bean
+		WebController webController() {
+			return new WebController();
+		}
+
+	}
+
 	@Configuration
 	public static class WebServerWithCustomMultipartResolver {
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/undertow/UndertowServletWebServerFactory.java

@@ -259,7 +259,6 @@ public class UndertowServletWebServerFactory extends AbstractServletWebServerFac
 		deployment.setClassLoader(getServletClassLoader());
 		deployment.setContextPath(getContextPath());
 		deployment.setDisplayName(getDisplayName());
-		deployment.setTempDir(new File(System.getProperty("java.io.tmpdir")));
 		deployment.setDeploymentName("spring-boot");
 		if (isRegisterDefaultServlet()) {
 			deployment.addServlet(Servlets.servlet("default", DefaultServlet.class));
@@ -267,6 +266,7 @@ public class UndertowServletWebServerFactory extends AbstractServletWebServerFac
 		configureErrorPages(deployment);
 		deployment.setServletStackTraces(ServletStackTraces.NONE);
 		deployment.setResourceManager(getDocumentRootResourceManager());
+		deployment.setTempDir(createTempDir("undertow"));
 		deployment.setEagerFilterInit(this.eagerInitFilters);
 		configureMimeMappings(deployment);
 		for (UndertowDeploymentInfoCustomizer customizer : this.deploymentInfoCustomizers) {