Set Jetty temp dir

Ensure that the system temp directory is passed to the Jetty
WebAppContext. This prevents `jsp` folders from appearing in the current
directory.

Fixes gh-2169

spring-boot/src/main/java/org/springframework/boot/context/embedded/jetty/JettyEmbeddedServletContainerFactory.java

@@ -221,6 +221,7 @@ public class JettyEmbeddedServletContainerFactory extends
 	protected final void configureWebAppContext(WebAppContext context,
 			ServletContextInitializer... initializers) {
 		Assert.notNull(context, "Context must not be null");
+		context.setTempDirectory(getTempDirectory());
 		setExtendedListenerTypes(context);
 		if (this.resourceLoader != null) {
 			context.setClassLoader(this.resourceLoader.getClassLoader());
@@ -246,6 +247,11 @@ public class JettyEmbeddedServletContainerFactory extends
 		postProcessWebAppContext(context);
 	}
 
+	private File getTempDirectory() {
+		String temp = System.getProperty("java.io.tmpdir");
+		return (temp == null ? null : new File(temp));
+	}
+
 	private void setExtendedListenerTypes(WebAppContext context) {
 		try {
 			context.getServletContext().setExtendedListenerTypes(true);