Allow Devtools to be enabled irrespective of the launching ClassLoader

Closes gh-21424

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/DefaultRestartInitializer.java

@@ -43,14 +43,38 @@ public URL[] getInitialUrls(Thread thread) {
 	}
 
 	/**
-	 * Returns if the thread is for a main invocation. By default checks the name of the
-	 * thread and the context classloader.
+	 * Returns if the thread is for a main invocation. By default {@link #isMain(Thread)
+	 * checks the name of the thread} and {@link #isDevelopmentClassLoader(ClassLoader)
+	 * the context classloader}.
 	 * @param thread the thread to check
 	 * @return {@code true} if the thread is a main invocation
+	 * @see #isMainThread
+	 * @see #isDevelopmentClassLoader(ClassLoader)
 	 */
 	protected boolean isMain(Thread thread) {
-		return thread.getName().equals("main")
-				&& thread.getContextClassLoader().getClass().getName().contains("AppClassLoader");
+		return isMainThread(thread) && isDevelopmentClassLoader(thread.getContextClassLoader());
+	}
+
+	/**
+	 * Returns whether the given {@code thread} is considered to be the main thread.
+	 * @param thread the thread to check
+	 * @return {@code true} if it's the main thread, otherwise {@code false}
+	 * @since 2.4.0
+	 */
+	protected boolean isMainThread(Thread thread) {
+		return thread.getName().equals("main");
+	}
+
+	/**
+	 * Returns whether the given {@code classLoader} is one that is typically used during
+	 * development.
+	 * @param classLoader the ClassLoader to check
+	 * @return {@code true} if it's a ClassLoader typically used during development,
+	 * otherwise {@code false}
+	 * @since 2.4.0
+	 */
+	protected boolean isDevelopmentClassLoader(ClassLoader classLoader) {
+		return classLoader.getClass().getName().contains("AppClassLoader");
 	}
 
 	/**

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartApplicationListener.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -64,7 +64,24 @@ private void onApplicationStartingEvent(ApplicationStartingEvent event) {
 		// It's too early to use the Spring environment but we should still allow
 		// users to disable restart using a System property.
 		String enabled = System.getProperty(ENABLED_PROPERTY);
-		if (enabled == null || Boolean.parseBoolean(enabled)) {
+		RestartInitializer restartInitializer = null;
+		if (enabled == null) {
+			restartInitializer = new DefaultRestartInitializer();
+		}
+		else if (Boolean.parseBoolean(enabled)) {
+			restartInitializer = new DefaultRestartInitializer() {
+
+				@Override
+				protected boolean isDevelopmentClassLoader(ClassLoader classLoader) {
+					return true;
+				}
+
+			};
+			logger.info(LogMessage.format(
+					"Restart enabled irrespective of application packaging due to System property '%s' being set to true",
+					ENABLED_PROPERTY));
+		}
+		if (restartInitializer != null) {
 			String[] args = event.getArgs();
 			DefaultRestartInitializer initializer = new DefaultRestartInitializer();
 			boolean restartOnInitialize = !AgentReloader.isActive();

spring-boot-project/spring-boot-devtools/src/test/java/org/springframework/boot/devtools/restart/RestartApplicationListenerTests.java

@@ -88,6 +88,14 @@ void disableWithSystemProperty(CapturedOutput output) {
 		assertThat(output).contains("Restart disabled due to System property");
 	}
 
+	@Test
+	void enableWithSystemProperty(CapturedOutput output) {
+		System.setProperty(ENABLED_PROPERTY, "true");
+		testInitialize(false);
+		assertThat(Restarter.getInstance()).hasFieldOrPropertyWithValue("enabled", true);
+		assertThat(output).contains("Restart enabled irrespective of application packaging due to System property");
+	}
+
 	private void testInitialize(boolean failed) {
 		Restarter.clearInstance();
 		RestartApplicationListener listener = new RestartApplicationListener();

spring-boot-project/spring-boot-docs/src/docs/asciidoc/using-spring-boot.adoc

@@ -542,7 +542,10 @@ To include devtools support, add the module dependency to your build, as shown i
 
 NOTE: Developer tools are automatically disabled when running a fully packaged application.
 If your application is launched from `java -jar` or if it is started from a special classloader, then it is considered a "`production application`".
-If that does not apply to you (i.e. if you run your application from a container), consider excluding devtools or set the `-Dspring.devtools.restart.enabled=false` system property.
+You can control this behavior by using the `spring.devtools.restart.enabled` system property.
+To enable devtools, irrespective of the classloader used to launch your application, set the `-Dspring.devtools.restart.enabled=true` system property.
+This must not be done in a production environment where running devtools is a security risk.
+To disable devtools, exclude the dependency or set the `-Dspring.devtools.restart.enabled=false` system property.
 
 TIP: Flagging the dependency as optional in Maven or using the `developmentOnly` configuration in Gradle (as shown above) prevents devtools from being transitively applied to other modules that use your project.