Enable fork more when devtools is present

This commit improves the run goal to automatically fork the process when
devtools is present and log a warning when fork has been disabled via
configuration since devtools will not work on a non-forked process.

We don't want devtools to kick in for integration tests so the logic has
been placed in `RunMojo` requiring a couple of protected methods to
override.

Closes gh-5137

spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/AbstractRunMojo.java

@@ -140,7 +140,8 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 
 	/**
 	 * Flag to indicate if the run processes should be forked. {@code fork } is
-	 * automatically enabled if an agent or jvmArguments are specified.
+	 * automatically enabled if an agent or jvmArguments are specified, or if
+	 * devtools is present.
 	 * @since 1.2
 	 */
 	@Parameter(property = "fork")
@@ -160,23 +161,6 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 	@Parameter(defaultValue = "false")
 	private boolean skip;
 
-	/**
-	 * Specify if the application process should be forked.
-	 * @return {@code true} if the application process should be forked
-	 */
-	protected boolean isFork() {
-		return (Boolean.TRUE.equals(this.fork)
-				|| (this.fork == null && (hasAgent() || hasJvmArgs())));
-	}
-
-	private boolean hasAgent() {
-		return (this.agent != null && this.agent.length > 0);
-	}
-
-	private boolean hasJvmArgs() {
-		return (this.jvmArguments != null && this.jvmArguments.length() > 0);
-	}
-
 	@Override
 	public void execute() throws MojoExecutionException, MojoFailureException {
 		if (this.skip) {
@@ -187,6 +171,32 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 		run(startClassName);
 	}
 
+	/**
+	 * Specify if the application process should be forked.
+	 * @return {@code true} if the application process should be forked
+	 */
+	protected boolean isFork() {
+		return (Boolean.TRUE.equals(this.fork)
+				|| (this.fork == null && enableForkByDefault()));
+	}
+
+	/**
+	 * Specify if fork should be enabled by default.
+	 * @return {@code true} if fork should be enabled by default
+	 * @see #logDisabledFork()
+	 */
+	protected boolean enableForkByDefault() {
+		return hasAgent() || hasJvmArgs();
+	}
+
+	private boolean hasAgent() {
+		return (this.agent != null && this.agent.length > 0);
+	}
+
+	private boolean hasJvmArgs() {
+		return (this.jvmArguments != null && this.jvmArguments.length() > 0);
+	}
+
 	private void findAgent() {
 		try {
 			if (this.agent == null || this.agent.length == 0) {
@@ -221,17 +231,26 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 			doRunWithForkedJvm(startClassName);
 		}
 		else {
-			if (hasAgent()) {
-				getLog().warn("Fork mode disabled, ignoring agent");
-			}
-			if (hasJvmArgs()) {
-				getLog().warn("Fork mode disabled, ignoring JVM argument(s) ["
-						+ this.jvmArguments + "]");
-			}
+			logDisabledFork();
 			runWithMavenJvm(startClassName, resolveApplicationArguments().asArray());
 		}
 	}
 
+	/**
+	 * Log a warning indicating that fork mode has been explicitly disabled
+	 * while some conditions are present that require to enable it.
+	 * @see #enableForkByDefault()
+	 */
+	protected void logDisabledFork() {
+		if (hasAgent()) {
+			getLog().warn("Fork mode disabled, ignoring agent");
+		}
+		if (hasJvmArgs()) {
+			getLog().warn("Fork mode disabled, ignoring JVM argument(s) ["
+					+ this.jvmArguments + "]");
+		}
+	}
+
 	private void doRunWithForkedJvm(String startClassName)
 			throws MojoExecutionException, MojoFailureException {
 		List<String> args = new ArrayList<String>();

spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RunMojo.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.maven;
 
+import java.net.URL;
 import java.net.URLClassLoader;
 import java.util.List;
 
@@ -41,6 +42,26 @@ public class RunMojo extends AbstractRunMojo {
 
 	private static final int EXIT_CODE_SIGINT = 130;
 
+	private static final String RESTARTER_CLASS_LOCATION = "org/springframework/boot/devtools/restart/Restarter.class";
+
+	/**
+	 * Devtools presence flag to avoid checking for it several times per execution.
+	 */
+	private Boolean hasDevtools;
+
+	@Override
+	protected boolean enableForkByDefault() {
+		return super.enableForkByDefault() || hasDevtools();
+	}
+
+	@Override
+	protected void logDisabledFork() {
+		super.logDisabledFork();
+		if (hasDevtools()) {
+			getLog().warn("Fork mode disabled, devtools will be disabled");
+		}
+	}
+
 	@Override
 	protected void runWithForkedJvm(List<String> args) throws MojoExecutionException {
 		try {
@@ -92,6 +113,24 @@ public class RunMojo extends AbstractRunMojo {
 		while (hasNonDaemonThreads);
 	}
 
+	private boolean hasDevtools() {
+		if (this.hasDevtools == null) {
+			this.hasDevtools = checkForDevtools();
+		}
+		return this.hasDevtools;
+	}
+
+	private boolean checkForDevtools() {
+		try {
+			URL[] urls = getClassPathUrls();
+			URLClassLoader classLoader = new URLClassLoader(urls);
+			return (classLoader.findResource(RESTARTER_CLASS_LOCATION) != null);
+		}
+		catch (Exception ex) {
+			return false;
+		}
+	}
+
 	private static final class RunProcessKiller implements Runnable {
 
 		private final RunProcess runProcess;