Fork application process with Maven by default

Closes gh-16945

Author: snicoll

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

@@ -829,8 +829,8 @@ and triggers a restart. In IntelliJ IDEA, building the project
 ====
 As long as forking is enabled, you can also start your application by using the supported
 build plugins (Maven and Gradle), since DevTools needs an isolated application
-classloader to operate properly. By default, Gradle and Maven do that when they detect
-DevTools on the classpath.
+classloader to operate properly. By default, the Gradle and Maven plugins fork the
+application process.
 
 ====
 

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/run-fork/pom.xml

@@ -22,9 +22,6 @@
 						<goals>
 							<goal>run</goal>
 						</goals>
-						<configuration>
-							<fork>true</fork>
-						</configuration>
 					</execution>
 				</executions>
 			</plugin>

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/run-profiles-fork/pom.xml

@@ -23,7 +23,6 @@
 							<goal>run</goal>
 						</goals>
 						<configuration>
-							<fork>true</fork>
 							<profiles>
 								<profile>foo</profile>
 								<profile>bar</profile>

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/run/pom.xml

@@ -22,6 +22,9 @@
 						<goals>
 							<goal>run</goal>
 						</goals>
+						<configuration>
+							<fork>false</fork>
+						</configuration>
 					</execution>
 				</executions>
 			</plugin>

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop-automatic-fork/invoker.properties

@@ -50,7 +50,6 @@
 					</execution>
 				</executions>
 				<configuration>
-					<fork>true</fork>
 					<jmxPort>${jmx.port}</jmxPort>
 				</configuration>
 			</plugin>

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop-automatic-fork/pom.xml

@@ -30,6 +30,9 @@
 						</goals>
 					</execution>
 				</executions>
+				<configuration>
+					<fork>false</fork>
+				</configuration>
 			</plugin>
 		</plugins>
 	</build>

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop-automatic-fork/src/main/java/org/test/SampleApplication.java

@@ -75,8 +75,7 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 	private boolean addResources = false;
 
 	/**
-	 * Path to agent jar. NOTE: the use of agents means that processes will be started by
-	 * forking a new JVM.
+	 * Path to agent jar. NOTE: a forked process is required to use this feature.
 	 * @since 1.0
 	 * @deprecated since 2.2.0 in favor of {@code agents}
 	 */
@@ -85,8 +84,7 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 	private File[] agent;
 
 	/**
-	 * Path to agent jars. NOTE: the use of agents means that processes will be started by
-	 * forking a new JVM.
+	 * Path to agent jars. NOTE: a forked process is required to use this feature.
 	 * @since 2.2
 	 */
 	@Parameter(property = "spring-boot.run.agents")
@@ -101,8 +99,7 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 
 	/**
 	 * Current working directory to use for the application. If not specified, basedir
-	 * will be used. NOTE: the use of working directory means that processes will be
-	 * started by forking a new JVM.
+	 * will be used. NOTE: a forked process is required to use this feature.
 	 * @since 1.5
 	 */
 	@Parameter(property = "spring-boot.run.workingDirectory")
@@ -111,25 +108,24 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 	/**
 	 * JVM arguments that should be associated with the forked process used to run the
 	 * application. On command line, make sure to wrap multiple values between quotes.
-	 * NOTE: the use of JVM arguments means that processes will be started by forking a
-	 * new JVM.
+	 * NOTE: a forked process is required to use this feature.
 	 * @since 1.1
 	 */
 	@Parameter(property = "spring-boot.run.jvmArguments")
 	private String jvmArguments;
 
 	/**
-	 * List of JVM system properties to pass to the process. NOTE: the use of system
-	 * properties means that processes will be started by forking a new JVM.
+	 * List of JVM system properties to pass to the process. NOTE: a forked process is
+	 * required to use this feature.
 	 * @since 2.1
 	 */
 	@Parameter
 	private Map<String, String> systemPropertyVariables;
 
 	/**
 	 * List of Environment variables that should be associated with the forked process
-	 * used to run the application. NOTE: the use of Environment variables means that
-	 * processes will be started by forking a new JVM.
+	 * used to run the application. NOTE: a forked process is required to use this
+	 * feature.
 	 * @since 2.1
 	 */
 	@Parameter
@@ -177,13 +173,13 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 	private File classesDirectory;
 
 	/**
-	 * Flag to indicate if the run processes should be forked. {@code fork} is
-	 * automatically enabled if an agent, jvmArguments or working directory are specified,
-	 * or if devtools is present.
+	 * Flag to indicate if the run processes should be forked. Disabling forking will
+	 * disable some features such as an agent, custom JVM arguments, devtools or
+	 * specifying the working directory to use.
 	 * @since 1.2
 	 */
-	@Parameter(property = "spring-boot.run.fork")
-	private Boolean fork;
+	@Parameter(property = "spring-boot.run.fork", defaultValue = "true")
+	private boolean fork;
 
 	/**
 	 * Flag to include the test classpath when running.
@@ -213,15 +209,16 @@ public void execute() throws MojoExecutionException, MojoFailureException {
 	 * @return {@code true} if the application process should be forked
 	 */
 	protected boolean isFork() {
-		return (Boolean.TRUE.equals(this.fork)
-				|| (this.fork == null && enableForkByDefault()));
+		return this.fork;
 	}
 
 	/**
 	 * Specify if fork should be enabled by default.
 	 * @return {@code true} if fork should be enabled by default
 	 * @see #logDisabledFork()
+	 * @deprecated as of 2.2.0 in favour of enabling forking by default.
 	 */
+	@Deprecated
 	protected boolean enableForkByDefault() {
 		return hasAgent() || hasJvmArgs() || hasEnvVariables()
 				|| hasWorkingDirectorySet();
@@ -264,7 +261,6 @@ private void run(String startClassName)
 	/**
 	 * 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 (getLog().isWarnEnabled()) {
@@ -273,9 +269,8 @@ protected void logDisabledFork() {
 			}
 			if (hasJvmArgs()) {
 				RunArguments runArguments = resolveJvmArguments();
-				getLog().warn("Fork mode disabled, ignoring JVM argument(s) [" + Arrays
-						.stream(runArguments.asArray()).collect(Collectors.joining(" "))
-						+ "]");
+				getLog().warn("Fork mode disabled, ignoring JVM argument(s) ["
+						+ String.join(" ", runArguments.asArray()) + "]");
 			}
 			if (hasWorkingDirectorySet()) {
 				getLog().warn(

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop-automatic-fork/verify.groovy

@@ -54,6 +54,7 @@ public class RunMojo extends AbstractRunMojo {
 	private Boolean hasDevtools;
 
 	@Override
+	@Deprecated
 	protected boolean enableForkByDefault() {
 		return super.enableForkByDefault() || hasDevtools();
 	}

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop-fork/pom.xml

@@ -67,8 +67,7 @@ public class StartMojo extends AbstractRunMojo {
 	private String jmxName = SpringApplicationAdminClient.DEFAULT_OBJECT_NAME;
 
 	/**
-	 * The port to use to expose the platform MBeanServer if the application needs to be
-	 * forked.
+	 * The port to use to expose the platform MBeanServer if the application is forked.
 	 */
 	@Parameter
 	private int jmxPort = 9001;

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop/pom.xml

@@ -6,10 +6,8 @@
  2014-05-14
  -----
 
-  By default, the <<<run>>> goal runs in the same process unless jvm arguments or an agent have been specified. You
-  can enable or disable forking explicitly using the <<<fork>>> attribute.
-
-  If you need to fork the process and debug it you can add the necessary JVM arguments to enable remote debugging. The
+  By default, the <<<run>>> goal runs your application in a forked process. If you need to
+  debug it, you should add the necessary JVM arguments to enable remote debugging. The
   following configuration suspend the process until a debugger has joined on port 5005:
 
 ---
@@ -38,8 +36,8 @@
 </project>
 ---
 
-  Note that since you specified some JVM arguments, the process is forked automatically. These arguments can be
-  specified on the command line as well, make sure to wrap that properly, that is:
+  These arguments can be specified on the command line as well, make sure to wrap that
+  properly, that is:
 
 ---
 mvn spring-boot:run -Dspring-boot.run.jvmArguments="-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005"

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

@@ -130,10 +130,11 @@ Usage
 mvn spring-boot:run
 ---
 
-  By default the application is executed directly from the Maven JVM. If you need to run
-  in a forked process you can use the 'fork' option. Forking will also occur if the
-  'jvmArguments', 'systemPropertyVariables', 'environmentVariables'  or 'agent' options
-  are specified, or if devtools is present.
+  By default the application is executed in a forked process. Although this is not
+  recommended, it is possible to execute the application directly from the Maven JVM by
+  disabling the <<<fork>>> property. Doing so means that <<<jvmArguments>>>,
+  <<<systemPropertyVariables>>>, <<<environmentVariables>>> and <<<agent>>> options are
+  ignored.
 
   If you need to specify some JVM arguments (i.e. for debugging purposes), you can use
   the <<<jvmArguments>>> parameter, see {{{./examples/run-debug.html}Debug the application}}