Annotate set-rootdir mojo as threadsafe and update code to make it so. Although I'm guessing concurrent set calls for the property would be ok (since they would set it to the same thing), I went ahead and actually made the code thread safe. My motivation for this is to remove "The following goals are not marked as thread-safe" warnings from my multi-thread scijava maven builds.

Author: trautmane

src/main/java/org/scijava/maven/plugin/SetRootDirPropertyMojo.java

@@ -45,7 +45,7 @@
  * 
  * @author Johannes Schindelin
  */
-@Mojo(name = "set-rootdir", defaultPhase = LifecyclePhase.VALIDATE)
+@Mojo(name = "set-rootdir", defaultPhase = LifecyclePhase.VALIDATE, threadSafe = true)
 public class SetRootDirPropertyMojo extends AbstractMojo {
 
 	/**
@@ -67,25 +67,40 @@ public class SetRootDirPropertyMojo extends AbstractMojo {
 
 	@Override
 	public void execute() throws MojoExecutionException {
-		if (currentProject.getProperties().getProperty(rootdirPropertyName) != null)
-		{
-			getLog().debug("Using previously defined rootdir");
-			return;
+		if (isRootdirPropertyNameDefined()) {
+			logUsingPreviouslyDefinedRootdir();
+		} else {
+			setRootdirPropertyName();
 		}
+	}
 
-		if (!isLocalProject(currentProject)) return;
+	private synchronized void setRootdirPropertyName() {
+		if (isRootdirPropertyNameDefined()) {
+			logUsingPreviouslyDefinedRootdir();
+		} else if (isLocalProject(currentProject)) {
+			MavenProject project = currentProject;
+			for (;;) {
+				final MavenProject parent = project.getParent();
+				if (parent == null || ! isLocalProject(parent)) {
+					break;
+				}
+				project = parent;
+			}
 
-		MavenProject project = currentProject;
-		for (;;) {
-			final MavenProject parent = project.getParent();
-			if (parent == null || !isLocalProject(parent)) break;
-			project = parent;
+			final String rootdir = project.getBasedir().getAbsolutePath();
+			getLog().info("Setting rootdir: " + rootdir);
+			for (final MavenProject reactorProject : reactorProjects) {
+				reactorProject.getProperties().setProperty(rootdirPropertyName, rootdir);
+			}
 		}
+	}
+
+	private boolean isRootdirPropertyNameDefined() {
+		return currentProject.getProperties().getProperty(rootdirPropertyName) != null;
+	}
 
-		final String rootdir = project.getBasedir().getAbsolutePath();
-		getLog().info("Setting rootdir: " + rootdir);
-		for (final MavenProject reactorProject : reactorProjects)
-			reactorProject.getProperties().setProperty(rootdirPropertyName, rootdir);
+	private void logUsingPreviouslyDefinedRootdir() {
+		getLog().debug("Using previously defined rootdir");
 	}
 
 	/**