Throw an exception if the result can't be validated.

src/main/java/org/jenkinsci/maven/plugins/hpi/TestInsertionMojo.java

@@ -10,6 +10,7 @@
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.attribute.FileTime;
+import javax.lang.model.SourceVersion;
 import org.apache.maven.artifact.Artifact;
 import org.apache.maven.artifact.versioning.ArtifactVersion;
 import org.apache.maven.artifact.versioning.DefaultArtifactVersion;
@@ -182,11 +183,16 @@ public void execute() throws MojoExecutionException, MojoFailureException {
         }
     }
 
-    static String legalizePackageName(@NonNull String input) {
+    static String legalizePackageName(@NonNull String input) throws MojoFailureException {
         String result = input.replace('-', '_');
         if (!result.isEmpty() && Character.isDigit(result.charAt(0))) {
             result = "_" + result;
         }
+        if (!SourceVersion.isName(result)) {
+            throw new MojoFailureException(
+                    "Could not convert " + input
+                            + " to a legal java package name. Please override \"injectedTestPackage\" with a valid java package name.");
+        }
         return result;
     }
 }

src/test/java/org/jenkinsci/maven/plugins/hpi/TestInsertionMojoTest.java

@@ -1,18 +1,23 @@
 package org.jenkinsci.maven.plugins.hpi;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThrows;
 
+import org.apache.maven.plugin.MojoExecutionException;
 import org.junit.Test;
 
 public class TestInsertionMojoTest {
 
     @Test
-    public void legalizePackageName() {
+    public void legalizePackageName() throws Exception {
         assertEquals(
                 "org.jenkinsci.maven.plugins.hpi",
                 TestInsertionMojo.legalizePackageName("org.jenkinsci.maven.plugins.hpi"));
         assertEquals(
                 "_123org.jenkins_ci.maven.plugins.hpi",
                 TestInsertionMojo.legalizePackageName("123org.jenkins-ci.maven.plugins.hpi"));
+        assertThrows(
+                MojoExecutionException.class,
+                () -> TestInsertionMojo.legalizePackageName("org.jenkinsci%maven.plugins.hpi"));
     }
 }