Warns the user if specified mainClass is not a valid Java class.

jib-core/src/main/java/com/google/cloud/tools/jib/builder/BuildConfiguration.java

@@ -30,6 +30,8 @@
 /** Immutable configuration options for the builder process. */
 public class BuildConfiguration {
 
+  public static final String VALID_JAVA_CLASS_REGEX = "([\\p{L}_$][\\p{L}\\p{N}_$]*\\.)*[\\p{L}_$][\\p{L}\\p{N}_$]*";
+
   public static class Builder {
 
     // All the parameters below are set to their default values.

jib-core/src/test/java/com/google/cloud/tools/jib/builder/BuildConfigurationTest.java

@@ -153,4 +153,14 @@ public void testBuilder_missingValues() {
           ex.getMessage());
     }
   }
+
+  @Test
+  public void testValidJavaClassRegex() {
+    Assert.assertTrue("my.Class".matches(BuildConfiguration.VALID_JAVA_CLASS_REGEX));
+    Assert.assertTrue("my.java_Class$valid".matches(BuildConfiguration.VALID_JAVA_CLASS_REGEX));
+    Assert.assertFalse("${start-class}".matches(BuildConfiguration.VALID_JAVA_CLASS_REGEX));
+    Assert.assertFalse("123not.Valid".matches(BuildConfiguration.VALID_JAVA_CLASS_REGEX));
+    Assert.assertFalse("{class}".matches(BuildConfiguration.VALID_JAVA_CLASS_REGEX));
+    Assert.assertFalse("not valid".matches(BuildConfiguration.VALID_JAVA_CLASS_REGEX));
+  }
 }

jib-gradle-plugin/src/main/java/com/google/cloud/tools/jib/gradle/BuildImageTask.java

@@ -122,6 +122,9 @@ public void buildImage() throws InvalidImageReferenceException, IOException {
         throw new GradleException("Could not find main class specified in a 'jar' task");
       }
     }
+    if (!mainClass.matches(BuildConfiguration.VALID_JAVA_CLASS_REGEX)) {
+      getLogger().warn("'mainClass' is not a valid Java class : " + mainClass);
+    }
 
     SourceFilesConfiguration sourceFilesConfiguration = getSourceFilesConfiguration();
 

jib-maven-plugin/src/main/java/com/google/cloud/tools/jib/maven/BuildImageMojo.java

@@ -131,6 +131,9 @@ public void execute() throws MojoExecutionException, MojoFailureException {
             "add a `mainClass` configuration to jib-maven-plugin");
       }
     }
+    if (!mainClass.matches(BuildConfiguration.VALID_JAVA_CLASS_REGEX)) {
+      getLog().warn("'mainClass' is not a valid Java class : " + mainClass);
+    }
 
     SourceFilesConfiguration sourceFilesConfiguration =
         projectProperties.getSourceFilesConfiguration();
@@ -293,7 +296,7 @@ private void validateParameters() throws MojoFailureException {
         throw new MojoFailureException("Invalid configuration parameters");
       }
     }
-    // Validates 'imageFormat'
+    // Validates 'imageFormat'.
     boolean validFormat = false;
     for (ImageFormat format : ImageFormat.values()) {
       if (imageFormat.equals(format.name())) {