Fix class cast exception in gradle plugin main class detection logic

jib-gradle-plugin/CHANGELOG.md

@@ -9,6 +9,8 @@ All notable changes to this project will be documented in this file.
 
 ### Fixed
 
+- Fixed `ClassCastException` when using non-`String` value (for example, [`Provider`](https://docs.gradle.org/current/javadoc/org/gradle/api/provider/Provider.html)) for `Main-Class` manifest attribute of the `jar` task. ([#3396](https://github.com/GoogleContainerTools/jib/issues/3396))
+
 ## 3.1.4
 
 ### Changed
@@ -62,8 +64,8 @@ All notable changes to this project will be documented in this file.
     jib {
       configurationName = 'myconfig'
     }
-```                                                                                                                                                                      
-  
+```
+
 ### Changed
 
 - [Switched the default base images](https://github.com/GoogleContainerTools/jib/blob/master/docs/default_base_image.md) from Distroless to [`adoptopenjdk:{8,11}-jre`](https://hub.docker.com/_/adoptopenjdk) and [`jetty`](https://hub.docker.com/_/jetty) (for WAR). ([#3124](https://github.com/GoogleContainerTools/jib/pull/3124))

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

@@ -72,6 +72,7 @@
 import org.gradle.api.logging.Logger;
 import org.gradle.api.plugins.JavaPluginConvention;
 import org.gradle.api.plugins.WarPlugin;
+import org.gradle.api.provider.Provider;
 import org.gradle.api.tasks.SourceSet;
 import org.gradle.api.tasks.TaskProvider;
 import org.gradle.jvm.tasks.Jar;
@@ -370,7 +371,22 @@ public String getMainClassFromJarPlugin() {
     if (jarTask == null) {
       return null;
     }
-    return (String) jarTask.getManifest().getAttributes().get("Main-Class");
+
+    Object value = jarTask.getManifest().getAttributes().get("Main-Class");
+
+    if (value instanceof Provider) {
+      value = ((Provider<?>) value).getOrNull();
+    }
+
+    if (value instanceof String) {
+      return (String) value;
+    }
+
+    if (value == null) {
+      return null;
+    }
+
+    return String.valueOf(value);
   }
 
   @Override

jib-gradle-plugin/src/test/java/com/google/cloud/tools/jib/gradle/GradleProjectPropertiesTest.java

@@ -66,6 +66,7 @@
 import org.gradle.api.logging.Logger;
 import org.gradle.api.plugins.JavaPlugin;
 import org.gradle.api.plugins.JavaPluginConvention;
+import org.gradle.api.provider.Property;
 import org.gradle.api.tasks.bundling.War;
 import org.gradle.jvm.tasks.Jar;
 import org.gradle.testfixtures.ProjectBuilder;
@@ -180,6 +181,27 @@ public void testGetMainClassFromJar_missing() {
     assertThat(gradleProjectProperties.getMainClassFromJarPlugin()).isNull();
   }
 
+  @Test
+  public void testGetMainClassFromJarAsProperty_success() {
+    Property<String> mainClass =
+        project.getObjects().property(String.class).value("some.main.class");
+
+    Jar jar = project.getTasks().withType(Jar.class).getByName("jar");
+    jar.setManifest(new DefaultManifest(null).attributes(ImmutableMap.of("Main-Class", mainClass)));
+
+    assertThat(gradleProjectProperties.getMainClassFromJarPlugin()).isEqualTo("some.main.class");
+  }
+
+  @Test
+  public void testGetMainClassFromJarAsPropertyWithValueNull_missing() {
+    Property<String> mainClass = project.getObjects().property(String.class).value((String) null);
+
+    Jar jar = project.getTasks().withType(Jar.class).getByName("jar");
+    jar.setManifest(new DefaultManifest(null).attributes(ImmutableMap.of("Main-Class", mainClass)));
+
+    assertThat(gradleProjectProperties.getMainClassFromJarPlugin()).isNull();
+  }
+
   @Test
   public void testIsWarProject() {
     project.getPlugins().apply("war");