Fail spotlessJava when the Gradle JVM does not export javac

The formatter checks that javac's internals are exported to it when
it loads, and inside the Spotless step that failure became a lint of
every file: spotlessJava succeeded, the build cache kept the lint, and
spotlessApply failed on it until the cache was cleared, even after the
flags were added.

spotlessJava now runs the same check before the step does, as an
action of its own, without loading the formatter: the flags export the
packages to every unnamed module, so the plugin's module answers for
the formatter's. spotlessJava fails with the message formatDiff gives,
nothing is stored in the build cache, and once the flags are there
the next build formats. A file the formatter cannot parse is still a
lint of that file.

Author: abashev

gradle-open-java-format/src/main/java/com/palantir/javaformat/gradle/FormatterServiceLoader.java

@@ -86,6 +86,14 @@ private static FormatterService loadUncached(Collection<File> jars) {
         return Iterables.getOnlyElement(ServiceLoader.load(FormatterService.class, classLoader));
     }
 
+    /**
+     * Fails with the fix spelt out when this JVM does not export javac's internals to the formatter, without loading
+     * it. The flags export them to every unnamed module, so the plugin's own module answers for the formatter's.
+     */
+    public static void checkJavacIsExported() {
+        checkJavacIsExportedTo(FormatterServiceLoader.class.getClassLoader());
+    }
+
     /**
      * Fails with the fix spelt out when this JVM does not export javac's internals to the formatter's class loader:
      * either the native binary, which runs outside the JVM, or the {@code --add-exports} flags on the Gradle JVM.

gradle-open-java-format/src/main/java/com/palantir/javaformat/gradle/PalantirJavaFormatSpotlessPlugin.java

@@ -17,8 +17,10 @@
 
 import com.diffplug.gradle.spotless.SpotlessExtension;
 import com.google.common.collect.ImmutableList;
+import org.gradle.api.Action;
 import org.gradle.api.Plugin;
 import org.gradle.api.Project;
+import org.gradle.api.Task;
 import org.gradle.api.plugins.JavaPluginExtension;
 
 public abstract class PalantirJavaFormatSpotlessPlugin implements Plugin<Project> {
@@ -49,7 +51,20 @@ public void apply(Project project) {
                         SpotlessExtension spotlessExtension =
                                 project.getExtensions().getByType(SpotlessExtension.class);
                         spotlessExtension.java(spotlessInterop);
+                        // Before the step runs: Spotless turns what a step throws into a lint of the file, which
+                        // the build cache would then hand back after the build is fixed.
+                        project.getTasks()
+                                .named("spotlessJava")
+                                .configure(spotlessJava -> spotlessJava.doFirst(new CheckJavacIsExported()));
                     }));
         });
     }
+
+    /** A class, not a lambda: the configuration cache stores it with the task. */
+    private static final class CheckJavacIsExported implements Action<Task> {
+        @Override
+        public void execute(Task _task) {
+            FormatterServiceLoader.checkJavacIsExported();
+        }
+    }
 }