Fix #2482: Warn about invalid Java migrations

flyway-core/src/main/java/org/flywaydb/core/Flyway.java

@@ -23,6 +23,7 @@
 import org.flywaydb.core.api.configuration.FluentConfiguration;
 import org.flywaydb.core.api.logging.Log;
 import org.flywaydb.core.api.logging.LogFactory;
+import org.flywaydb.core.api.migration.JavaMigration;
 import org.flywaydb.core.api.resolver.MigrationResolver;
 import org.flywaydb.core.internal.callback.CallbackExecutor;
 import org.flywaydb.core.internal.callback.DefaultCallbackExecutor;
@@ -371,7 +372,7 @@ public Void execute(MigrationResolver migrationResolver,
      * @return A new, fully configured, MigrationResolver instance.
      */
     private MigrationResolver createMigrationResolver(ResourceProvider resourceProvider,
-                                                      ClassProvider classProvider,
+                                                      ClassProvider<JavaMigration> classProvider,
                                                       SqlScriptExecutorFactory sqlScriptExecutorFactory,
                                                       SqlScriptFactory sqlScriptFactory) {
         return new CompositeMigrationResolver(resourceProvider, classProvider, configuration,
@@ -410,12 +411,14 @@ private MigrationResolver createMigrationResolver(ResourceProvider resourceProvi
 
 
         final ResourceProvider resourceProvider;
-        ClassProvider classProvider;
+        ClassProvider<JavaMigration> classProvider;
         if (!scannerRequired && configuration.isSkipDefaultResolvers() && configuration.isSkipDefaultCallbacks()) {
             resourceProvider = NoopResourceProvider.INSTANCE;
+            //noinspection unchecked
             classProvider = NoopClassProvider.INSTANCE;
         } else {
-            Scanner scanner = new Scanner(
+            Scanner<JavaMigration> scanner = new Scanner<>(
+                    JavaMigration.class,
                     Arrays.asList(configuration.getLocations()),
                     configuration.getClassLoader(),
                     configuration.getEncoding()

flyway-core/src/main/java/org/flywaydb/core/internal/clazz/ClassProvider.java

@@ -20,12 +20,11 @@
 /**
  * A facility to obtain classes.
  */
-public interface ClassProvider {
+public interface ClassProvider<I> {
     /**
-     * Retrieve all classes which implement this interface.
+     * Retrieve all classes which implement the specified interface.
      *
-     * @param implementedInterface The interface the matching classes should implement.
      * @return The non-abstract classes that were found.
      */
-    <I> Collection<Class<? extends I>> getClasses(Class<I> implementedInterface);
+    Collection<Class<? extends I>> getClasses();
 }

flyway-core/src/main/java/org/flywaydb/core/internal/clazz/NoopClassProvider.java

@@ -25,7 +25,7 @@ public enum NoopClassProvider implements ClassProvider {
     INSTANCE;
 
     @Override
-    public <I> Collection<Class<? extends I>> getClasses(Class<I> implementedInterface) {
+    public Collection<Class<?>> getClasses() {
         return Collections.emptyList();
     }
 }

flyway-core/src/main/java/org/flywaydb/core/internal/resolver/CompositeMigrationResolver.java

@@ -17,6 +17,7 @@
 
 import org.flywaydb.core.api.FlywayException;
 import org.flywaydb.core.api.configuration.Configuration;
+import org.flywaydb.core.api.migration.JavaMigration;
 import org.flywaydb.core.api.resolver.Context;
 import org.flywaydb.core.api.resolver.MigrationResolver;
 import org.flywaydb.core.api.resolver.ResolvedMigration;
@@ -62,7 +63,7 @@ public class CompositeMigrationResolver implements MigrationResolver {
      * @param customMigrationResolvers Custom Migration Resolvers.
      */
     public CompositeMigrationResolver(ResourceProvider resourceProvider,
-                                      ClassProvider classProvider,
+                                      ClassProvider<JavaMigration> classProvider,
                                       Configuration configuration,
                                       SqlScriptExecutorFactory sqlScriptExecutorFactory,
                                       SqlScriptFactory sqlScriptFactory,

flyway-core/src/main/java/org/flywaydb/core/internal/resolver/java/ScanningJavaMigrationResolver.java

@@ -36,7 +36,7 @@ public class ScanningJavaMigrationResolver implements MigrationResolver {
     /**
      * The Scanner to use.
      */
-    private final ClassProvider classProvider;
+    private final ClassProvider<JavaMigration> classProvider;
 
     /**
      * The configuration to inject (if necessary) in the migration classes.
@@ -49,7 +49,7 @@ public class ScanningJavaMigrationResolver implements MigrationResolver {
      * @param classProvider The class provider.
      * @param configuration The configuration to inject (if necessary) in the migration classes.
      */
-    public ScanningJavaMigrationResolver(ClassProvider classProvider, Configuration configuration) {
+    public ScanningJavaMigrationResolver(ClassProvider<JavaMigration> classProvider, Configuration configuration) {
         this.classProvider = classProvider;
         this.configuration = configuration;
     }
@@ -58,7 +58,7 @@ public ScanningJavaMigrationResolver(ClassProvider classProvider, Configuration
     public List<ResolvedMigration> resolveMigrations(Context context) {
         List<ResolvedMigration> migrations = new ArrayList<>();
 
-        for (Class<?> clazz : classProvider.getClasses(JavaMigration.class)) {
+        for (Class<?> clazz : classProvider.getClasses()) {
             JavaMigration javaMigration = ClassUtils.instantiate(clazz.getName(), configuration.getClassLoader());
             migrations.add(new ResolvedJavaMigration(javaMigration));
         }

flyway-core/src/main/java/org/flywaydb/core/internal/scanner/Scanner.java

@@ -31,18 +31,19 @@
 import java.nio.charset.Charset;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 
 /**
  * Scanner for Resources and Classes.
  */
-public class Scanner implements ResourceProvider, ClassProvider {
+public class Scanner<I> implements ResourceProvider, ClassProvider<I> {
     private static final Log LOG = LogFactory.getLog(Scanner.class);
 
     private final List<LoadableResource> resources = new ArrayList<>();
-    private final List<Class<?>> classes = new ArrayList<>();
+    private final List<Class<? extends I>> classes = new ArrayList<>();
 
-    public Scanner(Collection<Location> locations, ClassLoader classLoader, Charset encoding
+    public Scanner(Class<I> implementedInterface, Collection<Location> locations, ClassLoader classLoader, Charset encoding
 
 
 
@@ -59,9 +60,9 @@ public Scanner(Collection<Location> locations, ClassLoader classLoader, Charset
             if (location.isFileSystem()) {
                 resources.addAll(fileSystemScanner.scanForResources(location));
             } else {
-                ResourceAndClassScanner resourceAndClassScanner = android
-                        ? new AndroidScanner(classLoader, encoding, location)
-                        : new ClassPathScanner(classLoader, encoding, location);
+                ResourceAndClassScanner<I> resourceAndClassScanner = android
+                        ? new AndroidScanner<>(implementedInterface, classLoader, encoding, location)
+                        : new ClassPathScanner<>(implementedInterface, classLoader, encoding, location);
                 resources.addAll(resourceAndClassScanner.scanForResources());
                 classes.addAll(resourceAndClassScanner.scanForClasses());
             }
@@ -100,22 +101,12 @@ public Collection<LoadableResource> getResources(String prefix, String... suffix
     }
 
     /**
-     * Scans the classpath for concrete classes under the specified package implementing this interface.
+     * Scans the classpath for concrete classes under the specified package implementing the specified interface.
      * Non-instantiable abstract classes are filtered out.
      *
-     * @param implementedInterface The interface the matching classes should implement.
      * @return The non-abstract classes that were found.
      */
-    public <I> Collection<Class<? extends I>> getClasses(Class<I> implementedInterface) {
-        List<Class<? extends I>> result = new ArrayList<>();
-        for (Class<?> clazz : classes) {
-            if (!implementedInterface.isAssignableFrom(clazz)) {
-                continue;
-            }
-
-            //noinspection unchecked
-            result.add((Class<? extends I>) clazz);
-        }
-        return result;
+    public Collection<Class<? extends I>> getClasses() {
+        return Collections.unmodifiableCollection(classes);
     }
 }

flyway-core/src/main/java/org/flywaydb/core/internal/scanner/android/AndroidScanner.java

@@ -37,16 +37,18 @@
 /**
  * Class & resource scanner for Android.
  */
-public class AndroidScanner implements ResourceAndClassScanner {
+public class AndroidScanner<I> implements ResourceAndClassScanner<I> {
     private static final Log LOG = LogFactory.getLog(AndroidScanner.class);
 
     private final Context context;
 
+    private final Class<I> implementedInterface;
     private final ClassLoader clazzLoader;
     private final Charset encoding;
     private final Location location;
 
-    public AndroidScanner(ClassLoader clazzLoader, Charset encoding, Location location) {
+    public AndroidScanner(Class<I> implementedInterface, ClassLoader clazzLoader, Charset encoding, Location location) {
+        this.implementedInterface = implementedInterface;
         this.clazzLoader = clazzLoader;
         this.encoding = encoding;
         this.location = location;
@@ -74,10 +76,10 @@ public Collection<LoadableResource> scanForResources() {
     }
 
     @Override
-    public Collection<Class<?>> scanForClasses() {
+    public Collection<Class<? extends I>> scanForClasses() {
         String pkg = location.getPath().replace("/", ".");
 
-        List<Class<?>> classes = new ArrayList<>();
+        List<Class<? extends I>> classes = new ArrayList<>();
         String sourceDir = context.getApplicationInfo().sourceDir;
         DexFile dex = null;
         try {
@@ -86,7 +88,7 @@ public Collection<Class<?>> scanForClasses() {
             while (entries.hasMoreElements()) {
                 String className = entries.nextElement();
                 if (className.startsWith(pkg)) {
-                    Class<?> clazz = ClassUtils.loadClass(className, clazzLoader);
+                    Class<? extends I> clazz = ClassUtils.loadClass(implementedInterface, className, clazzLoader);
                     if (clazz != null) {
                         classes.add(clazz);
                     }

flyway-core/src/main/java/org/flywaydb/core/internal/scanner/classpath/ClassPathScanner.java

@@ -39,9 +39,10 @@
 /**
  * ClassPath scanner.
  */
-public class ClassPathScanner implements ResourceAndClassScanner {
+public class ClassPathScanner<I> implements ResourceAndClassScanner<I> {
     private static final Log LOG = LogFactory.getLog(ClassPathScanner.class);
 
+    private final Class<I> implementedInterface;
     /**
      * The ClassLoader for loading migrations on the classpath.
      */
@@ -70,7 +71,8 @@ public class ClassPathScanner implements ResourceAndClassScanner {
      *
      * @param classLoader The ClassLoader for loading migrations on the classpath.
      */
-    public ClassPathScanner(ClassLoader classLoader, Charset encoding, Location location) {
+    public ClassPathScanner(Class<I> implementedInterface, ClassLoader classLoader, Charset encoding, Location location) {
+        this.implementedInterface = implementedInterface;
         this.classLoader = classLoader;
         this.location = location;
 
@@ -87,14 +89,17 @@ public Collection<LoadableResource> scanForResources() {
     }
 
     @Override
-    public Collection<Class<?>> scanForClasses() {
+    public Collection<Class<? extends I>> scanForClasses() {
         LOG.debug("Scanning for classes at " + location);
 
-        List<Class<?>> classes = new ArrayList<>();
+        List<Class<? extends I>> classes = new ArrayList<>();
 
         for (LoadableResource resource : resources) {
             if (resource.getAbsolutePath().endsWith(".class")) {
-                Class<?> clazz = ClassUtils.loadClass(toClassName(resource.getAbsolutePath()), classLoader);
+                Class<? extends I> clazz = ClassUtils.loadClass(
+                        implementedInterface,
+                        toClassName(resource.getAbsolutePath()),
+                        classLoader);
                 if (clazz != null) {
                     classes.add(clazz);
                 }

flyway-core/src/main/java/org/flywaydb/core/internal/scanner/classpath/ResourceAndClassScanner.java

@@ -22,7 +22,7 @@
 /**
  * Scanner for both resources and classes.
  */
-public interface ResourceAndClassScanner {
+public interface ResourceAndClassScanner<I> {
     /**
      * Scans the classpath for resources under the configured location.
      *
@@ -31,10 +31,10 @@ public interface ResourceAndClassScanner {
     Collection<LoadableResource> scanForResources();
 
     /**
-     * Scans the classpath for concrete classes under the specified package implementing this interface.
+     * Scans the classpath for concrete classes under the specified package implementing the specified interface.
      * Non-instantiable abstract classes are filtered out.
      *
      * @return The non-abstract classes that were found.
      */
-    Collection<Class<?>> scanForClasses();
+    Collection<Class<? extends I>> scanForClasses();
 }

flyway-core/src/main/java/org/flywaydb/core/internal/util/ClassUtils.java

@@ -117,31 +117,44 @@ public static boolean isPresent(String className, ClassLoader classLoader) {
     /**
      * Loads the class with this name using the class loader.
      *
-     * @param className   The name of the class to load.
-     * @param classLoader The ClassLoader to use.
+     * @param implementedInterface The interface the class is expected to implement.
+     * @param className            The name of the class to load.
+     * @param classLoader          The ClassLoader to use.
      * @return the newly loaded class or {@code null} if it could not be loaded.
      */
-    public static Class<?> loadClass(String className, ClassLoader classLoader) {
+    public static <I> Class<? extends I> loadClass(Class<I> implementedInterface, String className, ClassLoader classLoader) {
         try {
             Class<?> clazz = classLoader.loadClass(className);
+
+            if (!implementedInterface.isAssignableFrom(clazz)) {
+                return null;
+            }
+
             if (Modifier.isAbstract(clazz.getModifiers()) || clazz.isEnum() || clazz.isAnonymousClass()) {
                 LOG.debug("Skipping non-instantiable class: " + className);
                 return null;
             }
 
             clazz.getDeclaredConstructor().newInstance();
             LOG.debug("Found class: " + className);
-            return clazz;
+            //noinspection unchecked
+            return (Class<? extends I>) clazz;
         } catch (Throwable e) {
             Throwable rootCause = ExceptionUtils.getRootCause(e);
-            LOG.debug("Skipping " + className + " (" + e.getClass().getSimpleName() + "): " + e.getMessage()
-                    + (rootCause == e ? "" :
-                    " caused by " + rootCause.getClass().getSimpleName() + ": " + rootCause.getMessage()
-                            + " at " + ExceptionUtils.getThrowLocation(rootCause)));
+            LOG.warn("Skipping " + className + ": " + formatThrowable(e) + (
+                    rootCause == e
+                            ? ""
+                            : " caused by " + formatThrowable(rootCause)
+                            + " at " + ExceptionUtils.getThrowLocation(rootCause)
+            ));
             return null;
         }
     }
 
+    private static String formatThrowable(Throwable e) {
+        return "(" + e.getClass().getSimpleName() + ": " + e.getMessage() + ")";
+    }
+
     /**
      * Retrieves the physical location on disk of this class.
      *