Added ThrowingEx.unwrapAsRuntimeOrRethrow for unwrapping exceptions e…

…asily, and used this logic to enhance the error messages thrown by KtLintStep.

lib/src/main/java/com/diffplug/spotless/ThrowingEx.java

@@ -87,11 +87,42 @@ public static RuntimeException asRuntime(Exception e) {
 		}
 	}
 
+	/**
+	 * Utility method for rethrowing an exception's cause with as few wrappers as possible.
+	 *
+	 * ```java
+	 * try {
+	 *     doSomething();
+	 * } catch (Throwable e) {
+	 *     throw unwrapAsRuntimeOrRethrow(e);
+	 * }
+	 * ```
+	 */
+	public static RuntimeException unwrapAsRuntimeOrRethrow(Throwable e) {
+		Throwable cause = e.getCause();
+		if (cause == null) {
+			return ifErrorRethrowElseAsRuntime(e);
+		} else {
+			return ifErrorRethrowElseAsRuntime(cause);
+		}
+	}
+
+	/** Rethrows errors, wraps and returns everything else as a runtime exception. */
+	private static RuntimeException ifErrorRethrowElseAsRuntime(Throwable e) {
+		if (e instanceof Error) {
+			throw (Error) e;
+		} else if (e instanceof RuntimeException) {
+			return (RuntimeException) e;
+		} else {
+			return new WrappedAsRuntimeException(e);
+		}
+	}
+
 	/** A RuntimeException specifically for the purpose of wrapping non-runtime Exceptions as RuntimeExceptions. */
 	public static class WrappedAsRuntimeException extends RuntimeException {
 		private static final long serialVersionUID = -912202209702586994L;
 
-		public WrappedAsRuntimeException(Exception e) {
+		public WrappedAsRuntimeException(Throwable e) {
 			super(e);
 		}
 	}

lib/src/main/java/com/diffplug/spotless/kotlin/KtLintStep.java

@@ -17,6 +17,7 @@
 
 import java.io.IOException;
 import java.io.Serializable;
+import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.lang.reflect.Proxy;
 import java.util.Collections;
@@ -25,6 +26,7 @@
 import com.diffplug.spotless.FormatterStep;
 import com.diffplug.spotless.JarState;
 import com.diffplug.spotless.Provisioner;
+import com.diffplug.spotless.ThrowingEx;
 
 /** Wraps up [ktlint](https://github.com/shyiko/ktlint) as a FormatterStep. */
 public class KtLintStep {
@@ -91,8 +93,12 @@ FormatterFunc createFormat() throws Exception {
 			Method formatterMethod = ktlintClass.getMethod("format", String.class, Iterable.class, function2Interface);
 
 			return input -> {
-				String formatted = (String) formatterMethod.invoke(ktlint, input, ruleSets, formatterCallback);
-				return formatted;
+				try {
+					String formatted = (String) formatterMethod.invoke(ktlint, input, ruleSets, formatterCallback);
+					return formatted;
+				} catch (InvocationTargetException e) {
+					throw ThrowingEx.unwrapAsRuntimeOrRethrow(e);
+				}
 			};
 		}
 	}