Improve JsonSimple exception messages

CHANGES.md

@@ -10,6 +10,8 @@ This document is intended for Spotless developers.
 We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (starting after version `1.27.0`).
 
 ## [Unreleased]
+### Changed
+* Improved exception messages for [JSON formatting](https://github.com/diffplug/spotless/pull/885) failures
 
 ## [2.15.0] - 2021-06-17
 

lib/src/main/java/com/diffplug/spotless/json/JsonSimpleStep.java

@@ -70,34 +70,29 @@ FormatterFunc toFormatter() {
 			}
 
 			return s -> {
-				String prettyPrinted = null;
 				if (s.isEmpty()) {
-					prettyPrinted = s;
+					return s;
 				}
-				if (s.startsWith("{")) {
-					try {
-						Object parsed = objectConstructor.newInstance(s);
-						prettyPrinted = objectToString.invoke(parsed, indentSpaces) + "\n";
-					} catch (InvocationTargetException ignored) {
-						// ignore if we cannot convert to JSON string
-					}
+				char first = s.charAt(0);
+				if (first == '{') {
+					return format(objectConstructor, objectToString, s);
 				}
-				if (s.startsWith("[")) {
-					try {
-						Object parsed = arrayConstructor.newInstance(s);
-						prettyPrinted = arrayToString.invoke(parsed, indentSpaces) + "\n";
-					} catch (InvocationTargetException ignored) {
-						// ignore if we cannot convert to JSON string
-					}
+				if (first == '[') {
+					return format(arrayConstructor, arrayToString, s);
 				}
 
-				if (prettyPrinted == null) {
-					throw new AssertionError("Invalid JSON file provided");
-				}
-
-				return prettyPrinted;
+				throw new AssertionError(String.format("Unable to determine JSON type, expected a '{' or '[' but found '%s'", first));
 			};
 		}
+
+		private String format(Constructor<?> constructor, Method toString, String input) throws Exception {
+			try {
+				Object parsed = constructor.newInstance(input);
+				return toString.invoke(parsed, indentSpaces) + "\n";
+			} catch (InvocationTargetException ex) {
+				throw new AssertionError("Unable to format JSON", ex.getCause());
+			}
+		}
 	}
 
 	private JsonSimpleStep() {

plugin-gradle/CHANGES.md

@@ -3,6 +3,8 @@
 We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (starting after version `3.27.0`).
 
 ## [Unreleased]
+### Changed
+* Improved exception messages for [JSON formatting](https://github.com/diffplug/spotless/pull/885) failures
 
 ## [5.14.0] - 2021-06-17
 

testlib/src/test/java/com/diffplug/spotless/json/JsonSimpleStepTest.java

@@ -73,12 +73,18 @@ public void handlesObjectWithNull() throws Exception {
 
 	@Test
 	public void handlesInvalidJson() {
-		assertThatThrownBy(() -> doWithResource(stepHarness, "invalidJson")).isInstanceOf(AssertionError.class).hasMessage("Invalid JSON file provided");
+		assertThatThrownBy(() -> doWithResource(stepHarness, "invalidJson"))
+				.isInstanceOf(AssertionError.class)
+				.hasMessage("Unable to format JSON")
+				.hasRootCauseMessage("Expected a ',' or '}' at 9 [character 0 line 3]");
 	}
 
 	@Test
 	public void handlesNotJson() {
-		assertThatThrownBy(() -> doWithResource(stepHarness, "notJson")).isInstanceOf(AssertionError.class).hasMessage("Invalid JSON file provided");
+		assertThatThrownBy(() -> doWithResource(stepHarness, "notJson"))
+				.isInstanceOf(AssertionError.class)
+				.hasMessage("Unable to determine JSON type, expected a '{' or '[' but found '#'")
+				.hasNoCause();
 	}
 
 	@Test