Add JVM-based JSON formatter

Currenly, if consumers of Spotless want to format their JSON files, they
need to do this by adding `prettier`, which increases the dependencies a
project needs.

To simplify things for consumers, as documented in diffplug#850, we can provide
a JVM-based JSON formatter, using `org.json`'s JSON formatting.

To follow how we've done this with other formatters, we can retrieve
`org.json`'s JAR at runtime, and retrieve the classes via Reflection,
instead of adding this as a `lib-extra` project, with an explicit
dependency.

To validate this fully, we want a few straightforward JSON files to
validate before/after, but we can also use [a sample Cucumber JSON
report] as a much more complex example of what happens, which we've
removed any `data` objects from its source, so the files are smaller.

We've added an `equality` test, but it doesn't really test too much
right now as there's nothing stored in the state.

[a sample Cucumber JSON report]: https://github.com/damianszczepanik/cucumber-reporting/raw/master/src/test/resources/json/sample.json

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

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2021 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.spotless.json;
+
+import java.io.IOException;
+import java.io.Serializable;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Method;
+import java.util.Objects;
+
+import com.diffplug.spotless.FormatterFunc;
+import com.diffplug.spotless.FormatterStep;
+import com.diffplug.spotless.JarState;
+import com.diffplug.spotless.Provisioner;
+
+public final class JsonFormatterStep {
+	private static final String MAVEN_COORDINATE = "org.json:json:";
+	private static final String DEFAULT_VERSION = "20210307";
+	private static final int INDENT = 4;
+
+	public static FormatterStep create(Provisioner provisioner) {
+		Objects.requireNonNull(provisioner, "provisioner cannot be null");
+		return FormatterStep.createLazy("json", () -> new State(provisioner), State::toFormatter);
+	}
+
+	private static final class State implements Serializable {
+		private static final long serialVersionUID = 1L;
+
+		private final JarState jarState;
+
+		private State(Provisioner provisioner) throws IOException {
+			this.jarState = JarState.from(MAVEN_COORDINATE + DEFAULT_VERSION, provisioner);
+		}
+
+		FormatterFunc toFormatter() {
+			return s -> {
+				ClassLoader classLoader = jarState.getClassLoader();
+				String prettyPrinted = null;
+				if (s.isEmpty()) {
+					prettyPrinted = s;
+				}
+				if (s.startsWith("{")) {
+					Class<?> jsonObject = classLoader.loadClass("org.json.JSONObject");
+					Class<?>[] type = new Class[]{String.class};
+					Constructor<?> constructor = jsonObject.getConstructor(type);
+					Method toString = jsonObject.getMethod("toString", int.class);
+					try {
+						Object parsed = constructor.newInstance(s);
+						prettyPrinted = toString.invoke(parsed, INDENT) + "\n";
+					} catch (Exception e) {
+						// ignore if we cannot convert to JSON string
+					}
+				}
+				if (s.startsWith("[")) {
+					Class<?> jsonArray = classLoader.loadClass("org.json.JSONArray");
+					Class<?>[] type = new Class[]{String.class};
+					Constructor<?> constructor = jsonArray.getConstructor(type);
+					Method toString = jsonArray.getMethod("toString", int.class);
+					try {
+						Object parsed = constructor.newInstance(s);
+						prettyPrinted = toString.invoke(parsed, INDENT) + "\n";
+					} catch (Exception e) {
+						// ignore if we cannot convert to JSON string
+					}
+				}
+
+				if (prettyPrinted == null) {
+					throw new AssertionError("Invalid JSON file provided");
+				}
+
+				return prettyPrinted;
+			};
+		}
+	}
+
+	private JsonFormatterStep() {
+		// cannot be directly instantiated
+	}
+}

lib/src/main/java/com/diffplug/spotless/json/package-info.java

@@ -0,0 +1,7 @@
+@ParametersAreNonnullByDefault
+@ReturnValuesAreNonnullByDefault
+package com.diffplug.spotless.extra.json.java;
+
+import javax.annotation.ParametersAreNonnullByDefault;
+
+import com.diffplug.spotless.annotations.ReturnValuesAreNonnullByDefault;