Add Gradle bindings for JSON formatter

As we've created a JVM JSON formatter as part of diffplug#850, we should make it
possible to use it natively in Gradle, which requires we add it as a new
supported type in `SpotlessExtension`.

When configured, we'll default to including any JSON files under the
`src` directory, while allowing it to be overriden if requested.

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/JsonExtension.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2016-2021 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.gradle.spotless;
+
+import javax.inject.Inject;
+
+import com.diffplug.spotless.json.JsonFormatterStep;
+
+public class JsonExtension extends FormatExtension {
+	static final String NAME = "json";
+
+	@Inject
+	public JsonExtension(SpotlessExtension spotless) {
+		super(spotless);
+		addStep(JsonFormatterStep.create(provisioner()));
+	}
+
+	@Override
+	protected void setupTask(SpotlessTask task) {
+		if (target == null) {
+			throw noDefaultTargetException();
+		}
+		super.setupTask(task);
+	}
+}

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessExtension.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2020 DiffPlug
+ * Copyright 2016-2021 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -169,6 +169,12 @@ public void python(Action<PythonExtension> closure) {
 		format(PythonExtension.NAME, PythonExtension.class, closure);
 	}
 
+	/** Configures the special JSON-specific extension. */
+	public void json(Action<JsonExtension> closure) {
+		requireNonNull(closure);
+		format(JsonExtension.NAME, JsonExtension.class, closure);
+	}
+
 	/** Configures a custom extension. */
 	public void format(String name, Action<FormatExtension> closure) {
 		requireNonNull(name, "name");

plugin-gradle/src/test/java/com/diffplug/gradle/spotless/JsonExtensionTest.java

@@ -0,0 +1,42 @@
+/*
+ * 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.gradle.spotless;
+
+import java.io.IOException;
+
+import org.junit.Test;
+
+public class JsonExtensionTest extends GradleIntegrationHarness {
+	@Test
+	public void formatsWhenTargetsAreSpecified() throws IOException {
+		setFile("build.gradle").toLines(
+				"buildscript { repositories { mavenCentral() } }",
+				"plugins {",
+				"    id 'java'",
+				"    id 'com.diffplug.spotless'",
+				"}",
+				"spotless {",
+				"    json {" +
+						"    target 'examples/**/*.json'" +
+						"}",
+				"}");
+		setFile("src/main/resources/example.json").toResource("json/nestedObjectBefore.json");
+		setFile("examples/main/resources/example.json").toResource("json/nestedObjectBefore.json");
+		gradleRunner().withArguments("spotlessApply").build();
+		assertFile("src/main/resources/example.json").sameAsResource("json/nestedObjectBefore.json");
+		assertFile("examples/main/resources/example.json").sameAsResource("json/nestedObjectAfter.json");
+	}
+}