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.

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

@@ -0,0 +1,28 @@
+/*
+ * 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;
+
+public class JsonDefaults {
+	private static final String INCLUDES = "src/**/*.json";
+
+	private JsonDefaults() {
+		// utility class, should not be instantiated
+	}
+
+	public static String includes() {
+		return INCLUDES;
+	}
+}

plugin-gradle/README.md

@@ -689,6 +689,26 @@ See the [javadoc](https://javadoc.io/static/com.diffplug.spotless/spotless-plugi
 
 <a name="retroactively-populating-year-range-from-git-history"></a>
 
+## JSON Files
+
+JSON files can be formatted for you, using the `json` formatter:
+
+```gradle
+spotless {
+  json {} // defaults to `src/**/*.json`
+}
+```
+
+Or if you'd like custom paths:
+
+```gradle
+spotless {
+  json {
+    target 'src/test/**/*.json
+  }
+}
+```
+
 ### Retroactively slurp years from git history
 
 If your project has not been rigorous with copyright headers, and you'd like to use git history to repair this retroactively, you can do so with `-PspotlessSetLicenseHeaderYearsFromGitHistory=true`.  When run in this mode, Spotless will do an expensive search through git history for each file, and set the copyright header based on the oldest and youngest commits for that file.  This is intended to be a one-off sort of thing.

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

@@ -0,0 +1,41 @@
+/*
+ * 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 java.util.*;
+
+import javax.inject.Inject;
+
+import com.diffplug.spotless.json.JsonDefaults;
+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) {
+			target = parseTarget(JsonDefaults.includes());
+		}
+		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 +1,60 @@
+/*
+ * 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 defaultsToSrcJsonFiles() throws IOException {
+		setFile("build.gradle").toLines(
+				"buildscript { repositories { mavenCentral() } }",
+				"plugins {",
+				"    id 'java'",
+				"    id 'com.diffplug.spotless'",
+				"}",
+				"spotless {",
+				"    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/nestedObjectAfter.json");
+		assertFile("examples/main/resources/example.json").sameAsResource("json/nestedObjectBefore.json");
+	}
+
+	@Test
+	public void canOverrideTarget() 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");
+	}
+}