[Java][native] add spotless gradle/mvn plugin (#14641)

* add spotless plugin to java native client

* apply spotless

* Revert "apply spotless"

This reverts commit 485d50bbde.

samples/client/echo_api/java/native/build.gradle

@@ -1,5 +1,6 @@
 apply plugin: 'idea'
 apply plugin: 'eclipse'
+apply plugin: 'com.diffplug.spotless'
 
 group = 'org.openapitools'
 version = '0.1.0'
@@ -8,6 +9,9 @@ buildscript {
     repositories {
         mavenCentral()
     }
+    dependencies {
+        classpath 'com.diffplug.spotless:spotless-plugin-gradle:6.11.0'
+    }
 }
 
 repositories {
@@ -79,3 +83,26 @@ dependencies {
     implementation "org.apache.httpcomponents:httpmime:$httpmime_version"
     testImplementation "junit:junit:$junit_version"
 }
+
+// Use spotless plugin to automatically format code, remove unused import, etc
+// To apply changes directly to the file, run `gradlew spotlessApply`
+// Ref: https://github.com/diffplug/spotless/tree/main/plugin-gradle
+spotless {
+    // comment out below to run spotless as part of the `check` task
+    enforceCheck false
+    format 'misc', {
+        // define the files (e.g. '*.gradle', '*.md') to apply `misc` to
+        target '.gitignore'
+        // define the steps to apply to those files
+        trimTrailingWhitespace()
+        indentWithSpaces() // Takes an integer argument if you don't like 4
+        endWithNewline()
+    }
+    java {
+        // don't need to set target, it is inferred from java
+        // apply a specific flavor of google-java-format
+        googleJavaFormat('1.8').aosp().reflowLongStrings()
+        removeUnusedImports()
+        importOrder()
+    }
+}