Standardize java source headers (#7205)

* Create and validate standardized java source header

Update Checkstyle to validate all java source files have a standard
header. Remove FixLiscenseHeaders.java unit test as it duplicates
Checkstyle functionality. Standard header is block java comment, instead
of dangling Javadoc comment. Create standard header template
(HEADER_JAVA) in project root. Reference header template in
build.gradle. Update CONTRIBUTING.md to exact text of header template.

* Convert Licenses from dangling Javadoc to block comment

Standardize License based off of HEADER_JAVA file. All headers are block
Java comments.

* Restoring FixLicenseHeaders.java and updating CONTRIBUTING date

* Updated License dates to 2016

Several files had 2017 dates for License. Checkstyle now specifically
looks for 2016 date in License comments.

* Fix bad logic for header check in FixLicenseHeaders test

Author: benjamintboyle

CONTRIBUTING.md

@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) 2016-present, RxJava Contributors.
+ *
+ * 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.
+ */

HEADER_JAVA

@@ -15,7 +15,7 @@ buildscript {
   ext.licenseVersion = "0.15.0"
   ext.jfrogExtractorVersion = "4.21.0"
   ext.bndVersion = "5.3.0"
-  ext.checkstyleVersion = "8.26"
+  ext.checkstyleVersion = "8.41"
   ext.vanniktechPublishPlugin = "0.14.2"
 
   // --------------------------------------
@@ -235,9 +235,11 @@ jacocoTestReport.dependsOn GCandMem
 build.dependsOn jacocoTestReport
 
 checkstyle {
-    configFile file("checkstyle.xml")
-    ignoreFailures = true
+    configFile = file("checkstyle.xml")
     toolVersion = checkstyleVersion
+    configProperties = [
+            "checkstyle.header.file": file("HEADER_JAVA")
+    ]
 }
 
 apply from: file("gradle/javadoc_cleanup.gradle")

build.gradle

@@ -1,43 +1,22 @@
 <?xml version="1.0"?>
 <!DOCTYPE module PUBLIC
-    "-//Puppy Crawl//DTD Check Configuration 1.2//EN"
-    "http://www.puppycrawl.com/dtds/configuration_1_2.dtd">
+        "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+        "https://checkstyle.org/dtds/configuration_1_3.dtd">
+
 <module name="Checker">
-<property name="severity" value="warning"/>
-  <module name="TreeWalker">
-    <module name="RegexpSinglelineJava">
-      <property name="severity" value="warning"/>
-      <property name="format" value="^(?!\s+\* $).*?\s+$"/>
-      <property name="message" value="Line has trailing spaces."/>
+
+    <!-- Headers -->
+    <module name="Header">
+        <property name="headerFile" value="${checkstyle.header.file}"/>
+        <property name="fileExtensions" value="java"/>
     </module>
-    <!-- 
-    <module name="ExplicitInitialization"/>
-    <module name="SummaryJavadoc"/>
-    <module name="ModifierOrder"/>
-    <module name="RedundantModifier"/>
-    <module name="NeedBraces"/>
-    <module name="EqualsHashCode"/>
-    <module name="FallThrough"/>
-    <module name="MultipleVariableDeclarations">
-      <property name="severity" value="ignore"/>
-      <metadata name="net.sf.eclipsecs.core.lastEnabledSeverity" value="inherit"/>
+
+    <module name="TreeWalker">
+        <module name="RegexpSinglelineJava">
+            <property name="severity" value="warning"/>
+            <property name="format" value="^(?!\s+\* $).*?\s+$"/>
+            <property name="message" value="Line has trailing spaces."/>
+        </module>
     </module>
-    <module name="PackageDeclaration"/>
-    <module name="FinalClass"/>
-    <module name="HideUtilityClassConstructor"/>
-    <module name="WhitespaceAround">
-      <property name="tokens" value="ASSIGN,BAND,BAND_ASSIGN,BOR,BOR_ASSIGN,BSR,BSR_ASSIGN,BXOR,BXOR_ASSIGN,COLON,DIV,DIV_ASSIGN,DO_WHILE,EQUAL,GE,GT,LAMBDA,LAND,LCURLY,LE,LITERAL_ASSERT,LITERAL_CATCH,LITERAL_DO,LITERAL_ELSE,LITERAL_FINALLY,LITERAL_FOR,LITERAL_IF,LITERAL_RETURN,LITERAL_SYNCHRONIZED,LITERAL_TRY,LITERAL_WHILE,LOR,LT,MINUS,MINUS_ASSIGN,MOD,MOD_ASSIGN,NOT_EQUAL,PLUS,PLUS_ASSIGN,QUESTION,SL,SLIST,SL_ASSIGN,SR,SR_ASSIGN,STAR,STAR_ASSIGN,TYPE_EXTENSION_AND"/>
-    </module>
-     -->
-  </module>
-  <!-- 
-  <module name="JavadocPackage">
-    <property name="severity" value="ignore"/>
-    <metadata name="net.sf.eclipsecs.core.lastEnabledSeverity" value="inherit"/>
-  </module>
-  <module name="FileTabCharacter">
-    <property name="eachLine" value="true"/>
-    <property name="fileExtensions" value=".java"/>
-  </module>
-   -->
+
 </module>

checkstyle.xml

@@ -1,4 +1,4 @@
-/**
+/*
  * Copyright (c) 2016-present, RxJava Contributors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in

src/jmh/java/io/reactivex/rxjava3/core/BinaryFlatMapPerf.java

@@ -1,4 +1,4 @@
-/**
+/*
  * Copyright (c) 2016-present, RxJava Contributors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in

src/jmh/java/io/reactivex/rxjava3/core/BlockingGetPerf.java

@@ -1,4 +1,4 @@
-/**
+/*
  * Copyright (c) 2016-present, RxJava Contributors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in

src/jmh/java/io/reactivex/rxjava3/core/BlockingPerf.java

@@ -1,4 +1,4 @@
-/**
+/*
  * Copyright (c) 2016-present, RxJava Contributors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in

src/jmh/java/io/reactivex/rxjava3/core/CallableAsyncPerf.java

@@ -1,4 +1,4 @@
-/**
+/*
  * Copyright (c) 2016-present, RxJava Contributors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in

src/jmh/java/io/reactivex/rxjava3/core/EachTypeFlatMapPerf.java

@@ -1,4 +1,4 @@
-/**
+/*
  * Copyright (c) 2016-present, RxJava Contributors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in