Implement VersionCollection in Java

build.gradle

@@ -103,10 +103,6 @@ subprojects {
  * in a branch if there are only betas and rcs in the branch so we have
  * *something* to test against. */
 VersionCollection versions = new VersionCollection(file('server/src/main/java/org/elasticsearch/Version.java').readLines('UTF-8'))
-if (versions.currentVersion != VersionProperties.elasticsearch) {
-  throw new GradleException("The last version in Versions.java [${versions.currentVersion}] does not match " +
-          "VersionProperties.elasticsearch [${VersionProperties.elasticsearch}]")
-}
 
 // build metadata from previous build, contains eg hashes for bwc builds
 String buildMetadataValue = System.getenv('BUILD_METADATA')
@@ -140,26 +136,16 @@ task verifyVersions {
     if (gradle.startParameter.isOffline()) {
       throw new GradleException("Must run in online mode to verify versions")
     }
-    // Read the list from maven central
-    Node xml
+    // Read the list from maven central. 
+    // Fetch the metadata an parse the xml into Version instances because it's more straight forward here 
+    // rather than bwcVersion ( VersionCollection ).
     new URL('https://repo1.maven.org/maven2/org/elasticsearch/elasticsearch/maven-metadata.xml').openStream().withStream { s ->
-        xml = new XmlParser().parse(s)
-    }
-    Set<Version> knownVersions = new TreeSet<>(xml.versioning.versions.version.collect { it.text() }.findAll { it ==~ /\d+\.\d+\.\d+/ }.collect { Version.fromString(it) })
-
-    // Limit the known versions to those that should be index compatible, and are not future versions
-    knownVersions = knownVersions.findAll { it.major >= bwcVersions.currentVersion.major - 1 && it.before(VersionProperties.elasticsearch) }
-
-    /* Limit the listed versions to those that have been marked as released.
-     * Versions not marked as released don't get the same testing and we want
-     * to make sure that we flip all unreleased versions to released as soon
-     * as possible after release. */
-    Set<Version> actualVersions = new TreeSet<>(bwcVersions.indexCompatible.findAll { false == it.snapshot })
-
-    // Finally, compare!
-    if (knownVersions.equals(actualVersions) == false) {
-      throw new GradleException("out-of-date released versions\nActual  :" + actualVersions + "\nExpected:" + knownVersions +
-        "\nUpdate Version.java. Note that Version.CURRENT doesn't count because it is not released.")
+      bwcVersions.compareToAuthoritative(
+              new XmlParser().parse(s)
+                      .versioning.versions.version
+                      .collect { it.text() }.findAll { it ==~ /\d+\.\d+\.\d+/ }
+                      .collect { Version.fromString(it) }
+      )
     }
   }
 }
@@ -251,20 +237,17 @@ subprojects {
     "org.elasticsearch.plugin:percolator-client:${version}": ':modules:percolator',
     "org.elasticsearch.plugin:rank-eval-client:${version}": ':modules:rank-eval',
   ]
-
-  bwcVersions.snapshotProjectNames.each { snapshotName ->
-    Version snapshot = bwcVersions.getSnapshotForProject(snapshotName)
-    if (snapshot != null ) {
-      String snapshotProject = ":distribution:bwc:${snapshotName}"
-      project(snapshotProject).ext.bwcVersion = snapshot
-      ext.projectSubstitutions["org.elasticsearch.distribution.deb:elasticsearch:${snapshot}"] = snapshotProject
-      ext.projectSubstitutions["org.elasticsearch.distribution.rpm:elasticsearch:${snapshot}"] = snapshotProject
-      ext.projectSubstitutions["org.elasticsearch.distribution.zip:elasticsearch:${snapshot}"] = snapshotProject
-      if (snapshot.onOrAfter('6.3.0')) {
-        ext.projectSubstitutions["org.elasticsearch.distribution.deb:elasticsearch-oss:${snapshot}"] = snapshotProject
-        ext.projectSubstitutions["org.elasticsearch.distribution.rpm:elasticsearch-oss:${snapshot}"] = snapshotProject
-        ext.projectSubstitutions["org.elasticsearch.distribution.zip:elasticsearch-oss:${snapshot}"] = snapshotProject
-      }
+  // substitute unreleased versions with projects that check out and build locally
+  bwcVersions.forPreviousUnreleased { VersionCollection.UnreleasedVersionInfo unreleasedVersion ->
+    Version unreleased = unreleasedVersion.version
+    String snapshotProject = ":distribution:bwc:${unreleasedVersion.gradleProjectName}"
+    ext.projectSubstitutions["org.elasticsearch.distribution.deb:elasticsearch:${unreleased}"] = snapshotProject
+    ext.projectSubstitutions["org.elasticsearch.distribution.rpm:elasticsearch:${unreleased}"] = snapshotProject
+    ext.projectSubstitutions["org.elasticsearch.distribution.zip:elasticsearch:${unreleased}"] = snapshotProject
+    if (unreleased.onOrAfter('6.3.0')) {
+      ext.projectSubstitutions["org.elasticsearch.distribution.deb:elasticsearch-oss:${unreleased}"] = snapshotProject
+      ext.projectSubstitutions["org.elasticsearch.distribution.rpm:elasticsearch-oss:${unreleased}"] = snapshotProject
+      ext.projectSubstitutions["org.elasticsearch.distribution.zip:elasticsearch-oss:${unreleased}"] = snapshotProject
     }
   }