Add Gradle toolchain configuration

Author: knutwalker

gradle/defaults-java.gradle

@@ -1,3 +1,34 @@
+ext.vendor = { ->
+    // By specifying the vendor, we force us (and CI) to
+    // use that specific JDK version. If it is not locally
+    // available, Gradle will provision the toolchain, i.e.
+    // download it.
+    if (project.hasProperty("javaLanguageVendor")) {
+        return JvmVendorSpec.matching(project.property('javaLanguageVendor'))
+    } else {
+        return JvmVendorSpec.matching("ADOPTIUM")
+    }
+}
+
+ext.languageVersion = { ->
+    if (project.hasProperty("javaLanguageVersion")) {
+        return JavaLanguageVersion.of(project.property('javaLanguageVersion'))
+    } else {
+        return JavaLanguageVersion.of(11)
+    }
+}
+
+allprojects {
+    plugins.withType(JavaPlugin) {
+        java {
+            toolchain {
+                vendor = vendor()
+                languageVersion = languageVersion()
+            }
+        }
+    }
+}
+
 allprojects {
     plugins.withType(JavaLibraryPlugin) {
         sourceCompatibility = JavaVersion.VERSION_11