Setup JDK 8 provisioning with java toolchains

Remove commented out JVM 6 target.

Author: ilya-g

README.md

@@ -362,8 +362,10 @@ git submodule init
 git submodule update
 ```
 
-The path to JDK 8 must be specified either with the environment variable `JDK_8` or 
-with the gradle property `JDK_8`. For local builds, you can use a later version of JDK if you don't have that 
-version installed.
+The project requires JDK 8 to build classes and to run tests. 
+Gradle will try to find it among the installed JDKs or [provision](https://docs.gradle.org/current/userguide/toolchains.html#sec:provisioning) it automatically if it couldn't be found.
+The path to JDK 8 can be additionally specified with the environment variable `JDK_8`. 
+<!-- For local builds, you can use a later version of JDK if you don't have that 
+version installed. -->
 
 After that, the project can be opened in IDEA and built with Gradle.

build.gradle.kts

@@ -14,15 +14,6 @@ infra {
     }
 }
 
-fun jdkPath(version: Int): String {
-    fun envOrProperty(name: String): String? = System.getenv(name) ?: findProperty(name) as String?
-
-    return envOrProperty("JDK_$version") ?:
-            version.takeIf { it < 9 }?.let { envOrProperty("JDK_1$version") } ?:
-            error("Specify path to JDK $version in JDK_$version environment variable or Gradle property")
-}
-//val JDK_6 by ext(jdkPath(6))
-val JDK_8 by ext(jdkPath(8))
 
 allprojects {
     repositories {

core/build.gradle.kts

@@ -17,10 +17,13 @@ base {
     archivesBaseName = "kotlinx-datetime" // doesn't work
 }
 
-//val JDK_6: String by project
-val JDK_8: String by project
 val serializationVersion: String by project
 
+java {
+    toolchain { languageVersion.set(JavaLanguageVersion.of(8)) }
+}
+logger.info("Using JDK 8 toolchain installed in ${javaToolchains.launcherFor(java.toolchain).get().metadata.installationPath}")
+
 kotlin {
     explicitApi()
 
@@ -51,29 +54,11 @@ kotlin {
             attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 8)
         }
         compilations.all {
-            kotlinOptions {
-                jvmTarget = "1.8"
-                jdkHome = JDK_8
-            }
+            // Set compilation options for JVM target here
         }
 
     }
 
-    /*
-    jvm("jvm6") {
-        this.withJava()
-        attributes {
-            attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 6)
-        }
-        compilations.all {
-            kotlinOptions {
-                jvmTarget = "1.6"
-                jdkHome = JDK_6
-            }
-        }
-    }
-     */
-
     js {
         nodejs {
 //            testTask { }
@@ -168,26 +153,12 @@ kotlin {
             }
         }
 
-        /*
-        val jvm6Main by getting {
-            dependencies {
-                api("org.jetbrains.kotlin:kotlin-stdlib")
-                api("org.threeten:threetenbp:1.4.0")
-
-            }
-        }
-        val jvm6Test by getting {
-            dependencies {
-                api("org.jetbrains.kotlin:kotlin-test-junit")
-            }
-        }
-        */
-
         val jvmMain by getting {
             dependencies {
                 api("org.jetbrains.kotlin:kotlin-stdlib")
             }
         }
+
         val jvmTest by getting {
             dependencies {
                 api("org.jetbrains.kotlin:kotlin-test-junit")
@@ -230,7 +201,6 @@ kotlin {
 tasks {
     named("jvmTest", Test::class) {
         // maxHeapSize = "1024m"
-//        executable = "$JDK_6/bin/java"
     }
 }
 

gradle.properties

@@ -1,4 +1,5 @@
 org.gradle.jvmargs=-Xmx1G -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8
+org.gradle.java.installations.fromEnv=JDK_8
 
 group=org.jetbrains.kotlinx
 version=0.3.1

serialization/build.gradle.kts

@@ -5,9 +5,12 @@ plugins {
     kotlin("plugin.serialization")
 }
 
-val JDK_8: String by project
 val serializationVersion: String by project
 
+java {
+    toolchain { languageVersion.set(JavaLanguageVersion.of(8)) }
+}
+
 kotlin {
     infra {
         target("linuxX64")
@@ -32,13 +35,6 @@ kotlin {
         attributes {
             attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 8)
         }
-        compilations.all {
-            kotlinOptions {
-                jvmTarget = "1.8"
-                jdkHome = JDK_8
-            }
-        }
-
     }
 
     js {