Allow overriding Java toolchain version with a gradle property

Author: ilya-g

README.md

@@ -365,7 +365,8 @@ git submodule update
 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. -->
+
+> For local builds, you can use a later version of JDK if you don't have that 
+version installed. Specify the version of this JDK with the `java.mainToolchainVersion` Gradle property.
 
 After that, the project can be opened in IDEA and built with Gradle.

build.gradle.kts

@@ -14,6 +14,7 @@ infra {
     }
 }
 
+val mainJavaToolchainVersion by ext(project.property("java.mainToolchainVersion"))
 
 allprojects {
     repositories {

core/build.gradle.kts

@@ -17,12 +17,15 @@ base {
     archivesBaseName = "kotlinx-datetime" // doesn't work
 }
 
+val mainJavaToolchainVersion: String by project
 val serializationVersion: String by project
 
 java {
-    toolchain { languageVersion.set(JavaLanguageVersion.of(8)) }
+    toolchain { languageVersion.set(JavaLanguageVersion.of(mainJavaToolchainVersion)) }
+    with(javaToolchains.launcherFor(toolchain).get().metadata) {
+        logger.info("Using JDK $languageVersion toolchain installed in $installationPath")
+    }
 }
-logger.info("Using JDK 8 toolchain installed in ${javaToolchains.launcherFor(java.toolchain).get().metadata.installationPath}")
 
 kotlin {
     explicitApi()

gradle.properties

@@ -8,6 +8,8 @@ versionSuffix=SNAPSHOT
 kotlinVersion=1.5.30
 serializationVersion=1.3.0
 
+java.mainToolchainVersion=8
+
 kotlin.mpp.enableGranularSourceSetsMetadata=true
 kotlin.mpp.enableCompatibilityMetadataVariant=true
 kotlin.js.compiler=both

serialization/build.gradle.kts

@@ -5,10 +5,11 @@ plugins {
     kotlin("plugin.serialization")
 }
 
+val mainJavaToolchainVersion: String by project
 val serializationVersion: String by project
 
 java {
-    toolchain { languageVersion.set(JavaLanguageVersion.of(8)) }
+    toolchain { languageVersion.set(JavaLanguageVersion.of(mainJavaToolchainVersion)) }
 }
 
 kotlin {