chore(deps): downgrade to Java 17

Java 19 is out of support, but Gradle current does not run on Java 20:
https://github.com/gradle/gradle/issues/23488

To avoid the complexity of running the build tool on a different JDK than the
application, we temporarily remove code that depends on experimental Java 19 or
Java 20 features.

Revert once the build can run on Java 20.

4 files changed, 7 insertions, 30 deletions

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index eecb62c1..6138fad1 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -27,10 +27,10 @@ jobs:
         uses: actions/checkout@v3
         with:
           fetch-depth: ${{ !steps.check-secret.outputs.is_SONAR_TOKEN_set && 1 || 0 }}  # Shallow clones should be disabled for a better relevancy of SonarCloud analysis
-      - name: Set up JDK 19
+      - name: Set up JDK 17
         uses: actions/setup-java@v3
         with:
-          java-version: 19
+          java-version: 17
           distribution: temurin
       - name: Cache Gradle packages
         uses: actions/cache@v3
diff --git a/README.md b/README.md
index 4dbc3bf5..f6e36e74 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@
 
 ### With Eclipse IDE
 
-1. Download and install a _Java 19_ compatible JDK. For Windows, prefer OpenJDK builds from [Adoptium](https://adoptium.net/).
+1. Download and install a _Java 17_ compatible JDK. For Windows, prefer OpenJDK builds from [Adoptium](https://adoptium.net/).
 
 2. Download and extract the [Eclipse IDE for Java and DSL Developers 2022-09](https://www.eclipse.org/downloads/packages/release/2022-09/r/eclipse-ide-java-and-dsl-developers) package.
 
@@ -23,16 +23,15 @@
 
 5. Open _Help > Eclipse Marketplace_ and install the following software:
     * _EclEmma Java Code Coverage_
-    * _Java 19 Support for Eclipse 2022-09 (4.25)_
     * _SonarLint_
 
 6. Open _Window > Preferences_ and set the following preferences:
     * _General > Workspace > Text file encoding_ should be _UTF-8_.
     * _General > Workspace > New text file line delimiter_ should be _Unix_.
-    * Add the JDK 19 to _Java > Installed JREs_.
-    * Make sure JDK 19 is selected for _JavaSE-19_ at _Java > Installed JREs > Execution Environments_.
+    * Add the JDK 17 to _Java > Installed JREs_.
+    * Make sure JDK 17 is selected for _JavaSE-17_ at _Java > Installed JREs > Execution Environments_.
     * Set _Gradle > Java home_ to the `JAVA_HOME` directory (the directory which contains the `bin` directory) of JDK 17. Here, Buildship will show a yellow warning sign, which can be safely ignored.
-    * Set _Java > Compiler > JDK Compliance > Compiler compliance level_ to _19_. The warning about using Java 16 system libraries during compilation should disappear.
+    * Set _Java > Compiler > JDK Compliance > Compiler compliance level_ to _17_.
 
 7. Clone the project Git repository but do not import it into Eclipse yet.
 
diff --git a/buildSrc/src/main/kotlin/tools/refinery/gradle/internal/java-conventions.gradle.kts b/buildSrc/src/main/kotlin/tools/refinery/gradle/internal/java-conventions.gradle.kts
index d8deffae..7c7cbabd 100644
--- a/buildSrc/src/main/kotlin/tools/refinery/gradle/internal/java-conventions.gradle.kts
+++ b/buildSrc/src/main/kotlin/tools/refinery/gradle/internal/java-conventions.gradle.kts
@@ -43,14 +43,10 @@ dependencies {
 }
 
 java.toolchain {
-        languageVersion.set(JavaLanguageVersion.of(19))
+        languageVersion.set(JavaLanguageVersion.of(17))
 }
 
 tasks {
-        withType(JavaCompile::class) {
-                options.release.set(17)
-        }
-
         test {
                 useJUnitPlatform {
                         excludeTags("slow")
diff --git a/subprojects/language-web/build.gradle.kts b/subprojects/language-web/build.gradle.kts
index e1d250ec..d398e002 100644
--- a/subprojects/language-web/build.gradle.kts
+++ b/subprojects/language-web/build.gradle.kts
@@ -29,13 +29,6 @@ dependencies {
 
 application {
         mainClass.set("tools.refinery.language.web.ServerLauncher")
-        // Enable JDK 19 preview features for virtual thread support.
-        applicationDefaultJvmArgs += "--enable-preview"
-}
-
-// Enable JDK 19 preview features for virtual thread support.
-fun enablePreview(task: JavaForkOptions) {
-        task.jvmArgs("--enable-preview")
 }
 
 tasks {
@@ -47,16 +40,6 @@ tasks {
                 }
         }
 
-        withType(JavaCompile::class) {
-                options.release.set(19)
-                // Enable JDK 19 preview features for virtual thread support.
-                options.compilerArgs.plusAssign("--enable-preview")
-        }
-
-        withType(Test::class) {
-                enablePreview(this)
-        }
-
         jar {
                 dependsOn(webapp)
                 from(webapp) {
@@ -82,7 +65,6 @@ tasks {
                 dependsOn(mainRuntimeClasspath)
                 classpath(mainRuntimeClasspath)
                 mainClass.set(application.mainClass)
-                enablePreview(this)
                 standardInput = System.`in`
                 val baseResource = webapp.incoming.artifacts.artifactFiles.first()
                 environment("BASE_RESOURCE", baseResource)