how to set the use --enable-preview compile and run flags from gradle?

Question

Looking to use records from Java 14 in a gradle build, but am getting:

thufir@dur:~/NetBeansProjects/FileWatcherHandler$ 
thufir@dur:~/NetBeansProjects/FileWatcherHandler$ gradle clean build

> Task :compileJava FAILED
/home/thufir/NetBeansProjects/FileWatcherHandler/src/main/java/net/bounceme/dur/files/FXOrder.java:3: error: records are a preview feature and are disabled by default.
public record FXOrder(int units) {}
       ^
  (use --enable-preview to enable records)
1 error

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':compileJava'.
> Compilation failed; see the compiler error output for details.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 641ms
2 actionable tasks: 1 executed, 1 up-to-date
thufir@dur:~/NetBeansProjects/FileWatcherHandler$            

compiling with javac looks okay:

thufir@dur:~/java$ 
thufir@dur:~/java$ ls
FXOrder.java
thufir@dur:~/java$ 
thufir@dur:~/java$ cat FXOrder.java 

public record FXOrder(int units) {}
thufir@dur:~/java$ 
thufir@dur:~/java$ javac --enable-preview -source 14 FXOrder.java 
Note: FXOrder.java uses preview language features.
Note: Recompile with -Xlint:preview for details.
thufir@dur:~/java$ 
thufir@dur:~/java$ ls
FXOrder.class  FXOrder.java
thufir@dur:~/java$ 

how to set those compile options in the following build file:

/*
 * This file was generated by the Gradle 'init' task.
 *
 * This generated file contains a sample Java project to get you started.
 * For more details take a look at the Java Quickstart chapter in the Gradle
 * User Manual available at https://docs.gradle.org/6.4.1/userguide/tutorial_java_projects.html
 */

plugins {
    // Apply the java plugin to add support for Java
    id 'java'

    // Apply the application plugin to add support for building a CLI application.
    id 'application'
}

repositories {
    // Use jcenter for resolving dependencies.
    // You can declare any Maven/Ivy/file repository here.
    jcenter()
}

dependencies {
    // This dependency is used by the application.
    implementation 'com.google.guava:guava:28.2-jre'

    // Use TestNG framework, also requires calling test.useTestNG() below
    testImplementation 'org.testng:testng:7.1.1'
}

application {
    // Define the main class for the application.
//    mainClassName = 'FileWatcherHandler.App'
    mainClassName = 'net.bounceme.dur.files.App'


}

test {
    // Use TestNG for unit tests
    useTestNG()
}

java version:

thufir@dur:~/java$ 
thufir@dur:~/java$ gradle --version

------------------------------------------------------------
Gradle 6.4.1
------------------------------------------------------------

Build time:   2020-05-15 19:43:40 UTC
Revision:     1a04183c502614b5c80e33d603074e0b4a2777c5

Kotlin:       1.3.71
Groovy:       2.5.10
Ant:          Apache Ant(TM) version 1.10.7 compiled on September 1 2019
JVM:          14.0.1 (AdoptOpenJDK 14.0.1+7)
OS:           Linux 5.4.0-29-generic amd64

thufir@dur:~/java$ 
thufir@dur:~/java$ java --version
openjdk 14.0.1 2020-04-14
OpenJDK Runtime Environment AdoptOpenJDK (build 14.0.1+7)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 14.0.1+7, mixed mode, sharing)
thufir@dur:~/java$ 
thufir@dur:~/java$ javac --version
javac 14.0.1
thufir@dur:~/java$ 
thufir@dur:~/java$ which java
/home/thufir/.sdkman/candidates/java/current/bin/java
thufir@dur:~/java$ 

Answer 1

To make this work you can modify the compileJava task and add this flag. Add this to your build.gradle :

compileJava {
    options.compilerArgs += ['--enable-preview']
}

This will make sure that your code will compile.

---

If you have other tasks which require compilation (for example compileTestJava) you can enable this flag for all tasks which have type JavaCompile :

tasks.withType(JavaCompile).all {
    options.compilerArgs += ['--enable-preview']
}

---

To enable this flag for test tasks you can do the following :

tasks.withType(Test).all {
    jvmArgs += '--enable-preview'
}

---

You also have to make sure to add this flag for the JVM that will run your code :

tasks.withType(JavaExec) {
    jvmArgs += '--enable-preview'
}

This is described in the corresponding JEP :

Developers who wish to use preview language features in their programs must explicitly enable them in the compiler and the runtime system