Problems passing system properties and parameters when running Java class via Gradle

Figured it out. The main issue is that when Gradle forks a new Java process, it does not automatically pass the environment variable values along to the new environment. One has to explicitly pass these variables via the systemProperties property of the task or plugin.

The other issue was understanding how to pass command-line args; these are via the args property on the task or plugin. As with the Maven exec-maven-plugin, they should be passed in on the command line via yet another system property, as a space-delimited list that then needs to be split() before setting args, which accepts List objects. I've named the property exec.args, which is the old Maven name.

It seems both the javaExec and application plugin approach are valid. One might favor the application plugin approach if one wants to use some of its other features (automatically putting together a distribution, etc.)

Here are the solutions:

JavaExec Approach

Command Line:

gradle execute -Dmyvariable=myvalue -Dexec.args="arg1 arg2 arg3"

build.gradle:

task execute (type:JavaExec) {

    main = "com.myCompany.MyMain"
    classpath = sourceSets.main.runtimeClasspath 

    /* Can pass all the properties: */
    systemProperties System.getProperties()

    /* Or just each by name: */
    systemProperty "myvariable", System.getProperty("myvariable")

    /* Need to split the space-delimited value in the exec.args */
    args System.getProperty("exec.args", "").split()    
}

Application Plugin Approach

Command Line:

gradle run -Dmyvariable=myvalue -Dexec.args="arg1 arg2 arg3"

build.gradle:

apply plugin: 'application'
mainClassName = "com.mycompany.MyMain"
run {    
    /* Can pass all the properties: */
    systemProperties System.getProperties()

    /* Or just each by name: */
    systemProperty "myvariable", System.getProperty("myvariable")

    /* Need to split the space-delimited value in the exec.args */
    args System.getProperty("exec.args", "").split()    
}

For those who might not want to pollute your application's system properties by passing unrelated Gradle props, I recommend namespacing your arguments.

tasks.withType(JavaExec) {
    System.properties.each { k,v->
        if (k.startsWith("prefix.")) {
            systemProperty k - "prefix.", v
        }
    }
}

java ... -Dprefix.my.prop=true will pass my.prop

I'm new to gradle so I needed this and what is working for me with gradle 4.6 seems a little easier for the command line. Instead of parsing 1 arg string you can pass an array of args, and I found a way to pass in all property with one line as well. Combined below:

apply plugin: 'java'
apply plugin: 'org.springframework.boot'    <- for my project

task runApp(type: JavaExec) {
  classpath = sourceSets.main.runtimeClasspath

  main = 'testit.TomcatApp'

  // arguments to pass to the application
  //  args 'myarg1 -rest'    <- came in as 1 string

  args = ["--myarg1 with spaces even", "--myarg2"]

  // and to pass in all -D system property args:
  systemProperties = System.properties
}

gradle run -Dwhatever=xxx -Dmyarg2=hey

// Java reading them:
public static void main(String[] args) {
    for ( int i = 0; i < args.length; i++ )
        {
        logger.info( "** args [" + i + "] =" + args[i] + "=" );
        }
    logger.info( "** -Dwhatever =" + System.getProperty("whatever") + "=" );
    logger.info( "** -Dmyarg2 =" + System.getProperty("myarg2") + "=" );

[main] INFO testit.TomcatApp - ** args [0] =--myarg1 with spaces even=
[main] INFO testit.TomcatApp - ** args [1] =--myarg2=
[main] INFO testit.TomcatApp - ** -Dwhatever =xxx=
[main] INFO testit.TomcatApp - ** -Dmyarg2 =hey=