Android Studio: JavaExec: Could not find or load main class

Question

As part of my build script, I would like to run some Java code before the APK is packaged.

When I try to build my project, I get the error:

Error: Could not find or load main class net.bgreco.myapp.build.BuildScript

BuildScript.java is located in app/src/main/java/net/bgreco/myapp/build/.

The BuildScript.class does get compiled successfully before I receive the error message above, and I can run it from the command line with no errors.

app/build.gradle :

task runBuildScript(type: JavaExec) {
    main = 'net.bgreco.myapp.build.BuildScript'
}

tasks.whenTaskAdded { theTask ->
    if (theTask.name.contains('package')) {
        theTask.dependsOn 'runBuildScript'
    }
}

bgreco · Accepted Answer · 2018-09-16T23:12:31.617

With some help from this answer, I got it working by modifying the classpath as below. I also needed to reference some of the Android libraries so I included the path to android.jar.

app/build.gradle :

task runBuildScript(type: JavaExec) {
    String flavor, buildType
    (flavor, buildType) = getCurrentFlavorAndBuildType()
    classpath files("build/intermediates/classes/" + flavor + "/" + buildType, "${android.getSdkDirectory().getAbsolutePath()}\\platforms\\${android.compileSdkVersion}\\android.jar")
    main = 'net.bgreco.myapp.build.BuildScript'
    args = [flavor] // Not needed unless you want to know the current flavor from within the BuildScript
}

tasks.whenTaskAdded { theTask ->
    if (theTask.name.contains('package')) {
        theTask.dependsOn 'runBuildScript'
    }
}

def getCurrentFlavorAndBuildType() {
    Gradle gradle = getGradle()
    String tskReqStr = gradle.getStartParameter().getTaskRequests().toString()

    Pattern pattern;

    if (tskReqStr.contains("assemble"))
        pattern = Pattern.compile("assemble(\\w+)(Release|Debug)")
    else
        pattern = Pattern.compile("generate(\\w+)(Release|Debug)")

    Matcher matcher = pattern.matcher(tskReqStr)

    if (matcher.find())
        return [matcher.group(1).toLowerCase(), matcher.group(2).toLowerCase()]
    else {
        println "NO MATCH FOUND"
        return ["", ""];
    }
}

Android Studio: JavaExec: Could not find or load main class

1 Answers1