How to make Retrolambda work with both Mac OS X and Windows. Thanks to @Takhion - https://gist.github.com/Takhion/5c0f6c0c5aba9db5a488

build.gradle

import org.gradle.internal.os.OperatingSystem;
String getJavaHome(String version) {
    def stdout = new ByteArrayOutputStream()
    exec {
        commandLine "/usr/libexec/java_home", "-v", version
        standardOutput = stdout;
    }
    return stdout.toString().trim()
}

task findOs {
    doLast {
        if (OperatingSystem.current().isMacOsX()) {
            println("Adding retrolmbda")
            retrolambda {
                jdk System.getenv("JAVA8_HOME") ?: getJavaHome("1.8")
                oldJdk System.getenv("JAVA7_HOME") ?: getJavaHome("1.7")
            }
        }
    }
}
preBuild.dependsOn findOs