add jar to the classpath at runtime in jjs (the hacky way)

27 June 2015

Normally, the JVM does not allow extending the classpath during runtime. But due to the demand, people found ways around that restriction. To bring one of the work-around to jjs I wrote the following javascript function.

function addUrlToClasspath(pathName){
	var/*java.net.URLClassLoader*/ sysloader = /*(java.net.URLClassLoader) */ java.lang.ClassLoader.getSystemClassLoader();
  var/*java.lang.Class*/ sysclass = java.net.URLClassLoader.class;
     var ClassArray = Java.type("java.lang.Class[]");
     var parameters = new ClassArray(1);
     parameters[0]= java.net.URL.class;
     var/*java.lang.reflect.Method*/ method = sysclass.getDeclaredMethod("addURL", parameters);
     method.setAccessible(true);
     var ObjectArray = Java.type("java.lang.Object[]");
     var array = new ObjectArray(1);
  var/*java.io.File*/ f = new java.io.File(pathName);
  if(f.isFile()){
	var/*java.net.URL*/ u = f.toURL();
    array[0]=u;
    //if(u.toString().endsWith(".jar"))
      method.invoke(sysloader, array);
  }else{
  	var/*File[]*/ listOfFiles = f.listFiles();
  	if(listOfFiles !=null)
  	for (var i = 0; i < listOfFiles.length; i++) {
      if (listOfFiles[i].isFile()) {
        var/*java.net.URL*/ u = listOfFiles[i].toURL();
    	array[0]=u;
      	method.invoke(sysloader, array);
      }
    }
  }
}

So now, every time you need to add a jar file to the class path you can just do the following.

addUrlToClasspath("/Users/jens/tmp/trivium-core.jar");
var mainType =  Java.type("io.trivium.Start");
var args =[];
mainType.main(args);