java jar getClass().getClassLoader().getResourceAsStream return nullpointerexception

Question

Image: This is my Project directory

In MainRun class, i have the following code:

package bin;
public class MainRun{
   public static void main(String[] args){

    compileCode("Square.java");

    int length = 0;
    MCAMClassLoader classLoader = new MCAMClassLoader();
    Class<?> c= classLoader.findClass(bin.Square);

    try{
        Shape myClassObject = (Shape)c.newInstance();
        Method method = c.getMethod(getLength);
        Object result = method.invoke(myClassObject);
        length = (int)result;
    }
    catch(Exception e){
        e.printStackTrace();
    }
    System.out.println(length);
  }

  public void compileCode(String _filename){
     String[] args = new String[] {"-d", "C:\Users\Tan\Downloads\MCAM", "C:\Users\Tan\Downloads\MCAM\src\"+_filename};
    com.sun.tools.javac.Main javac = new com.sun.tools.javac.Main();
    javac.compile(args);
  }
}

In my custom ClassLoader, i have the following code:

package bin;
public class MCAMClassLoader extends ClassLoader{
  @Override
  public Class<?> findClass(String name) 
  {
      byte[] bt = loadClassData(name);
      return defineClass(name, bt, 0, bt.length);
  }

  private byte[] loadClassData(String className) {

    InputStream is =  this.getClass().getClassLoader().getResourceAsStream(className.replace(".", "/")+".class"); //it will end up with bin/Square.class
    ByteArrayOutputStream byteSt = new ByteArrayOutputStream();

    int len =0;
    try 
    {
        while((len=is.read())!=-1)
        {
            byteSt.write(len);
        }
    } catch (IOException e) {
         e.printStackTrace();
    }

    return byteSt.toByteArray();
  }
}

And i have my Square class added and compiled on run time. Square.class will be located in bin folder.

package bin;
public class Square extends Shape{

  public Square(){
    length = 8;
  }

  public int getLength(){
    return length;
  }
}

These codes work perfectly fine when it was executed by .bat on console. Only when i build those files into a jar file and run it, I will get the error on the console: NullPointerException which fall on the line "while((len=is.read())!=-1)".

Note: in my MANIFEST.MF

Manifest-Version: 1.0
Main-Class: bin.MainRun
Class-Path: .\lib\xmlbeans-2.6.0.jar .\lib\tools.jar

Why? Am I using the ClassLoader incorrectly? Something else? Please advise and thanks in advance

Answer 1

There are (often) no directories inside jar files. Therefore it will return null.

If you want to get the file you could get that resource directly:

InputStream fileInputStream = getClass().getResourceAsStream(File.pathSeparator+className+".class");