Executing Singleton class with Custom Class Loaders

[sriram sundararajan]

Hi All,
I am trying to develop a custom class loader to load a singleton class.

Below are the code snippets involved.

I compiled Singleton1.java and got the class file in the same folder.

When i am trying to run ChkSingletonCL.java, i am getting a java.lang.IllegalAccessException
@ line 45. How can I run the Singleton class with custom class loader.

The Error msg. is "java.lang.IllegalAccessException: Class sripy.classLoader.ChkSingletonCL can not access a member of class Singleton1 with modifiers "private"

Basically i want to test the Singleton behaviours in the custom class loaders.

Singleton1.java

  package sample.designPattern;
    
    public class Singleton1 {
        private String strName = "None";
        private Singleton1(){}
        private static Singleton1 ins = null;
        public static Singleton1 getInstance(){
            if(ins == null){
            synchronized(Singleton1.class) {
                    if(ins == null){
                        ins = new Singleton1();
                    }
                 }
            }
            return ins;
        }
    
        public void set(String s){
            this.strName = s;
        }
        public String get(){
            return this.strName;
        }
    
    }

ChkSingletonCL.java

    package sample.classLoader;
    
    
    import java.io.FileInputStream;
    import java.lang.reflect.Method;
    
    
    class CustomClassLoader1 extends ClassLoader {
    
        public Class findClass(String name) {
            byte[] b = null;
            try {
                b = loadClassData(name);
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println(b.length);
            return defineClass(name, b, 0, b.length);
        }
    
        private byte[] loadClassData(String name) throws Exception {
            //load the class data from Class file not in classpath
            String cf = "C:\\work\\java\\coreSamples\\coreSamples\\src\\sample\\designPattern\\" + getPath(name) + ".class";
    
            FileInputStream fis = new FileInputStream(cf);
            int len = fis.available();
            byte b[] = new byte[len];
            fis.read(b, 0, len);
            return b;
        }
    
        private String getPath(String name) {
            return name.replace(".", "/");
        }
    
    }
    
    public class ChkSingletonCL {
    
        public static void main(String[] args) {
            try {
                System.out.println(ChkSingletonCL.class.getClassLoader());
                ClassLoader loader = new CustomClassLoader1();
                Class c = loader.loadClass("Singleton1");
                Object obj = c.newInstance();
                Method m = c.getMethod("getName", new Class[]{});
                System.out.println("Name: " + m.invoke(obj, new Object[]{}));
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

regards
sri

[Campbell Ritchie]

Question too difficult for "beginning". Moving thread.

[Ireneusz Kordal]

Hi,

first, your Singleton1 class doesn't have a getName() method, so you must declare it first before invoking it from the main().

class Singleton1{
     ...... 
   private String getName(){
          return this.strName;
    } 
...

Second, constructor Singleton() is private, therefore you cannot use Class.newInstance() method because it works only for public default constructor.
Third, method getName() is private, so you cannot use class.getMethod(name,...) method because it works only for public methods.
See javadoc for details.

Try this code:

            //Object obj = c.newInstance();
            Constructor<?>[] cons = c.getDeclaredConstructors();
            cons[0].setAccessible(true);
            Object obj = cons[0].newInstance(new Object[]{});

            //Method m = c.getMethod("getName", new Class[]{});  
            Method[] methods = c.getDeclaredMethods();
            for (Method m : methods) {
                if (m.getName().contentEquals("getName")) {
                    m.setAccessible(true);
                    System.out.println("Name: " + m.invoke(obj, new Object[]{}));
                }
            } 

[sriram sundararajan]

Hi Ireneusz,
Thanks i can now able to call the singleton class. I think its high time for me to tune up my reflection knowledge. Thanks again.