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crivens

newInstance and ClassCastException

12 posts in this topic

I have an abstract class Entity and a derived class, let's call it Fred. I'm using newInstance() but I want to store the new instance in a HashMap<String, Entity>.  When I do    (Entity) klass.newInstance();      I'm getting an exception that I can't cast to Entity but I don't understand why. 

 

To further complicate matters, the class definition for Fred is created dynamically at runtime using a Javassist class loader.

 

Any suggestions appreciated.

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Hopefully this is a small working example:
 

package cloud;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtField;
import javassist.CtMethod;
import javassist.CtNewMethod;
import javassist.Loader;
import javassist.Modifier;
import javassist.NotFoundException;

public class TestBuilder {

	private Loader classLoader;
	private ClassPool pool;
	private CtClass abstractEntityCtClass;

	public TestBuilder() {

	}

	public void buildClass(String name, String msg) {

		System.out.println("Building class " + name);

		// Create CtClass.
		CtClass ctClass = pool.makeClass(name);

		// Set superclass to cloud.Entity.
		try {
			ctClass.setSuperclass(abstractEntityCtClass);
		} catch (CannotCompileException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
			return;
		}

		// Add field.
		String fieldName = "age";
		try {
			
			// Create field.
			CtField field = new CtField(CtClass.intType, fieldName, ctClass);
			field.setModifiers(Modifier.PUBLIC);
			ctClass.addField(field);
			
			String s = "public void say() { System.out.println(\"" + msg + "\");}";

			ctClass.addMethod(CtNewMethod.make(
					s, ctClass));

		} catch (CannotCompileException e1) {
			// TODO Auto-generated catch block
			e1.printStackTrace();
			return;
		}

		// Add property.
		String getterName = "getAge";
		String type = CtClass.intType.getName();

		StringBuffer sb = new StringBuffer();
		sb.append("public ").append(type).append(" ").append(getterName)
				.append("(){").append("return this.").append(fieldName)
				.append(";").append("}");

		try {
			CtMethod method = CtMethod.make(sb.toString(), ctClass);
			ctClass.addMethod(method);
		} catch (CannotCompileException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
			return;
		}
		
		System.out.println("ctClass " + ctClass);
	}

	public Object createInstance(String name) {

		System.out.println("Creating class instance " + name);

		Object o = null;

		// Create and add to list.
		Class<? extends SimpleEntity> newKlass;
		try {
			newKlass = (Class<? extends SimpleEntity>) classLoader.loadClass(name);
			o = newKlass.newInstance();
			SimpleEntity e = (SimpleEntity)o;
			// objects.add(o);
			System.out.println("Created instance " + o);
		} catch (ClassNotFoundException e1) {
			// TODO Auto-generated catch block
			e1.printStackTrace();
			return o;
		} catch (InstantiationException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
			return o;
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
			return o;
		}
		
		Method method;
		try {
			method = o.getClass().getMethod("say");
			method.invoke(o, null);
		} catch (NoSuchMethodException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (SecurityException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalArgumentException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InvocationTargetException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

		return o;

	}

	public void build() {

		String name = "foo.Bob";

		// Create new class pool and loader.
		buildPool();

		// Build the class.
		buildClass(name, "fred");

		// Create instance.
		Object o = createInstance(name);

		// Create new class pool and loader.
		buildPool();

		// Rebuild the class.
		buildClass(name, "bob");

		// Create an instance of the rebuilt class.
		Object o2 = createInstance(name);

	}
	
	private void buildPool() {

		// Create class loader and pool.
		this.pool = new ClassPool(ClassPool.getDefault());
		this.classLoader = new Loader();
		this.classLoader.setClassPool(this.pool);


		// Get the cloud.AbstractEntity class.
		try {
			abstractEntityCtClass = ClassPool.getDefault().get(
					"cloud.SimpleEntity");
			System.out.println("Abstract Entity " + abstractEntityCtClass);
		} catch (NotFoundException e) {
			e.printStackTrace();
			System.exit(-2);
		}

	}

	/**
	 * @param args
	 */
	public static void main(String[] args) {

		TestBuilder builder = new TestBuilder();
		builder.build();

	}

}

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Forgot the SimpleEntity class:

 

package cloud;

import java.io.Serializable;

public abstract class SimpleEntity implements Serializable {

}

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I think my problems is because I'm using multiple class loaders though I'm not sure how to fix it yet.

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Class loader for Entity sun.misc.Launcher$AppClassLoader@151cc2a8

Class loader for objcet javassist.Loader@9246bec

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I tried changing to this:

 

this.classLoader = new Loader(ClassLoader.getSystemClassLoader(), this.pool);

 

But it still doesn't work.

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I wonder if Javassist's class loader (Loader) isn't delegating to the parent class loader (the system one).

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This may not be quite what you're looking for , but it's a tweaked version of your sample that creates an object that is recognised as a subclass of SimpleEntity. Main changes (I think!) have been setting up parent class loaders and using defaults wherever possible. I'm not sure if your attempt to call createPool twice will cause problems either...

 

 

package cloud;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtField;
import javassist.CtMethod;
import javassist.CtNewMethod;
import javassist.Loader;
import javassist.Modifier;
import javassist.NotFoundException;

public class TestBuilder
{

	private Loader classLoader;
	private ClassPool pool;
	private CtClass abstractEntityCtClass;

	public TestBuilder()
	{

	}

	public void buildClass(String name, String msg)
	{

		System.out.println("Building class " + name);

		// Create CtClass.
		CtClass ctClass = pool.makeClass(name);

		// Set superclass to cloud.Entity.
		try
		{
			ctClass.setSuperclass(abstractEntityCtClass);
		}
		catch (CannotCompileException e)
		{
			// TODO Auto-generated catch block
			e.printStackTrace();
			return;
		}

		// Add field.
		String fieldName = "age";
		try
		{

			// Create field.
			CtField field = new CtField(CtClass.intType, fieldName, ctClass);
			field.setModifiers(Modifier.PUBLIC);
			ctClass.addField(field);

			String s = "public void say() { System.out.println(\"" + msg + "\");}";

			ctClass.addMethod(CtNewMethod.make(s, ctClass));

		}
		catch (CannotCompileException e1)
		{
			// TODO Auto-generated catch block
			e1.printStackTrace();
			return;
		}

		// Add property.
		String getterName = "getAge";
		String type = CtClass.intType.getName();

		StringBuffer sb = new StringBuffer();
		sb.append("public ").append(type).append(" ").append(getterName).append("(){").append("return this.")
				.append(fieldName).append(";").append("}");

		try
		{
			CtMethod method = CtMethod.make(sb.toString(), ctClass);
			ctClass.addMethod(method);
		}
		catch (CannotCompileException e)
		{
			// TODO Auto-generated catch block
			e.printStackTrace();
			return;
		}

		System.out.println("ctClass " + ctClass);
	}

	public Object createInstance(String name)
	{

		System.out.println("Creating class instance " + name);

		Object o = null;

		// Create and add to list.
		//Class<? extends SimpleEntity> newKlass;
		Class newKlass;
		try
		{
			//newKlass = (Class<? extends SimpleEntity>) classLoader.loadClass(name);
			//newKlass = classLoader.loadClass(name);
			CtClass newCtClass = pool.getCtClass(name);
			
			//o = newKlass.newInstance();
			Class c1 = newCtClass.toClass();
			o = c1.newInstance();
			
			if(o instanceof SimpleEntity)
			{
				int foo = 1;
			}


			int ibreak = 0;
			SimpleEntity e = (SimpleEntity) o;
			// objects.add(o);
			System.out.println("Created instance " + o);
		}
		catch(Throwable t)
		{
			t.printStackTrace();
		}
//		catch (ClassNotFoundException e1)
//		{
//			// TODO Auto-generated catch block
//			e1.printStackTrace();
//			return o;
//		}

		Method method;
		try
		{
			method = o.getClass().getMethod("say");
			method.invoke(o, null);
		}
		catch (NoSuchMethodException e)
		{
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		catch (SecurityException e)
		{
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		catch (IllegalAccessException e)
		{
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		catch (IllegalArgumentException e)
		{
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		catch (InvocationTargetException e)
		{
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

		return o;

	}

	public void build()
	{

		String name = "foo.Bob";

		// Create new class pool and loader.
		buildPool();

		// Build the class.
		buildClass(name, "fred");

		// Create instance.
		Object o = createInstance(name);

		// Create new class pool and loader.
		//buildPool();

		// Rebuild the class.
		//buildClass(name, "bob");

		// Create an instance of the rebuilt class.
		//Object o2 = createInstance(name);

	}

	private void buildPool()
	{

		// Create class loader and pool.
		//this.pool = new ClassPool(ClassPool.getDefault());
		this.pool = ClassPool.getDefault();
		//this.classLoader = new Loader(ClassLoader.getSystemClassLoader(),this.pool);
		//		this.classLoader = Loader.getSystemClassLoader();
		//		this.classLoader.setClassPool(this.pool);

		//pool.insertClassPath(new ClassClassPath(this.getClass()));

		// Get the cloud.AbstractEntity class.
		try
		{
			abstractEntityCtClass = pool.get("cloud.SimpleEntity");
			System.out.println("Abstract Entity " + abstractEntityCtClass);
		}
		catch (NotFoundException e)
		{
			e.printStackTrace();
			System.exit(-2);
		}

	}

	/**
	 * @param args
	 */
	public static void main(String[] args)
	{

		TestBuilder builder = new TestBuilder();
		builder.build();

	}
}
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