类池。get(className)抛出RuntimeException无法找到类

我正在尝试使用Javassist编写一个简单的插装代理。

public class Agent implements ClassFileTransformer {

    protected Instrumentation instrumentation;
    protected ClassPool classPool;

    public Agent(Instrumentation instrumentation){

        this.instrumentation = instrumentation;
        this.classPool = ClassPool.getDefault();
        this.instrumentation.addTransformer(this);
    }

    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {

        try {

            System.out.printf("%s.transform: %s\n", this.getClass().getCanonicalName(), className);

            classPool.insertClassPath(new ByteArrayClassPath(className, classfileBuffer));

            CtClass ctClass = classPool.get(className);  /* <- throws error */
        }
        catch (Exception ex){
            System.out.println(ex);
            return null;
        }
    }
}

但我发现以下错误:

我试图手动从类路径添加路径,但也没有帮助。即

String[] cpEntries = System.getProperty("java.class.path").split(File.pathSeparator);
for (String cpEntry : classpathEntries)
    classPool.insertClassPath(cpEntry);

我错过了什么?谢谢