Debugging dynamic proxy creation mutates invocationhandler state

Question

Given the following dynamic java proxy with an invocation handler that has internal state which changes during invocation:

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class Main {
    public static void main(String[] args) {
        Test test = (Test) Proxy.newProxyInstance(
                Test.class.getClassLoader(),
                new Class[] { Test.class },
                new MyInvocationHandler(true));
        test.doWork();
        test.doWork();
    }

    public interface Test {
        void doWork();
    }

    public static class MyInvocationHandler implements InvocationHandler {
        private boolean flag;
        public MyInvocationHandler(boolean flag) {
            this.flag = flag;
        }
        @Override
        public Object invoke(Object o, Method method, Object[] objects) throws Throwable {
            if ("doWork".equals(method.getName())) {
                System.out.println("Flag is " + flag);
            }
            if (flag) {
                flag = false;
                // Do somemthing
            }
            return null;
        }
    }
}

I expect this to write the following to stdout:

Flag is true

Flag is false

However when debugging step by step, it outputs:

Flag is false

Flag is false

I'm using Java 11.0.5 and IntelliJ IDEA for debugging.

Does someone know what happened here?

Answer 1

'toString()' object view option is known to cause side effects during debug as it will call the methods which are not invoked during the normal app execution. If your app state depends on these methods, you should disable the corresponding option (File | Settings | Build, Execution, Deployment | Debugger | Data Views | Java: Enable 'toString()' object view):