Question

1. How can synthetic fields be created in Java?

2. Can synthetic fields in java only be created at runtime?
   If not: Is there a standard-compliant way to this at compile time (without changing some bytes in the class file)

Answer 1

They’re created by the compiler when “oddities” of the language require them. A simple example of this is using an inner class:

public class Test
{
    class Inner
    {
    }
}

The Test.Inner class will have a synthetic field to represent the appropriate instance of the Test class.

We can extend this code slightly to show that field:

import java.lang.reflect.*;

public class Test
{
    public static void main(String[] args)
    {
        for (Field field : Inner.class.getDeclaredFields())
        {
            System.out.println(field.getName() + ": " + field.isSynthetic());
        }
    }

    class Inner
    {
    }
}

With my compiler, that prints:

this$0: true