I tried to use java reflection to set a non-static field value of class, but got the “java.lang.IllegalArgumentException”

if I add the “static” modifier for this field. All codes will pass without any exception.

Here is my codes:

        Reflections reflections = new Reflections("abc.test", new SubTypesScanner(false));
        Set<Class<?>> classes = reflections.getSubTypesOf(Object.class);
        for (Class clazz : classes) {
            try {
                Optional<Field> optional = Arrays.stream(clazz.getDeclaredFields())
                        .filter(f -> VariableResolver.class.isAssignableFrom(f.getType()))
                        .findFirst();
                if (!optional.isPresent()) {
                    continue;
                }

                Field field = optional.get();
                field.setAccessible(true);
                field.set(clazz, new DefaultVariableResolver());
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        }

Here is the class file and the field.

public class TestClass {
    VariableResolver resolver;  //The code can only pass if I add 'static' modifier for this field.

    public void method(){
        String a = resolver.get("test");
    }
}

The exception is:

Caused by: java.lang.IllegalArgumentException: Can not set test.util.VariableResolver field test.TestClass.resolver to java.lang.Class
    at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:167)
    at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:171)
    at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:58)
    at java.base/jdk.internal.reflect.UnsafeObjectFieldAccessorImpl.set(UnsafeObjectFieldAccessorImpl.java:75)
    at java.base/java.lang.reflect.Field.set(Field.java:799)