Question

I would like to clarify something.

With “plain” Java reflection techniques (without using a library) afaik it is not possible to get a reference to a private field (I mean the java.lang.reflect.Field object, no the field value).

For example, if I have this class:

public class MyClass {
  private String field1;
}

If I attempt to execute this:

Field field = MyClass.class.getField("field1");

I will get a NoSuchFieldException exception, as expected.

With the Guava Reflection library, if I try to execute this:

Object o = new MyClass();
Property property = Properties.getPropertyByName(o, "field1");
Field f = property.getField();

I get the following exception:

java.lang.IllegalStateException: Unknown property: field1 in class MyClass

And this was also expected. However, if I add a getter method, like this:

public class MyClass {
  private String field1;
  public String getField1() {return field1;}
}

Then the Guava-reflection code is working.
I have to confess I am a bit loss about this. I understand that a reflection library could use a getter to return the value of a private instance variable, but the Field object itself just because a getter exists ?. Does someone has an idea how does this happen ?

Answer 1

You can reflect on private fields using standard java reflection, which is probably what Guava is doing under the hood:

Class<?> c = ... some class ...
Field field = c.getDeclaredField("name");
field.setAccessible(true);
Object value = field.get(object);

getDeclaredField allows you to obtain private fields.
setAccessible is needed to prevent security issues.

Anyway, as a best practice, consider using reflection on public members only, so work with getters/setters if possible.

Hope that helps.