Question

Imagine I have the following situation:

Test1.java

import java.lang.ref.WeakReference;
public class Test1
{
    public WeakReference fieldName;

    public init()
    {
        fieldName = new WeakReference(this);
        Test2.setWeakRef(fieldName);
    }
}

Test2.java

import java.lang.ref.WeakReference;
public class Test2
{    
    public static setWeakRef(WeakReference weakRef)
    {
        //at this point I got weakRef in an other class.. now, how do I get the field name this reference was created with? So that it returns exactly "fieldName", because that's the name I gave it in Test1.java?
    }
}

At the location of the comment I received the weak reference created in an other class. How would I retreive the field name that this weak reference was created with, in this case “fieldName”?

Thanks in advance.

Answer 1

The only way would be using reflection. This is true for fields of any type, not only for weak references.

Field[] fields = Test1.getDeclaredFields();

for (Field field : fields) {
  if (field.getType().equals(WeakReference.class) {
    System.out.println(field.getName());
  }
}

Of course this example works only if you have a single WeakReference field in Test1.

Update: In case you have multiple reference fields, you need access to the object itself, in order to get and compare the actual value of a specific field with the value received. Like in this modified code example:

public class Test2 {    
  public static setWeakRef(Test1 instance, WeakReference weakRef) {
    Field[] fields = Test1.getDeclaredFields();

    for (Field field : fields) {
      if (field.getType().equals(WeakReference.class) {
        WeakReference value = (WeakReference) field.get(instance);
        if (value == weakRef) {
          System.out.println(field.getName());
        }
      }
    }
  }
}

Note that exception handling is left out for the sake of simplicity.