WeakReference to String and String constants

Question

I have come across this example from wikipedia regarding weak reference:

import java.lang.ref.WeakReference;

public class ReferenceTest {
        public static void main(String[] args) throws InterruptedException {

            WeakReference r = new WeakReference(new String("I'm here"));
            WeakReference sr = new WeakReference("I'm here");
            System.out.println("before gc: r=" + r.get() + ", static=" + sr.get());
            System.gc();
            Thread.sleep(100);

            // only r.get() becomes null
            System.out.println("after gc: r=" + r.get() + ", static=" + sr.get());

        }
}

I don't understand in this scenario why only r.get() returns null but not the sr.get(). Can someone let me know the reason?

Many thanks.

Answer 1

the literal "I'm here" is a compile time constant string and as such gets placed in the constant string pool, which (up until java 7) was never garbage collected. that means sr points to an object that will never be garbage collected. r, on the other hand, points to a copy of that string, which is not in any const pool and so is eligible for collection.

see the documentation for String.intern() for some more details on this string pool