Android: the GC doesn't respect SoftReferences?

Question

It seams that Dalvik\'s garbage collector doesn\'t respect SoftReferences and removes them as soon as possible, just like WeakReferences. I\'m not 100% sure yet, but despite the

Answer 1

After not receiving an answer I decided to make my own study. I've made a simple test to exercise the GC against SoftReferences.

public class TestSoftReference extends TestCase {

    public void testSoftRefsAgainstGc_1() {        testGcWithSoftRefs(1);    }

    public void testSoftRefsAgainstGc_2() {        testGcWithSoftRefs(2);    }

    public void testSoftRefsAgainstGc_3() {        testGcWithSoftRefs(3);    }

    public void testSoftRefsAgainstGc_4() {        testGcWithSoftRefs(4);    }

    public void testSoftRefsAgainstGc_5() {        testGcWithSoftRefs(5);    }

    public void testSoftRefsAgainstGc_6() {        testGcWithSoftRefs(6);    }

    public void testSoftRefsAgainstGc_7() {        testGcWithSoftRefs(7);    }


    private static final int SR_COUNT = 1000;

    private void testGcWithSoftRefs(final int gc_count) {
        /* "Integer(i)" is a referrent. It is important to have it referenced
         * only from the SoftReference and from nothing else. */
        final ArrayList<SoftReference<Integer>> list = new ArrayList<SoftReference<Integer>>(SR_COUNT);
        for (int i = 0; i < SR_COUNT; ++i) {
            list.add(new SoftReference<Integer>(new Integer(i)));
        }

        /* Test */
        for (int i = 0; i < gc_count; ++i) {
            System.gc();

            try {
                Thread.sleep(200);
            } catch (final InterruptedException e) {
            }
        }

        /* Check */
        int dead = 0;
        for (final SoftReference<Integer> ref : list) {
            if (ref.get() == null) {
                ++dead;
            }
        }
        assertEquals(0, dead);
    }
}

The idea is that I make few runs of the same code increasing stress on SoftReferences each time (by running more GC passes).

Results are pretty interesting: All runs pass just fine except for one!

On Android 1.5 device:
testSoftRefsAgainstGc_1()  FAILED!  AssertionFailedError: expected:0 but was:499
testSoftRefsAgainstGc_2()  passed
testSoftRefsAgainstGc_3()  passed
testSoftRefsAgainstGc_4()  passed
testSoftRefsAgainstGc_5()  passed
testSoftRefsAgainstGc_6()  passed
testSoftRefsAgainstGc_7()  passed


On Android 1.6 device:
testSoftRefsAgainstGc_1()  passed
testSoftRefsAgainstGc_2()  FAILED!  AssertionFailedError: expected:0 but was:499
testSoftRefsAgainstGc_3()  passed
testSoftRefsAgainstGc_4()  passed
testSoftRefsAgainstGc_5()  passed
testSoftRefsAgainstGc_6()  passed
testSoftRefsAgainstGc_7()  passed

On Android 2.2 device:
All pass.

These test results are stable. I've tried many times and every time it is the same. So I believe it is indeed a bug in garbage collector.

CONCLUSION

So, what we learn out of this... Using SoftReferences in your code is pointless for Android 1.5-1.6 devices. For these devices you will not get the behavior you expect. I didn't try for 2.1, however.

Answer 2

@JBM I've tried your TestCase on Nexus S (android4.2.2), all tests are failed. GC is more aggressive against SoftReference on android4.2.2

Answer 3

I reported this issue to Google: https://code.google.com/p/android/issues/detail?id=20015