How to make the java system release Soft References?

Question

I'm going to use a SoftReference-based cache (a pretty simple thing by itself). However, I've came across a problem when writing a test for it.

The objective of the test is to check if the cache does request the previously cached object from the server again after the memory cleanup occurs.

Here I find the problem how to make system to release soft referenced objects. Calling System.gc() is not enough because soft references will not be released until the memory is low. I'm running this unit test on the PC so the memory budget for the VM could be pretty large.

================== Added later ==============================

Thank you all who took care to answer!

After considering all pro's and contra's I've decided to go the brute force way as advised by nanda and jarnbjo. It appeared, however, that JVM is not that dumb - it won't even attempt garbage collecting if you ask for a block which alone is bigger than VM's memory budget. So I've modified the code like this:

    /* Force releasing SoftReferences */
    try {
        final List<long[]> memhog = new LinkedList<long[]>();
        while(true) {
            memhog.add(new long[102400]);
        }
    }
    catch(final OutOfMemoryError e) {
        /* At this point all SoftReferences have been released - GUARANTEED. */
    }

    /* continue the test here */

Answer 1

This piece of code forces the JVM to flush all SoftReferences. And it's very fast to do.

It's working better than the Integer.MAX_VALUE approach, since here the JVM really tries to allocate that much memory.

try {
    Object[] ignored = new Object[(int) Runtime.getRuntime().maxMemory()];
} catch (OutOfMemoryError e) {
    // Ignore
}

I now use this bit of code everywhere I need to unit test code using SoftReferences.

Update: This approach will indeed work only with less than 2G of max memory.

Also, one need to be very careful with SoftReferences. It's so easy to keep a hard reference by mistake that will negate the effect of SoftReferences.

Here is a simple test that shows it working every time on OSX. Would be interested in knowing if JVM's behavior is the same on Linux and Windows.

for (int i = 0; i < 1000; i++) {
    SoftReference<Object> softReference = new SoftReferencelt<Object>(new Object());
    if (null == softReference.get()) {
        throw new IllegalStateException("Reference should NOT be null");
    }

    try {
        Object[] ignored = new Object[(int) Runtime.getRuntime().maxMemory()];
    } catch (OutOfMemoryError e) {
        // Ignore
    }

    if (null != softReference.get()) {
        throw new IllegalStateException("Reference should be null");
    }

    System.out.println("It worked!");
}

Answer 2

An improvement that will work for more than 2G max memory. It loops until an OutOfMemory error occurs.

@Test
public void shouldNotHoldReferencesToObject() {
    final SoftReference<T> reference = new SoftReference<T>( ... );

    // Sanity check
    assertThat(reference.get(), not(equalTo(null)));

    // Force an OoM
    try {
        final ArrayList<Object[]> allocations = new ArrayList<Object[]>();
        int size;
        while( (size = Math.min(Math.abs((int)Runtime.getRuntime().freeMemory()),Integer.MAX_VALUE))>0 )
            allocations.add( new Object[size] );
    } catch( OutOfMemoryError e ) {
        // great!
    }

    // Verify object has been garbage collected
    assertThat(reference.get(), equalTo(null));

}