确定何时运行Android GC

WeakReference<GcWatcher> mGcWatcher
        = new WeakReference<GcWatcher>(new GcWatcher());
long mLastGcTime;

class GcWatcher {
    @Override
    protected void finalize() throws Throwable {
        handleGc();
        mLastGcTime = SystemClock.uptimeMillis();
        mGcWatcher = new WeakReference<GcWatcher>(new GcWatcher());
    }
}