Use a process-wide counter instead of identity hash code for phantom …

…references in the precise leak detector

The added int field hides in alignment padding on most JVM configurations, so the objects won't actually take up more space from this added field.

src/main/java/stormpot/internal/PreciseLeakDetector.java

@@ -20,6 +20,7 @@
 import java.lang.ref.ReferenceQueue;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.LongAdder;
 
 public final class PreciseLeakDetector {
@@ -30,11 +31,11 @@ public final class PreciseLeakDetector {
   public PreciseLeakDetector() {
     referenceQueue = new ReferenceQueue<>();
     leakedObjectCount = new LongAdder();
-    refs = new IdentityHashSet();
+    refs = new CountedPhantomRefHashSet();
   }
 
   public void register(BSlot<?> slot) {
-    PhantomReference<Object> ref = new PhantomReference<>(slot.obj, referenceQueue);
+    CountedPhantomRef<Object> ref = new CountedPhantomRef<>(slot.obj, referenceQueue);
     slot.leakCheck = ref;
     synchronized (refs) {
       refs.add(ref);
@@ -74,4 +75,22 @@ public long countLeakedObjects() {
     accumulateLeaks();
     return leakedObjectCount.sum();
   }
+
+  private static final class CountedPhantomRef<T> extends PhantomReference<T> {
+    private static final AtomicInteger COUNTER = new AtomicInteger();
+
+    private final int id; // This hides in alignment padding on most JVMs, including Lilliput.
+
+    CountedPhantomRef(T referent, ReferenceQueue<? super T> q) {
+      super(referent, q);
+      id = COUNTER.getAndIncrement();
+    }
+  }
+
+  private static final class CountedPhantomRefHashSet extends IdentityHashSet {
+    @Override
+    protected int hashOf(Object obj) {
+      return ((CountedPhantomRef<?>) obj).id;
+    }
+  }
 }