bind: use single Ref for null objects

The aim of the RefTracker is to "... pin Java objects so
they don't get GCed while the only reference to them is
held by Go code." But in the case of null objects there is
nothing to GC. Therefore we do return a single Ref which
contains a null object, but we ignore the logic that is used
to pin for GC purposes.

Change-Id: If3771ec0180d09485963c3297abccb39a1a8d9ab
Reviewed-on: https://go-review.googlesource.com/13647
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/Seq.java

@@ -216,12 +216,17 @@ public class Seq {
 	static final RefTracker tracker = new RefTracker();
 
 	static final class RefTracker {
+		private static final int REF_OFFSET = 42;
+
+		// use single Ref for null Seq.Object
+		private static final Ref nullRef = new Ref(REF_OFFSET - 1, null);
+
 		// Next Java object reference number.
 		//
 		// Reference numbers are positive for Java objects,
 		// and start, arbitrarily at a different offset to Go
 		// to make debugging by reading Seq hex a little easier.
-		private int next = 42; // next Java object ref
+		private int next = REF_OFFSET; // next Java object ref
 
 		// Java objects that have been passed to Go. refnum -> Ref
 		// The Ref obj field is non-null.
@@ -237,7 +242,10 @@ public class Seq {
 				// We don't keep track of the Go object.
 				return;
 			}
-			// Count Java objects passed to Go.
+			if (refnum == nullRef.refnum) {
+				return;
+			}
+			// Count how many times this ref's Java object is passed to Go.
 			if (ref.refcnt == Integer.MAX_VALUE) {
 				throw new RuntimeException("refnum " + refnum + " overflow");
 			}
@@ -259,6 +267,9 @@ public class Seq {
 				log.severe("dec request for Go object "+ refnum);
 				return;
 			}
+			if (refnum == nullRef.refnum) {
+				return;
+			}
 			// Java objects are removed on request of Go.
 			Ref obj = javaObjs.get(refnum);
 			if (obj == null) {
@@ -271,7 +282,9 @@ public class Seq {
 		}
 
 		synchronized Ref createRef(Seq.Object o) {
-			// TODO(crawshaw): use single Ref for null.
+			if (o == null) {
+				return nullRef;
+			}
 			if (next == Integer.MAX_VALUE) {
 				throw new RuntimeException("createRef overflow for " + o);
 			}
@@ -293,6 +306,9 @@ public class Seq {
 		// When we have real code, examine the tradeoffs.
 		synchronized Ref get(int refnum) {
 			if (refnum > 0) {
+				if (refnum == nullRef.refnum) {
+					return nullRef;
+				}
 				Ref ref = javaObjs.get(refnum);
 				if (ref == null) {
 					throw new RuntimeException("unknown java Ref: "+refnum);