Class RefTracker

bind/java/Seq.java:154–248

	static final RefTracker tracker = new RefTracker();

	static final class RefTracker {
		private static final int REF_OFFSET = 42;

		// 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 = REF_OFFSET; // next Java object ref

		// Java objects that have been passed to Go. refnum -> Ref
		// The Ref obj field is non-null.
		// This map pins Java objects so they don't get GCed while the
		// only reference to them is held by Go code.
		private final RefMap javaObjs = new RefMap();

		// Java objects to refnum
		private final IdentityHashMap<Object, Integer> javaRefs = new IdentityHashMap<>();

		// inc increments the reference count of a Java object when it
		// is sent to Go. inc returns the refnum for the object.
		synchronized int inc(Object o) {
			if (o == null) {
				return NULL_REFNUM;
			}
			if (o instanceof Proxy) {
				return ((Proxy)o).incRefnum();
			}
			Integer refnumObj = javaRefs.get(o);
			if (refnumObj == null) {
				if (next == Integer.MAX_VALUE) {
					throw new RuntimeException("createRef overflow for " + o);
				}
				refnumObj = next++;
				javaRefs.put(o, refnumObj);
			}
			int refnum = refnumObj;
			Ref ref = javaObjs.get(refnum);
			if (ref == null) {
				ref = new Ref(refnum, o);
				javaObjs.put(refnum, ref);
			}
			ref.inc();
			return refnum;
		}

		synchronized void incRefnum(int refnum) {
			Ref ref = javaObjs.get(refnum);
			if (ref == null) {
				throw new RuntimeException("referenced Java object is not found: refnum="+refnum);
			}
			ref.inc();
		}

		// dec decrements the reference count of a Java object when
		// Go signals a corresponding proxy object is finalized.
		// If the count reaches zero, the Java object is removed
		// from the javaObjs map.