#vulnerable code 
                                public static void releaseEngine() {
        if (engine != null) {
            try {
                engine.close();
            } catch (Exception e) {
                throw new GraphvizException("Problem closing engine", e);
            }
        }
        engine = null;
        engineQueue = null;
    } 
                            #location 4 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                public static void releaseEngine() {
        synchronized (Graphviz.class) {
            if (engine != null) {
                doReleaseEngine(engine);
            }
            if (engineQueue != null) {
                for (final GraphvizEngine engine : engineQueue) {
                    doReleaseEngine(engine);
                }
            }
        }
        engine = null;
        engineQueue = null;
    }