#vulnerable code 
                                @Override
    public Object invoke(MethodInvocation invocation) throws Throwable {
        if (instance == null) {
            throw new NullPointerException("target instance is null may be not initial correct.");
        }
        Object result = invocation.getMethod().invoke(instance, invocation.getArguments());
        return result;
    } 
                            #location 6 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                @Override
    public Object invoke(MethodInvocation invocation) throws Throwable {
        Method method = invocation.getMethod();
        ProtobufRPC protobufPRC = method.getAnnotation(ProtobufRPC.class);
        if (protobufPRC == null) {
            throw new IllegalAccessError("Target method is not marked annotation @ProtobufPRC. method name :"
                    + method.getDeclaringClass().getName() + "." + method.getName());
        }
        String serviceName = protobufPRC.serviceName();
        String methodName = protobufPRC.methodName();
        if (StringUtils.isEmpty(methodName)) {
            methodName = method.getName();
        }
        String methodSignature = serviceName + '!' + methodName;
        Object instance = instancesMap.get(methodSignature);
        if (instance == null) {
            throw new NullPointerException("target instance is null may be not initial correct.");
        }
        Object result = invocation.getMethod().invoke(instance, invocation.getArguments());
        return result;
    }