Currently dubbo support java6, java7 and java8.

All unit test pass in java9, we should consider add official support for java9 and even java10.
Currently travis support java9. java10 is in progress (https://github.com/travis-ci/travis-ci/issues/9368)

As discussed in mail list, the first step is to add java9 in travis ci.
return 0F;
        } else if (returnType == double.class || returnType == Double.class) {
            return 0D;
        } else if (returnType.isArray()) {
            return Array.newInstance(returnType.getComponentType(), 0);
        } else if (returnType.isAssignableFrom(ArrayList.class)) {
            return new ArrayList<Object>(0);
        } else if (returnType.isAssignableFrom(HashSet.class)) {
            return new HashSet<Object>(0);
        } else if (returnType.isAssignableFrom(HashMap.class)) {
            return new HashMap<Object, Object>(0);
        } else if (String.class.equals(returnType)) {
            return "";
        } else if (!returnType.isInterface()) {
            try {
                Object value = emptyInstances.get(returnType);
                if (value == null) {
                    value = returnType.newInstance();
                    emptyInstances.put(returnType, value);
                }
                Class<?> cls = value.getClass();
                while (cls != null && cls != Object.class) {
                    Field[] fields = cls.getDeclaredFields();
                    for (Field field : fields) {
                        Object property = getEmptyObject(field.getType(), emptyInstances, level + 1);
                        if (property != null) {
                            try {
                                if (!field.isAccessible()) {
                                    field.setAccessible(true);
                                }