java - 如何跳过责任链中的空值？

Question

我了解责任链模式。我有这样一个问题。从我的责任链中可以看出，如果该方法没有将语言环境返回给我，那么它会返回 null。如果返回 null，如何转到链中的下一项？

public abstract class StandardLocaleHandler {

            protected StandardLocaleHandler localeHandler;


            public StandardLocaleHandler() {
                this.localeHandler = null;
            }

            protected abstract Locale getTrueLocale(HttpServletRequest req, HttpServletResponse resp, List<String> localeList, String defaultLocale, Integer cookieAge);

            public void setNext(StandardLocaleHandler localeHandler) {
                this.localeHandler = localeHandler;
            }

            public StandardLocaleHandler getNext() {
                return localeHandler;
            }
        }

        public class GetLocaleByAvailable extends StandardLocaleHandler {

            @Override
            protected Locale getTrueLocale(HttpServletRequest req, HttpServletResponse resp, List<String> localeList, String defaultLocale, Integer cookieAge) {
                if (isNull(req.getSession().getAttribute(LANG_ATTRIBUTE)) && isNull(req.getCookies())) {
                    return setAvailable(req, resp, localeList, defaultLocale, cookieAge);
                }
                return null;
            }
        }

    public class GetLocaleBySession extends StandardLocaleHandler {

            @Override
            protected Locale getTrueLocale(HttpServletRequest req, HttpServletResponse resp, List<String> localeList, String defaultLocale, Integer cookieAge) {
                if (nonNull(req.getSession().getAttribute(LANG_ATTRIBUTE))) {
                    LOG.debug(req.getParameter(LANG_ATTRIBUTE));
                    return new Locale((String) req.getSession().getAttribute(LANG_ATTRIBUTE));
                }
                return null;
            }
        }

我是这样形成我的责任链的:

public class ChainBuilder {

    private List<StandardLocaleHandler> localeHandlers = new ArrayList<>();

    public void addToFilterList(StandardLocaleHandler filter) {
        if (!localeHandlers.contains(filter)) {
            localeHandlers.add(filter);
        } else {
            throw new IllegalArgumentException("Already in the list");
        }
    }

    public StandardLocaleHandler createChainOfResponsibility() {
        for (int i = 0; i < localeHandlers.size() - 1; i++) {
            localeHandlers.get(i).setNext(localeHandlers.get(i + 1));
        }
        return localeHandlers.get(0);
    }
}

ChainBuilder builder = new ChainBuilder();
        builder.addToFilterList(new GetLocaleByAvailable());
        builder.addToFilterList(new GetLocaleByParam());
        builder.addToFilterList(new GetLocaleBySession());
        builder.addToFilterList(new GetLocaleByCookie());

        StandardLocaleHandler handler = builder.createChainOfResponsibility();
        return handler.getTrueLocale(req, resp, localeList, defaultLocale, cookieAge);

如果返回 null，如何转到链中的下一项？

Answer 1

public abstract class StandardLocaleHandler {
    public final Locale getTrueLocale() {
        Locale local = getTrueLocaleInternal();
        return local == null && localeHandler != null ? localeHandler.getTrueLocale() : local;
    }

    protected abstract Locale getTrueLocaleInternal();
}

public class GetLocaleByAvailable extends StandardLocaleHandler {

    @Override
    protected Locale getTrueLocaleInternal() {
        // TODO logic
        return null;
    }
}