Spring Mobile是如何判断访问设备的类型的

Spring最近换域名了，去转转，发现了一个有意思的项目：spring mobile。

http://projects.spring.io/spring-mobile/

这个项目有很多实用的功能，如识别访问我们网站的设备是什么类型的（手机，平板，PC），据域名或者url来切换不同访问内容，据不同的访问设备转到不同的view中。

识别访问设备类型的核心代码在org.springframework.mobile.device.LiteDeviceResolver 类中，这个类实际上根据http请求头部的User-Agent的内容来判断设备到底是哪种类型的。

可以看到，这个类实际上是很简单的。不过字符串的匹配算法可能可以优化下。

public class LiteDeviceResolver implements DeviceResolver {

    private final List<String> mobileUserAgentPrefixes = new ArrayList<String>();

    private final List<String> mobileUserAgentKeywords = new ArrayList<String>();

    private final List<String> tabletUserAgentKeywords = new ArrayList<String>();

    private final List<String> normalUserAgentKeywords = new ArrayList<String>();

    public LiteDeviceResolver() {
        init();
    }

    public LiteDeviceResolver(List<String> normalUserAgentKeywords) {
        init();
        this.normalUserAgentKeywords.addAll(normalUserAgentKeywords);
    }

    public Device resolveDevice(HttpServletRequest request) {
        String userAgent = request.getHeader("User-Agent");
        // UserAgent keyword detection of Normal devices
        if (userAgent != null) {
            userAgent = userAgent.toLowerCase();
            for (String keyword : normalUserAgentKeywords) {
                if (userAgent.contains(keyword)) {
                    return resolveFallback(request);
                }
            }
        }
        // UAProf detection
        if (request.getHeader("x-wap-profile") != null || request.getHeader("Profile") != null) {
            return LiteDevice.MOBILE_INSTANCE;
        }
        // User-Agent prefix detection
        if (userAgent != null && userAgent.length() >= 4) {
            String prefix = userAgent.substring(0, 4).toLowerCase();
            if (mobileUserAgentPrefixes.contains(prefix)) {
                return LiteDevice.MOBILE_INSTANCE;
            }
        }
        // Accept-header based detection
        String accept = request.getHeader("Accept");
        if (accept != null && accept.contains("wap")) {
            return LiteDevice.MOBILE_INSTANCE;
        }
        // UserAgent keyword detection for Mobile and Tablet devices
        if (userAgent != null) {
            userAgent = userAgent.toLowerCase();
            // Android special case
            if (userAgent.contains("android") && !userAgent.contains("mobile")) {
                return LiteDevice.TABLET_INSTANCE;
            }
            // Kindle Fire special case
            if (userAgent.contains("silk") && !userAgent.contains("mobile")) {
                return LiteDevice.TABLET_INSTANCE;
            }
            for (String keyword : tabletUserAgentKeywords) {
                if (userAgent.contains(keyword)) {
                    return LiteDevice.TABLET_INSTANCE;
                }
            }
            for (String keyword : mobileUserAgentKeywords) {
                if (userAgent.contains(keyword)) {
                    return LiteDevice.MOBILE_INSTANCE;
                }
            }
        }
        // OperaMini special case
        @SuppressWarnings("rawtypes")
        Enumeration headers = request.getHeaderNames();
        while (headers.hasMoreElements()) {
            String header = (String) headers.nextElement();
            if (header.contains("OperaMini")) {
                return LiteDevice.MOBILE_INSTANCE;
            }
        }
        return resolveFallback(request);
    }

    // subclassing hooks

    /**
     * List of user agent prefixes that identify mobile devices.
     * Used primarily to match by operator or handset manufacturer.
     */
    protected List<String> getMobileUserAgentPrefixes() {
        return mobileUserAgentPrefixes;
    }

    /**
     * List of user agent keywords that identify mobile devices.
     * Used primarily to match by mobile platform or operating system.
     */
    protected List<String> getMobileUserAgentKeywords() {
        return mobileUserAgentKeywords;
    }

    /**
     * List of user agent keywords that identify tablet devices.
     * Used primarily to match by tablet platform or operating system.
     */
    protected List<String> getTabletUserAgentKeywords() {
        return tabletUserAgentKeywords;
    }

    /**
     * List of user agent keywords that identify normal devices.
     * Any items in this list take precedence over the mobile and
     * tablet user agent keywords, effectively overriding those.
     */
    protected List<String> getNormalUserAgentKeywords() {
        return normalUserAgentKeywords;
    }

    /**
     * Initialize this device resolver implementation.
     * Registers the known set of device signature strings.
     * Subclasses may override to register additional strings.
     */
    protected void init() {
        getMobileUserAgentPrefixes().addAll(Arrays.asList(KNOWN_MOBILE_USER_AGENT_PREFIXES));
        getMobileUserAgentKeywords().addAll(Arrays.asList(KNOWN_MOBILE_USER_AGENT_KEYWORDS));
        getTabletUserAgentKeywords().addAll(Arrays.asList(KNOWN_TABLET_USER_AGENT_KEYWORDS));
    }

    /**
     * Fallback called if no mobile device is matched by this resolver.
     * The default implementation of this method returns a "normal" {@link Device} that is neither mobile or a tablet.
     * Subclasses may override to try additional mobile or tablet device matching before falling back to a "normal" device.
     */
    protected Device resolveFallback(HttpServletRequest request) {
        return LiteDevice.NORMAL_INSTANCE;
    }

    // internal helpers

    private static final String[] KNOWN_MOBILE_USER_AGENT_PREFIXES = new String[] { "w3c ", "w3c-", "acs-", "alav",
            "alca", "amoi", "audi", "avan", "benq", "bird", "blac", "blaz", "brew", "cell", "cldc", "cmd-", "dang",
            "doco", "eric", "hipt", "htc_", "inno", "ipaq", "ipod", "jigs", "kddi", "keji", "leno", "lg-c", "lg-d",
            "lg-g", "lge-", "lg/u", "maui", "maxo", "midp", "mits", "mmef", "mobi", "mot-", "moto", "mwbp", "nec-",
            "newt", "noki", "palm", "pana", "pant", "phil", "play", "port", "prox", "qwap", "sage", "sams", "sany",
            "sch-", "sec-", "send", "seri", "sgh-", "shar", "sie-", "siem", "smal", "smar", "sony", "sph-", "symb",
            "t-mo", "teli", "tim-", "tosh", "tsm-", "upg1", "upsi", "vk-v", "voda", "wap-", "wapa", "wapi", "wapp",
            "wapr", "webc", "winw", "winw", "xda ", "xda-" };

    private static final String[] KNOWN_MOBILE_USER_AGENT_KEYWORDS = new String[] { "blackberry", "webos", "ipod",
            "lge vx", "midp", "maemo", "mmp", "mobile", "netfront", "hiptop", "nintendo DS", "novarra", "openweb",
            "opera mobi", "opera mini", "palm", "psp", "phone", "smartphone", "symbian", "up.browser", "up.link",
            "wap", "windows ce" };

    private static final String[] KNOWN_TABLET_USER_AGENT_KEYWORDS = new String[] { "ipad", "playbook", "hp-tablet",
            "kindle" };

}