java - volley中的entry.softTtl和entry.ttl有什么区别？

Question

在 HttpHeaderParser 类中:

public static Cache.Entry parseCacheHeaders(NetworkResponse response) {
    long now = System.currentTimeMillis();

    Map<String, String> headers = response.headers;

    long serverDate = 0;
    long serverExpires = 0;
    long softExpire = 0;
    long maxAge = 0;
    boolean hasCacheControl = false;

    String serverEtag = null;
    String headerValue;

    headerValue = headers.get("Date");
    if (headerValue != null) {
        serverDate = parseDateAsEpoch(headerValue);
    }

    headerValue = headers.get("Cache-Control");
    if (headerValue != null) {
        hasCacheControl = true;
        String[] tokens = headerValue.split(",");
        for (int i = 0; i < tokens.length; i++) {
            String token = tokens[i].trim();
            if (token.equals("no-cache") || token.equals("no-store")) {
                return null;
            } else if (token.startsWith("max-age=")) {
                try {
                    maxAge = Long.parseLong(token.substring(8));
                } catch (Exception e) {
                }
            } else if (token.equals("must-revalidate") || token.equals("proxy-revalidate")) {
                maxAge = 0;
            }
        }
    }

    headerValue = headers.get("Expires");
    if (headerValue != null) {
        serverExpires = parseDateAsEpoch(headerValue);
    }

    serverEtag = headers.get("ETag");

    // Cache-Control takes precedence over an Expires header, even if both exist and Expires
    // is more restrictive.
    if (hasCacheControl) {
        softExpire = now + maxAge * 1000;
    } else if (serverDate > 0 && serverExpires >= serverDate) {
        // Default semantic for Expire header in HTTP specification is softExpire.
        softExpire = now + (serverExpires - serverDate);
    }

    Cache.Entry entry = new Cache.Entry();
    entry.data = response.data;
    entry.etag = serverEtag;
    entry.softTtl = softExpire;
    entry.ttl = entry.softTtl;
    entry.serverDate = serverDate;
    entry.responseHeaders = headers;

    return entry;
}

entry.softTtl = softExpire;

entry.ttl = entry.softTtl;

这两个变量有相同的值，为什么？

在类 CacheDispatcher 中

@Override
public void run() {
    ...
    ...

    // If it is completely expired, just send it to the network.
    if (entry.isExpired()) {
        request.addMarker("cache-hit-expired");
        request.setCacheEntry(entry);
        mNetworkQueue.put(request);
        continue;
    }

    ...

    if (!entry.refreshNeeded()) {
        // Completely unexpired cache hit. Just deliver the response.
        mDelivery.postResponse(request, response);
    } else {
        ...

        // Post the intermediate response back to the user and have
        // the delivery then forward the request along to the network.
        mDelivery.postResponse(request, response, new Runnable() {
            @Override
            public void run() {
                try {
                    mNetworkQueue.put(request);
                } catch (InterruptedException e) {
                    // Not much we can do about this.
                }
            }
        });
    }

    ...
}

由于值相同，如何区分 entry.isExpired() 和 entry.refreshNeeded() 的转换？

Answer 1

看看这篇文章:Group Google : Volley Users - soft and hard ttl on cache

refreshNeeded() and isExpired() are not exactly the same. One compares to the ttl value and the other to softTtl. This can be used to implement request semantics where you will return a response from cache even if it is "soft" expired, but will then go to the network and refresh, returning a new response if the data has changed.