Volley-DiskBasedCache的内部类CacheHeader的源码分析

Android控件跟框架 Android源码分析

DiskBasedCache中定义了一个CacheHeader内部类，表示缓存信息的摘要，存储在缓存文件的头部，网络请求的缓存写入与读取都是通过CacheHeadler类来操作的。与Cache接口的内部类Cache.Entry相似。

CacheHeader的源码：

    static class CacheHeader {
        /**缓存文件的大小 */
        public long size;
        /**缓存文件的key，也就是url */
        public String key;
        /** 用于缓存新鲜度验证的 ETag */
        public String etag;
        /** 缓存产生的时间 */
        public long serverDate;
        /** 缓存文件在服务器端最后的修改时间 */
        public long lastModified;
        /** 缓存过期时间*/
        public long ttl;
        /** 缓存的新鲜度时间 */
        public long softTtl;
        /** 缓存对应的首部信息 */
        public Map<String, String> responseHeaders;
        private CacheHeader() { }
        public CacheHeader(String key, Entry entry) {
            this.key = key;
            this.size = entry.data.length;
            this.etag = entry.etag;
            this.serverDate = entry.serverDate;
            this.lastModified = entry.lastModified;
            this.ttl = entry.ttl;
            this.softTtl = entry.softTtl;
            this.responseHeaders = entry.responseHeaders;
        }
        public static CacheHeader readHeader(InputStream is) throws IOException {
            CacheHeader entry = new CacheHeader();
            int magic = readInt(is);
            if (magic != CACHE_MAGIC) {
                // don't bother deleting, it'll get pruned eventually
                throw new IOException();
            }
            entry.key = readString(is);
            entry.etag = readString(is);
            if (entry.etag.equals("")) {
                entry.etag = null;
            }
            entry.serverDate = readLong(is);
            entry.lastModified = readLong(is);
            entry.ttl = readLong(is);
            entry.softTtl = readLong(is);
            entry.responseHeaders = readStringStringMap(is);
            return entry;
        }
        public Entry toCacheEntry(byte[] data) {
            Entry e = new Entry();
            e.data = data;
            e.etag = etag;
            e.serverDate = serverDate;
            e.lastModified = lastModified;
            e.ttl = ttl;
            e.softTtl = softTtl;
            e.responseHeaders = responseHeaders;
            return e;
        }
        public boolean writeHeader(OutputStream os) {
            try {
                writeInt(os, CACHE_MAGIC);
                writeString(os, key);
                writeString(os, etag == null ? "" : etag);
                writeLong(os, serverDate);
                writeLong(os, lastModified);
                writeLong(os, ttl);
                writeLong(os, softTtl);
                writeStringStringMap(responseHeaders, os);
                os.flush();
                return true;
            } catch (IOException e) {
                VolleyLog.d("%s", e.toString());
                return false;
            }
        }
}

readHeader(InputStream is)读头信息：

public static CacheHeader readHeader(InputStream is) throws IOException {
            CacheHeader entry = new CacheHeader();
            int magic = readInt(is);
            if (magic != CACHE_MAGIC) {
                // don't bother deleting, it'll get pruned eventually
                throw new IOException();
            }
            entry.key = readString(is);
            entry.etag = readString(is);
            if (entry.etag.equals("")) {
                entry.etag = null;
            }
            entry.serverDate = readLong(is);
            entry.lastModified = readLong(is);
            entry.ttl = readLong(is);
            entry.softTtl = readLong(is);
            entry.responseHeaders = readStringStringMap(is);
            return entry;
}

在方法体的第三行，验证输入流对应的缓存文件的合法性。如果读出的magic不等于写入的magic，终止读写，如果合法，从缓存文件中读出结构数据，组成对象返回。

writeHeader(OutputStream os)与上面方法相反，这里将maic和一些属性信息写入到缓存文件。注意写文件也是按照结构来写。

public boolean writeHeader(OutputStream os) {
            try {
                writeInt(os, CACHE_MAGIC);
                writeString(os, key);
                writeString(os, etag == null ? "" : etag);
                writeLong(os, serverDate);
                writeLong(os, lastModified);
                writeLong(os, ttl);
                writeLong(os, softTtl);
                writeStringStringMap(responseHeaders, os);
                os.flush();
                return true;
            } catch (IOException e) {
                VolleyLog.d("%s", e.toString());
                return false;
            }
}

toCacheEntry(byte[] data)方法,此方法用于将头信息和数据信息封装成Entry（Cache的单位数据）返回，代码很简单。

 public Entry toCacheEntry(byte[] data) {
            Entry e = new Entry();
            e.data = data;
            e.etag = etag;
            e.serverDate = serverDate;
            e.lastModified = lastModified;
            e.ttl = ttl;
            e.softTtl = softTtl;
            e.responseHeaders = responseHeaders;
            return e;
}

readInt 和writeInt
readLong和 writeLong
readString和writeString
readStringStringMap和writeStringStringMap
这些方法是DiskBasedCache中定义的方法，主要用于头信息读写有结构的数据，其中夹杂着一些自定义的位运算，这部分的源码没看懂。

Volley的cache之硬盘缓存--DiskBasedCache（作者的其他博文也值得学习）
Volley源码解读