2 zhangshexin zhangsheXIN_ 于 2014.05.21 14:29 提问

关于左移右移与或的作用

希望 有耐心的大牛给解惑


public class DiskBasedCache implements Cache {

/** Map of the Key, CacheHeader pairs */
private final Map<String, CacheHeader> mEntries =
        new LinkedHashMap<String, CacheHeader>(16, .75f, true);

/** Total amount of space currently used by the cache in bytes. */
private long mTotalSize = 0;

/** The root directory to use for the cache. */
private final File mRootDirectory;

/** The maximum size of the cache in bytes. */
private final int mMaxCacheSizeInBytes;

/** Default maximum disk usage in bytes. */
private static final int DEFAULT_DISK_USAGE_BYTES = 5 * 1024 * 1024;

/** High water mark percentage for the cache */
private static final float HYSTERESIS_FACTOR = 0.9f;

/** Magic number for current version of cache file format. */
private static final int CACHE_MAGIC = 0x20120504;

/**
 * Constructs an instance of the DiskBasedCache at the specified directory.
 * @param rootDirectory The root directory of the cache.
 * @param maxCacheSizeInBytes The maximum size of the cache in bytes.
 */
public DiskBasedCache(File rootDirectory, int maxCacheSizeInBytes) {
    mRootDirectory = rootDirectory;
    mMaxCacheSizeInBytes = maxCacheSizeInBytes;
}

/**
 * Constructs an instance of the DiskBasedCache at the specified directory using
 * the default maximum cache size of 5MB.
 * @param rootDirectory The root directory of the cache.
 */
public DiskBasedCache(File rootDirectory) {
    this(rootDirectory, DEFAULT_DISK_USAGE_BYTES);
}

/**
 * Clears the cache. Deletes all cached files from disk.
 */
@Override
public synchronized void clear() {
    File[] files = mRootDirectory.listFiles();
    if (files != null) {
        for (File file : files) {
            file.delete();
        }
    }
    mEntries.clear();
    mTotalSize = 0;
    VolleyLog.d("Cache cleared.");
}

/**
 * Returns the cache entry with the specified key if it exists, null otherwise.
 */
@Override
public synchronized Entry get(String key) {
    CacheHeader entry = mEntries.get(key);
    // if the entry does not exist, return.
    if (entry == null) {
        return null;
    }

    File file = getFileForKey(key);
    CountingInputStream cis = null;
    try {
        cis = new CountingInputStream(new FileInputStream(file));
        CacheHeader.readHeader(cis); // eat header
        byte[] data = streamToBytes(cis, (int) (file.length() - cis.bytesRead));
        return entry.toCacheEntry(data);
    } catch (IOException e) {
        VolleyLog.d("%s: %s", file.getAbsolutePath(), e.toString());
        remove(key);
        return null;
    } finally {
        if (cis != null) {
            try {
                cis.close();
            } catch (IOException ioe) {
                return null;
            }
        }
    }
}

/**
 * Initializes the DiskBasedCache by scanning for all files currently in the
 * specified root directory. Creates the root directory if necessary.
 */
@Override
public synchronized void initialize() {
    if (!mRootDirectory.exists()) {
        if (!mRootDirectory.mkdirs()) {
            VolleyLog.e("Unable to create cache dir %s", mRootDirectory.getAbsolutePath());
        }
        return;
    }

    File[] files = mRootDirectory.listFiles();
    if (files == null) {
        return;
    }
    for (File file : files) {
        FileInputStream fis = null;
        try {
            fis = new FileInputStream(file);
            CacheHeader entry = CacheHeader.readHeader(fis);
            entry.size = file.length();
            putEntry(entry.key, entry);
        } catch (IOException e) {
            if (file != null) {
               file.delete();
            }
        } finally {
            try {
                if (fis != null) {
                    fis.close();
                }
            } catch (IOException ignored) { }
        }
    }
}

/**
 * Invalidates an entry in the cache.
 * @param key Cache key
 * @param fullExpire True to fully expire the entry, false to soft expire
 */
@Override
public synchronized void invalidate(String key, boolean fullExpire) {
    Entry entry = get(key);
    if (entry != null) {
        entry.softTtl = 0;
        if (fullExpire) {
            entry.ttl = 0;
        }
        put(key, entry);
    }

}

/**
 * Puts the entry with the specified key into the cache.
 */
@Override
public synchronized void put(String key, Entry entry) {
    pruneIfNeeded(entry.data.length);
    File file = getFileForKey(key);
    try {
        FileOutputStream fos = new FileOutputStream(file);
        CacheHeader e = new CacheHeader(key, entry);
        e.writeHeader(fos);
        fos.write(entry.data);
        fos.close();
        putEntry(key, e);
        return;
    } catch (IOException e) {
    }
    boolean deleted = file.delete();
    if (!deleted) {
        VolleyLog.d("Could not clean up file %s", file.getAbsolutePath());
    }
}

/**
 * Removes the specified key from the cache if it exists.
 */
@Override
public synchronized void remove(String key) {
    boolean deleted = getFileForKey(key).delete();
    removeEntry(key);
    if (!deleted) {
        VolleyLog.d("Could not delete cache entry for key=%s, filename=%s",
                key, getFilenameForKey(key));
    }
}

/**
 * Creates a pseudo-unique filename for the specified cache key.
 * @param key The key to generate a file name for.
 * @return A pseudo-unique filename.
 */
private String getFilenameForKey(String key) {
    int firstHalfLength = key.length() / 2;
    String localFilename = String.valueOf(key.substring(0, firstHalfLength).hashCode());
    localFilename += String.valueOf(key.substring(firstHalfLength).hashCode());
    return localFilename;
}

/**
 * Returns a file object for the given cache key.
 */
public File getFileForKey(String key) {
    return new File(mRootDirectory, getFilenameForKey(key));
}

/**
 * Prunes the cache to fit the amount of bytes specified.
 * @param neededSpace The amount of bytes we are trying to fit into the cache.
 */
private void pruneIfNeeded(int neededSpace) {
    if ((mTotalSize + neededSpace) < mMaxCacheSizeInBytes) {
        return;
    }
    if (VolleyLog.DEBUG) {
        VolleyLog.v("Pruning old cache entries.");
    }

    long before = mTotalSize;
    int prunedFiles = 0;
    long startTime = SystemClock.elapsedRealtime();

    Iterator<Map.Entry<String, CacheHeader>> iterator = mEntries.entrySet().iterator();
    while (iterator.hasNext()) {
        Map.Entry<String, CacheHeader> entry = iterator.next();
        CacheHeader e = entry.getValue();
        boolean deleted = getFileForKey(e.key).delete();
        if (deleted) {
            mTotalSize -= e.size;
        } else {
           VolleyLog.d("Could not delete cache entry for key=%s, filename=%s",
                   e.key, getFilenameForKey(e.key));
        }
        iterator.remove();
        prunedFiles++;

        if ((mTotalSize + neededSpace) < mMaxCacheSizeInBytes * HYSTERESIS_FACTOR) {
            break;
        }
    }

    if (VolleyLog.DEBUG) {
        VolleyLog.v("pruned %d files, %d bytes, %d ms",
                prunedFiles, (mTotalSize - before), SystemClock.elapsedRealtime() - startTime);
    }
}

/**
 * Puts the entry with the specified key into the cache.
 * @param key The key to identify the entry by.
 * @param entry The entry to cache.
 */
private void putEntry(String key, CacheHeader entry) {
    if (!mEntries.containsKey(key)) {
        mTotalSize += entry.size;
    } else {
        CacheHeader oldEntry = mEntries.get(key);
        mTotalSize += (entry.size - oldEntry.size);
    }
    mEntries.put(key, entry);
}

/**
 * Removes the entry identified by 'key' from the cache.
 */
private void removeEntry(String key) {
    CacheHeader entry = mEntries.get(key);
    if (entry != null) {
        mTotalSize -= entry.size;
        mEntries.remove(key);
    }
}

/**
 * Reads the contents of an InputStream into a byte[].
 * */
private static byte[] streamToBytes(InputStream in, int length) throws IOException {
    byte[] bytes = new byte[length];
    int count;
    int pos = 0;
    while (pos < length && ((count = in.read(bytes, pos, length - pos)) != -1)) {
        pos += count;
    }
    if (pos != length) {
        throw new IOException("Expected " + length + " bytes, read " + pos + " bytes");
    }
    return bytes;
}

/**
 * Handles holding onto the cache headers for an entry.
 */
// Visible for testing.
static class CacheHeader {
    /** The size of the data identified by this CacheHeader. (This is not
     * serialized to disk. */
    public long size;

    /** The key that identifies the cache entry. */
    public String key;

    /** ETag for cache coherence. */
    public String etag;

    /** Date of this response as reported by the server. */
    public long serverDate;

    /** TTL for this record. */
    public long ttl;

    /** Soft TTL for this record. */
    public long softTtl;

    /** Headers from the response resulting in this cache entry. */
    public Map<String, String> responseHeaders;

    private CacheHeader() { }

    /**
     * Instantiates a new CacheHeader object
     * @param key The key that identifies the cache entry
     * @param entry The cache entry.
     */
    public CacheHeader(String key, Entry entry) {
        this.key = key;
        this.size = entry.data.length;
        this.etag = entry.etag;
        this.serverDate = entry.serverDate;
        this.ttl = entry.ttl;
        this.softTtl = entry.softTtl;
        this.responseHeaders = entry.responseHeaders;
    }

    /**
     * Reads the header off of an InputStream and returns a CacheHeader object.
     * @param is The InputStream to read from.
     * @throws IOException
     */
    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.ttl = readLong(is);
        entry.softTtl = readLong(is);
        entry.responseHeaders = readStringStringMap(is);
        return entry;
    }

    /**
     * Creates a cache entry for the specified data.
     */
    public Entry toCacheEntry(byte[] data) {
        Entry e = new Entry();
        e.data = data;
        e.etag = etag;
        e.serverDate = serverDate;
        e.ttl = ttl;
        e.softTtl = softTtl;
        e.responseHeaders = responseHeaders;
        return e;
    }


    /**
     * Writes the contents of this CacheHeader to the specified OutputStream.
     */
    public boolean writeHeader(OutputStream os) {
        try {
            writeInt(os, CACHE_MAGIC);
            writeString(os, key);
            writeString(os, etag == null ? "" : etag);
            writeLong(os, serverDate);
            writeLong(os, ttl);
            writeLong(os, softTtl);
            writeStringStringMap(responseHeaders, os);
            os.flush();
            return true;
        } catch (IOException e) {
            VolleyLog.d("%s", e.toString());
            return false;
        }
    }

}

private static class CountingInputStream extends FilterInputStream {
    private int bytesRead = 0;

    private CountingInputStream(InputStream in) {
        super(in);
    }

    @Override
    public int read() throws IOException {
        int result = super.read();
        if (result != -1) {
            bytesRead++;
        }
        return result;
    }

    @Override
    public int read(byte[] buffer, int offset, int count) throws IOException {
        int result = super.read(buffer, offset, count);
        if (result != -1) {
            bytesRead += result;
        }
        return result;
    }
}

/*
 * Homebrewed simple serialization system used for reading and writing cache
 * headers on disk. Once upon a time, this used the standard Java
 * Object{Input,Output}Stream, but the default implementation relies heavily
 * on reflection (even for standard types) and generates a ton of garbage.
 */

/**
 * Simple wrapper around {@link InputStream#read()} that throws EOFException
 * instead of returning -1.
 */
private static int read(InputStream is) throws IOException {
    int b = is.read();
    if (b == -1) {
        throw new EOFException();
    }
    return b;
}

static void writeInt(OutputStream os, int n) throws IOException {
    os.write((n >> 0) & 0xff);
    os.write((n >> 8) & 0xff);
    os.write((n >> 16) & 0xff);
    os.write((n >> 24) & 0xff);
}

static int readInt(InputStream is) throws IOException {
    int n = 0;
    n |= (read(is) << 0);
    n |= (read(is) << 8);
    n |= (read(is) << 16);
    n |= (read(is) << 24);
    return n;
}

static void writeLong(OutputStream os, long n) throws IOException {
    os.write((byte)(n >>> 0));
    os.write((byte)(n >>> 8));
    os.write((byte)(n >>> 16));
    os.write((byte)(n >>> 24));
    os.write((byte)(n >>> 32));
    os.write((byte)(n >>> 40));
    os.write((byte)(n >>> 48));
    os.write((byte)(n >>> 56));
}

static long readLong(InputStream is) throws IOException {
    long n = 0;
    n |= ((read(is) & 0xFFL) << 0);
    n |= ((read(is) & 0xFFL) << 8);
    n |= ((read(is) & 0xFFL) << 16);
    n |= ((read(is) & 0xFFL) << 24);
    n |= ((read(is) & 0xFFL) << 32);
    n |= ((read(is) & 0xFFL) << 40);
    n |= ((read(is) & 0xFFL) << 48);
    n |= ((read(is) & 0xFFL) << 56);
    return n;
}

static void writeString(OutputStream os, String s) throws IOException {
    byte[] b = s.getBytes("UTF-8");
    writeLong(os, b.length);
    os.write(b, 0, b.length);
}

static String readString(InputStream is) throws IOException {
    int n = (int) readLong(is);
    byte[] b = streamToBytes(is, n);
    return new String(b, "UTF-8");
}

static void writeStringStringMap(Map<String, String> map, OutputStream os) throws IOException {
    if (map != null) {
        writeInt(os, map.size());
        for (Map.Entry<String, String> entry : map.entrySet()) {
            writeString(os, entry.getKey());
            writeString(os, entry.getValue());
        }
    } else {
        writeInt(os, 0);
    }
}

static Map<String, String> readStringStringMap(InputStream is) throws IOException {
    int size = readInt(is);
    Map<String, String> result = (size == 0)
            ? Collections.<String, String>emptyMap()
            : new HashMap<String, String>(size);
    for (int i = 0; i < size; i++) {
        String key = readString(is).intern();
        String value = readString(is).intern();
        result.put(key, value);
    }
    return result;
}

这里面有几个方法的作用我不太理解,希望有大牛可以给解释一下,谢谢。
不明的方法是
static void writeInt(OutputStream os, int n) throws IOException {
os.write((n >> 0) & 0xff);
os.write((n >> 8) & 0xff);
os.write((n >> 16) & 0xff);
os.write((n >> 24) & 0xff);
}

static int readInt(InputStream is) throws IOException {
    int n = 0;
    n |= (read(is) << 0);
    n |= (read(is) << 8);
    n |= (read(is) << 16);
    n |= (read(is) << 24);
    return n;
}

static void writeLong(OutputStream os, long n) throws IOException {
    os.write((byte)(n >>> 0));
    os.write((byte)(n >>> 8));
    os.write((byte)(n >>> 16));
    os.write((byte)(n >>> 24));
    os.write((byte)(n >>> 32));
    os.write((byte)(n >>> 40));
    os.write((byte)(n >>> 48));
    os.write((byte)(n >>> 56));
}

static long readLong(InputStream is) throws IOException {
    long n = 0;
    n |= ((read(is) & 0xFFL) << 0);
    n |= ((read(is) & 0xFFL) << 8);
    n |= ((read(is) & 0xFFL) << 16);
    n |= ((read(is) & 0xFFL) << 24);
    n |= ((read(is) & 0xFFL) << 32);
    n |= ((read(is) & 0xFFL) << 40);
    n |= ((read(is) & 0xFFL) << 48);
    n |= ((read(is) & 0xFFL) << 56);
    return n;
}

我不明白为什么需要作与或操作,为什么要左移右移,这是定好的计算方式???

1个回答

zhangsheXIN_
zhangsheXIN_   2014.05.27 11:25

已解决,…………………………谢谢

Csdn user default icon
上传中...
上传图片
插入图片
准确详细的回答,更有利于被提问者采纳,从而获得C币。复制、灌水、广告等回答会被删除,是时候展现真正的技术了!