硬盘缓存方案DiskLruCache源码解析

Posted 2020-06-14 weiers

前面研究了LruCache，它作为现在用的最多的内存缓存方案已经在很多开源缓存框架中使用，同样的还有硬盘缓存方案也就是DiskLruCache，通常的做法就是使用内存和硬盘二级缓存。

使用方法

1.存储：

DiskLruCache diskLruCache= open(File directory, int appVersion, int valueCount, long maxSize);
DiskLruCache.Editor editor = diskLruCache.edit(key);
OuputStream ouputStream = editor.newOutputStream(0);

然后往该ouputStream中写入收即可。如果是写入字符串可以使用，其实就是封装了往输出流中写的代码。

editor.set(int index, String value);

2.访问

DiskLruCache.Snapshot snapShot = mDiskLruCache.get(key);  
if (snapShot != null) {  
    InputStream is = snapShot.getInputStream(0);  
}

如果是文本，可以直接获取

snapShot.getString(int index)

3.close() 和open方法对应，用于关闭DiskLruCache里面的方法，journalWriter被置null，就不能再更新日志文件了。

4.delete() 删除缓存目录下的所有文件，用于清空缓存。

二级缓存的框架

综合前面的内存缓存LruCache和硬盘缓存DiskLruCache，内存和硬盘二级缓存的大概框架可以如下代码

Bitmap bitmap = getBitmap(generateKey(url))
if (bitmap == null) {
	downLoadFromNetOnAsyncTask(url);
	// set the bitmap from menory
}

Bitmap getBitmap(String key){
	Bitmap bitmap = null;
	if ((bitmap = lruCache.get(key)) == null) {
		DiskLruCache.Snapshot snapShot = diskLruCache.get(key);  
		if (snapShot != null) {  
		    InputStream is = snapShot.getInputStream(0);  
		    bitmap = BitmapFactory.decodeStream(is); 
		    lruCache.put(bitmap);
		}
	}
	return bitmap;
}
void downLoadFromNetOnAsyncTask(String url) {
	// download a picture via Thread
	DiskLruCache.Editor editor = diskLruCache.edit(generateKey(url));
	OuputStream ouputStream = editor.newOutputStream(0);
	storeToMemoryAndDisk();
}

日志文件格式

This cache uses a journal file named "journal". A typical journal file
looks like this:
libcore.io.DiskLruCache
1
100
2

CLEAN 3400330d1dfc7f3f7f4b8d4d803dfcf6 832 21054
DIRTY 335c4c6028171cfddfbaae1a9c313c52
CLEAN 335c4c6028171cfddfbaae1a9c313c52 3934 2342
REMOVE 335c4c6028171cfddfbaae1a9c313c52
DIRTY 1ab96a171faeeee38496d8b330771a7a
CLEAN 1ab96a171faeeee38496d8b330771a7a 1600 234
READ 335c4c6028171cfddfbaae1a9c313c52
READ 3400330d1dfc7f3f7f4b8d4d803dfcf6

The first five lines of the journal form its header. They are the constant string "libcore.io.DiskLruCache", the disk cache's version,the application's version, the value count, and a blank line.

Each of the subsequent lines in the file is a record of the state of a cache entry. Each line contains space-separated values: a state, a key,and optional state-specific values.

• DIRTY lines track that an entry is actively being created or updated.Every successful DIRTY action should be followed by a CLEAN or REMOVE action. DIRTY lines without a matching CLEAN or REMOVE indicate thattemporary files may need to be deleted.
• CLEAN lines track a cache entry that has been successfully published and may be read. A publish line is followed by the lengths of each of its values.
• READ lines track accesses for LRU.
• REMOVE lines track entries that have been deleted.

源码分析

public final class DiskLruCache implements Closeable {
    static final String JOURNAL_FILE = "journal";
    static final String JOURNAL_FILE_TMP = "journal.tmp";
    static final String MAGIC = "libcore.io.DiskLruCache";
    static final String VERSION_1 = "1";
    static final long ANY_SEQUENCE_NUMBER = -1;
    private static final String CLEAN = "CLEAN";
    private static final String DIRTY = "DIRTY";
    private static final String REMOVE = "REMOVE";
    private static final String READ = "READ";

    private final File directory;
    private final File journalFile;
    private final File journalFileTmp;
    private final int appVersion;
    private final long maxSize;
    // 一个键对应几个文件
    private final int valueCount;
    private long size = 0;
    // 全局操作日志文件
    private Writer journalWriter;
    private final LinkedHashMap<String, Entry> lruEntries
            = new LinkedHashMap<String, Entry>(0, 0.75f, true);
    private int redundantOpCount;

    // 用来标识被成功提交的序号
    private long nextSequenceNumber = 0;

    // 用一个线程来处理在冗余操作大于2000后重构日志
    private final ExecutorService executorService = new ThreadPoolExecutor(0, 1,
            60L, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>());
    private final Callable<Void> cleanupCallable = new Callable<Void>() {
        @Override public Void call() throws Exception {
            synchronized (DiskLruCache.this) {
                if (journalWriter == null) {
                    return null; // closed
                }
                trimToSize();
                if (journalRebuildRequired()) {
                    rebuildJournal();
                    redundantOpCount = 0;
                }
            }
            return null;
        }
    };

    private DiskLruCache(File directory, int appVersion, int valueCount, long maxSize) {
        this.directory = directory;
        this.appVersion = appVersion;
        this.journalFile = new File(directory, JOURNAL_FILE);
        this.journalFileTmp = new File(directory, JOURNAL_FILE_TMP);
        this.valueCount = valueCount;
        this.maxSize = maxSize;
    }

    /
    public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)
            throws IOException {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        if (valueCount <= 0) {
            throw new IllegalArgumentException("valueCount <= 0");
        }
        // prefer to pick up where we left off
        DiskLruCache cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
        // 日志文件存在即解析
        if (cache.journalFile.exists()) {
            try {
                cache.readJournal();
                cache.processJournal();
                cache.journalWriter = new BufferedWriter(new FileWriter(cache.journalFile, true));
                return cache;
            } catch (IOException journalIsCorrupt) {
                Libcore.logW("DiskLruCache " + directory + " is corrupt: "
                        + journalIsCorrupt.getMessage() + ", removing");
                cache.delete();
            }
        }

        // 不存在则创建新的日志文件，然后重建日志文件。
        directory.mkdirs();
        cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
        cache.rebuildJournal();
        return cache;
    }

	// 读日志文件
    private void readJournal() throws IOException {
        InputStream in = new BufferedInputStream(new FileInputStream(journalFile));
        try {
            String magic = Streams.readAsciiLine(in);
            String version = Streams.readAsciiLine(in);
            String appVersionString = Streams.readAsciiLine(in);
            String valueCountString = Streams.readAsciiLine(in);
            String blank = Streams.readAsciiLine(in);
            if (!MAGIC.equals(magic)
                    || !VERSION_1.equals(version)
                    || !Integer.toString(appVersion).equals(appVersionString)
                    || !Integer.toString(valueCount).equals(valueCountString)
                    || !"".equals(blank)) {
                throw new IOException("unexpected journal header: ["
                        + magic + ", " + version + ", " + valueCountString + ", " + blank + "]");
            }

            while (true) {
                try {
                	// 循环读取每一行
                    readJournalLine(Streams.readAsciiLine(in));
                } catch (EOFException endOfJournal) {
                    break;
                }
            }
        } finally {
            IoUtils.closeQuietly(in);
        }
    }

    // 读每一行，根据每行的字符串构建Entry
    private void readJournalLine(String line) throws IOException {
        String[] parts = line.split(" ");
        if (parts.length < 2) {
            throw new IOException("unexpected journal line: " + line);
        }

        String key = parts[1];
        if (parts[0].equals(REMOVE) && parts.length == 2) {
            lruEntries.remove(key);
            return;
        }
        // 如果存在对key的操作会使其移动到链表尾
        Entry entry = lruEntries.get(key);
        if (entry == null) {
        	// 如果不存在则添加
            entry = new Entry(key);
            lruEntries.put(key, entry);
        }

        if (parts[0].equals(CLEAN) && parts.length == 2 + valueCount) {
            entry.readable = true;
            entry.currentEditor = null;
            entry.setLengths(Arrays.copyOfRange(parts, 2, parts.length));
        } else if (parts[0].equals(DIRTY) && parts.length == 2) {
            entry.currentEditor = new Editor(entry);
        } else if (parts[0].equals(READ) && parts.length == 2) {
            // this work was already done by calling lruEntries.get()
            // 如果为READ则什么都不需要做。上面这句翻译一下就是说这里要做的工作已经在调用lruEntries.get()时做过了
            // 遇到READ其实就是再次访问该key，因此上面调用get的时候已经将其移动到最近使用的位置了
        } else {
            throw new IOException("unexpected journal line: " + line);
        }
    }

    // 处理日志，将添加到Map中的所有键所占的磁盘空间加起来。赋值给size
    private void processJournal() throws IOException {
    	// 如果有日志文件的备份文件存在则删除它
        deleteIfExists(journalFileTmp);
        for (Iterator<Entry> i = lruEntries.values().iterator(); i.hasNext();) {
            Entry entry = i.next();
            if (entry.currentEditor == null) {
                for (int t = 0; t < valueCount; t++) {
                    size += entry.lengths[t];
                }
            } else {
            	// 当前条目正在被编辑，删除正在编辑的文件并将currentEditor赋值为null
                entry.currentEditor = null;
                for (int t = 0; t < valueCount; t++) {
                    deleteIfExists(entry.getCleanFile(t));
                    deleteIfExists(entry.getDirtyFile(t));
                }
                i.remove();
            }
        }
    }
    // 创建新的日志文件，忽略掉冗余的信息，也就是对hashmap里面的元素进行遍历重新写日志文件
    private synchronized void rebuildJournal() throws IOException {
        if (journalWriter != null) {
            journalWriter.close();
        }
        // 创建临时日志文件
        Writer writer = new BufferedWriter(new FileWriter(journalFileTmp));
        writer.write(MAGIC);
        writer.write("\\n");
        writer.write(VERSION_1);
        writer.write("\\n");
        writer.write(Integer.toString(appVersion));
        writer.write("\\n");
        writer.write(Integer.toString(valueCount));
        writer.write("\\n");
        writer.write("\\n");
        // 遍历Map写入日志文件
        for (Entry entry : lruEntries.values()) {
            if (entry.currentEditor != null) {
                writer.write(DIRTY + ' ' + entry.key + '\\n');
            } else {
                writer.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\\n');
            }
        }
        writer.close();
        // 重命名
        journalFileTmp.renameTo(journalFile);
        journalWriter = new BufferedWriter(new FileWriter(journalFile, true));
    }

    private static void deleteIfExists(File file) throws IOException {
        Libcore.deleteIfExists(file);
    }

    // 获取某个key对应的快照，通过该快照可以恢复到该缓存的文件
    public synchronized Snapshot get(String key) throws IOException {
        checkNotClosed();
        validateKey(key);
        // 对它的访问会让他移动到Map的最近使用过的位置
        Entry entry = lruEntries.get(key);
        if (entry == null) {
            return null;
        }

        if (!entry.readable) {
            return null;
        }

        /*
         * Open all streams eagerly to guarantee that we see a single published
         * snapshot. If we opened streams lazily then the streams could come
         * from different edits.
         */
        InputStream[] ins = new InputStream[valueCount];
        try {
            for (int i = 0; i < valueCount; i++) {
            	// 通过干净的文件创建的流，于是如果获取某个键对应的文件还未被编辑完成，文件不存在，
            	// 那么返回null，因此可以在从硬盘获取时根据返回值是否为null判断硬盘是否有该缓存
                ins[i] = new FileInputStream(entry.getCleanFile(i));
            }
        } catch (FileNotFoundException e) {
            // a file must have been deleted manually!
            return null;
        }

        redundantOpCount++;
        journalWriter.append(READ + ' ' + key + '\\n');
        if (journalRebuildRequired()) {
            executorService.submit(cleanupCallable);
        }
        return new Snapshot(key, entry.sequenceNumber, ins);
    }

    /**
     * Returns an editor for the entry named {@code key}, or null if another
     * edit is in progress.
     */
    public Editor edit(String key) throws IOException {
        return edit(key, ANY_SEQUENCE_NUMBER);
    }

    // 当DiskLruCache类调用edit方法传入的都是ANY_SEQUENCE_NUMBER，Snapshot调用edit的时候入当前Entry的序列号
    // 目的是当Snapshot调用edit的时候如果该条目的序列号已经改变了(在持有这个Snapshot后又成功commit了)就会返回null
    // 这也就是为什么Snapshot命名为快照的含义。
    private synchronized Editor edit(String key, long expectedSequenceNumber) throws IOException {
        checkNotClosed();
        validateKey(key);
        Entry entry = lruEntries.get(key);
        if (expectedSequenceNumber != ANY_SEQUENCE_NUMBER
                && (entry == null || entry.sequenceNumber != expectedSequenceNumber)) {
            return null; // snapshot is stale
        }
        if (entry == null) {
            entry = new Entry(key);
            // 只要edit，就put进去，这样在commit的时候如果出错就lrucache.remove，并且添加REMOVE记录。
            // 成功不用再put。除了put进HashMap中的记录，其余的全部属于冗余的。其他版本用总行数-lrucache.size
            lruEntries.put(key, entry);
        } else if (entry.currentEditor != null) {
            return null; // another edit is in progress
        }

        Editor editor = new Editor(entry);
        entry.currentEditor = editor;
        // 写入一条脏数记录
        journalWriter.write(DIRTY + ' ' + key + '\\n');
        journalWriter.flush();
        return editor;
    }

    public File getDirectory() {
        return directory;
    }

    public long maxSize() {
        return maxSize;
    }

    public synchronized long size() {
        return size;
    }

    private synchronized void completeEdit(Editor editor, boolean success) throws IOException {
        Entry entry = editor.entry;
        if (entry.currentEditor != editor) {
            throw new IllegalStateException();
        }
        // readabe为false也即该条目还未被成功写入成为CLEAN，也就是首次创建缓存文件
        // 也就是CLEAN必须和DIRTY配对，如果脏的文件不存在就出现异常edit didn't create file
        if (success && !entry.readable) {
            for (int i = 0; i < valueCount; i++) {
                if (!entry.getDirtyFile(i).exists()) {
                    editor.abort();
                    // 一定要在同一个线程里面创建newOutputStream和提交commit，不然可能会出问题
                    throw new IllegalStateException("edit didn't create file " + i);
                }
            }
        }
        // 如果sucess也就是写I/O未出现异常那么将脏的文件重命名成干净的文件然后更新entry的中的文件大小字段，并且更新已经占用的磁盘空间size。
        // 如果false也就是写I/O出错，就将脏的文件删除
        for (int i = 0; i < valueCount; i++) {
            File dirty = entry.getDirtyFile(i);
            if (success) {
                if (dirty.exists()) {
                    File clean = entry.getCleanFile(i);
                    dirty.renameTo(clean);
                    long oldLength = entry.lengths[i];
                    long newLength = clean.length();
                    entry.lengths[i] = newLength;
                    size = size - oldLength + newLength;
                }
            } else {
            	// 将脏的文件删除
                deleteIfExists(dirty);
            }
        }
        redundantOpCount++;
        // 提交完成后不论成功与否，将该条目的currentEditor置为null，以便其他地方可以在同一个key上进行再次edit
        entry.currentEditor = null;
        // readable为true指这个文件已经被成功创建过了。
        // 有两地方对readable赋值为true。一个是在解析日志文件时遇到clean时，此时即在再次访问该条目，要写入到日志文件
        // 另一处就是这里，如果写入成功了success，那么将置为readble置为true，此时还要写入到日志文件
        //　这里条件指要么之前该条目已经被成功的写入过，或者这次成功写入或者都为true
        if (entry.readable | success) {
            entry.readable = true;
            journalWriter.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\\n');
            if (success) {
            	// 每次提交成功就重新赋值该Entry的序列号加一。
                entry.sequenceNumber = nextSequenceNumber++;
            }
        } else {
        	// 首次写入并且写入失败
            lruEntries.remove(entry.key);
            journalWriter.write(REMOVE + ' ' + entry.key + '\\n');
        }
        // 判断是否达到重构日志的条件，执行之。
        if (size > maxSize || journalRebuildRequired()) {
        	// 在一个新的线程里面重构日志
            executorService.submit(cleanupCallable);
        }
    }

    // 判断是否达到重构日志的要求：冗余操作大于有效数据的数目并且大于2000
    private boolean journalRebuildRequired() {
        final int redundantOpCompactThreshold = 2000;
        return redundantOpCount >= redundantOpCompactThreshold
                && redundantOpCount >= lruEntries.size();
    }

    /**
     * Drops the entry for {@code key} if it exists and can be removed. Entries
     * actively being edited cannot be removed.
     *
     * @return true if an entry was removed.
     */
    public synchronized boolean remove(String key) throws IOException {
        checkNotClosed();
        validateKey(key);
        Entry entry = lruEntries.get(key);
        // 正处于编辑状态还未被提交则不能被remove
        if (entry == null || entry.currentEditor != null) {
            return false;
        }

        // 循环删除对应的文件
        for (int i = 0; i < valueCount; i++) {
            File file = entry.getCleanFile(i);
            if (!file.delete()) {
                throw new IOException("failed to delete " + file);
            }
            size -= entry.lengths[i];
            entry.lengths[i] = 0;
        }

        redundantOpCount++;
        // 添加REMOVE记录
        journalWriter.append(REMOVE + ' ' + key + '\\n');
        // 从LinkedHashMap中移除
        lruEntries.remove(key);

        // 判断重构条件
        if (journalRebuildRequired()) {
            executorService.submit(cleanupCallable);
        }

        return true;
    }

    public boolean isClosed() {
        return journalWriter == null;
    }

    private void checkNotClosed() {
        if (journalWriter == null) {
            throw new IllegalStateException("cache is closed");
        }
    }

    public synchronized void flush() throws IOException {
        checkNotClosed();
        trimToSize();
        journalWriter.flush();
    }

	// 关闭DiskLruCache
    public synchronized void close() throws IOException {
        if (journalWriter == null) {
            return; // already closed
        }
        // 终止所有的正处于编辑状态的条目
        for (Entry entry : new ArrayList<Entry>(lruEntries.values())) {
            if (entry.currentEditor != null) {
                entry.currentEditor.abort();
            }
        }
        // 删除最近最少使用的条目
        trimToSize();
        journalWriter.close();
        journalWriter = null;
    }

    // 删除最近最少使用的条目，包括从Map删除和从磁盘删除文件
    private void trimToSize() throws IOException {
        while (size > maxSize) {
            Map.Entry<String, Entry> toEvict = lruEntries.entrySet().iterator().next();
            remove(toEvict.getKey());
        }
    }

    // 删除所有的缓存文件，清除缓存用
    public void delete() throws IOException {
        close();
        IoUtils.deleteContents(directory);
    }

    private void validateKey(String key) {
        if (key.contains(" ") || key.contains("\\n") || key.contains("\\r")) {
            throw new IllegalArgumentException(
                    "keys must not contain spaces or newlines: \\"" + key + "\\"");
        }
    }

    private static String inputStreamToString(InputStream in) throws IOException {
        return Streams.readFully(new InputStreamReader(in, Charsets.UTF_8));
    }

    // DiskLruCache的get方法返回的对象的封装
    // 其实和Entry几乎一样的。为什么要从新封装一个Snapshot类呢？
   	// 1.这个类里面封装了对应的缓存的文件的流，通过该流可以读取到里面的数据
   	// 2.这里面封装的sequenceNumber用于判断该快照是否已经过期，获取到该快照后，如果该key又经过了commit，
   	// 它的序列号已经增加，于是该快照就过期了，调用edit方法返回null
    public final class Snapshot implements Closeable {
    	// 和Entry中的key一样
        private final String key;

        //　顾名思义是序列号的意思，这个值是在调用get方法获得快照的时候从Entry里面读取的。
        private final long sequenceNumber;

        // 对应的输入流的数组，通过它可以读取到文件里面的数据
        private final InputStream[] ins;

        private Snapshot(String key, long sequenceNumber, InputStream[] ins) {
            this.key = key;
            this.sequenceNumber = sequenceNumber;
            this.ins = ins;
        }

        // 通过快照过去对该条目的editor，可以有下面情况
        // 1.如果是已经成功编辑完的则获取一个新的editor
        // 2.该快照在创建后已经被改变了，返回null
        // 3.另一个editor正在编辑还未commit，返回null
        public Editor edit() throws IOException {
            return DiskLruCache.this.edit(key, sequenceNumber);
        }

        /**
         * Returns the unbuffered stream with the value for {@code index}.
         */
        // 获取xxx.index文件对应的流
        public InputStream getInputStream(int index) {
            return ins[index];
        }

        // 如果某个index的流里面是文本，则通过getString获得文本
        public String getString(int index) throws IOException {
            return inputStreamToString(getInputStream(index));
        }

        @Override public void close() {
            for (InputStream in : ins) {
                IoUtils.closeQuietly(in);
            }
        }
    }

    // 通过DiskLruCache的edit方法获取该Editor对象，用来完成对Entry的编辑
    public final class Editor {
    	// 每个editor编辑一个entry条目
        private final Entry entry;

        // 标识在I/O过程中是否有错误发生
        private boolean hasErrors;

        private Editor(Entry entry) {
            this.entry = entry;
        }

        // 获取某个xxx.index文件的流，即用来读取改文件
        public InputStream newInputStream(int index) throws IOException {
            synchronized (DiskLruCache.this) {
                if (entry.currentEditor != this) {
                    throw new IllegalStateException();
                }
                if (!entry.readable) {
                    return null;
                }
                return new FileInputStream(entry.getCleanFile(index));
            }
        }

        // 以String的方式获取最上一次提交的值，也就是获取该条目中的xxx.index文件的内容。
        // 一般用于改文件保存字符串时使用
        public String getString(int index) throws IOException {
            InputStream in = newInputStream(index);
            return in != null ? inputStreamToString(in) : null;
        }

        // 返回一个OutputStream，也就是被封装成FaultHidingOutputStream，不会再抛出I/O异常
        // The returned output stream does not throw IOExceptions.
        public OutputStream newOutputStream(int index) throws IOException {
            synchronized (DiskLruCache.this) {
                if (entry.currentEditor != this) {
                    throw new IllegalStateException();
                }
                // 注意这里用的是getDirtyFile
                return new FaultHidingOutputStream(new FileOutputStream(entry.getDirtyFile(index)));
            }
        }

        // 用于往xx.index.tmp文件里面写文本
        // 一般用于改文件保存字符串时使用
        // 其实只是在上面newOutputStream的基础上增加了往文件里面写数据的代码，毕竟写几个字符串简单嘛
        // 如果是从网络获取较大的图片或者文件，还是自己拿到这个OutputStream然后网里面写比较好
        public void set(int index, String value) throws IOException {
            Writer writer = null;
            try {
                writer = new OutputStreamWriter(newOutputStream(index), Charsets.UTF_8);
                writer.write(value);
            } finally {
                IoUtils.closeQuietly(writer);
            }
        }

        /**
         * Commits this edit so it is visible to readers.  This releases the
         * edit lock so another edit may be started on the same key.
         */
        // 每次edit后写完数据需要调用commit函数
        // 调用commit的原因：
        // 1.根据流的写过程是否出错调用completeEdit函数执行不同的逻辑
        // 2.在completeEdit函数里最重要的一点是entry.currentEditor = null;另一个edit才能在同一个key上编辑写入，这一点参考323行，每次调用edit方法如果在该key对应的条目的currentEditor部位null说明有一个edit正在编辑它，就要返回null，即现在不允许在对他进行编辑
        public void commit() throws IOException {
            if (hasErrors) {
            	// 出现I/O错误
                completeEdit(this, false);
                remove(entry.key); // the previous entry is stale
            } else {
            	// 正常情况
                completeEdit(this, true);
            }
        }

        // 终止edit数据
        public void abort() throws IOException {
            completeEdit(this, false);
        }

		// 封装的OutputStream，好处是屏蔽掉所有可能出现I/O异常的地方
		// 如果出现I/O异常则将hasErrors赋值为false，这样后面处理逻辑简单
		// 只需要将其理解为不抛出异常而是置位hasErroes标志的OutputStream即可
        private final class FaultHidingOutputStream extends FilterOutputStream {
            private FaultHidingOutputStream(OutputStream out) {
                super(out);
            }

            @Override public void write(int oneByte) {
                try {
                    out.write(oneByte);
                } catch (IOException e) {
                    hasErrors = true;
                }
            }

            @Override public void write(byte[] buffer, int offset, int length) {
                try {
                    out.write(buffer, offset, length);
                } catch (IOException e) {
                    hasErrors = true;
                }
            }

            @Override public void close() {
                try {
                    out.close();
                } catch (IOException e) {
                    hasErrors = true;
                }
            }

            @Override public void flush() {
                try {
                    out.flush();
                } catch (IOException e) {
                    hasErrors = true;
                }
            }
        }
    }

    // LinkedHashMap中的value条目，封装了一些简单的信息
    private final class Entry {
    	// key，一般是url的MD5
        private final String key;

        // 所对应的文件的大小的数组，如：12876 1567
        private final long[] lengths;

        // 如果该条目被提交过一次即为true
        private boolean readable;

        // 该条目所对应的editor，如果正在被edit不为null，否则(已经被写完了，commited)为null
        private Editor currentEditor;

        /** The sequence number of the most recently committed edit to this entry. */
        private long sequenceNumber;

        private Entry(String key) {
            this.key = key;
            this.lengths = new long[valueCount];
        }

        // 获取该条目对应的文件的大小的数组的字符串，用于写journal
        public String getLengths() throws IOException {
            StringBuilder result = new StringBuilder();
            for (long size : lengths) {
                result.append(' ').append(size);
            }
            return result.toString();
        }

        //读取journal文件，解析每行的条目填充该entry
        private void setLengths(String[] strings) throws IOException {
            if (strings.length != valueCount) {
                throw invalidLengths(strings);
            }

            try {
                for (int i = 0; i < strings.length; i++) {
                    lengths[i] = Long.parseLong(strings[i]);
                }
            } catch (NumberFormatException e) {
                throw invalidLengths(strings);
            }
        }

        // 当journal文件的某一行所对应的文件的个数的和valueCount不匹配时调用
        private IOException invalidLengths(String[] strings) throws IOException {
            throw new IOException("unexpected journal line: " + Arrays.toString(strings));
        }

        // 获取“干净的”文件：xxx.i
        public File getCleanFile(int i) {
            return new File(directory, key + "." + i);
        }

        // 获取“脏的”文件：xxx.i.tmp
        public File getDirtyFile(int i) {
            return new File(directory, key + "." + i + ".tmp");
        }
    }
}