Android4.0以上源码中的DiskLruCache类简介

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


package com.example.android.bitmapfun.util;


import java.io.BufferedInputStream;
import java.io.BufferedWriter;
import java.io.Closeable;
import java.io.EOFException;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.Reader;
import java.io.StringWriter;
import java.io.Writer;
import java.lang.reflect.Array;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;


/**
 ******************************************************************************
 * Taken from the JB source code, can be found in:
 * libcore/luni/src/main/java/libcore/io/DiskLruCache.java
 * or direct link:
 * https://android.googlesource.com/platform/libcore/+/android-4.1.1_r1/luni/src/main/java/libcore/io/DiskLruCache.java
 ******************************************************************************
 *
 * A cache that uses a bounded amount of space on a filesystem. Each cache
 * entry has a string key and a fixed number of values. Values are byte
 * sequences, accessible as streams or files. Each value must be between {@code
 * 0} and {@code Integer.MAX_VALUE} bytes in length.
 * 一个使用空间大小有边界的文件cache，每一个entry包含一个key和values。values是byte序列，按文件或者流来访问的。
 * 每一个value的长度在0---Integer.MAX_VALUE之间。
 *
 * <p>The cache stores its data in a directory on the filesystem. This
 * directory must be exclusive to the cache; the cache may delete or overwrite
 * files from its directory. It is an error for multiple processes to use the
 * same cache directory at the same time.
 * cache使用目录文件存储数据。文件路径必须是唯一的，可以删除和重写目录文件。多个进程同时使用同样的文件目录是不正确的
 * 
 * <p>This cache limits the number of bytes that it will store on the
 * filesystem. When the number of stored bytes exceeds the limit, the cache will
 * remove entries in the background until the limit is satisfied. The limit is
 * not strict: the cache may temporarily exceed it while waiting for files to be
 * deleted. The limit does not include filesystem overhead or the cache
 * journal so space-sensitive applications should set a conservative limit.
 * cache限制了大小，当超出空间大小时，cache就会后台删除entry直到空间没有达到上限为止。空间大小限制不是严格的，
 * cache可能会暂时超过limit在等待文件删除的过程中。cache的limit不包括文件系统的头部和日志，
 * 所以空间大小敏感的应用应当设置一个保守的limit大小
 *
 * <p>Clients call {@link #edit} to create or update the values of an entry. An
 * entry may have only one editor at one time; if a value is not available to be
 * edited then {@link #edit} will return null.
 * <ul>
 *     <li>When an entry is being <strong>created</strong> it is necessary to
 *         supply a full set of values; the empty value should be used as a
 *         placeholder if necessary.
 *     <li>When an entry is being <strong>edited</strong>, it is not necessary
 *         to supply data for every value; values default to their previous
 *         value.
 * </ul>
 * Every {@link #edit} call must be matched by a call to {@link Editor#commit}
 * or {@link Editor#abort}. Committing is atomic: a read observes the full set
 * of values as they were before or after the commit, but never a mix of values.
 *调用edit（）来创建或者更新entry的值，一个entry同时只能有一个editor；如果值不可被编辑就返回null。
 *当entry被创建时必须提供一个value。空的value应当用占位符表示。当entry被编辑的时候，必须提供value。
 *每次调用必须有匹配Editor commit或abort，commit是原子操作，读必须在commit前或者后，不会造成值混乱。
 *
 * <p>Clients call {@link #get} to read a snapshot of an entry. The read will
 * observe the value at the time that {@link #get} was called. Updates and
 * removals after the call do not impact ongoing reads.
 * 调用get来读entry的快照。当get调用时读者读其值，更新或者删除不会影响先前的读
 *
 * <p>This class is tolerant of some I/O errors. If files are missing from the
 * filesystem, the corresponding entries will be dropped from the cache. If
 * an error occurs while writing a cache value, the edit will fail silently.
 * Callers should handle other problems by catching {@code IOException} and
 * responding appropriately.
 * 该类可以容忍一些I/O errors。如果文件丢失啦，相应的entry就会被drop。写cache时如果error发生，edit将失败。
 * 调用者应当相应的处理其它问题
 */
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 static final Charset UTF_8 = Charset.forName("UTF-8");
    private static final int IO_BUFFER_SIZE = 8 * 1024;//8K


    /*
     * This cache uses a journal file named "journal". A typical journal file
     * looks like this:
     *     libcore.io.DiskLruCache
     *     1           //the disk cache's version
     *     100         //the application's version
     *     2           //value count
     *
     *    //state  key                            optional
     *     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.
     *   o 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 that
     *     temporary files may need to be deleted.
     *     Dirty是entry被创建或者更新，每一个dirty应当被clean或remove action，如果有一行dirty没有
     *     匹配的clean或Remove action，就表示临时文件需要被删除。
     *   o 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.
     *     Clean entry已经成功的发布并且可能会被读过。一个发布行
     *   o READ lines track accesses for LRU.
     *   Read表示LRU访问
     *   o REMOVE lines track entries that have been deleted.
     *   Remove表示entry已经被删除
     *   
     * The journal file is appended to as cache operations occur. The journal may
     * occasionally be compacted by dropping redundant lines. A temporary file named
     * "journal.tmp" will be used during compaction; that file should be deleted if
     * it exists when the cache is opened.
     * 日志文件在cache操作发生时添加，日志可能O尔删除的冗余行来压缩。一个临时的名字为journal.tmp的文件将被使用
     * 在压缩期间。当cache被opened的时候文件应当被删除。
     */


    private final File directory;
    private final File journalFile;//日志文件
    private final File journalFileTmp;//日志文件临时文件
    private final int appVersion;//应用ersion
    private final long maxSize;//最大空间
    private final int valueCount;//key对应的value的个数
    private long size = 0;
    private Writer journalWriter;
    private final LinkedHashMap<String, Entry> lruEntries
            = new LinkedHashMap<String, Entry>(0, 0.75f, true);
    private int redundantOpCount;


    /**
     * To differentiate between old and current snapshots, each entry is given
     * a sequence number each time an edit is committed. A snapshot is stale if
     * its sequence number is not equal to its entry's sequence number.
     * 区分老的和当前的快照，每一个实体在每次编辑被committed时都被赋予一个序列号。
     * 一个快照的序列号如果不等于entry的序列号那它就是废弃的。
     */
    private long nextSequenceNumber = 0;


    //数组拷贝
    /* From java.util.Arrays */
    @SuppressWarnings("unchecked")
    private static <T> T[] copyOfRange(T[] original, int start, int end) {
        final int originalLength = original.length; // For exception priority compatibility.
        if (start > end) {
            throw new IllegalArgumentException();
        }
        if (start < 0 || start > originalLength) {
            throw new ArrayIndexOutOfBoundsException();
        }
        final int resultLength = end - start;
        final int copyLength = Math.min(resultLength, originalLength - start);
        final T[] result = (T[]) Array
                .newInstance(original.getClass().getComponentType(), resultLength);
        System.arraycopy(original, start, result, 0, copyLength);
        return result;
    }


    /**
     * Returns the remainder of 'reader' as a string, closing it when done.
     * 返回String的值，然后close
     */
    public static String readFully(Reader reader) throws IOException {
        try {
            StringWriter writer = new StringWriter();
            char[] buffer = new char[1024];
            int count;
            while ((count = reader.read(buffer)) != -1) {
                writer.write(buffer, 0, count);
            }
            return writer.toString();
        } finally {
            reader.close();
        }
    }


    /**
     * Returns the ASCII characters up to but not including the next "\r\n", or
     * "\n".
     *
     * @throws java.io.EOFException if the stream is exhausted before the next newline
     *     character.
     *  读取输入流中返回的某行ASCII码字符
     */
    public static String readAsciiLine(InputStream in) throws IOException {
        // TODO: support UTF-8 here instead


        StringBuilder result = new StringBuilder(80);
        while (true) {
            int c = in.read();
            if (c == -1) {
                throw new EOFException();
            } else if (c == '\n') {
                break;
            }


            result.append((char) c);
        }
        int length = result.length();
        if (length > 0 && result.charAt(length - 1) == '\r') {
            result.setLength(length - 1);
        }
        return result.toString();
    }


    /**
     * Closes 'closeable', ignoring any checked exceptions. Does nothing if 'closeable' is null.
     * closeable关闭
     */
    public static void closeQuietly(Closeable closeable) {
        if (closeable != null) {
            try {
                closeable.close();
            } catch (RuntimeException rethrown) {
                throw rethrown;
            } catch (Exception ignored) {
            }
        }
    }


    /**
     * Recursively delete everything in {@code dir}.
     * 递归删除dir
     */
    // TODO: this should specify paths as Strings rather than as Files
    public static void deleteContents(File dir) throws IOException {
        File[] files = dir.listFiles();
        if (files == null) {
            throw new IllegalArgumentException("not a directory: " + dir);
        }
        for (File file : files) {
            if (file.isDirectory()) {
                deleteContents(file);
            }
            if (!file.delete()) {
                throw new IOException("failed to delete file: " + file);
            }
        }
    }


    /** This cache uses a single background thread to evict entries.
     *  后台单线程回收entry  
     */
    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();//回收到满足maxsize
                if (journalRebuildRequired()) {
                    rebuildJournal();
                    redundantOpCount = 0;
                }
            }
            return null;
        }
    };


    //构造器
    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;
    }


    /**
     * Opens the cache in {@code directory}, creating a cache if none exists
     * there.
     * 创建cache
     * 
     * @param directory a writable directory
     * @param appVersion
     * @param valueCount the number of values per cache entry. Must be positive.
     * 每一个key相对应的value的数目
     * @param maxSize the maximum number of bytes this cache should use to store
     * @throws IOException if reading or writing the cache directory fails
     */
    public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)
            throws IOException {
        if (maxSize <= 0) {//maxsize必须大于0
            throw new IllegalArgumentException("maxSize <= 0");
        }
        if (valueCount <= 0) {//valuecount也必须大于0
            throw new IllegalArgumentException("valueCount <= 0");
        }


        // prefer to pick up where we left off优先处理先前的cache
        DiskLruCache cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
        if (cache.journalFile.exists()) {
            try {
                cache.readJournal();
                cache.processJournal();
                cache.journalWriter = new BufferedWriter(new FileWriter(cache.journalFile, true),
                        IO_BUFFER_SIZE);
                return cache;
            } catch (IOException journalIsCorrupt) {
//                System.logW("DiskLruCache " + directory + " is corrupt: "
//                        + journalIsCorrupt.getMessage() + ", removing");
                cache.delete();
            }
        }


        // create a new empty cache创建一个空新的cache
        directory.mkdirs();
        cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
        cache.rebuildJournal();
        return cache;
    }


    //读取日志信息
    private void readJournal() throws IOException {
        InputStream in = new BufferedInputStream(new FileInputStream(journalFile), IO_BUFFER_SIZE);
        try {
            String magic = readAsciiLine(in);
            String version = readAsciiLine(in);
            String appVersionString = readAsciiLine(in);
            String valueCountString = readAsciiLine(in);
            String blank = 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 {
                    readJournalLine(readAsciiLine(in));//读取日志信息
                } catch (EOFException endOfJournal) {
                    break;
                }
            }
        } finally {
            closeQuietly(in);//关闭输入流
        }
    }


    //读取日志中某行日志信息
    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;
        }


        Entry entry = lruEntries.get(key);
        if (entry == null) {
            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(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()
        } else {
            throw new IOException("unexpected journal line: " + line);
        }
    }


    /**
     * Computes the initial size and collects garbage as a part of opening the
     * cache. Dirty entries are assumed to be inconsistent and will be deleted.
     * 处理日志
     * 计算初始化cache的初始化大小和收集垃圾。Dirty entry假定不一致将会被删掉。
     */
    private void processJournal() throws IOException {
        deleteIfExists(journalFileTmp);//删除日志文件
        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 {
                entry.currentEditor = null;
                for (int t = 0; t < valueCount; t++) {
                    deleteIfExists(entry.getCleanFile(t));
                    deleteIfExists(entry.getDirtyFile(t));
                }
                i.remove();
            }
        }
    }


    /**
     * Creates a new journal that omits redundant information. This replaces the
     * current journal if it exists.
     * 创建一个新的删掉冗余信息的日志。替换当前的日志
     */
    private synchronized void rebuildJournal() throws IOException {
        if (journalWriter != null) {
            journalWriter.close();
        }


        Writer writer = new BufferedWriter(new FileWriter(journalFileTmp), IO_BUFFER_SIZE);
        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");


        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();
        journalFileTmp.renameTo(journalFile);
        journalWriter = new BufferedWriter(new FileWriter(journalFile, true), IO_BUFFER_SIZE);
    }


    //文件若存在删除
    private static void deleteIfExists(File file) throws IOException {
//        try {
//            Libcore.os.remove(file.getPath());
//        } catch (ErrnoException errnoException) {
//            if (errnoException.errno != OsConstants.ENOENT) {
//                throw errnoException.rethrowAsIOException();
//            }
//        }
        if (file.exists() && !file.delete()) {
            throw new IOException();
        }
    }


    /**
     * Returns a snapshot of the entry named {@code key}, or null if it doesn't
     * exist is not currently readable. If a value is returned, it is moved to
     * the head of the LRU queue.
     * 返回key对应的entry的snapshot，当key相应的entry不存在或者当前不可读时返回null。
     * 如果返回相应的值，它就会被移动到LRU队列的头部。
     */
    public synchronized Snapshot get(String key) throws IOException {
        checkNotClosed();//检查cache是否已关闭
        validateKey(key);//验证key格式的正确性
        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++) {
                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);
    }


    //有key和序列号生成一个editor
    private synchronized Editor edit(String key, long expectedSequenceNumber) throws IOException {
        checkNotClosed();//检查cache关闭与否
        validateKey(key);//验证key格式正确性
        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);
            lruEntries.put(key, entry);
        } else if (entry.currentEditor != null) {
            return null; // another edit is in progress
        }


        Editor editor = new Editor(entry);
        entry.currentEditor = editor;


        // flush the journal before creating files to prevent file leaks
        journalWriter.write(DIRTY + ' ' + key + '\n');
        journalWriter.flush();
        return editor;
    }


    /**
     * Returns the directory where this cache stores its data.
     */
    public File getDirectory() {
        return directory;
    }


    /**
     * Returns the maximum number of bytes that this cache should use to store
     * its data.
     */
    public long maxSize() {
        return maxSize;
    }


    /**
     * Returns the number of bytes currently being used to store the values in
     * this cache. This may be greater than the max size if a background
     * deletion is pending.
     */
    public synchronized long size() {
        return size;
    }


    //完成Edit动作
    private synchronized void completeEdit(Editor editor, boolean success) throws IOException {
        Entry entry = editor.entry;
        if (entry.currentEditor != editor) {
            throw new IllegalStateException();
        }


        // if this edit is creating the entry for the first time, every index must have a value
        if (success && !entry.readable) {
            for (int i = 0; i < valueCount; i++) {
                if (!entry.getDirtyFile(i).exists()) {
                    editor.abort();
                    throw new IllegalStateException("edit didn't create file " + i);
                }
            }
        }


        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 {
                deleteIfExists(dirty);
            }
        }


        redundantOpCount++;
        entry.currentEditor = null;
        if (entry.readable | success) {
            entry.readable = true;
            journalWriter.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');
            if (success) {
                entry.sequenceNumber = nextSequenceNumber++;
            }
        } else {
            lruEntries.remove(entry.key);
            journalWriter.write(REMOVE + ' ' + entry.key + '\n');
        }


        if (size > maxSize || journalRebuildRequired()) {
            executorService.submit(cleanupCallable);
        }
    }


    /**
     * We only rebuild the journal when it will halve the size of the journal
     * and eliminate at least 2000 ops.
     * 当日志大小减半并且删掉至少2000项时重新构造日志
     */
    private boolean journalRebuildRequired() {
        final int REDUNDANT_OP_COMPACT_THRESHOLD = 2000;
        return redundantOpCount >= REDUNDANT_OP_COMPACT_THRESHOLD
                && redundantOpCount >= lruEntries.size();
    }


    /**
     * Drops the entry for {@code key} if it exists and can be removed. Entries
     * actively being edited cannot be removed.
     * 删除key相应的entry，被编辑的Entry不能被remove
     * @return true if an entry was removed.
     */
    public synchronized boolean remove(String key) throws IOException {
        checkNotClosed();//检查cache是否已经关闭
        validateKey(key);//验证key格式的正确性
        Entry entry = lruEntries.get(key);
        if (entry == null || entry.currentEditor != null) {
            return false;
        }


        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++;
        journalWriter.append(REMOVE + ' ' + key + '\n');
        lruEntries.remove(key);


        if (journalRebuildRequired()) {
            executorService.submit(cleanupCallable);
        }


        return true;
    }


    /**
     * Returns true if this cache has been closed.
     * 判断cache是否已经关闭
     */
    public boolean isClosed() {
        return journalWriter == null;
    }


    //检查cache是否已经关闭
    private void checkNotClosed() {
        if (journalWriter == null) {
            throw new IllegalStateException("cache is closed");
        }
    }


    /**
     * Force buffered operations to the filesystem.
     */
    public synchronized void flush() throws IOException {
        checkNotClosed();//检查cache是否关闭
        trimToSize();//满足最大空间limit
        journalWriter.flush();
    }


    /**
     * Closes this cache. Stored values will remain on the filesystem.
     * 关闭cache。
     */
    public synchronized void close() throws IOException {
        if (journalWriter == null) {
            return; // already closed
        }
        for (Entry entry : new ArrayList<Entry>(lruEntries.values())) {
            if (entry.currentEditor != null) {
                entry.currentEditor.abort();
            }
        }
        trimToSize();
        journalWriter.close();
        journalWriter = null;
    }


    //回收删除某些entry到空间大小满足maxsize
    private void trimToSize() throws IOException {
        while (size > maxSize) {
//            Map.Entry<String, Entry> toEvict = lruEntries.eldest();
            final Map.Entry<String, Entry> toEvict = lruEntries.entrySet().iterator().next();
            remove(toEvict.getKey());
        }
    }


    /**
     * Closes the cache and deletes all of its stored values. This will delete
     * all files in the cache directory including files that weren't created by
     * the cache.
     * 关闭删除cache
     */
    public void delete() throws IOException {
        close();
        deleteContents(directory);
    }


    //验证key格式的正确性
    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 + "\"");
        }
    }


    //字符串形式读出输入流的内容
    private static String inputStreamToString(InputStream in) throws IOException {
        return readFully(new InputStreamReader(in, UTF_8));
    }


    /**
     * A snapshot of the values for an entry.
     * entry的快照
     */
    public final class Snapshot implements Closeable {
        private final String key;//key
        private final long sequenceNumber;//序列号（同文件名称）
        private final InputStream[] ins;//两个修改的文件输入流


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


        /**
         * Returns an editor for this snapshot's entry, or null if either the
         * entry has changed since this snapshot was created or if another edit
         * is in progress.
         * 返回entry快照的editor，如果entry已经更新了或者另一个edit正在处理过程中返回null。
         */
        public Editor edit() throws IOException {
            return DiskLruCache.this.edit(key, sequenceNumber);
        }


        /**
         * Returns the unbuffered stream with the value for {@code index}.
         */
        public InputStream getInputStream(int index) {
            return ins[index];
        }


        /**
         * Returns the string value for {@code index}.
         */
        public String getString(int index) throws IOException {
            return inputStreamToString(getInputStream(index));
        }


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


    /**
     * Edits the values for an entry.
     * entry编辑器
     */
    public final class Editor {
        private final Entry entry;
        private boolean hasErrors;


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


        /**
         * Returns an unbuffered input stream to read the last committed value,
         * or null if no value has been committed.
         * 返回一个最后提交的entry的不缓存输入流，如果没有值被提交过返回null
         */
        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));
            }
        }


        /**
         * Returns the last committed value as a string, or null if no value
         * has been committed.
         * 返回最后提交的entry的文件内容，字符串形式
         */
        public String getString(int index) throws IOException {
            InputStream in = newInputStream(index);
            return in != null ? inputStreamToString(in) : null;
        }


        /**
         * Returns a new unbuffered output stream to write the value at
         * {@code index}. If the underlying output stream encounters errors
         * when writing to the filesystem, this edit will be aborted when
         * {@link #commit} is called. The returned output stream does not throw
         * IOExceptions.
         * 返回一个新的无缓冲的输出流，写文件时如果潜在的输出流存在错误，这个edit将被废弃。
         */
        public OutputStream newOutputStream(int index) throws IOException {
            synchronized (DiskLruCache.this) {
                if (entry.currentEditor != this) {
                    throw new IllegalStateException();
                }
                return new FaultHidingOutputStream(new FileOutputStream(entry.getDirtyFile(index)));
            }
        }


        /**
         * Sets the value at {@code index} to {@code value}.
         * 设置entry的value的文件的内容
         */
        public void set(int index, String value) throws IOException {
            Writer writer = null;
            try {
                writer = new OutputStreamWriter(newOutputStream(index), UTF_8);
                writer.write(value);
            } finally {
                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.
         * commit提交编辑的结果，释放edit锁然后其它edit可以启动
         */
        public void commit() throws IOException {
            if (hasErrors) {
                completeEdit(this, false);
                remove(entry.key); // the previous entry is stale
            } else {
                completeEdit(this, true);
            }
        }


        /**
         * Aborts this edit. This releases the edit lock so another edit may be
         * started on the same key.
         * 废弃edit，释放edit锁然后其它edit可以启动
         */
        public void abort() throws IOException {
            completeEdit(this, false);
        }


        //包装的输出流类
        private 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;
                }
            }
        }
    }


    /*
     * Entry 最终类
     */
    private final class Entry {
        private final String key;


        /** Lengths of this entry's files. */
        private final long[] lengths;//每一个cache文件的长度


        /** True if this entry has ever been published */
        private boolean readable;


        /** The ongoing edit or null if this entry is not being edited. */
        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];
        }


        public String getLengths() throws IOException {
            StringBuilder result = new StringBuilder();
            for (long size : lengths) {
                result.append(' ').append(size);
            }
            return result.toString();
        }


        /**
         * Set lengths using decimal numbers like "10123".
         * 设置每一个cache文件的长度（即lengths[i]的长度）
         */
        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);
            }
        }


        private IOException invalidLengths(String[] strings) throws IOException {
            throw new IOException("unexpected journal line: " + Arrays.toString(strings));
        }


        public File getCleanFile(int i) {
            return new File(directory, key + "." + i);
        }


        public File getDirtyFile(int i) {
            return new File(directory, key + "." + i + ".tmp");
        }
    }
}