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");
}
}
}