DiskLruCache 閱讀筆記
DiskLruCache使用名爲 journal 的日誌文件來記錄對緩存的操做。
* 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
libcore.io.DiskLruCache 是一種聲明，表示該文件是使用 DiskLruCache 生成的。
1 表示緩存的版本號
100 表示應用的版本號
2 表示每一個 key 值對應的緩存文件的個數，一般使用 1 便可。
CLEAN 表示記錄了對緩存文件的操做已經完成，表示一個無錯誤的操做。
DIRTY 表示記錄了正在操做緩存文件的操做，該記錄後面必需要跟一個 CLEAN 和 REMOVE 記錄，用來表示該操做成功（CLEAN）或者失敗（REMOVE）了。
READ 表示記錄了一個獲取緩存文件的操做。
REMOVE 表示記錄了一條刪除緩存文件的操做。
java
3400330d1dfc7f3f7f4b8d4d803dfcf6 這樣的字符串表示緩存文件的文件名，也是 key 值。CLEAN 記錄後面跟着的多個數字表示緩存文件的大小，key 對應多個緩存文件，就有多個數字。緩存
先看構造函數：app
private DiskLruCache(File directory, int appVersion, int valueCount, long maxSize, int maxFileCount) {
this.directory = directory;
this.appVersion = appVersion;
this.journalFile = new File(directory, JOURNAL_FILE);
this.journalFileTmp = new File(directory, JOURNAL_FILE_TEMP);
this.journalFileBackup = new File(directory, JOURNAL_FILE_BACKUP);
this.valueCount = valueCount;
this.maxSize = maxSize;
this.maxFileCount = maxFileCount;
}
主要是給成員變量賦值，初始化版本號，緩存容量等，構造函數私有，只能經過別的方式來獲取實例對象。less
使用 DiskLruCache ，只能使用 open() 方法：ide
/**
* Opens the cache in {@code directory}, creating a cache if none exists
* there.
*
* @param directory a writable directory
* @param valueCount the number of values per cache entry. Must be positive.
* @param maxSize the maximum number of bytes this cache should use to store
* @param maxFileCount the maximum file count this cache should store
* @throws IOException if reading or writing the cache directory fails
*/
public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize, int maxFileCount)
throws IOException {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
if (maxFileCount <= 0) {
throw new IllegalArgumentException("maxFileCount <= 0");
}
if (valueCount <= 0) {
throw new IllegalArgumentException("valueCount <= 0");
}
// If a bkp file exists, use it instead.
File backupFile = new File(directory, JOURNAL_FILE_BACKUP);
if (backupFile.exists()) {
File journalFile = new File(directory, JOURNAL_FILE);
// If journal file also exists just delete backup file.
if (journalFile.exists()) {
backupFile.delete();
} else {
renameTo(backupFile, journalFile, false);
}
}
// Prefer to pick up where we left off.
DiskLruCache cache = new DiskLruCache(directory, appVersion, valueCount, maxSize, maxFileCount);
if (cache.journalFile.exists()) {
try {
cache.readJournal();
cache.processJournal();
cache.journalWriter = new BufferedWriter(
new OutputStreamWriter(new FileOutputStream(cache.journalFile, true), Util.US_ASCII));
return cache;
} catch (IOException journalIsCorrupt) {
System.out
.println("DiskLruCache "
+ directory
+ " is corrupt: "
+ journalIsCorrupt.getMessage()
+ ", removing");
cache.delete();
}
}
// Create a new empty cache.
directory.mkdirs();
cache = new DiskLruCache(directory, appVersion, valueCount, maxSize, maxFileCount);
cache.rebuildJournal();
return cache;
}
1.先檢查設置是否正常。函數
2.若是日誌備份文件存在，日誌文件不存在，將備份文件替重命名爲日誌文件，日誌文件存在，刪除備份文件。ui
3.實例化 DiskLruCache，日誌文件存在，則讀取並處理日誌文件，不然從新建立一個日誌文件。this
private void readJournal() throws IOException {
StrictLineReader reader = new StrictLineReader(new FileInputStream(journalFile), Util.US_ASCII);
try {
String magic = reader.readLine();
String version = reader.readLine();
String appVersionString = reader.readLine();
String valueCountString = reader.readLine();
String blank = reader.readLine();
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 + "]");
}
int lineCount = 0;
while (true) {
try {
readJournalLine(reader.readLine());
lineCount++;
} catch (EOFException endOfJournal) {
break;
}
}
redundantOpCount = lineCount - lruEntries.size();
} finally {
Util.closeQuietly(reader);
}
}
一行一行的讀取日誌文件：3d
1.先看日誌文件頭是否格式匹配，不然拋出異常。日誌
2.讀取每一行記錄
private void readJournalLine(String line) throws IOException {
int firstSpace = line.indexOf(' ');
if (firstSpace == -1) {
throw new IOException("unexpected journal line: " + line);
}
int keyBegin = firstSpace + 1;
int secondSpace = line.indexOf(' ', keyBegin);
final String key;
if (secondSpace == -1) {
key = line.substring(keyBegin);
if (firstSpace == REMOVE.length() && line.startsWith(REMOVE)) {
lruEntries.remove(key);
return;
}
} else {
key = line.substring(keyBegin, secondSpace);
}
Entry entry = lruEntries.get(key);
if (entry == null) {
entry = new Entry(key);
lruEntries.put(key, entry);
}
if (secondSpace != -1 && firstSpace == CLEAN.length() && line.startsWith(CLEAN)) {
String[] parts = line.substring(secondSpace + 1).split(" ");
entry.readable = true;
entry.currentEditor = null;
entry.setLengths(parts);
} else if (secondSpace == -1 && firstSpace == DIRTY.length() && line.startsWith(DIRTY)) {
entry.currentEditor = new Editor(entry);
} else if (secondSpace == -1 && firstSpace == READ.length() && line.startsWith(READ)) {
// This work was already done by calling lruEntries.get().
} else {
throw new IOException("unexpected journal line: " + line);
}
}
1.若是是 REMOVE 記錄，從 lruEntries 中刪除 key 對應的 Entry。
2.根據 key 獲取 Entry，若是爲 null，新建一個放入 lruEntries 中。
3.記錄爲 CLEAN，DIRTY，READ ，對 Entry 作相應的操做。
/**
* 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.
*/
private void processJournal() throws IOException {
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];
fileCount++;
}
} else {
entry.currentEditor = null;
for (int t = 0; t < valueCount; t++) {
deleteIfExists(entry.getCleanFile(t));
deleteIfExists(entry.getDirtyFile(t));
}
i.remove();
}
}
}
1.刪除臨時日誌文件
2.entry.currentEditor 爲 null，表示 CLEAN 或者 READ 記錄，能夠算出緩存文件的總大小，緩存文件的個數。
3.entry.currentEditor 不爲 null，表示 DIRTY 記錄，將對應的文件刪除。
/**
* Creates a new journal that omits redundant information. This replaces the
* current journal if it exists.
*/
private synchronized void rebuildJournal() throws IOException {
if (journalWriter != null) {
journalWriter.close();
}
Writer writer = new BufferedWriter(
new OutputStreamWriter(new FileOutputStream(journalFileTmp), Util.US_ASCII));
try {
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');
}
}
} finally {
writer.close();
}
if (journalFile.exists()) {
renameTo(journalFile, journalFileBackup, true);
}
renameTo(journalFileTmp, journalFile, false);
journalFileBackup.delete();
journalWriter = new BufferedWriter(
new OutputStreamWriter(new FileOutputStream(journalFile, true), Util.US_ASCII));
}
根據日誌文件的格式，生成一個日誌文件。
/**
* 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.
*/
public synchronized Snapshot get(String key) throws IOException {
checkNotClosed();
validateKey(key);
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.
File[] files = new File[valueCount];
InputStream[] ins = new InputStream[valueCount];
try {
File file;
for (int i = 0; i < valueCount; i++) {
file = entry.getCleanFile(i);
files[i] = file;
ins[i] = new FileInputStream(file);
}
} catch (FileNotFoundException e) {
// A file must have been deleted manually!
for (int i = 0; i < valueCount; i++) {
if (ins[i] != null) {
Util.closeQuietly(ins[i]);
} else {
break;
}
}
return null;
}
redundantOpCount++;
journalWriter.append(READ + ' ' + key + '\n');
if (journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}
return new Snapshot(key, entry.sequenceNumber, files, ins, entry.lengths);
}
根據 key 值獲取緩存文件，返回的是 Snapshot 對象，同時生成一個 READ 記錄。
/** A snapshot of the values for an entry. */
public final class Snapshot implements Closeable {
private final String key;
private final long sequenceNumber;
private File[] files;
private final InputStream[] ins;
private final long[] lengths;
private Snapshot(String key, long sequenceNumber, File[] files, InputStream[] ins, long[] lengths) {
this.key = key;
this.sequenceNumber = sequenceNumber;
this.files = files;
this.ins = ins;
this.lengths = lengths;
}
/**
* 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.
*/
public Editor edit() throws IOException {
return DiskLruCache.this.edit(key, sequenceNumber);
}
/** Returns file with the value for {@code index}. */
public File getFile(int index) {
return files[index];
}
/** 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));
}
/** Returns the byte length of the value for {@code index}. */
public long getLength(int index) {
return lengths[index];
}
public void close() {
for (InputStream in : ins) {
Util.closeQuietly(in);
}
}
}
Snapshot 類，封裝了 key 和緩存文件的關係。
最重要的方法：
public InputStream getInputStream(int index) {
return ins[index];
}
根據這個方法，就能夠獲取緩文件的輸入流，參數 index 是由 key 值對應的緩存文件個數決定的，緩存文件個數是1，那麼這裏顯然傳0。
/**
* 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);
}
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);
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;
}
寫緩存文件，返回一個 Editor 對象，同時生成一個 DIRTY 記錄。
/** Edits the values for an entry. */
public final class Editor {
private final Entry entry;
private final boolean[] written;
private boolean hasErrors;
private boolean committed;
private Editor(Entry entry) {
this.entry = entry;
this.written = (entry.readable) ? null : new boolean[valueCount];
}
/**
* Returns an unbuffered input stream to read the last committed value,
* or null if no value has been committed.
*/
public InputStream newInputStream(int index) throws IOException {
synchronized (DiskLruCache.this) {
if (entry.currentEditor != this) {
throw new IllegalStateException();
}
if (!entry.readable) {
return null;
}
try {
return new FileInputStream(entry.getCleanFile(index));
} catch (FileNotFoundException e) {
return null;
}
}
}
/**
* Returns the last committed value as a string, or null if no value
* has been committed.
*/
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.
*/
public OutputStream newOutputStream(int index) throws IOException {
synchronized (DiskLruCache.this) {
if (entry.currentEditor != this) {
throw new IllegalStateException();
}
if (!entry.readable) {
written[index] = true;
}
File dirtyFile = entry.getDirtyFile(index);
FileOutputStream outputStream;
try {
outputStream = new FileOutputStream(dirtyFile);
} catch (FileNotFoundException e) {
// Attempt to recreate the cache directory.
directory.mkdirs();
try {
outputStream = new FileOutputStream(dirtyFile);
} catch (FileNotFoundException e2) {
// We are unable to recover. Silently eat the writes.
return NULL_OUTPUT_STREAM;
}
}
return new FaultHidingOutputStream(outputStream);
}
}
/** Sets the value at {@code index} to {@code value}. */
public void set(int index, String value) throws IOException {
Writer writer = null;
try {
writer = new OutputStreamWriter(newOutputStream(index), Util.UTF_8);
writer.write(value);
} finally {
Util.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.
*/
public void commit() throws IOException {
if (hasErrors) {
completeEdit(this, false);
remove(entry.key); // The previous entry is stale.
} else {
completeEdit(this, true);
}
committed = true;
}
/**
* Aborts this edit. This releases the edit lock so another edit may be
* started on the same key.
*/
public void abort() throws IOException {
completeEdit(this, false);
}
public void abortUnlessCommitted() {
if (!committed) {
try {
abort();
} catch (IOException ignored) {
}
}
}
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;
}
}
}
}
Editor 能夠看作一個幫助類。拿到 Editor 對象後，就能夠獲取緩存的輸出流：
/**
* 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.
*/
public OutputStream newOutputStream(int index) throws IOException {
synchronized (DiskLruCache.this) {
if (entry.currentEditor != this) {
throw new IllegalStateException();
}
if (!entry.readable) {
written[index] = true;
}
File dirtyFile = entry.getDirtyFile(index);
FileOutputStream outputStream;
try {
outputStream = new FileOutputStream(dirtyFile);
} catch (FileNotFoundException e) {
// Attempt to recreate the cache directory.
directory.mkdirs();
try {
outputStream = new FileOutputStream(dirtyFile);
} catch (FileNotFoundException e2) {
// We are unable to recover. Silently eat the writes.
return NULL_OUTPUT_STREAM;
}
}
return new FaultHidingOutputStream(outputStream);
}
}
能夠看到是輸出到一個 DirtyFile 文件。獲取到了輸出流，就能夠寫緩存了，寫完緩存以後，須要調用 commit 方法。
/**
* Commits this edit so it is visible to readers. This releases the
* edit lock so another edit may be started on the same key.
*/
public void commit() throws IOException {
if (hasErrors) {
completeEdit(this, false);
remove(entry.key); // The previous entry is stale.
} else {
completeEdit(this, true);
}
committed = true;
}
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 (!editor.written[i]) {
editor.abort();
throw new IllegalStateException("Newly created entry didn't create value for index " + i);
}
if (!entry.getDirtyFile(i).exists()) {
editor.abort();
return;
}
}
}
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;
fileCount++;
}
} 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');
}
journalWriter.flush();
if (size > maxSize || fileCount > maxFileCount || journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}
}
completeEdit() 方法將 DirtyFile 從新命名爲 CleanFile，若是寫入成功，則記錄一條 CLEAN，不然記錄 REMOVE。
這就是 DIRTY 記錄後面必定會跟一條 CLEAN 或者 REMOVE 的緣由。
/**
* 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);
if (entry == null || entry.currentEditor != null) {
return false;
}
for (int i = 0; i < valueCount; i++) {
File file = entry.getCleanFile(i);
if (file.exists() && !file.delete()) {
throw new IOException("failed to delete " + file);
}
size -= entry.lengths[i];
fileCount--;
entry.lengths[i] = 0;
}
redundantOpCount++;
journalWriter.append(REMOVE + ' ' + key + '\n');
lruEntries.remove(key);
if (journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}
return true;
}
刪除緩存，記錄一條 REMOVE。
大體的方法就是這些了，還有一些瑣碎的方法，跟使用到 LruCache 的其餘類，都差很少。
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