Android4.1.1中的LruDiskCache源码

最新推荐文章于 2023-09-22 21:12:15 发布

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本文介绍了一种基于文件系统的 LRU 缓存实现——DiskLruCache，它使用固定数量的值来存储缓存项，并限制缓存占用的空间大小。缓存通过日志文件跟踪缓存条目的状态，支持创建、编辑、读取和删除操作。

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/*
*
* 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 libcore.io;
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.Writer;
import java.nio.charset.Charsets;
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;
/**
* 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.
*
* 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.
*
* 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.
*
* 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 created 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 edited, 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.
*
* 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.
*
* 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.
*/
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";
/*
* 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.
* 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.
* 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.
* o READ lines track accesses for LRU.
* o REMOVE lines track entries that have been deleted.
*
* 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.
*/
private final File directory;
private final File journalFile;
private final File journalFileTmp;
private final int appVersion;
private final long maxSize;
private final int valueCount;
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.
*/
private long nextSequenceNumber = 0;
/** This cache uses a single background thread to evict entries. */
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;
}
/**
* Opens the cache in {@code directory}, creating a cache if none exists
* there.
*
* @param directory a writable directory
* @param appVersion
* @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
* @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) {
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);
if (cache.journalFile.exists()) {
try {
cache.readJournal();
cache.processJournal();
cache.journalWriter = new BufferedWriter(new FileWriter(cache.journalFile, true));
return cache;
} catch (IOException journalIsCorrupt) {
System.logW("DiskLruCache " + directory + " is corrupt: "
+ journalIsCorrupt.getMessage() + ", removing");
cache.delete();
}
}
// create a new empty cache
directory.mkdirs();
cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
cache.rebuildJournal();
return cache;
}
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 {
readJournalLine(Streams.readAsciiLine(in));
} catch (EOFException endOfJournal) {
break;
}
}
} finally {
IoUtils.closeQuietly(in);
}
}
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(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()
} 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.
*/
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];
}
} 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.
*/
private synchronized void rebuildJournal() throws IOException {
if (journalWriter != null) {
journalWriter.close();
}
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");
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));
}
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();
}
}
}
/**
* 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.
*/
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);
}
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;
}
/**
* 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;
}
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.
*/
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.
*
* @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.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.
*/
public boolean isClosed() {
return journalWriter == null;
}
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();
trimToSize();
journalWriter.flush();
}
/**
* Closes this cache. Stored values will remain on the filesystem.
*/
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;
}
private void trimToSize() throws IOException {
while (size > maxSize) {
Map.Entry<String, Entry> toEvict = lruEntries.eldest();
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.
*/
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));
}
/**
* A snapshot of the values for an entry.
*/
public final class Snapshot implements Closeable {
private final String key;
private final long sequenceNumber;
private final InputStream[] ins;
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.
*/
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) {
IoUtils.closeQuietly(in);
}
}
}
/**
* Edits the values for an entry.
*/
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.
*/
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.
*/
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();
}
return new FaultHidingOutputStream(new FileOutputStream(entry.getDirtyFile(index)));
}
}
/**
* 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), 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.
*/
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.
*/
public void abort() throws IOException {
completeEdit(this, false);
}
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;
}
}
}
}
private final class Entry {
private final String key;
/** Lengths of this entry's files. */
private final long[] lengths;
/** 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".
*/
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");
}
}
}