Android4，系统学Android从零开始

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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.

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*/
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();
}

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}

/
* 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();
}

自我介绍一下，小编13年上海交大毕业，曾经在小公司待过，也去过华为、OPPO等大厂，18年进入阿里一直到现在。

深知大多数初中级安卓工程师，想要提升技能，往往是自己摸索成长，但自己不成体系的自学效果低效又漫长，而且极易碰到天花板技术停滞不前！

因此收集整理了一份《2024年最新Android移动开发全套学习资料》送给大家，初衷也很简单，就是希望能够帮助到想自学提升又不知道该从何学起的朋友，同时减轻大家的负担。
[外链图片转存中…(img-OWI0FRCb-63)]
[外链图片转存中…(img-P9DV0VpD-63)]
[外链图片转存中…(img-dzmqOyw4-63)]
[外链图片转存中…(img-mC24F0Z9-64)]

由于文件比较大，这里只是将部分目录截图出来，每个节点里面都包含大厂面经、学习笔记、源码讲义、实战项目、讲解视频
如果你觉得这些内容对你有帮助，可以添加下面V无偿领取！（备注Android）
[外链图片转存中…(img-cWofcRqd-64)]

这里我就分享一份资料，希望可以帮助到大家提升进阶。

如果你有需要的话，可以点击Android学习PDF+架构视频+面试文档+源码笔记获取免费领取方式

喜欢本文的话，不妨给我点个小赞、评论区留言或者转发支持一下呗~

Android4，系统学Android从零开始

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