1 Glide缓存与复用机制简介
1.1 Glide的资源状态可以分为四种
- Active Resources：有其他View正在展示这张图片
- Memory cache：该图片被存进内存中
- Resource：经过decode、transformed后的缓存
- Data：原始的没有修改过的数据缓存
Glide读取缓存也是依次从上面四种状态的缓存中读取，如果都未能找到图片，则Glide会返回到原始资源以取回数据（原始文件，Uri, Url等）
1.2 Glide中Bitmap复用机制
复用：
将已经不需要使用的数据空间重新拿来使用，减少内存抖动（指在短时间内有大量的对象被创建或者被回收的现象）
原理：
inMutable是Glide能够复用Bitmap的基石，是BitmapFactory提供的一个参数，表示该Bitmap是可变的，支持复用的。BitmapFactory.Options中提供了两个属性：inMutable、inBitmap。当进行Bitmap复用时，需要设置inMutable为true，inBitmap设置被复用的已经存在的Bitmap。Bitmap复用池使用LRU算法实现。
Bitmap复用使用条件：
- 在Android 4.4之前，仅支持相同大小的Bitmap，inSampleSize必须为1，而且必须采用jpeg或png格式。
- 在Android 4.4之后只有一个限制，就是被复用的Bitmap尺寸要大于 新的bitmap，简单来说就是大图可以给小图复用。
2 缓存源码流程
在Glide源码分析-网络图片加载主流程分析一文中，我们已经知道memory cache和disk cache在Glide创建的时候也被创建了，Glide创建的代码在GlideBuilder.build(Context)方法
@NonNull
Glide build(@NonNull Context context) {
if (memoryCache == null) {
memoryCache = new LruResourceCache(memorySizeCalculator.getMemoryCacheSize());
}
if (diskCacheFactory == null) {
diskCacheFactory = new InternalCacheDiskCacheFactory(context);
}
if (engine == null) {
engine =
new Engine(
memoryCache,
diskCacheFactory,
...);
}
return new Glide(
...
memoryCache,
...);
}
2.1 memoryCache
通过代码可以看到 memoryCache 被放入 Engine 和 Glide 实例中。在Engine中利用memoryCache进行存取操作，Glide 实例中的memoryCache是用来在内存紧张的时候，通知memoryCache释放内存。Glide实现了ComponentCallbacks2接口，在Glide创建完成后，通过applicationContext.registerComponentCallbacks(glide)似的 Glide 实例可以监听内存紧张的信号。
// Glide
@Override
public void onTrimMemory(int level) {
trimMemory(level);
}
public void trimMemory(int level) {
// Engine asserts this anyway when removing resources, fail faster and consistently
Util.assertMainThread();
// memory cache needs to be trimmed before bitmap pool to trim re-pooled Bitmaps too. See #687.
memoryCache.trimMemory(level);
bitmapPool.trimMemory(level);
arrayPool.trimMemory(level);
}
memoryCache是一个使用LRU(least recently used)算法实现的内存缓存类LruResourceCache，继承至LruCache类，并实现了MemoryCache接口。LruCache定义了LRU算法实现相关的操作，而MemoryCache定义的是内存缓存相关的操作。
LruCache 的实现是利用了 LinkedHashMap 的这种数据结构的一个特性（ accessOrder＝true 基于访问顺序 ）再加上对 LinkedHashMap 的数据操作上锁实现的缓存策略。
当调用 put()方法时，就会在集合中添加元素，并调用
trimToSize()判断缓存是否已满，如果满了就用 LinkedHashMap 的迭代器删除队尾元素，即近期最少访问的元素。
当调用 get()方法访问缓存对象时，就会调用 LinkedHashMap 的 get()方法获得对应集合元素，同时会更新该元素到队头。
2.2 diskCacheFactory
diskCacheFactory是创建DiskCache的Factory，DiskCache接口定义
public interface DiskCache {
interface Factory {
/** 250 MB of cache. */
int DEFAULT_DISK_CACHE_SIZE = 250 * 1024 * 1024;
String DEFAULT_DISK_CACHE_DIR = "image_manager_disk_cache";
@Nullable
DiskCache build();
}
interface Writer {
boolean write(@NonNull File file);
}
@Nullable
File get(Key key);
void put(Key key, Writer writer);
@SuppressWarnings("unused")
void delete(Key key);
void clear();
}
接着再来看下DiskCache.Factory的默认实现：InternalCacheDiskCacheFactory
public final class InternalCacheDiskCacheFactory extends DiskLruCacheFactory {
public InternalCacheDiskCacheFactory(Context context) {
this(context, DiskCache.Factory.DEFAULT_DISK_CACHE_DIR,
DiskCache.Factory.DEFAULT_DISK_CACHE_SIZE);
}
public InternalCacheDiskCacheFactory(Context context, long diskCacheSize) {
this(context, DiskCache.Factory.DEFAULT_DISK_CACHE_DIR, diskCacheSize);
}
public InternalCacheDiskCacheFactory(final Context context, final String diskCacheName,
long diskCacheSize) {
super(new CacheDirectoryGetter() {
@Override
public File getCacheDirectory() {
File cacheDirectory = context.getCacheDir();
if (cacheDirectory == null) {
return null;
}
if (diskCacheName != null) {
return new File(cacheDirectory, diskCacheName);
}
return cacheDirectory;
}
}, diskCacheSize);
}
}
由以上代码可以看出：默认会创建一个250M的缓存目录，其路径为/data/data/{package}/cache/image_manager_disk_cache/
继续看其父类DiskLruCacheFactory的代码
public class DiskLruCacheFactory implements DiskCache.Factory {
private final long diskCacheSize;
private final CacheDirectoryGetter cacheDirectoryGetter;
public interface CacheDirectoryGetter {
File getCacheDirectory();
}
...
public DiskLruCacheFactory(CacheDirectoryGetter cacheDirectoryGetter, long diskCacheSize) {
this.diskCacheSize = diskCacheSize;
this.cacheDirectoryGetter = cacheDirectoryGetter;
}
@Override
public DiskCache build() {
File cacheDir = cacheDirectoryGetter.getCacheDirectory();
if (cacheDir == null) {
return null;
}
if (!cacheDir.mkdirs() && (!cacheDir.exists() || !cacheDir.isDirectory())) {
return null;
}
return DiskLruCacheWrapper.create(cacheDir, diskCacheSize);
}
}
DiskLruCacheFactory.build()方法会返回一个DiskLruCacheWrapper类的实例，看下DiskLruCacheWrapper的实现
public class DiskLruCacheWrapper implements DiskCache {
private static final String TAG = "DiskLruCacheWrapper";
private static final int APP_VERSION = 1;
private static final int VALUE_COUNT = 1;
private static DiskLruCacheWrapper wrapper;
private final SafeKeyGenerator safeKeyGenerator;
private final File directory;
private final long maxSize;
private final DiskCacheWriteLocker writeLocker = new DiskCacheWriteLocker();
private DiskLruCache diskLruCache;
@SuppressWarnings("deprecation")
public static DiskCache create(File directory, long maxSize) {
return new DiskLruCacheWrapper(directory, maxSize);
}
@Deprecated
@SuppressWarnings({"WeakerAccess", "DeprecatedIsStillUsed"})
protected DiskLruCacheWrapper(File directory, long maxSize) {
this.directory = directory;
this.maxSize = maxSize;
this.safeKeyGenerator = new SafeKeyGenerator();
}
private synchronized DiskLruCache getDiskCache() throws IOException {
if (diskLruCache == null) {
diskLruCache = DiskLruCache.open(directory, APP_VERSION, VALUE_COUNT, maxSize);
}
return diskLruCache;
}
@Override
public File get(Key key) {
String safeKey = safeKeyGenerator.getSafeKey(key);
File result = null;
try {
final DiskLruCache.Value value = getDiskCache().get(safeKey);
if (value != null) {
result = value.getFile(0);
}
} catch (IOException e) {
...
}
return result;
}
@Override
public void put(Key key, Writer writer) {
String safeKey = safeKeyGenerator.getSafeKey(key);
writeLocker.acquire(safeKey);
try {
try {
DiskLruCache diskCache = getDiskCache();
Value current = diskCache.get(safeKey);
...
DiskLruCache.Editor editor = diskCache.edit(safeKey);
...
try {
File file = editor.getFile(0);
if (writer.write(file)) {
editor.commit();
}
} finally {
editor.abortUnlessCommitted();
}
} catch (IOException e) {
...
}
} finally {
writeLocker.release(safeKey);
}
}
...
}
顾名思义，里面包装了一个DiskLruCache，该类主要是为DiskLruCache提供了一个根据Key生成safeKey的SafeKeyGenerator以及写锁DiskCacheWriteLocker。
回到GlideBuilder.build(Context)中，diskCacheFactory会被传进Engine中，在Engine的构造方法中会被包装成为一个LazyDiskCacheProvider，在被需要的时候调用getDiskCache()方法，这样就会调用factory的build()方法返回一个DiskCache。代码如下：
private static class LazyDiskCacheProvider implements DecodeJob.DiskCacheProvider {
private final DiskCache.Factory factory;
private volatile DiskCache diskCache;
LazyDiskCacheProvider(DiskCache.Factory factory) {
this.factory = factory;
}
...
@Override
public DiskCache getDiskCache() {
if (diskCache == null) {
synchronized (this) {
if (diskCache == null) {
diskCache = factory.build();
}
if (diskCache == null) {
diskCache = new DiskCacheAdapter();
}
}
}
return diskCache;
}
}
LazyDiskCacheProvider会在Engine后面的初始化流程中作为入参传到DecodeJobFactory的构造器。在DecodeJobFactory创建DecodeJob时也会作为入参会传进去，DecodeJob中会以全局变量保存此LazyDiskCacheProvider，在资源加载完毕并展示后，会进行缓存的存储。同时，DecodeJob也会在DecodeHelper初始化时，将此DiskCacheProvider设置进去，供ResourceCacheGenerator、DataCacheGenerator读取缓存，供SourceGenerator写入缓存。
2.3 ActiveResources
ActiveResources在Engine的构造器中被创建，在ActiveResources的构造器中会启动一个后台优先级级别（THREAD_PRIORITY_BACKGROUND）的线程，在该线程中会调用cleanReferenceQueue()方法一直循环清除ReferenceQueue中的将要被GC的Resource。
final class ActiveResources {
private final boolean isActiveResourceRetentionAllowed;
private final Executor monitorClearedResourcesExecutor;
@VisibleForTesting
final Map<Key, ResourceWeakReference> activeEngineResources = new HashMap<>();
private final ReferenceQueue<EngineResource<?>> resourceReferenceQueue = new ReferenceQueue<>();
private volatile boolean isShutdown;
ActiveResources(boolean isActiveResourceRetentionAllowed) {
this(
isActiveResourceRetentionAllowed,
java.util.concurrent.Executors.newSingleThreadExecutor(
new ThreadFactory() {
@Override
public Thread newThread(@NonNull final Runnable r) {
return new Thread(
new Runnable() {
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
r.run();
}
},
"glide-active-resources");
}
}));
}
@VisibleForTesting
ActiveResources(
boolean isActiveResourceRetentionAllowed, Executor monitorClearedResourcesExecutor) {
this.isActiveResourceRetentionAllowed = isActiveResourceRetentionAllowed;
this.monitorClearedResourcesExecutor = monitorClearedResourcesExecutor;
monitorClearedResourcesExecutor.execute(
new Runnable() {
@Override
public void run() {
cleanReferenceQueue();
}
});
}
@SuppressWarnings("WeakerAccess")
@Synthetic void cleanReferenceQueue() {
while (!isShutdown) {
try {
ResourceWeakReference ref = (ResourceWeakReference) resourceReferenceQueue.remove();
cleanupActiveReference(ref);
// This section for testing only.
DequeuedResourceCallback current = cb;
if (current != null) {
current.onResourceDequeued();
}
// End for testing only.
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}
}
}
先来看看ActiveResources的activate方法（保存）、deactivate方法（删除）的方法
synchronized void activate(Key key, EngineResource<?> resource) {
ResourceWeakReference toPut =
new ResourceWeakReference(
key, resource, resourceReferenceQueue, isActiveResourceRetentionAllowed);
ResourceWeakReference removed = activeEngineResources.put(key, toPut);
if (removed != null) {
removed.reset();
}
}
synchronized void deactivate(Key key) {
ResourceWeakReference removed = activeEngineResources.remove(key);
if (removed != null) {
removed.reset();
}
}
activate方法会将参数封装成为一个ResourceWeakReference，然后放入map中，如果对应的key之前有值，那么调用之前值的reset方法进行清除。deactivate方法先在map中移除，然后调用resource的reset方法进行清除。ResourceWeakReference继承WeakReference，内部只是保存了Resource的一些属性。
static final class ResourceWeakReference extends WeakReference<EngineResource<?>> {
@SuppressWarnings("WeakerAccess") @Synthetic final Key key;
@SuppressWarnings("WeakerAccess") @Synthetic final boolean isCacheable;
@Nullable @SuppressWarnings("WeakerAccess") @Synthetic Resource<?> resource;
@Synthetic
@SuppressWarnings("WeakerAccess")
ResourceWeakReference(
@NonNull Key key,
@NonNull EngineResource<?> referent,
@NonNull ReferenceQueue<? super EngineResource<?>> queue,
boolean isActiveResourceRetentionAllowed) {
super(referent, queue);
this.key = Preconditions.checkNotNull(key);
this.resource =
referent.isCacheable() && isActiveResourceRetentionAllowed
? Preconditions.checkNotNull(referent.getResource()) : null;
isCacheable = referent.isCacheable();
}
}
构造方法中调用了super(referent, queue)，这样做可以让将要被GC的对象放入到ReferenceQueue中。而ActiveResources.cleanReferenceQueue()方法会一直尝试从queue中获取将要被GC的resource，然后调用cleanupActiveReference方法将resource从activeEngineResources中移除。cleanupActiveReference源码如下：
void cleanupActiveReference(@NonNull ResourceWeakReference ref) {
synchronized (listener) {
synchronized (this) {
// 移除active资源
activeEngineResources.remove(ref.key);
if (!ref.isCacheable || ref.resource == null) {
return;
}
// 构造新的 Resource
EngineResource<?> newResource =
new EngineResource<>(ref.resource, /*isCacheable=*/ true, /*isRecyclable=*/ false);
newResource.setResourceListener(ref.key, listener);
// 回调Engine的onResourceReleased方法
// 这会导致此资源从active变成memory cache状态
listener.onResourceReleased(ref.key, newResource);
}
}
}
Engine实现了EngineResource.ResourceListener，此处的listener就是Engine，最终会回调Engine.onResourceReleased
@Override
public synchronized void onResourceReleased(Key cacheKey, EngineResource<?> resource) {
activeResources.deactivate(cacheKey);
if (resource.isCacheable()) {
cache.put(cacheKey, resource);
} else {
resourceRecycler.recycle(resource);
}
}
如果资源可以被缓存，则缓存到 memory cache，否则对资源进行回收。
2.4 磁盘缓存读取
了解了上述三种缓存后我们分析下缓存的存取代码。我们看下
public synchronized <R> LoadStatus load(...) {
EngineKey key = keyFactory.buildKey(model, signature, width, height, transformations,
resourceClass, transcodeClass, options);
EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
if (active != null) {
cb.onResourceReady(active, DataSource.MEMORY_CACHE);
return null;
}
EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
if (cached != null) {
cb.onResourceReady(cached, DataSource.MEMORY_CACHE);
return null;
}
EngineJob<?> current = jobs.get(key, onlyRetrieveFromCache);
if (current != null) {
current.addCallback(cb, callbackExecutor);
return new LoadStatus(cb, current);
}
EngineJob<R> engineJob =
engineJobFactory.build(...);
DecodeJob<R> decodeJob =
decodeJobFactory.build(...);
jobs.put(key, engineJob);
engineJob.addCallback(cb, callbackExecutor);
engineJob.start(decodeJob);
return new LoadStatus(cb, engineJob);
}
缓存需要根据EngineKey去存取，先看下EngineKey的构造方法
EngineKey(
Object model,
Key signature,
int width
int height,
Map<Class<?>, Transformation<?>> transformations,
Class<?> resourceClass,
Class<?> transcodeClass,
Options options)
-
model
load方法传的参数 -
signature
BaseRequestOptions的成员变量，默认会是EmptySignature.obtain()
在加载本地resource资源时会变成ApplicationVersionSignature.obtain(context) -
width、height
如果没有指定override(int size)，那么将得到view的size -
transformations
默认会基于ImageView的scaleType设置对应的四个Transformation；
如果指定了transform，那么就基于该值进行设置 -
resourceClass
解码后的资源，如果没有asBitmap、asGif，一般会是Object -
transcodeClass
最终要转换成的数据类型，根据as方法确定，加载本地res或者网络URL，都会调用asDrawable，所以为Drawable -
options
如果没有设置过transform，此处会根据ImageView的scaleType默认指定一个option
所以，在多次加载同一个model的过程中，只要上述任何一个参数有改变，都不会认为是同一个key。
回到Engine.load方法，从缓存加载成功后的回调cb.onResourceReady(cached, DataSource.MEMORY_CACHE);可以看到：active状态的资源和memory cache状态的资源都是DataSource.MEMORY_CACHE，并且加载的资源都是 EngineResource 对象，该对象内部采用了引用计数去判断资源是否被释放，如果引用计数为0，那么会调用listener.onResourceReleased(key, this)方法通知外界此资源已经释放了。这里的listener是ResourceListener类型的接口，只有一个onResourceReleased(Key key, EngineResource<?> resource)方法，Engine实现了该接口，此处的listener就是Engine。在Engine.onResourceReleased方法中会判断资源是否可缓存，可缓存则将此资源放入memory cache中，否则回收掉该资源，代码如下：
public synchronized void onResourceReleased(Key cacheKey, EngineResource<?> resource) {
// 从activeResources中移除
activeResources.deactivate(cacheKey);
if (resource.isCacheable()) {
// 存入 MemoryCache
cache.put(cacheKey, resource);
} else {
resourceRecycler.recycle(resource);
}
}
继续回到Engine.load方法，先来看下active资源获取的方法
@Nullable
private EngineResource<?> loadFromActiveResources(Key key, boolean isMemoryCacheable) {
// 设置skipMemoryCache(true)，则isMemoryCacheable为false，跳过ActiveResources
if (!isMemoryCacheable) {
return null;
}
EngineResource<?> active = activeResources.get(key);
if (active != null) {
// 命中缓存，引用计数+1
active.acquire();
}
return active;
}
继续分析cached资源获取的方法，如果从active资源中没有获取到缓存，则继续从内存缓存中查找
private EngineResource<?> loadFromCache(Key key, boolean isMemoryCacheable) {
// 设置skipMemoryCache(true)，则isMemoryCacheable为false，跳过ActiveResources
if (!isMemoryCacheable) {
return null;
}
EngineResource<?> cached = getEngineResourceFromCache(key);
if (cached != null) {
// 命中缓存，引用计数+1
cached.acquire();
// 将此资源从memoryCache中移到activeResources中
activeResources.activate(key, cached);
}
return cached;
}
如果从memoryCache中获取到资源则将此资源从memoryCache中移到activeResources中。第一次加载的时候activeResources和memoryCache中都没有缓存的，后面继续通过DecodeJob和EngineJob去加载资源。DecoceJob实现了Runnable接口，然后会被EngineJob.start方法提交到对应的线程池中去执行。在DecoceJob的run方法中，会依次从ResourceCacheGenerator和DataCacheGenerator中去取缓存数据，当这两者都取不到的情况下，会交给SourceGenerator加载网络图片或者本地资源。resource资源和data资源都是磁盘缓存中的资源。
先看下 ResourceCacheGenerator.startNext
@Override
public boolean startNext() {
// list里面只有一个GlideUrl对象
List<Key> sourceIds = helper.getCacheKeys();
if (sourceIds.isEmpty()) {
return false;
}
// 获得了三个可以到达的registeredResourceClasses
// GifDrawable、Bitmap、BitmapDrawable
List<Class<?>> resourceClasses = helper.getRegisteredResourceClasses();
if (resourceClasses.isEmpty()) {
if (File.class.equals(helper.getTranscodeClass())) {
return false;
}
throw new IllegalStateException(
"Failed to find any load path from " + helper.getModelClass() + " to "
+ helper.getTranscodeClass());
}
// 遍历sourceIds中的每一个key、resourceClasses中每一个class，以及其他的一些值组成key
// 尝试在磁盘缓存中以key找到缓存文件
while (modelLoaders == null || !hasNextModelLoader()) {
resourceClassIndex++;
if (resourceClassIndex >= resourceClasses.size()) {
sourceIdIndex++;
if (sourceIdIndex >= sourceIds.size()) {
return false;
}
resourceClassIndex = 0;
}
Key sourceId = sourceIds.get(sourceIdIndex);
Class<?> resourceClass = resourceClasses.get(resourceClassIndex);
Transformation<?> transformation = helper.getTransformation(resourceClass);
// PMD.AvoidInstantiatingObjectsInLoops Each iteration is comparatively expensive anyway,
// we only run until the first one succeeds, the loop runs for only a limited
// number of iterations on the order of 10-20 in the worst case.
// 构造key
currentKey =
new ResourceCacheKey(// NOPMD AvoidInstantiatingObjectsInLoops
helper.getArrayPool(),
sourceId,
helper.getSignature(),
helper.getWidth(),
helper.getHeight(),
transformation,
resourceClass,
helper.getOptions());
// 查找缓存文件
cacheFile = helper.getDiskCache().get(currentKey);
// 如果找到了缓存文件，循环条件则会为false，退出循环
if (cacheFile != null) {
sourceKey = sourceId;
// 1. 找出注入时以File.class为modelClass的注入代码
// 2. 调用所有注入的factory.build方法得到ModelLoader
// 3 .过滤掉不可能处理model的ModelLoader
// 此时的modelLoaders值为：
// [ByteBufferFileLoader, FileLoader, FileLoader, UnitModelLoader]
modelLoaders = helper.getModelLoaders(cacheFile);
modelLoaderIndex = 0;
}
}
// 如果找到了缓存文件，hasNextModelLoader()方法则会为true，可以执行循环
// 没有找到缓存文件，则不会进入循环，会直接返回false
loadData = null;
boolean started = false;
while (!started && hasNextModelLoader()) {
ModelLoader<File, ?> modelLoader = modelLoaders.get(modelLoaderIndex++);
// 在循环中会依次判断某个ModelLoader能不能加载此文件
loadData = modelLoader.buildLoadData(cacheFile,
helper.getWidth(), helper.getHeight(), helper.getOptions());
if (loadData != null && helper.hasLoadPath(loadData.fetcher.getDataClass())) {
started = true;
// 如果某个ModelLoader可以，那么就调用其fetcher进行加载数据
// 加载成功或失败会通知自身
loadData.fetcher.loadData(helper.getPriority(), this);
}
}
return started;
}
该方法的相关注释代码里都有标明。找缓存时key的类型为ResourceCacheKey，我们先来看下ResourceCacheKey的构成
currentKey =
new ResourceCacheKey(// NOPMD AvoidInstantiatingObjectsInLoops
helper.getArrayPool(),
sourceId,
helper.getSignature(),
helper.getWidth(),
helper.getHeight(),
transformation,
resourceClass,
helper.getOptions());
ResourceCacheKey(
ArrayPool arrayPool,
Key sourceKey,
Key signature,
int width,
int height,
Transformation<?> appliedTransformation,
Class<?> decodedResourceClass,
Options options)
-
arrayPool
默认值是LruArrayPool，不参与key的equals方法 -
sourceKey
如果请求的是URL，此处就是GlideUrl（GlideUrl implements Key） -
signature
BaseRequestOptions的成员变量，默认会是EmptySignature.obtain()
在加载本地resource资源时会变成ApplicationVersionSignature.obtain(context) -
width、height
如果没有指定override(int size)，那么将得到view的size -
appliedTransformation
默认会根据ImageView的scaleType设置对应的BitmapTransformation；
如果指定了transform，那么就会是指定的值 -
decodedResourceClass
可以被编码成的资源类型，如BitmapDrawable等 -
options
如果没有设置过transform，此处会根据ImageView的scaleType默认指定一个option
在ResourceCacheKey中，arrayPool并没有参与equals方法。
生成ResourceCacheKey之后会根据key去磁盘缓存中查找cacheFile = helper.getDiskCache().get(currentKey);
helper.getDiskCache()返回DiskCache接口，它的实现类是DiskLruCacheWrapper，看下DiskLruCacheWrapper.get方法
@Override
public File get(Key key) {
String safeKey = safeKeyGenerator.getSafeKey(key);
...
File result = null;
try {
final DiskLruCache.Value value = getDiskCache().get(safeKey);
if (value != null) {
result = value.getFile(0);
}
} catch (IOException e) {
...
}
return result;
}
这里调用SafeKeyGenerator生成了一个String类型的SafeKey，实际上就是对原始key中每个字段都使用SHA-256加密，然后将得到的字节数组转换为16进制的字符串。生成SafeKey后，接着根据SafeKey去DiskCache里面找对应的缓存文件，然后返回文件。
回到ResourceCacheGenerator.startNext方法中，如果找到了缓存会调用loadData.fetcher.loadData(helper.getPriority(), this);这里的 fetcher 是 ByteBufferFetcher，ByteBufferFetcher的loadData方法中最终会执行callback.onDataReady(result)这里callback是ResourceCacheGenerator
public void onDataReady(Object data) {
cb.onDataFetcherReady(sourceKey, data, loadData.fetcher, DataSource.RESOURCE_DISK_CACHE,
currentKey);
}
ResourceCacheGenerator的onDataReady方法又会回调DecodeJob的onDataFetcherReady方法进行后续的解码操作。
如果ResourceCacheGenerator没有找到缓存，就会交给DataCacheGenerator继续查找缓存。该类大体流程和ResourceCacheGenerator一样，有点不同的是，DataCacheGenerator的构造器有两个构造器，其中的DataCacheGenerator(List<Key>, DecodeHelper<?>, FetcherReadyCallback)构造器是给SourceGenerator准备的。因为如果没有磁盘缓存，那么从源头加载后，肯定需要进行磁盘缓存操作的。所以，SourceGenerator会将加载后的资源保存到磁盘中，然后转交给DataCacheGenerator从磁盘中取出交给ImageView展示。
看下DataCacheGenerator.startNext
public boolean startNext() {
while (modelLoaders == null || !hasNextModelLoader()) {
sourceIdIndex++;
if (sourceIdIndex >= cacheKeys.size()) {
return false;
}
Key sourceId = cacheKeys.get(sourceIdIndex);
...
Key originalKey = new DataCacheKey(sourceId, helper.getSignature());
cacheFile = helper.getDiskCache().get(originalKey);
...
while (!started && hasNextModelLoader()) {
ModelLoader<File, ?> modelLoader = modelLoaders.get(modelLoaderIndex++);
loadData =
modelLoader.buildLoadData(cacheFile, helper.getWidth(), helper.getHeight(),
helper.getOptions());
if (loadData != null && helper.hasLoadPath(loadData.fetcher.getDataClass())) {
started = true;
loadData.fetcher.loadData(helper.getPriority(), this);
}
}
return started;
}
这里的originalKey是DataCacheKey类型的，DataCacheKey构造方法如下
DataCacheKey(Key sourceKey, Key signature)
这里的sourceKey和signature与ResourceCacheKey中的两个变量一致，从这里就可以看出：DataCache缓存的是原始的数据，ResourceCache缓存的是是被解码、转换后的数据。
如果DataCacheGenerator没有取到缓存，那么会交给SourceGenerator从源头加载。看下SourceGenerator的startNext方法
@Override
public boolean startNext() {
// 首次运行dataToCache为null
if (dataToCache != null) {
Object data = dataToCache;
dataToCache = null;
cacheData(data);
}
// 首次运行sourceCacheGenerator为null
if (sourceCacheGenerator != null && sourceCacheGenerator.startNext()) {
return true;
}
sourceCacheGenerator = null;
loadData = null;
boolean started = false;
while (!started && hasNextModelLoader()) {
loadData = helper.getLoadData().get(loadDataListIndex++);
if (loadData != null
&& (helper.getDiskCacheStrategy().isDataCacheable(loadData.fetcher.getDataSource())
|| helper.hasLoadPath(loadData.fetcher.getDataClass()))) {
started = true;
loadData.fetcher.loadData(helper.getPriority(), this);
}
}
return started;
}
加载成功后依然会回调SourceGenerator的onDataReady方法
@Override
public void onDataReady(Object data) {
DiskCacheStrategy diskCacheStrategy = helper.getDiskCacheStrategy();
if (data != null && diskCacheStrategy.isDataCacheable(loadData.fetcher.getDataSource())) {
dataToCache = data;
// cb 为 DecodeJob
cb.reschedule();
} else {
// cb 为 DecodeJob
cb.onDataFetcherReady(loadData.sourceKey, data, loadData.fetcher,
loadData.fetcher.getDataSource(), originalKey);
}
}
先判断获取到的数据是否需要进行磁盘缓存，如果需要磁盘缓存，则经过DecodeJob、EngineJob的调度，重新调用SourceGenerator.startNext方法，此时dataToCache已经被赋值，则会调用cacheData(data);进行磁盘缓存的写入，并转交给DataCacheGenerator完成后续的处理；否则就通知DecodeJob已经加载成功。
先看下SourceGenerator的startNext方法中调用的SourceGenerator.cacheData(data)
private void cacheData(Object dataToCache) {
long startTime = LogTime.getLogTime();
try {
Encoder<Object> encoder = helper.getSourceEncoder(dataToCache);
DataCacheWriter<Object> writer =
new DataCacheWriter<>(encoder, dataToCache, helper.getOptions());
originalKey = new DataCacheKey(loadData.sourceKey, helper.getSignature());
helper.getDiskCache().put(originalKey, writer);
...
} finally {
loadData.fetcher.cleanup();
}
sourceCacheGenerator =
new DataCacheGenerator(Collections.singletonList(loadData.sourceKey), helper, this);
}
cacheData方法先构建了一个DataCacheKey将data写入了磁盘，然后new了一个DataCacheGenerator赋值给sourceCacheGenerator。回到startNext继续向下执行，此时sourceCacheGenerator不为空，就调用其startNext()方法从磁盘中加载刚写入磁盘的数据，并返回true让DecodeJob停止尝试获取数据。此时，从磁盘缓存中读取数据的逻辑已经完成，接下来是写磁盘缓存。
假如SourceGenerator的onDataReady方法中的磁盘缓存策略不可用，则会回调DecodeJob.onDataFetcherReady方法
// DecodeJob
@Override
public void onDataFetcherReady(Key sourceKey, Object data, DataFetcher<?> fetcher,
DataSource dataSource, Key attemptedKey) {
this.currentSourceKey = sourceKey;
this.currentData = data;
this.currentFetcher = fetcher;
this.currentDataSource = dataSource;
this.currentAttemptingKey = attemptedKey;
if (Thread.currentThread() != currentThread) {
runReason = RunReason.DECODE_DATA;
callback.reschedule(this);
} else {
GlideTrace.beginSection("DecodeJob.decodeFromRetrievedData");
try {
decodeFromRetrievedData();
} finally {
GlideTrace.endSection();
}
}
}
private void decodeFromRetrievedData() {
...
Resource<R> resource = null;
try {
resource = decodeFromData(currentFetcher, currentData, currentDataSource);
} catch (GlideException e) {
e.setLoggingDetails(currentAttemptingKey, currentDataSource);
throwables.add(e);
}
if (resource != null) {
notifyEncodeAndRelease(resource, currentDataSource);
} else {
runGenerators();
}
}
decodeFromRetrievedData();后续的方法调用链在之前的文章中分析过，主要做的事情就是：将原始的data数据转变为可以供ImageView显示的resource数据并将其显示在ImageView上。
将原始的data数据转变为resource数据后，会调用DecodeJob.onResourceDecoded(dataSource, decoded)
@Synthetic
@NonNull
<Z> Resource<Z> onResourceDecoded(DataSource dataSource,
@NonNull Resource<Z> decoded) {
@SuppressWarnings("unchecked")
Class<Z> resourceSubClass = (Class<Z>) decoded.get().getClass();
Transformation<Z> appliedTransformation = null;
Resource<Z> transformed = decoded;
// 不是 resource cache时要transform
if (dataSource != DataSource.RESOURCE_DISK_CACHE) {
appliedTransformation = decodeHelper.getTransformation(resourceSubClass);
transformed = appliedTransformation.transform(glideContext, decoded, width, height);
}
// TODO: Make this the responsibility of the Transformation.
if (!decoded.equals(transformed)) {
decoded.recycle();
}
final EncodeStrategy encodeStrategy;
final ResourceEncoder<Z> encoder;
if (decodeHelper.isResourceEncoderAvailable(transformed)) {
encoder = decodeHelper.getResultEncoder(transformed);
encodeStrategy = encoder.getEncodeStrategy(options);
} else {
encoder = null;
encodeStrategy = EncodeStrategy.NONE;
}
Resource<Z> result = transformed;
boolean isFromAlternateCacheKey = !decodeHelper.isSourceKey(currentSourceKey);
if (diskCacheStrategy.isResourceCacheable(isFromAlternateCacheKey, dataSource,
encodeStrategy)) {
if (encoder == null) {
throw new Registry.NoResultEncoderAvailableException(transformed.get().getClass());
}
final Key key;
switch (encodeStrategy) {
case SOURCE:
key = new DataCacheKey(currentSourceKey, signature);
break;
case TRANSFORMED:
key =
new ResourceCacheKey(
decodeHelper.getArrayPool(),
currentSourceKey,
signature,
width,
height,
appliedTransformation,
resourceSubClass,
options);
break;
default:
throw new IllegalArgumentException("Unknown strategy: " + encodeStrategy);
}
LockedResource<Z> lockedResult = LockedResource.obtain(transformed);
deferredEncodeManager.init(key, encoder, lockedResult);
result = lockedResult;
}
return result;
}
然后是此过程中的磁盘缓存过程，影响的因素有encodeStrategy、DiskCacheStrategy.isResourceCacheable。encodeStrategy根据resource数据的类型来判断，如果是Bitmap或BitmapDrawable，那么就是TRANSFORMED；如果是GifDrawable，那么就是SOURCE。磁盘缓存策略默认是DiskCacheStrategy.AUTOMATIC。源码如下：
public static final DiskCacheStrategy AUTOMATIC = new DiskCacheStrategy() {
public boolean isDataCacheable(DataSource dataSource) {
return dataSource == DataSource.REMOTE;
}
public boolean isResourceCacheable(boolean isFromAlternateCacheKey, DataSource dataSource, EncodeStrategy encodeStrategy) {
return (isFromAlternateCacheKey && dataSource == DataSource.DATA_DISK_CACHE || dataSource == DataSource.LOCAL) && encodeStrategy == EncodeStrategy.TRANSFORMED;
}
public boolean decodeCachedResource() {
return true;
}
public boolean decodeCachedData() {
return true;
}
};
只有dataSource为DataSource.LOCAL且encodeStrategy为EncodeStrategy.TRANSFORMED时，才允许缓存。也就是只有本地的resource数据为Bitmap或BitmapDrawable的资源才可以缓存。
在DecodeJob.onResourceDecoded中会调用deferredEncodeManager.init(key, encoder, lockedResult);去初始化deferredEncodeManager。
在DecodeJob的decodeFromRetrievedData();中拿到resource数据后会调用notifyEncodeAndRelease(resource, currentDataSource)利用deferredEncodeManager对象进行磁盘缓存的写入
private void notifyEncodeAndRelease(Resource<R> resource, DataSource dataSource) {
...
// 通知回调，资源已经就绪
notifyComplete(result, dataSource);
stage = Stage.ENCODE;
try {
if (deferredEncodeManager.hasResourceToEncode()) {
deferredEncodeManager.encode(diskCacheProvider, options);
}
} finally {
if (lockedResource != null) {
lockedResource.unlock();
}
}
onEncodeComplete();
}
deferredEncodeManager.encode行磁盘缓存的写入
// DecodeJob
private static class DeferredEncodeManager<Z> {
private Key key;
private ResourceEncoder<Z> encoder;
private LockedResource<Z> toEncode;
@Synthetic
DeferredEncodeManager() { }
// We just need the encoder and resource type to match, which this will enforce.
@SuppressWarnings("unchecked")
<X> void init(Key key, ResourceEncoder<X> encoder, LockedResource<X> toEncode) {
this.key = key;
this.encoder = (ResourceEncoder<Z>) encoder;
this.toEncode = (LockedResource<Z>) toEncode;
}
void encode(DiskCacheProvider diskCacheProvider, Options options) {
GlideTrace.beginSection("DecodeJob.encode");
try {
// 存入磁盘缓存
diskCacheProvider.getDiskCache().put(key,
new DataCacheWriter<>(encoder, toEncode, options));
} finally {
toEncode.unlock();
GlideTrace.endSection();
}
}
boolean hasResourceToEncode() {
return toEncode != null;
}
void clear() {
key = null;
encoder = null;
toEncode = null;
}
}
diskCacheProvider.getDiskCache()获取到DiskLruCacheWrapper，并调用DiskLruCacheWrapper的put写入。DiskLruCacheWrapper在写入的时候会使用到写锁DiskCacheWriteLocker，锁对象由对象池WriteLockPool创建，写锁WriteLock实现是一个不公平锁ReentrantLock。
在缓存写入前，会判断key对应的value存不存在，若存在则不写入。缓存的真正写入会由DataCacheWriter交给ByteBufferEncoder和StreamEncoder两个具体类来写入，前者负责将ByteBuffer写入到文件，后者负责将InputStream写入到文件。
到目前为止，磁盘缓存的读写流程都已分析完成。
2.5 内存缓存：ActiveResource与MemoryCache读取
回到DecodeJob.notifyEncodeAndRelease方法中，经过notifyComplete、EngineJob.onResourceReady、notifyCallbacksOfResult方法中。
在该方法中一方面会将原始的resource包装成一个EngineResource，然后通过回调传给Engine.onEngineJobComplete
@Override
public synchronized void onEngineJobComplete(
EngineJob<?> engineJob, Key key, EngineResource<?> resource) {
// 设置资源的回调为自己，这样在资源释放时会通知自己的回调方法
if (resource != null) {
resource.setResourceListener(key, this);
// 将资源放入activeResources中，资源变为active状态
if (resource.isCacheable()) {
activeResources.activate(key, resource);
}
}
// 将engineJob从Jobs中移除
jobs.removeIfCurrent(key, engineJob);
}
在这里会将资源放入activeResources中，资源变为active状态。后面会使用Executors.mainThreadExecutor()调用SingleRequest.onResourceReady回调进行资源的显示。在触发回调前后各有一个地方会对engineResource进行acquire()和release()操作，这两个操作分别发生在notifyCallbacksOfResult()方法的incrementPendingCallbacks、decrementPendingCallbacks()调用中
@Synthetic
void notifyCallbacksOfResult() {
ResourceCallbacksAndExecutors copy;
Key localKey;
EngineResource<?> localResource;
synchronized (this) {
...
engineResource = engineResourceFactory.build(resource, isCacheable);
...
hasResource = true;
copy = cbs.copy();
incrementPendingCallbacks(copy.size() + 1);
localKey = key;
localResource = engineResource;
}
listener.onEngineJobComplete(this, localKey, localResource);
for (final ResourceCallbackAndExecutor entry : copy) {
entry.executor.execute(new CallResourceReady(entry.cb));
}
decrementPendingCallbacks();
}
synchronized void incrementPendingCallbacks(int count) {
...
if (pendingCallbacks.getAndAdd(count) == 0 && engineResource != null) {
engineResource.acquire();
}
}
synchronized void decrementPendingCallbacks() {
...
int decremented = pendingCallbacks.decrementAndGet();
if (decremented == 0) {
if (engineResource != null) {
engineResource.release();
}
release();
}
}
private class CallResourceReady implements Runnable {
private final ResourceCallback cb;
CallResourceReady(ResourceCallback cb) {
this.cb = cb;
}
@Override
public void run() {
synchronized (EngineJob.this) {
if (cbs.contains(cb)) {
// Acquire for this particular callback.
engineResource.acquire();
callCallbackOnResourceReady(cb);
removeCallback(cb);
}
decrementPendingCallbacks();
}
}
}
CallResourceReady的run方法中也会调用engineResource.acquire()，上面的代码调用结束后，engineResource的引用计数为1。engineResource的引用计数会在RequestManager.onDestory方法中最终调用SingleRequest.clear()方法，SingleRequest.clear()内部调用releaseResource()、Engine.release 进行释放，这样引用计数就变为0。引用计数就变为0后会通知Engine将此资源从active状态变成memory cache状态。如果我们再次加载资源时可以从memory cache中加载，那么资源又会从memory cache状态变成active状态。也就是说，在资源第一次显示后，我们关闭页面，资源会由active变成memory cache；然后我们再次进入页面，加载时会命中memory cache，从而又变成active状态。
2.6 总结
四种缓存状态
-
memory cache和disk cache在Glide创建的时候也被创建。
-
disk cache默认会创建一个250M的缓存目录(/data/data/{package}/cache/image_manager_disk_cache/)。
-
ActiveResources在Engine的构造器中被创建，内部维护了一个 Map<Key, ResourceWeakReference>类型的activeEngineResources用来存储包裹EngineResource的ResourceWeakReference，ResourceWeakReference构造器中会传入一个ReferenceQueue，在ActiveResources的构造器中会启动一个后台线程，在该线程中会循环从activeEngineResources清除ReferenceQueue中的将要被GC的Resource。
-
ActiveResources被引用后，其内部的引用计数会+1，当被释放后，其内部的引用计数会-1，当引用计数为0，则表示该ActiveResources不再被引用，会将资源放入LruResourceCache中。
-
首先从ActiveResources中获取缓存资源，获取不到再从LruResourceCache中查找。第一次没有缓存，会从网络下载图片成功后会存入磁盘缓存，然后再从磁盘缓存获取资源存入ActiveResources中并将其交给ImageView展示。
-
缓存查找中涉及的三个类：ResourceCacheGenerator、DataCacheGenerator、SourceGenerator。按照先后关系依次调用他们的startNext()方法查找缓存，ResourceCacheGenerator获取downsample、transform后的资源文件的缓存文件；DataCacheGenerator获取原始的没有修改过的资源文件的缓存文件；SourceGenerator获取原始源数据。
-
ActiveResources引用计数就变为0后会通知Engine将此资源从active状态变成memory cache状态。如果我们再次加载资源时可以从memory cache中加载，那么资源又会从memory cache状态变成active状态。也就是说，在资源第一次显示后，我们关闭页面，资源会由active变成memory cache；然后我们再次进入页面，加载时会命中memory cache，从而又变成active状态。
-
之所以需要ActiveResources，因为它用弱引用包装资源，随时可能被回收，memory的强引用频繁读写可能造成内存激增频繁GC，而造成内存抖动。资源在使用过程中保存在ActiveResources中，而ActiveResources是弱引用，随时被系统回收，不会造成内存过多使用和泄漏。
3 Bitmap复用池复用源码流程
在Glide源码分析-网络图片加载主流程分析文章中分析过Glide加载图片主要流程源码，在ByteBufferBitmapDecoder.decode方法中，先将ByteBuffer转换成InputStream，然后在调用Downsampler.decode方法进行解码，代码如下：
// ByteBufferBitmapDecoder
@Override
public Resource<Bitmap> decode(@NonNull ByteBuffer source, int width, int height,
@NonNull Options options)
throws IOException {
InputStream is = ByteBufferUtil.toStream(source);
return downsampler.decode(is, width, height, options);
}
继续跟进Downsampler.decode方法
public Resource<Bitmap> decode(InputStream is, int requestedWidth, int requestedHeight,
Options options, DecodeCallbacks callbacks) throws IOException {
Preconditions.checkArgument(is.markSupported(), "You must provide an InputStream that supports"
+ " mark()");
byte[] bytesForOptions = byteArrayPool.get(ArrayPool.STANDARD_BUFFER_SIZE_BYTES, byte[].class);
// getDefaultOptions()中将inMutable设置为true
BitmapFactory.Options bitmapFactoryOptions = getDefaultOptions();
// inTempStorage是一个bitmap解析的参数，带入一个buffer，创建临时文件，将图片存储的临时缓存空间
bitmapFactoryOptions.inTempStorage = bytesForOptions;
DecodeFormat decodeFormat = options.get(DECODE_FORMAT);
DownsampleStrategy downsampleStrategy = options.get(DownsampleStrategy.OPTION);
boolean fixBitmapToRequestedDimensions = options.get(FIX_BITMAP_SIZE_TO_REQUESTED_DIMENSIONS);
boolean isHardwareConfigAllowed =
options.get(ALLOW_HARDWARE_CONFIG) != null && options.get(ALLOW_HARDWARE_CONFIG);
try {
Bitmap result = decodeFromWrappedStreams(is, bitmapFactoryOptions,
downsampleStrategy, decodeFormat, isHardwareConfigAllowed, requestedWidth,
requestedHeight, fixBitmapToRequestedDimensions, callbacks);
return BitmapResource.obtain(result, bitmapPool);
} finally {
releaseOptions(bitmapFactoryOptions);
byteArrayPool.put(bytesForOptions);
}
}
在getDefaultOptions()方法中会将inMutable设置为true，代码如下：
private static synchronized BitmapFactory.Options getDefaultOptions() {
BitmapFactory.Options decodeBitmapOptions;
synchronized (OPTIONS_QUEUE) {
decodeBitmapOptions = OPTIONS_QUEUE.poll();
}
if (decodeBitmapOptions == null) {
decodeBitmapOptions = new BitmapFactory.Options();
resetOptions(decodeBitmapOptions);
}
return decodeBitmapOptions;
}
private static void resetOptions(BitmapFactory.Options decodeBitmapOptions) {
decodeBitmapOptions.inTempStorage = null;
decodeBitmapOptions.inDither = false;
decodeBitmapOptions.inScaled = false;
decodeBitmapOptions.inSampleSize = 1;
decodeBitmapOptions.inPreferredConfig = null;
decodeBitmapOptions.inJustDecodeBounds = false;
decodeBitmapOptions.inDensity = 0;
decodeBitmapOptions.inTargetDensity = 0;
decodeBitmapOptions.outWidth = 0;
decodeBitmapOptions.outHeight = 0;
decodeBitmapOptions.outMimeType = null;
decodeBitmapOptions.inBitmap = null;
decodeBitmapOptions.inMutable = true;
}
回到Downsampler.decode方法中，继续调用decodeFromWrappedStreams方法返回Bitmap，跟进decodeFromWrappedStreams
// DownSampler
private Bitmap decodeFromWrappedStreams(InputStream is,
BitmapFactory.Options options, DownsampleStrategy downsampleStrategy,
DecodeFormat decodeFormat, boolean isHardwareConfigAllowed, int requestedWidth,
int requestedHeight, boolean fixBitmapToRequestedDimensions,
DecodeCallbacks callbacks) throws IOException {
long startTime = LogTime.getLogTime();
// 计算原始大小
int[] sourceDimensions = getDimensions(is, options, callbacks, bitmapPool);
int sourceWidth = sourceDimensions[0];
int sourceHeight = sourceDimensions[1];
String sourceMimeType = options.outMimeType;
...
// 计算方向
int orientation = ImageHeaderParserUtils.getOrientation(parsers, is, byteArrayPool);
int degreesToRotate = TransformationUtils.getExifOrientationDegrees(orientation);
boolean isExifOrientationRequired = TransformationUtils.isExifOrientationRequired(orientation);
// 计算目标大小
int targetWidth = requestedWidth == Target.SIZE_ORIGINAL ? sourceWidth : requestedWidth;
int targetHeight = requestedHeight == Target.SIZE_ORIGINAL ? sourceHeight : requestedHeight;
// 计算类型
ImageType imageType = ImageHeaderParserUtils.getType(parsers, is, byteArrayPool);
...
// 使用复用池
if ((options.inSampleSize == 1 || isKitKatOrGreater) && shouldUsePool(imageType)) {
int expectedWidth;
int expectedHeight;
...
if (expectedWidth > 0 && expectedHeight > 0) {
setInBitmap(options, bitmapPool, expectedWidth, expectedHeight);
}
}
Bitmap downsampled = decodeStream(is, options, callbacks, bitmapPool);
callbacks.onDecodeComplete(bitmapPool, downsampled);
...
return rotated;
}
private static void setInBitmap(
BitmapFactory.Options options, BitmapPool bitmapPool, int width, int height) {
@Nullable Bitmap.Config expectedConfig = null;
...
options.inBitmap = bitmapPool.getDirty(width, height, expectedConfig);
}
setInBitmap方法里面的bitmapPool就是LruBitmapPool，在Glide构造器里面被初始化，LruBitmapPool 就是Glide提供Bitmap复用池,真正的实现类是LruPoolStrategy
public class LruBitmapPool implements BitmapPool {
private final LruPoolStrategy strategy;
...
public synchronized void put(Bitmap bitmap) {
if (bitmap == null) {
throw new NullPointerException("Bitmap must not be null");
}
if (bitmap.isRecycled()) {
throw new IllegalStateException("Cannot pool recycled bitmap");
}
if (!bitmap.isMutable() || strategy.getSize(bitmap) > maxSize
|| !allowedConfigs.contains(bitmap.getConfig())) {
bitmap.recycle();
return;
}
final int size = strategy.getSize(bitmap);
strategy.put(bitmap);
tracker.add(bitmap);
puts++;
currentSize += size;
dump();
evict();
}
public Bitmap get(int width, int height, Bitmap.Config config) {
Bitmap result = getDirtyOrNull(width, height, config);
if (result != null) {
result.eraseColor(Color.TRANSPARENT);
} else {
result = createBitmap(width, height, config);
}
return result;
}
}
看下LruBitmapPool.getDirty方法
// LruBitmapPool
public Bitmap getDirty(int width, int height, Bitmap.Config config) {
// 优先获取
Bitmap result = getDirtyOrNull(width, height, config);
if (result == null) { // 若没有，新建一个bitmap
result = createBitmap(width, height, config);
}
return result;
}
如果可以使用bitmap池，就会调用bitmapPool的getDirty()，最后赋值给inBitmap，总结一下：setInBitmap方法主要就是从LruBitmapPool中获取可以被复用的Bitmap返回，并赋值给BitmapFactory.Options的inBitmap。
那么可以被复用的Bitmap是什么时候加入Bitmap复用池呢？当Resource资源没有被引用并且不可被缓存的时候，会调用recycle()方法进行回收，在BitmapDrawableResource的recycle()方法被调用的时候，会将当前BitmapDrawableResource的bitmap放入复用池，代码如下：
// BitmapDrawableResource
@Override
public void recycle() {
bitmapPool.put(drawable.getBitmap());
}
LruBitmapPool内部利用了Lru算法，每次操作都自动检测是否删除多余的缓存
// LruBitmapPool
private void evict() {
trimToSize(maxSize);
}
private synchronized void trimToSize(long size) {
while (currentSize > size) {
final Bitmap removed = strategy.removeLast();
// TODO: This shouldn't ever happen, see #331.
if (removed == null) {
if (Log.isLoggable(TAG, Log.WARN)) {
Log.w(TAG, "Size mismatch, resetting");
dumpUnchecked();
}
currentSize = 0;
return;
}
tracker.remove(removed);
currentSize -= strategy.getSize(removed);
evictions++;
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "Evicting bitmap=" + strategy.logBitmap(removed));
}
dump();
removed.recycle();
}
}
再来看看Bitmap复用池对Bitmap具体的存取逻辑的类LruPoolStrategy，在LruBitmapPool的put方法中会调用LruPoolStrategy的put方法，在LruBitmapPool的get方法中会调用getDirtyOrNull方法进而调用LruPoolStrategy的get方法，LruPoolStrategy时一个接口，在LruBitmapPool中为LruPoolStrategy类型的全局变量strategy其赋值的地方在LruBitmapPool构造器中
LruBitmapPool(long maxSize, LruPoolStrategy strategy, Set<Bitmap.Config> allowedConfigs) {
this.initialMaxSize = maxSize;
this.maxSize = maxSize;
this.strategy = strategy;
this.allowedConfigs = allowedConfigs;
this.tracker = new NullBitmapTracker();
}
public LruBitmapPool(long maxSize) {
this(maxSize, getDefaultStrategy(), getDefaultAllowedConfigs());
}
private static LruPoolStrategy getDefaultStrategy() {
final LruPoolStrategy strategy;
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
strategy = new SizeConfigStrategy();
} else {
strategy = new AttributeStrategy();
}
return strategy;
}
当sdk版本在19之上，会创建SizeConfigStrategy实例并赋值给strategy，那么继续看下SizeConfigStrategy的put和get方法相关代码
public class SizeConfigStrategy implements LruPoolStrategy {
private static final int MAX_SIZE_MULTIPLE = 8;
private final KeyPool keyPool = new KeyPool();
private final GroupedLinkedMap<Key, Bitmap> groupedMap = new GroupedLinkedMap<>();
private final Map<Bitmap.Config, NavigableMap<Integer, Integer>> sortedSizes = new HashMap<>();
// 存入
public void put(Bitmap bitmap) {
int size = Util.getBitmapByteSize(bitmap);
Key key = keyPool.get(size, bitmap.getConfig());
groupedMap.put(key, bitmap);
NavigableMap<Integer, Integer> sizes = getSizesForConfig(bitmap.getConfig());
Integer current = sizes.get(key.size);
sizes.put(key.size, current == null ? 1 : current + 1);
}
// 取出
public Bitmap get(int width, int height, Bitmap.Config config) {
int size = Util.getBitmapByteSize(width, height, config);
// 查找出最合适的bitmap
Key bestKey = findBestKey(size, config);
// 取出
Bitmap result = groupedMap.get(bestKey);
if (result != null) {
// Decrement must be called before reconfigure.
decrementBitmapOfSize(bestKey.size, result);
result.reconfigure(width, height,
result.getConfig() != null ? result.getConfig() : Bitmap.Config.ARGB_8888);
}
return result;
}
}
findBestKey()方法就是通过对size进行匹配，找出最合适size的Bitmap的key，上面有提到过：在Android 4.4之后复用Bitmap有一个限制，就是被复用的Bitmap尺寸要大于新的Bitmap尺寸，findBestKey()方法就是实现这个逻辑
private Key findBestKey(int size, Bitmap.Config config) {
Key result = keyPool.get(size, config);
for (Bitmap.Config possibleConfig : getInConfigs(config)) {
NavigableMap<Integer, Integer> sizesForPossibleConfig = getSizesForConfig(possibleConfig);
// 返回大于或等于指定size的最小的符合要求的size
Integer possibleSize = sizesForPossibleConfig.ceilingKey(size);
if (possibleSize != null && possibleSize <= size * MAX_SIZE_MULTIPLE) {
if (possibleSize != size
|| (possibleConfig == null ? config != null : !possibleConfig.equals(config))) {
keyPool.offer(result);
result = keyPool.get(possibleSize, possibleConfig);
}
break;
}
}
return result;
}
总结
- Glide中使用Bitmap复用池来减少内存抖动
- Bitmap复用池实现类是LruBitmapPool，通过LRU算法来管理Bitmap复用池
- 具体的存储和取出符合要求的可被复用的Bitmap的逻辑在SizeConfigStrategy中，主要是通过NavigableMap数据结构的ceilingKey(K key)方法取出大于或等于新的Bitmap的size的最优size对应的key，进而取出最优的待复用的Bitmap
参考链接
https://muyangmin.github.io/glide-docs-cn/doc/caching.html
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