参考
作者：BlackFlag
链接：[https://www.jianshu.com/p/57123450a9c8]
基本概念
image.png
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使用
implementation 'com.github.bumptech.glide:glide:3.8.0'
annotationProcessor 'com.github.bumptech.glide:compiler:3.8.0'
Glide.with(this).load(url).into(imageView);
Glide.with(getApplicationContext()) //指定Context
.load(url) //指定图片的URL
.placeholder(R.mipmap.ic_launcher) //指定图片未成功加载前显示的图片
.error(R.mipmap.ic_launcher) //指定图片加载失败显示的图片
.override(300, 300) //指定图片的尺寸
.fitCenter() //指定图片缩放类型为
.centerCrop() //指定图片缩放类型为
.skipMemoryCache(true) //跳过内存缓存
.crossFade(1000) //设置渐变式显示的时间
.diskCacheStrategy(DiskCacheStrategy.NONE) //跳过磁盘缓存
.diskCacheStrategy(DiskCacheStrategy.SOURCE) //仅仅只缓存原理的全分辨率的图像
.diskCacheStrategy(DiskCacheStrategy.RESULT) //仅仅缓存最终的图像
.diskCacheStrategy(DiskCacheStrategy.ALL) //缓存所有版本的图像
.priority(Priority.HIGH) //指定优先级.Glide将会为他们作为一个准则，并尽可能的处理这些请求，但是并不能保证100%实施
.into(imageView); //指定显示图片的ImageView
1、with(*)方法
public static RequestManager with(Context context) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(context);
}
public static RequestManager with(Activity activity) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(activity);
}
public static RequestManager with(FragmentActivity activity) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(activity);
}
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
public static RequestManager with(android.app.Fragment fragment) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(fragment);
}
public static RequestManager with(Fragment fragment) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(fragment);
}
1、获取RequestManagerRetriever.get()得到retriever
2、通过retriever.get(context)获得RequestManager对象并返回
3、with通过传入不同的参数，Context、Activity、Fragment等等，主要是通过相应的生命周期是管理加载图片，避免了消耗多余的资源，也避免了在Activity销毁之后加载图片从而导致的空指针问题。
2、RequestManagerRetriever
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上面这个方法主要是通过传入context的不同类型来做不同的操作。context可以是Application、FragmentActivity、Activity或者是ContextWrapper。getApplicationManager(Context context)通过单例模式创建并返回了 applicationManager
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如果不是主线程或者版本过低，还是通过get(Application)方法，否则通过fragmentGet(activity, fm)方法。
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fragmentGet 在当前的activity创建没有界面的fragment并add进activity，并将这个fragment与RequestManager进行绑定，实现对activity生命周期的监听
总结
- 通过RequestManagerRetriever的get获取RequestManagerRetriever对象
- 通过retriever.get(context)获取RequestManager，在get(context)方法中通过对context类型的判断做不同的处理：
- context是Application，通过getApplicationManager(Context context) 创建并返回一个RequestManager对象
- context是Activity，通过fragmentGet(activity, fm)在当前activity创建并添加一个没有界面的fragment，从而实现图片加载与activity的生命周期相绑定，之后创建并返回一个RequestManager对象
3、load(url)
image.png
通过RequestManager调用load方法——>DrawableTypeRequest<String>) fromString().load(string)
先看fromString方法
public DrawableTypeRequest<String> fromString() {
return loadGeneric(String.class);
}
image.png
创建并返回了一个DrawableTypeRequest。
接下来看load方法
@Override
public DrawableRequestBuilder<ModelType> load(ModelType model) {
super.load(model);
return this;
}
返回DrawableTypeRequest<String>对象。跟进父类的load看看
* @param model The model to load data for, or null.
* @return This request builder.
*/
public GenericRequestBuilder<ModelType, DataType, ResourceType, TranscodeType> load(ModelType model) {
this.model = model;
isModelSet = true;
return this;
}
DrawableRequestBuilder是GenericRequestBuilder子类，前者是Drawable请求的构建者，后者是通用的请求构建者。这个load方法其实是把我们传入的String类型的URL存入了内部的model成员变量中，再将数据来源是否已经设置的标志位 isModelSet 设置为true，意味着我们在调用 Glide.with(context).load(url) 之后数据来源已经设置成功了。
Glide.with(getApplicationContext()) //指定Context
.load(url) //指定图片的URL
.placeholder(R.mipmap.ic_launcher) //指定图片未成功加载前显示的图片
.error(R.mipmap.ic_launcher) //指定图片加载失败显示的图片
.override(300, 300) //指定图片的尺寸
.fitCenter() //指定图片缩放类型为
.centerCrop() //指定图片缩放类型为
.skipMemoryCache(true) //跳过内存缓存
.crossFade(1000) //设置渐变式显示的时间
.diskCacheStrategy(DiskCacheStrategy.NONE) //跳过磁盘缓存
.diskCacheStrategy(DiskCacheStrategy.SOURCE) //仅仅只缓存原理的全分辨率的图像
.diskCacheStrategy(DiskCacheStrategy.RESULT) //仅仅缓存最终的图像
.diskCacheStrategy(DiskCacheStrategy.ALL) //缓存所有版本的图像
.priority(Priority.HIGH) //指定优先级.Glide将会为他们作为一个准则，并尽可能的处理这些请求，但是并不能保证100%实施
.into(imageView); //指定显示图片的ImageView
4、placeholder
DrawableRequestBuilder.class
/**
* {@inheritDoc}
*/
@Override
public DrawableRequestBuilder<ModelType> placeholder(int resourceId) {
super.placeholder(resourceId);
return this;
}
GenericRequestBuilder.class
/**
* Sets an Android resource id for a {@link android.graphics.drawable.Drawable} resource to display while a resource
* is loading.
*
* @param resourceId The id of the resource to use as a placeholder
* @return This request builder.
*/
public GenericRequestBuilder<ModelType, DataType, ResourceType, TranscodeType> placeholder(
int resourceId) {
this.placeholderId = resourceId;
return this;
}
GenericRequestBuilder通过builder模式为成员属性进行赋值
5、into(imageView)方法
into之前的步骤其实就是创建了一个Request（加载图片的配置请求）。info方法便是真正的执行。
DrawableRequestBuilder.class
@Override
public Target<GlideDrawable> into(ImageView view) {
return super.into(view);
}
GenericRequestBuilder.class
/**
* Sets the {@link ImageView} the resource will be loaded into, cancels any existing loads into the view, and frees
* any resources Glide may have previously loaded into the view so they may be reused.
*
* @see Glide#clear(android.view.View)
*
* @param view The view to cancel previous loads for and load the new resource into.
* @return The {@link com.bumptech.glide.request.target.Target} used to wrap the given {@link ImageView}.
*/
public Target<TranscodeType> into(ImageView view) {
Util.assertMainThread();
if (view == null) {
throw new IllegalArgumentException("You must pass in a non null View");
}
if (!isTransformationSet && view.getScaleType() != null) {
switch (view.getScaleType()) {
case CENTER_CROP:
applyCenterCrop();
break;
case FIT_CENTER:
case FIT_START:
case FIT_END:
applyFitCenter();
break;
//$CASES-OMITTED$
default:
// Do nothing.
}
}
return into(glide.buildImageViewTarget(view, transcodeClass));
}
最终还是走到了GenericRequestBuilder.class中的into方法中。重点看
return into(glide.buildImageViewTarget(view, transcodeClass));
返回了Target<TranscodeType>对象。
Glide.class
<R> Target<R> buildImageViewTarget(ImageView imageView, Class<R> transcodedClass) {
return imageViewTargetFactory.buildTarget(imageView, transcodedClass);
}
/**
* A factory responsible for producing the correct type of {@link com.bumptech.glide.request.target.Target} for a given
* {@link android.view.View} subclass.
*/
public class ImageViewTargetFactory {
@SuppressWarnings("unchecked")
public <Z> Target<Z> buildTarget(ImageView view, Class<Z> clazz) {
if (GlideDrawable.class.isAssignableFrom(clazz)) {
return (Target<Z>) new GlideDrawableImageViewTarget(view);
} else if (Bitmap.class.equals(clazz)) {
return (Target<Z>) new BitmapImageViewTarget(view);
} else if (Drawable.class.isAssignableFrom(clazz)) {
return (Target<Z>) new DrawableImageViewTarget(view);
} else {
throw new IllegalArgumentException("Unhandled class: " + clazz
+ ", try .as*(Class).transcode(ResourceTranscoder)");
}
}
}
通过对图片来源类型的判断，创建并返回与图片来源对应的imageViewTarget。
GenericRequestBuilder.class
/**
* Set the target the resource will be loaded into.
*
* @see Glide#clear(com.bumptech.glide.request.target.Target)
*
* @param target The target to load the resource into.
* @return The given target.
*/
public <Y extends Target<TranscodeType>> Y into(Y target) {
Util.assertMainThread();
if (target == null) {
throw new IllegalArgumentException("You must pass in a non null Target");
}
//确保数据来源已经确定，即已经调用了load(url)方法
if (!isModelSet) {
throw new IllegalArgumentException("You must first set a model (try #load())");
}
//获取当前target已经绑定的Request对象
Request previous = target.getRequest();
//如果当前target已经绑定了Request对象，则清空这个Request对象
if (previous != null) {
previous.clear();
//停止绑定到当前target的上一个Request的图片请求处理
requestTracker.removeRequest(previous);
previous.recycle();
}
//创建Request对象
Request request = buildRequest(target);
target.setRequest(request);
lifecycle.addListener(target);
//执行request
requestTracker.runRequest(request);
return target;
}
我们梳理一下方法中的逻辑：
- 获取当前target中的Request对象，如果存在，则清空并终止这个Request对象的执行
- 创建新的Request对象并与当前target绑定
- 执行新创建的图片处理请求Request
在没有Glide之前，我们处理ListView中的图片加载其实是一件比较麻烦的事情。由于ListView中Item的复用机制，会导致网络图片加载的错位或者闪烁。那我们解决这个问题的办法也很简单，就是给当前的ImageView设置tag，这个tag可以是图片的URL等等。当从网络中获取到图片时判断这个ImageVIew中的tag是否是这个图片的URL，如果是就加载图片，如果不是则跳过。
在有了Glide之后，我们处理ListView或者Recyclerview中的图片加载就很无脑了，根本不需要作任何多余的操作，直接正常使用就行了。这其中的原理是Glide给我们处理了这些判断，我们来看一下Glide内部是如何处理的：
@Override
public void setRequest(Request request) {
setTag(request);
}
@Override
public Request getRequest() {
Object tag = getTag();
Request request = null;
if (tag != null) {
if (tag instanceof Request) {
request = (Request) tag;
} else {
throw new IllegalArgumentException("You must not call setTag() on a view Glide is targeting");
}
}
return request;
}
private void setTag(Object tag) {
if (tagId == null) {
isTagUsedAtLeastOnce = true;
view.setTag(tag);
} else {
view.setTag(tagId, tag);
}
}
private Object getTag() {
if (tagId == null) {
return view.getTag();
} else {
return view.getTag(tagId);
}
}
target.getRequest() 和 target.setRequest(Request request) 本质上还是通过setTag和getTag来做的处理，这也印证了我们上面所说。
继续回到into方法中，在创建并绑定了Request后，关键的就是 requestTracker.runRequest(request) 来执行我们创建的请求了。
/**
* Starts tracking the given request.
*/
public void runRequest(Request request) {
//将请求加入请求集合
requests.add(request);
if (!isPaused) {
//如果处于非暂停状态，开始执行请求
request.begin();
} else {
//如果处于暂停状态，将请求添加到等待集合
pendingRequests.add(request);
}
}
request.begin() 实际调用的是Request的子类 GenericRequest 的begin方法
/**
* {@inheritDoc}
*/
@Override
public void begin() {
startTime = LogTime.getLogTime();
if (model == null) {
onException(null);
return;
}
status = Status.WAITING_FOR_SIZE;
if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
onSizeReady(overrideWidth, overrideHeight);
} else {
target.getSize(this);
}
if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
//开始加载图片，先显示占位图
target.onLoadStarted(getPlaceholderDrawable());
}
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished run method in " + LogTime.getElapsedMillis(startTime));
}
}
- 获取图片的长宽尺寸，如果长宽已经确定，走
onSizeReady(overrideWidth, overrideHeight)流程；如果未确定，先获取长宽，再走onSizeReady(overrideWidth, overrideHeight) - 图片开始加载，首先显示占位图
可以明白，主要的逻辑还是在 onSizeReady 这个方法中
/**
* A callback method that should never be invoked directly.
*/
@Override
public void onSizeReady(int width, int height) {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
}
if (status != Status.WAITING_FOR_SIZE) {
return;
}
status = Status.RUNNING;
width = Math.round(sizeMultiplier * width);
height = Math.round(sizeMultiplier * height);
ModelLoader<A, T> modelLoader = loadProvider.getModelLoader();
final DataFetcher<T> dataFetcher = modelLoader.getResourceFetcher(model, width, height);
if (dataFetcher == null) {
onException(new Exception("Failed to load model: \'" + model + "\'"));
return;
}
ResourceTranscoder<Z, R> transcoder = loadProvider.getTranscoder();
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
}
loadedFromMemoryCache = true;
loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
priority, isMemoryCacheable, diskCacheStrategy, this);
loadedFromMemoryCache = resource != null;
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
}
}
重点看loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder, priority, isMemoryCacheable, diskCacheStrategy, this);
public <T, Z, R> LoadStatus load(Key signature, int width, int height, DataFetcher<T> fetcher,
DataLoadProvider<T, Z> loadProvider, Transformation<Z> transformation, ResourceTranscoder<Z, R> transcoder,
Priority priority, boolean isMemoryCacheable, DiskCacheStrategy diskCacheStrategy, ResourceCallback cb) {
Util.assertMainThread();
long startTime = LogTime.getLogTime();
final String id = fetcher.getId();
EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),
loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),
transcoder, loadProvider.getSourceEncoder());
//使用LruCache获取缓存
EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
if (cached != null) {
//从缓存中获取资源成功
cb.onResourceReady(cached);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Loaded resource from cache", startTime, key);
}
return null;
}
//从弱引用中获取缓存
EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
if (active != null) {
//从缓存中获取资源成功
cb.onResourceReady(active);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Loaded resource from active resources", startTime, key);
}
return null;
}
//开启线程从网络中加载图片......
EngineJob current = jobs.get(key);
if (current != null) {
current.addCallback(cb);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Added to existing load", startTime, key);
}
return new LoadStatus(cb, current);
}
EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
DecodeJob<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(key, width, height, fetcher, loadProvider, transformation,
transcoder, diskCacheProvider, diskCacheStrategy, priority);
EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);
jobs.put(key, engineJob);
engineJob.addCallback(cb);
engineJob.start(runnable);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Started new load", startTime, key);
}
return new LoadStatus(cb, engineJob);
}
load方法位于 Engine 类中。load方法内部会从三个来源获取图片数据，我们最熟悉的就是LruCache了。如何获取数据过于复杂，这里就不再展开分析，我们这里主要关注图片数据获取到之后的操作。获取到图片数据之后，通过 cb.onResourceReady(cached) 来处理，我们来看一下这个回调的具体实现：
/**
* A callback method that should never be invoked directly.
*/
@SuppressWarnings("unchecked")
@Override
public void onResourceReady(Resource<?> resource) {
if (resource == null) {
onException(new Exception("Expected to receive a Resource<R> with an object of " + transcodeClass
+ " inside, but instead got null."));
return;
}
Object received = resource.get();
if (received == null || !transcodeClass.isAssignableFrom(received.getClass())) {
releaseResource(resource);
onException(new Exception("Expected to receive an object of " + transcodeClass
+ " but instead got " + (received != null ? received.getClass() : "") + "{" + received + "}"
+ " inside Resource{" + resource + "}."
+ (received != null ? "" : " "
+ "To indicate failure return a null Resource object, "
+ "rather than a Resource object containing null data.")
));
return;
}
if (!canSetResource()) {
releaseResource(resource);
// We can't set the status to complete before asking canSetResource().
status = Status.COMPLETE;
return;
}
onResourceReady(resource, (R) received);
}
跟进onResourceReady(resource, (R) received);
/**
* Internal {@link #onResourceReady(Resource)} where arguments are known to be safe.
*
* @param resource original {@link Resource}, never <code>null</code>
* @param result object returned by {@link Resource#get()}, checked for type and never <code>null</code>
*/
private void onResourceReady(Resource<?> resource, R result) {
// We must call isFirstReadyResource before setting status.
boolean isFirstResource = isFirstReadyResource();
status = Status.COMPLETE;
this.resource = resource;
if (requestListener == null || !requestListener.onResourceReady(result, model, target, loadedFromMemoryCache,
isFirstResource)) {
GlideAnimation<R> animation = animationFactory.build(loadedFromMemoryCache, isFirstResource);
target.onResourceReady(result, animation);
}
notifyLoadSuccess();
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("Resource ready in " + LogTime.getElapsedMillis(startTime) + " size: "
+ (resource.getSize() * TO_MEGABYTE) + " fromCache: " + loadedFromMemoryCache);
}
}
target.onResourceReady(result, animation);最终回调到
/**
* A target for display {@link Drawable} objects in {@link ImageView}s.
*/
public class DrawableImageViewTarget extends ImageViewTarget<Drawable> {
public DrawableImageViewTarget(ImageView view) {
super(view);
}
@Override
protected void setResource(Drawable resource) {
view.setImageDrawable(resource);
}
}
本质是通过 setResource(Drawable resource) 来实现的，在这个方法的内部调用了Android内部最常用的加载图片的方法 view.setImageDrawable(resource) 。
到此为止，into方法基本已经分析完了，我们忽略了网络图片获取的过程，专注于获取图片后的处理。现在来对into方法做个总结：
- 将imageview包装成imageViewTarget
- 清除这个imageViewTarget之前绑定的请求，绑定新的请求
- 执行新的请求
- 获取图片数据之后，成功则会调用ImageViewTarget中的onResourceReady()方法，失败则会调用ImageViewTarget中的onLoadFailed();二者的本质都是通过调用Android中的imageView.setImageDrawable(drawable)来实现对imageView的图片加载
6、LruCache源码分析
在Glide源码分析的最后我们再来分析一下LruCache的源码，这个LruCache来自于 android.support.v4.util 中：
public class LruCache<K, V> {
//存储缓存容器
private final LinkedHashMap<K, V> map;
/** Size of this cache in units. Not necessarily the number of elements. */
//当前缓存的总大小
private int size;
//最大缓存大小
private int maxSize;
//添加到缓存的个数
private int putCount;
//创建的个数
private int createCount;
//移除的个数
private int evictionCount;
//命中个数
private int hitCount;
//未命中个数
private int missCount;
/**
* @param maxSize for caches that do not override {@link #sizeOf}, this is
* the maximum number of entries in the cache. For all other caches,
* this is the maximum sum of the sizes of the entries in this cache.
*/
public LruCache(int maxSize) {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
this.maxSize = maxSize;
this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
}
/**
* Sets the size of the cache.
* @param maxSize The new maximum size.
*
* @hide
*/
//重新设置最大缓存
public void resize(int maxSize) {
//确保最大缓存大于0
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
synchronized (this) {
this.maxSize = maxSize;
}
//对当前的缓存做一些操作以适应新的最大缓存大小
trimToSize(maxSize);
}
/**
* Returns the value for {@code key} if it exists in the cache or can be
* created by {@code #create}. If a value was returned, it is moved to the
* head of the queue. This returns null if a value is not cached and cannot
* be created.
*/
//获取缓存
public final V get(K key) {
if (key == null) {
throw new NullPointerException("key == null");
}
V mapValue;
synchronized (this) {
//如果可以获取key对应的value
mapValue = map.get(key);
if (mapValue != null) {
//命中数加一
hitCount++;
return mapValue;
}
//如果根据key获取的value为null，未命中数加一
missCount++;
}
/*
* Attempt to create a value. This may take a long time, and the map
* may be different when create() returns. If a conflicting value was
* added to the map while create() was working, we leave that value in
* the map and release the created value.
*/
V createdValue = create(key);
if (createdValue == null) {
return null;
}
synchronized (this) {
createCount++;
mapValue = map.put(key, createdValue);
if (mapValue != null) {
// There was a conflict so undo that last put
map.put(key, mapValue);
} else {
size += safeSizeOf(key, createdValue);
}
}
if (mapValue != null) {
entryRemoved(false, key, createdValue, mapValue);
return mapValue;
} else {
trimToSize(maxSize);
return createdValue;
}
}
/**
* Caches {@code value} for {@code key}. The value is moved to the head of
* the queue.
*
* @return the previous value mapped by {@code key}.
*/
public final V put(K key, V value) {
if (key == null || value == null) {
throw new NullPointerException("key == null || value == null");
}
V previous;
synchronized (this) {
//添加到缓存的个数加一
putCount++;
//更新当前缓存大小
size += safeSizeOf(key, value);
previous = map.put(key, value);
if (previous != null) {
//如果之前map中对应key存在value不为null，由于重复的key新添加的value会覆盖上一个value，所以当前缓存大小应该再减去之前value的大小
size -= safeSizeOf(key, previous);
}
}
if (previous != null) {
entryRemoved(false, key, previous, value);
}
//根据缓存最大值调整缓存
trimToSize(maxSize);
return previous;
}
/**
* @param maxSize the maximum size of the cache before returning. May be -1
* to evict even 0-sized elements.
*/
//根据最大缓存大小对map中的缓存做调整
private void trimToSize(int maxSize) {
while (true) {
K key;
V value;
synchronized (this) {
if (size < 0 || (map.isEmpty() && size != 0)) {
throw new IllegalStateException(getClass().getName()
+ ".sizeOf() is reporting inconsistent results!");
}
//当前缓存大小小于最大缓存，或LinkedHashMap为空时跳出循环
if (size <= maxSize) {
break;
}
// BEGIN LAYOUTLIB CHANGE
// get the last item in the linked list.
// This is not efficient, the goal here is to minimize the changes
// compared to the platform version.
Map.Entry<K, V> toEvict = null;
//遍历LinkedHashMap,删除顶部的（也就是最先添加的）元素，直到当前缓存大小小于最大缓存，或LinkedHashMap为空
for (Map.Entry<K, V> entry : map.entrySet()) {
toEvict = entry;
}
// END LAYOUTLIB CHANGE
if (toEvict == null) {
break;
}
key = toEvict.getKey();
value = toEvict.getValue();
map.remove(key);
size -= safeSizeOf(key, value);
evictionCount++;
}
entryRemoved(true, key, value, null);
}
}
/**
* Removes the entry for {@code key} if it exists.
*
* @return the previous value mapped by {@code key}.
*/
public final V remove(K key) {
if (key == null) {
throw new NullPointerException("key == null");
}
V previous;
synchronized (this) {
previous = map.remove(key);
if (previous != null) {
size -= safeSizeOf(key, previous);
}
}
if (previous != null) {
entryRemoved(false, key, previous, null);
}
return previous;
}
/**
* Called for entries that have been evicted or removed. This method is
* invoked when a value is evicted to make space, removed by a call to
* {@link #remove}, or replaced by a call to {@link #put}. The default
* implementation does nothing.
*
* <p>The method is called without synchronization: other threads may
* access the cache while this method is executing.
*
* @param evicted true if the entry is being removed to make space, false
* if the removal was caused by a {@link #put} or {@link #remove}.
* @param newValue the new value for {@code key}, if it exists. If non-null,
* this removal was caused by a {@link #put}. Otherwise it was caused by
* an eviction or a {@link #remove}.
*/
protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
/**
* Called after a cache miss to compute a value for the corresponding key.
* Returns the computed value or null if no value can be computed. The
* default implementation returns null.
*
* <p>The method is called without synchronization: other threads may
* access the cache while this method is executing.
*
* <p>If a value for {@code key} exists in the cache when this method
* returns, the created value will be released with {@link #entryRemoved}
* and discarded. This can occur when multiple threads request the same key
* at the same time (causing multiple values to be created), or when one
* thread calls {@link #put} while another is creating a value for the same
* key.
*/
protected V create(K key) {
return null;
}
private int safeSizeOf(K key, V value) {
int result = sizeOf(key, value);
if (result < 0) {
throw new IllegalStateException("Negative size: " + key + "=" + value);
}
return result;
}
/**
* Returns the size of the entry for {@code key} and {@code value} in
* user-defined units. The default implementation returns 1 so that size
* is the number of entries and max size is the maximum number of entries.
*
* <p>An entry's size must not change while it is in the cache.
*/
protected int sizeOf(K key, V value) {
return 1;
}
/**
* Clear the cache, calling {@link #entryRemoved} on each removed entry.
*/
public final void evictAll() {
trimToSize(-1); // -1 will evict 0-sized elements
}
/**
* For caches that do not override {@link #sizeOf}, this returns the number
* of entries in the cache. For all other caches, this returns the sum of
* the sizes of the entries in this cache.
*/
public synchronized final int size() {
return size;
}
/**
* For caches that do not override {@link #sizeOf}, this returns the maximum
* number of entries in the cache. For all other caches, this returns the
* maximum sum of the sizes of the entries in this cache.
*/
public synchronized final int maxSize() {
return maxSize;
}
/**
* Returns the number of times {@link #get} returned a value that was
* already present in the cache.
*/
public synchronized final int hitCount() {
return hitCount;
}
/**
* Returns the number of times {@link #get} returned null or required a new
* value to be created.
*/
public synchronized final int missCount() {
return missCount;
}
/**
* Returns the number of times {@link #create(Object)} returned a value.
*/
public synchronized final int createCount() {
return createCount;
}
/**
* Returns the number of times {@link #put} was called.
*/
public synchronized final int putCount() {
return putCount;
}
/**
* Returns the number of values that have been evicted.
*/
public synchronized final int evictionCount() {
return evictionCount;
}
/**
* Returns a copy of the current contents of the cache, ordered from least
* recently accessed to most recently accessed.
*/
public synchronized final Map<K, V> snapshot() {
return new LinkedHashMap<K, V>(map);
}
@Override public synchronized final String toString() {
int accesses = hitCount + missCount;
int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
maxSize, hitCount, missCount, hitPercent);
}
}
image.png
LruCache(最近最少使用算法)内部主要靠一个LinkedHashMap来存储缓存，这里使用LinkedHashMap而不使用普通的HashMap正是看中了它的顺序性，即LinkedHashMap中元素的存储顺序就是我们存入的顺序，而HashMap则无法保证这一点。关键在于 trimToSize(int maxSize) 这个方法内部，在它的内部开启了一个循环，遍历LinkedHashMap,删除顶部的（也就是最先添加的）元素，直到当前缓存大小小于最大缓存，或LinkedHashMap为空。这里需要注意的是由于LinkedHashMap的特点，它的存储顺序就是存放的顺序，所以位于顶部的元素就是最近最少使用的元素，正是由于这个特点，从而实现了当缓存不足时优先删除最近最少使用的元素。
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