Android源码分析-资源加载机制

转载请注明出处：http://blog.csdn.net/singwhatiwanna/article/details/23387079 （来自singwhatiwanna的csdn博客）

前言

我们知道，在activity内部访问资源（字符串，图片等）是很简单的，只要getResources然后就可以得到Resources对象，有了Resources对象就可以访问各种资源了，这很简单，不过本文不是介绍这个的，本文主要介绍在这套逻辑之下的资源加载机制

资源加载机制

很明确，不同的Context得到的都是同一份资源。这是很好理解的，请看下面的分析
得到资源的方式为context.getResources，而真正的实现位于ContextImpl中的getResources方法，在ContextImpl中有一个成员 private Resources mResources，它就是getResources方法返回的结果，mResources的赋值代码为：
mResources = mResourcesManager.getTopLevelResources(mPackageInfo.getResDir(),
Display.DEFAULT_DISPLAY, null, compatInfo, activityToken);
下面看一下ResourcesManager的getTopLevelResources方法，这个方法的思想是这样的：在ResourcesManager中，所有的资源对象都被存储在ArrayMap中，首先根据当前的请求参数去查找资源，如果找到了就返回，否则就创建一个资源对象放到ArrayMap中。有一点需要说明的是为什么会有多个资源对象，原因很简单，因为res下可能存在多个适配不同设备、不同分辨率、不同系统版本的目录，按照android系统的设计，不同设备在访问同一个应用的时候访问的资源可以不同，比如drawable-hdpi和drawable-xhdpi就是典型的例子。

[java] view plain copy print?public Resources getTopLevelResources(String resDir, int displayId,
Configuration overrideConfiguration, CompatibilityInfo compatInfo, IBinder token) {
final float scale = compatInfo.applicationScale;
ResourcesKey key = new ResourcesKey(resDir, displayId, overrideConfiguration, scale,
token);
Resources r;
synchronized (this) {
// Resources is app scale dependent.
if (false) {
Slog.w(TAG, ”getTopLevelResources: ” + resDir + “ / ” + scale);
}
WeakReference<Resources> wr = mActiveResources.get(key);
r = wr != null ? wr.get() : null;
//if (r != null) Slog.i(TAG, “isUpToDate ” + resDir + ”: ” + r.getAssets().isUpToDate());
if (r != null && r.getAssets().isUpToDate()) {
if (false) {
Slog.w(TAG, ”Returning cached resources ” + r + “ ” + resDir
+ ”: appScale=” + r.getCompatibilityInfo().applicationScale);
}
return r;
}
}

//if (r != null) {
// Slog.w(TAG, “Throwing away out-of-date resources!!!! ”
// + r + ” ” + resDir);
//}

AssetManager assets = new AssetManager();
if (assets.addAssetPath(resDir) == 0) {
return null;
}

//Slog.i(TAG, “Resource: key=” + key + ”, display metrics=” + metrics);
DisplayMetrics dm = getDisplayMetricsLocked(displayId);
Configuration config;
boolean isDefaultDisplay = (displayId == Display.DEFAULT_DISPLAY);
final boolean hasOverrideConfig = key.hasOverrideConfiguration();
if (!isDefaultDisplay || hasOverrideConfig) {
config = new Configuration(getConfiguration());
if (!isDefaultDisplay) {
applyNonDefaultDisplayMetricsToConfigurationLocked(dm, config);
}
if (hasOverrideConfig) {
config.updateFrom(key.mOverrideConfiguration);
}
} else {
config = getConfiguration();
}
r = new Resources(assets, dm, config, compatInfo, token);
if (false) {
Slog.i(TAG, ”Created app resources ” + resDir + “ ” + r + “: ”
+ r.getConfiguration() + ” appScale=”
+ r.getCompatibilityInfo().applicationScale);
}

synchronized (this) {
WeakReference<Resources> wr = mActiveResources.get(key);
Resources existing = wr != null ? wr.get() : null;
if (existing != null && existing.getAssets().isUpToDate()) {
// Someone else already created the resources while we were
// unlocked; go ahead and use theirs.
r.getAssets().close();
return existing;
}

// XXX need to remove entries when weak references go away
mActiveResources.put(key, new WeakReference<Resources>(r));
return r;
}
}

public Resources getTopLevelResources(String resDir, int displayId,
Configuration overrideConfiguration, CompatibilityInfo compatInfo, IBinder token) {
final float scale = compatInfo.applicationScale;
ResourcesKey key = new ResourcesKey(resDir, displayId, overrideConfiguration, scale,
token);
Resources r;
synchronized (this) {
// Resources is app scale dependent.
if (false) {
Slog.w(TAG, "getTopLevelResources: " + resDir + " / " + scale);
}
WeakReference<Resources> wr = mActiveResources.get(key);
r = wr != null ? wr.get() : null;
//if (r != null) Slog.i(TAG, "isUpToDate " + resDir + ": " + r.getAssets().isUpToDate());
if (r != null && r.getAssets().isUpToDate()) {
if (false) {
Slog.w(TAG, "Returning cached resources " + r + " " + resDir
+ ": appScale=" + r.getCompatibilityInfo().applicationScale);
}
return r;
}
}

//if (r != null) {
//    Slog.w(TAG, "Throwing away out-of-date resources!!!! "
//            + r + " " + resDir);
//}

AssetManager assets = new AssetManager();
if (assets.addAssetPath(resDir) == 0) {
return null;
}

//Slog.i(TAG, "Resource: key=" + key + ", display metrics=" + metrics);
DisplayMetrics dm = getDisplayMetricsLocked(displayId);
Configuration config;
boolean isDefaultDisplay = (displayId == Display.DEFAULT_DISPLAY);
final boolean hasOverrideConfig = key.hasOverrideConfiguration();
if (!isDefaultDisplay || hasOverrideConfig) {
config = new Configuration(getConfiguration());
if (!isDefaultDisplay) {
applyNonDefaultDisplayMetricsToConfigurationLocked(dm, config);
}
if (hasOverrideConfig) {
config.updateFrom(key.mOverrideConfiguration);
}
} else {
config = getConfiguration();
}
r = new Resources(assets, dm, config, compatInfo, token);
if (false) {
Slog.i(TAG, "Created app resources " + resDir + " " + r + ": "
+ r.getConfiguration() + " appScale="
+ r.getCompatibilityInfo().applicationScale);
}

synchronized (this) {
WeakReference<Resources> wr = mActiveResources.get(key);
Resources existing = wr != null ? wr.get() : null;
if (existing != null && existing.getAssets().isUpToDate()) {
// Someone else already created the resources while we were
// unlocked; go ahead and use theirs.
r.getAssets().close();
return existing;
}

// XXX need to remove entries when weak references go away
mActiveResources.put(key, new WeakReference<Resources>(r));
return r;
}
}

根据上述代码中资源的请求机制，再加上ResourcesManager采用单例模式，这样就保证了不同的ContextImpl访问的是同一套资源，注意，这里说的同一套资源未必是同一个资源，因为资源可能位于不同的目录，但它一定是我们的应用的资源，或许这样来描述更准确，在设备参数和显示参数不变的情况下，不同的ContextImpl访问到的是同一份资源。设备参数不变是指手机的屏幕和android版本不变，显示参数不变是指手机的分辨率和横竖屏状态。也就是说，尽管Application、Activity、Service都有自己的ContextImpl，并且每个ContextImpl都有自己的mResources成员，但是由于它们的mResources成员都来自于唯一的ResourcesManager实例，所以它们看似不同的mResources其实都指向的是同一块内存(C语言的概念)，因此，它们的mResources都是同一个对象（在设备参数和显示参数不变的情况下）。在横竖屏切换的情况下且应用中为横竖屏状态提供了不同的资源，处在横屏状态下的ContextImpl和处在竖屏状态下的ContextImpl访问的资源不是同一个资源对象。

代码：单例模式的ResourcesManager类
[java] view plain copy print?public static ResourcesManager getInstance() {
synchronized (ResourcesManager.class) {
if (sResourcesManager == null) {
sResourcesManager = new ResourcesManager();
}
return sResourcesManager;
}
}

public static ResourcesManager getInstance() {
synchronized (ResourcesManager.class) {
if (sResourcesManager == null) {
sResourcesManager = new ResourcesManager();
}
return sResourcesManager;
}
}

Resources对象的创建过程

通过阅读Resources类的源码可以知道，Resources对资源的访问实际上是通过AssetManager来实现的，那么如何创建一个Resources对象呢，有人会问，我为什么要去创建一个Resources对象呢，直接getResources不就可以了吗？我要说的是在某些特殊情况下你的确需要去创建一个资源对象，比如动态加载apk。很简单，首先看一下它的几个构造方法：

[java] view plain copy print?/**
* Create a new Resources object on top of an existing set of assets in an
* AssetManager.
*
* @param assets Previously created AssetManager.
* @param metrics Current display metrics to consider when
* selecting/computing resource values.
* @param config Desired device configuration to consider when
* selecting/computing resource values (optional).
*/
public Resources(AssetManager assets, DisplayMetrics metrics, Configuration config) {
this(assets, metrics, config, CompatibilityInfo.DEFAULT_COMPATIBILITY_INFO, null);
}

/**
* Creates a new Resources object with CompatibilityInfo.
*
* @param assets Previously created AssetManager.
* @param metrics Current display metrics to consider when
* selecting/computing resource values.
* @param config Desired device configuration to consider when
* selecting/computing resource values (optional).
* @param compatInfo this resource’s compatibility info. Must not be null.
* @param token The Activity token for determining stack affiliation. Usually null.
* @hide
*/
public Resources(AssetManager assets, DisplayMetrics metrics, Configuration config,
CompatibilityInfo compatInfo, IBinder token) {
mAssets = assets;
mMetrics.setToDefaults();
if (compatInfo != null) {
mCompatibilityInfo = compatInfo;
}
mToken = new WeakReference<IBinder>(token);
updateConfiguration(config, metrics);
assets.ensureStringBlocks();
}

/**
* Create a new Resources object on top of an existing set of assets in an
* AssetManager.
*
* @param assets Previously created AssetManager.
* @param metrics Current display metrics to consider when
*                selecting/computing resource values.
* @param config Desired device configuration to consider when
*               selecting/computing resource values (optional).
*/
public Resources(AssetManager assets, DisplayMetrics metrics, Configuration config) {
this(assets, metrics, config, CompatibilityInfo.DEFAULT_COMPATIBILITY_INFO, null);
}

/**
* Creates a new Resources object with CompatibilityInfo.
*
* @param assets Previously created AssetManager.
* @param metrics Current display metrics to consider when
*                selecting/computing resource values.
* @param config Desired device configuration to consider when
*               selecting/computing resource values (optional).
* @param compatInfo this resource's compatibility info. Must not be null.
* @param token The Activity token for determining stack affiliation. Usually null.
* @hide
*/
public Resources(AssetManager assets, DisplayMetrics metrics, Configuration config,
CompatibilityInfo compatInfo, IBinder token) {
mAssets = assets;
mMetrics.setToDefaults();
if (compatInfo != null) {
mCompatibilityInfo = compatInfo;
}
mToken = new WeakReference<IBinder>(token);
updateConfiguration(config, metrics);
assets.ensureStringBlocks();
}

除了这两个构造方法还有一个私有的无参方法，由于是私有的，所以没法访问。上面两个构造方法，从简单起见，我们应该采用第一个
public Resources(AssetManager assets, DisplayMetrics metrics, Configuration config)
它接受3个参数，第一个是AssetManager，后面两个是和设备相关的配置参数，我们可以直接用当前应用的配置就好，所以，问题的关键在于如何创建AssetManager，下面请看分析，为了创建一个我们自己的AssetManager，我们先去看看系统是怎么创建的。还记得getResources的底层实现吗，在ResourcesManager的getTopLevelResources方法中有这么两句：

[java] view plain copy print?AssetManager assets = new AssetManager();
if (assets.addAssetPath(resDir) == 0) {
return null;
}

AssetManager assets = new AssetManager();
if (assets.addAssetPath(resDir) == 0) {
return null;
}

这两句就是创建一个AssetManager对象，后面会用这个对象来创建Resources对象，ok，AssetManager就是这么创建的，assets.addAssetPath(resDir)这句话的意思是把资源目录里的资源都加载到AssetManager对象中，具体的实现在jni中，大家感兴趣自己去了解下。而资源目录就是我们的res目录，当然resDir可以是一个目录也可以是一个zip文件。有没有想过，如果我们把一个未安装的apk的路径传给这个方法，那么apk中的资源是不是就被加载到AssetManager对象里面了呢？事实证明，的确是这样，具体情况可以参见Android apk动态加载机制的研究（二）：资源加载和activity生命周期管理这篇文章。addAssetPath方法的定义如下，注意到它的注释里面有一个{@hide}关键字，这意味着即使它是public的，但是外界仍然无法访问它，因为android sdk导出的时候会自动忽略隐藏的api，因此只能通过反射来调用。

[java] view plain copy print?/**
* Add an additional set of assets to the asset manager. This can be
* either a directory or ZIP file. Not for use by applications. Returns
* the cookie of the added asset, or 0 on failure.
* {@hide}
*/
public final int addAssetPath(String path) {
int res = addAssetPathNative(path);
return res;
}

/**
* Add an additional set of assets to the asset manager.  This can be
* either a directory or ZIP file.  Not for use by applications.  Returns
* the cookie of the added asset, or 0 on failure.
* {@hide}
*/
public final int addAssetPath(String path) {
int res = addAssetPathNative(path);
return res;
}

有了AssetManager对象后，我们就可以创建自己的Resources对象了，代码如下:

[java] view plain copy print?try {
AssetManager assetManager = AssetManager.class.newInstance();
Method addAssetPath = assetManager.getClass().getMethod(”addAssetPath”, String.class);
addAssetPath.invoke(assetManager, mDexPath);
mAssetManager = assetManager;
} catch (Exception e) {
e.printStackTrace();
}
Resources currentRes = this.getResources();
mResources = new Resources(mAssetManager, currentRes.getDisplayMetrics(),
currentRes.getConfiguration());

try {
AssetManager assetManager = AssetManager.class.newInstance();
Method addAssetPath = assetManager.getClass().getMethod("addAssetPath", String.class);
addAssetPath.invoke(assetManager, mDexPath);
mAssetManager = assetManager;
} catch (Exception e) {
e.printStackTrace();
}
Resources currentRes = this.getResources();
mResources = new Resources(mAssetManager, currentRes.getDisplayMetrics(),
currentRes.getConfiguration());

有了Resources对象，我们就可以通过Resources对象来访问里面的各种资源了，通过这种方法，我们可以完成一些特殊的功能，比如换肤、换语言包、动态加载apk等，欢迎大家交流。