Android资源访问机制—获取Resources对象

前人指路:http://willsunforjava.iteye.com/blog/1663355



modify base on android-4.4.4_r2.0.1


我们知道在开发中,需要应用程序资源,如应用工程中assets和res目录下的图片,layout,values等,或者需要系统内置的资源。我们获取这些资源的入口对象都是Resources对象,并博文将分析如何获取Resources对象。

 

获取Resources的过程:

(1)将framework/framework-res.apk和应用资源apk装载为Resources对象。

(2)获取Resources对象

获取Resources对象有两种方式,第一种通过Context,第二种通过PackageManager。

 

1. 通过Context获取Resources对象

 

在一个Acitvity或者一个Service中,我们直接this.getResources()方法,就可以获得Reousrces对象。其实Acitivity或者Service本质上就是一个Context.getResources()方法来自Context,而真正实现Context接口是ContextImpl类,所以调用的实际上时ContextImpl类的getResources()方法。

 

我们查看ContextImpl类源码可以看到,getResources方法直接返回内部的mResources变量,而对该变量的赋值在私有的构造方法中。

 

core/java/android/app/ContextImpl.java

    private ContextImpl(ContextImpl container, ActivityThread mainThread,
            LoadedApk packageInfo, IBinder activityToken, UserHandle user, boolean restricted,
            Display display, Configuration overrideConfiguration) {
        mOuterContext = this;
        mMainThread = mainThread;
        mActivityToken = activityToken;
        mRestricted = restricted;
        if (user == null) {
            user = Process.myUserHandle();
        }
        mUser = user;
        mPackageInfo = packageInfo;
        mContentResolver = new ApplicationContentResolver(this, mainThread, user);
        mResourcesManager = ResourcesManager.getInstance();
        mDisplay = display;
        mOverrideConfiguration = overrideConfiguration;
        final int displayId = getDisplayId();
        CompatibilityInfo compatInfo = null;
        if (container != null) {
            compatInfo = container.getDisplayAdjustments(displayId).getCompatibilityInfo();
        }
        if (compatInfo == null && displayId == Display.DEFAULT_DISPLAY) {
            compatInfo = packageInfo.getCompatibilityInfo();
        }
        mDisplayAdjustments.setCompatibilityInfo(compatInfo);
        mDisplayAdjustments.setActivityToken(activityToken);
        Resources resources = packageInfo.getResources(mainThread);
        if (resources != null) {
            if (activityToken != null
                    || displayId != Display.DEFAULT_DISPLAY
                    || overrideConfiguration != null
                    || (compatInfo != null && compatInfo.applicationScale
                            != resources.getCompatibilityInfo().applicationScale)) {
                resources = mResourcesManager.getTopLevelResources(
                        packageInfo.getResDir(), displayId,
                        overrideConfiguration, compatInfo, activityToken);
            }
        }
        mResources = resources;
        if (container != null) {
            mBasePackageName = container.mBasePackageName;
            mOpPackageName = container.mOpPackageName;
        } else {
            mBasePackageName = packageInfo.mPackageName;
            ApplicationInfo ainfo = packageInfo.getApplicationInfo();
            if (ainfo.uid == Process.SYSTEM_UID && ainfo.uid != Process.myUid()) {
                // Special case: system components allow themselves to be loaded in to other
                // processes.  For purposes of app ops, we must then consider the context as
                // belonging to the package of this process, not the system itself, otherwise
                // the package+uid verifications in app ops will fail.
                mOpPackageName = ActivityThread.currentPackageName();
            } else {
                mOpPackageName = mBasePackageName;
            }
        }
    }


 mResources又是调用LoadedApk的getResources方法进行赋值。代码如下。

 

Java代码  
  1. public Resources getResources(ActivityThread mainThread) {  
  2.     if (mResources == null) {  
  3.         mResources = mainThread.getTopLevelResources(mResDir, this);  
  4.     }  
  5.     return mResources;  
  6. }  

 从代码中可以看到,最终mResources的赋值是由AcitivtyThread的getTopLevelResources方法返回。代码如下。

 

Java代码  
  1. Resources getTopLevelResources(String resDir, CompatibilityInfo compInfo) {  
  2.     ResourcesKey key = new ResourcesKey(resDir, compInfo.applicationScale);  
  3.     Resources r;  
  4.     synchronized (mPackages) {  
  5.         // Resources is app scale dependent.  
  6.         if (false) {  
  7.             Slog.w(TAG, "getTopLevelResources: " + resDir + " / "  
  8.                     + compInfo.applicationScale);  
  9.         }  
  10.         WeakReference wr = mActiveResources.get(key);  
  11.         r = wr != null ? wr.get() : null;  
  12.           
  13.         if (r != null && r.getAssets().isUpToDate()) {  
  14.             if (false) {  
  15.                 Slog.w(TAG, "Returning cached resources " + r + " " + resDir  
  16.                         + ": appScale=" + r.getCompatibilityInfo().applicationScale);  
  17.             }  
  18.             return r;  
  19.         }  
  20.     }  
  21.   
  22.     AssetManager assets = new AssetManager();  
  23.     if (assets.addAssetPath(resDir) == 0) {  
  24.         return null;  
  25.     }  
  26.   
  27.     DisplayMetrics metrics = getDisplayMetricsLocked(false);  
  28.     r = new Resources(assets, metrics, getConfiguration(), compInfo);  
  29.     if (false) {  
  30.         Slog.i(TAG, "Created app resources " + resDir + " " + r + ": "  
  31.                 + r.getConfiguration() + " appScale="  
  32.                 + r.getCompatibilityInfo().applicationScale);  
  33.     }  
  34.       
  35.     synchronized (mPackages) {  
  36.         WeakReference wr = mActiveResources.get(key);  
  37.         Resources existing = wr != null ? wr.get() : null;  
  38.         if (existing != null && existing.getAssets().isUpToDate()) {  
  39.             // Someone else already created the resources while we were  
  40.             // unlocked; go ahead and use theirs.  
  41.             r.getAssets().close();  
  42.             return existing;  
  43.         }  
  44.         // XXX need to remove entries when weak references go away  
  45.         mActiveResources.put(key, new WeakReference(r));  
  46.         return r;  
  47.     }  
  48. }  

以上代码中,mActiveResources对象内部保存了该应用程序所使用到的所有Resources对象,其类型为Hash>,可以看出这些Resources对象都是以一个弱引用的方式保存,以便在内存紧张时可以释放Resources所占内存。

ResourcesKey的构造需要resDir和compInfo.applicationScale。resdDir变量的含义是资源文件所在路径,实际指的是APK程序所在路径,比如可以是:/data/app/com.haii.android.xxx-1.apk,该apk会对应/data/dalvik-cache目录下的:data@[email protected]@classes.dex文件。

所以,如果一个应用程序没有访问该程序以外的资源,那么mActiveResources变量中就仅有一个Resources对象。这也从侧面说明,mActiveResources内部可能包含多个Resources对象,条件是必须有不同的ResourceKey,也就是必须有不同的resDir,这就意味着一个应用程序可以访问另外的APK文件,并从中读取读取其资源。(PS:其实目前的“换肤”就是采用加载不同的资源apk实现主题切换的)

如果mActivityResources对象中没有包含所要的Resources对象,那么,就重新建立一个Resources对象

 

Java代码  
  1. r = new Resources(assets, metrics, getConfiguration(), compInfo);  

 可以看出构造一个Resources需要一个AssetManager对象,一个DisplayMetrics对象,一个Configuration对象,一个CompatibilityInfo对象,后三者传入的对象都与设备或者Android平台相关的参数,因为资源的使用与这些信息总是相关。还有一个AssetManager对象,其实它并不是访问项目中res/assets下的资源,而是访问res下所有的资源。以上代码中的addAssetPath(resDir)非常关键,它为所创建的AssetManager对象添加一个资源路径。

AssetManager类的构造函数如下:

 

Java代码  
  1. public AssetManager() {  
  2.     synchronized (this) {  
  3.         if (DEBUG_REFS) {  
  4.             mNumRefs = 0;  
  5.             incRefsLocked(this.hashCode());  
  6.         }  
  7.         init();  
  8.         if (localLOGV) Log.v(TAG, "New asset manager: " + this);  
  9.         ensureSystemAssets();  
  10.     }  
  11. }  

 构造方法中调用两个方法init()和ensureSystemAssets(),init方法是一个native实现。AssetManager.java对应的C++文件是android_util_AssetManager.cpp(注意不是AssetManager.cpp,它是C++层内部使用的cpp文件,与Java层无关)。下面看一下init()的native实现。

 

Cpp代码  
  1. static void android_content_AssetManager_init(JNIEnv* env, jobject clazz)  
  2. {  
  3.     AssetManager* am = new AssetManager();  
  4.     if (am == NULL) {  
  5.         jniThrowException(env, "java/lang/OutOfMemoryError""");  
  6.         return;  
  7.     }  
  8.   
  9.     am->addDefaultAssets();  
  10.   
  11.     LOGV("Created AssetManager %p for Java object %p\n", am, clazz);  
  12.     env->SetIntField(clazz, gAssetManagerOffsets.mObject, (jint)am);  
  13. }  

首先创建一个C++类的AssetManager对象,然后调用am->addDefaultAssets()方法,该方法的作用就是把framework的资源文件添加到这个AssetManager对象的路径中。最后调用setInitField()方法把C++创建的AssetManager对象的引用保存到Java端的mObject变量中,这种方式是常用的C++层与Java层通信的方式。

addDefaultAssets代码如下:

 

Java代码  
  1. bool AssetManager::addDefaultAssets()  
  2. {  
  3.     const char* root = getenv("ANDROID_ROOT");  
  4.     LOG_ALWAYS_FATAL_IF(root == NULL, "ANDROID_ROOT not set");  
  5.   
  6.     String8 path(root);  
  7.     path.appendPath(kSystemAssets);  
  8.   
  9.     return addAssetPath(path, NULL);  
  10. }  

该函数首先获取Android的根目录,getenv是一个Linux系统调用,用户同样可以使用以下终端命令获取该值。


 

获得根目录后,再与kSystemAssets路径进行组合,该变量的定义如下:

 

Cpp代码  
  1. static const char* kSystemAssets = "framework/framework-res.apk";  

 所以最终获得的路径文件名称为/system/framework/framework-res.apk,这正式framework对应的资源文件。

分析完了AssetManager的init方法,再来看一下ensureSystemAssets方法。

 

Java代码  
  1. private static void ensureSystemAssets() {  
  2.     synchronized (sSync) {  
  3.         if (sSystem == null) {  
  4.             AssetManager system = new AssetManager(true);  
  5.             system.makeStringBlocks(false);  
  6.             sSystem = system;  
  7.         }  
  8.     }  
  9. }  

 该方法实际上仅在framework启动时就已经调用了,因为sSystem是一个静态的AssetManager对象,该变量在Zygote启动时已经赋值了,以后都不为空,所以该方法形同虚设。

由此可以知道,Resources对象内部的AssetManager对象除了包含应用程序本身的资源路径外,还包含了framework的资源路径,这就是为什么应用程序仅使用Resources对象就可以访问应用资源和系统资源的原因。如

 

Java代码  
  1. Resources res = getResources();  
  2. Drawable btnPic = res.getDrawable(android.R.drawable.btn_default_small);  

那么如何AssetManager如何区分访问的是系统资源还是应用资源呢?当使用getXXX(int id)访问资源时,如果id值小于0x10000000时,AssetManager会认为要访问的是系统资源。因为aapt在对系统资源进行编译时,所有的资源id都被编译为小于该值的一个int值,而当访问应用资源时,id值都大于0x70000000。 

创建好了Resources对象后,就把该变量缓存到mActiveResources中,以便以后继续使用。

访问Resources内部的整个流程如下图。

 

 

2. 通过PackageManager获取Resources对象

    文件路径/frameworks/base/+/android-4.4.4_r2.0.1/core/java/android/content/pm/PackageManager.java

    packageManager为抽象类,,跟Resource相关的方法有:

    

    public abstract Resources getResourcesForActivity(ComponentName activityName)
            throws NameNotFoundException;
    public abstract Resources getResourcesForApplication(ApplicationInfo app)
    public abstract Resources getResourcesForApplication(String appPackageName)
            throws NameNotFoundException;

    /** @hide */
    public abstract Resources getResourcesForApplicationAsUser(String appPackageName, int userId)
            throws NameNotFoundException;
            throws NameNotFoundException;

具体的实现类为ApplicationPackageManager.java

    @Override public Resources getResourcesForActivity(
        ComponentName activityName) throws NameNotFoundException {
        return getResourcesForApplication(
            getActivityInfo(activityName, 0).applicationInfo);
    }
    @Override public Resources getResourcesForApplication(
        ApplicationInfo app) throws NameNotFoundException {
        if (app.packageName.equals("system")) {
            return mContext.mMainThread.getSystemContext().getResources();
        }
        Resources r = mContext.mMainThread.getTopLevelResources(
                app.uid == Process.myUid() ? app.sourceDir : app.publicSourceDir,
                        Display.DEFAULT_DISPLAY, null, mContext.mPackageInfo);
        if (r != null) {
            return r;
        }
        throw new NameNotFoundException("Unable to open " + app.publicSourceDir);
    }
    @Override public Resources getResourcesForApplication(
        String appPackageName) throws NameNotFoundException {
        return getResourcesForApplication(
            getApplicationInfo(appPackageName, 0));
    }
    /** @hide */
    @Override
    public Resources getResourcesForApplicationAsUser(String appPackageName, int userId)
            throws NameNotFoundException {
        if (userId < 0) {
            throw new IllegalArgumentException(
                    "Call does not support special user #" + userId);
        }
        if ("system".equals(appPackageName)) {
            return mContext.mMainThread.getSystemContext().getResources();
        }
        try {
            ApplicationInfo ai = mPM.getApplicationInfo(appPackageName, 0, userId);
            if (ai != null) {
                return getResourcesForApplication(ai);
            }
        } catch (RemoteException e) {
            throw new RuntimeException("Package manager has died", e);
        }
        throw new NameNotFoundException("Package " + appPackageName + " doesn't exist");
    }


1.包名为system的情况,mContext.mMainThread.getSystemContext().getResources()

ActivityThread.getSystemContext

    public ContextImpl getSystemContext() {
        synchronized (this) {
            if (mSystemContext == null) {
                mSystemContext = ContextImpl.createSystemContext(this);
            }
            return mSystemContext;
        }
    }


ContextImpl.createSystemContext

   static ContextImpl createSystemContext(ActivityThread mainThread) {
        LoadedApk packageInfo = new LoadedApk(mainThread);
        ContextImpl context = new ContextImpl(null, mainThread,
                packageInfo, null, null, false, null, null);
        context.mResources.updateConfiguration(context.mResourcesManager.getConfiguration(),
                context.mResourcesManager.getDisplayMetricsLocked(Display.DEFAULT_DISPLAY));
        return context;
    }
    static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo) {
        if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
        return new ContextImpl(null, mainThread,
                packageInfo, null, null, false, null, null);
    }
    static ContextImpl createActivityContext(ActivityThread mainThread,
            LoadedApk packageInfo, IBinder activityToken) {
        if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
        if (activityToken == null) throw new IllegalArgumentException("activityInfo");
        return new ContextImpl(null, mainThread,
                packageInfo, activityToken, null, false, null, null);
    }
    private ContextImpl(ContextImpl container, ActivityThread mainThread,
            LoadedApk packageInfo, IBinder activityToken, UserHandle user, boolean restricted,
            Display display, Configuration overrideConfiguration) {
        mOuterContext = this;
        mMainThread = mainThread;
        mActivityToken = activityToken;
        mRestricted = restricted;
        if (user == null) {
            user = Process.myUserHandle();
        }
        mUser = user;
        mPackageInfo = packageInfo;
        mContentResolver = new ApplicationContentResolver(this, mainThread, user);
        mResourcesManager = ResourcesManager.getInstance();
        mDisplay = display;
        mOverrideConfiguration = overrideConfiguration;
        final int displayId = getDisplayId();
        CompatibilityInfo compatInfo = null;
        if (container != null) {
            compatInfo = container.getDisplayAdjustments(displayId).getCompatibilityInfo();
        }
        if (compatInfo == null && displayId == Display.DEFAULT_DISPLAY) {
            compatInfo = packageInfo.getCompatibilityInfo();
        }
        mDisplayAdjustments.setCompatibilityInfo(compatInfo);
        mDisplayAdjustments.setActivityToken(activityToken);
        Resources resources = packageInfo.getResources(mainThread);
        if (resources != null) {
            if (activityToken != null
                    || displayId != Display.DEFAULT_DISPLAY
                    || overrideConfiguration != null
                    || (compatInfo != null && compatInfo.applicationScale
                            != resources.getCompatibilityInfo().applicationScale)) {
                resources = mResourcesManager.getTopLevelResources(
                        packageInfo.getResDir(), displayId,
                        overrideConfiguration, compatInfo, activityToken);
            }
        }
        mResources = resources;
        if (container != null) {
            mBasePackageName = container.mBasePackageName;
            mOpPackageName = container.mOpPackageName;
        } else {
            mBasePackageName = packageInfo.mPackageName;
            ApplicationInfo ainfo = packageInfo.getApplicationInfo();
            if (ainfo.uid == Process.SYSTEM_UID && ainfo.uid != Process.myUid()) {
                // Special case: system components allow themselves to be loaded in to other
                // processes.  For purposes of app ops, we must then consider the context as
                // belonging to the package of this process, not the system itself, otherwise
                // the package+uid verifications in app ops will fail.
                mOpPackageName = ActivityThread.currentPackageName();
            } else {
                mOpPackageName = mBasePackageName;
            }
        }
    }


最终的mResource是通过packageinfo.getResources和mResoucesManager.getTopLevelResources得到

packageInfo为core/java/android/app/LoadedApk.java

    public Resources getResources(ActivityThread mainThread) {
        if (mResources == null) {
            mResources = mainThread.getTopLevelResources(mResDir,
                    Display.DEFAULT_DISPLAY, null, this);
        }
        return mResources;
    }


core/java/android/app/ResourcesManager.java

    /**
     * Creates the top level Resources for applications with the given compatibility info.
     *
     * @param resDir the resource directory.
     * @param compatInfo the compability info. Must not be null.
     * @param token the application token for determining stack bounds.
     */
    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 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 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(r));
            return r;
        }
    }


这里我们可以看到创建的Resouces保存到ResourcesManager中的mActiveResources中,相同的包名存对应的key,存在对应的Resources

final ArrayMap > mActiveResources
            = new ArrayMap >();



2.通用情况mContext.mMainThread.getTopLevelResources

    /**
     * Creates the top level resources for the given package.
     */
    Resources getTopLevelResources(String resDir,
            int displayId, Configuration overrideConfiguration,
            LoadedApk pkgInfo) {
        return mResourcesManager.getTopLevelResources(resDir, displayId, overrideConfiguration,
                pkgInfo.getCompatibilityInfo(), null);
    }


 
  

结果同样通过ResourcesManager.getTopLevelResources来获取Resources




结论:

1.获取Resources,最终都是通过ResourcesManager获取

2.ResourcesManager中使用

ArrayMap > mActiveResources保存Resources的键值对
在内存不足是,弱引用会被回收


PS:

在Resources被回收后,会重新创建Resources对象
->Activity.
getResources

->ContextThemeWrapper.getResources

->ContextWrapper.createConfigurationContext

->ContextImpl.createConfigurationContext


ContextThemeWrapper.getResources

    public Resources getResources() {
        if (mResources != null) {
            return mResources;
        }
        if (mOverrideConfiguration == null) {
            mResources = super.getResources();
            return mResources;
        } else {
            Context resc = createConfigurationContext(mOverrideConfiguration);
            mResources = resc.getResources();
            return mResources;
        }
    }


ContextWrapper.createConfigurationContext

    public Context createConfigurationContext(Configuration overrideConfiguration) {
        return mBase.createConfigurationContext(overrideConfiguration);
    }


ContextImpl.createConfigurationContext

    public Context createConfigurationContext(Configuration overrideConfiguration) {
        if (overrideConfiguration == null) {
            throw new IllegalArgumentException("overrideConfiguration must not be null");
        }
        return new ContextImpl(this, mMainThread, mPackageInfo, mActivityToken,
                mUser, mRestricted, mDisplay, overrideConfiguration);
    }


最后调用ContextImpl的构造方法创建Context

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