1、相关类图

QQ截图20160818093629.png

这里主要说明一下，为什么Resources和AssetManager都有一个mSystem属性和getSystem()方法，这是因为我们的应用不但要使用我们自己的资源，还要使用系统资源，也就是framework-res.apk中的资源，所以这里的mSystem是用来获取系统资源的。

2、初始化过程

在Activity启动过程中了解到，当zogyte进程fork一个应用进程用于运行子进程时，会跳转到ActivityThread.main()中进行一系列操作，这其中有一步就是创建应用程序运行的上下文环境ContextImpl:
ActivityThread.ApplicationThread.handleBindApplication()

 private void handleBindApplication(AppBindData data) {
      //..........
      // Context初始化(ContextImpl)
      final ContextImpl appContext = ContextImpl.createAppContext(this/*ActivityThread*/, data.info/*LoadedApk*/);
      //........
  }

      /**
    *ActivityThread mainThread = mainThread
    *LoadedApk packageInfo = packageInfo
    *boolean restricted = false
    *int createDisplayWithId = Display.INVALID_DISPLAY
    */
    private ContextImpl(ContextImpl container, ActivityThread mainThread,
            LoadedApk packageInfo, IBinder activityToken, UserHandle user, boolean restricted,
            Display display, Configuration overrideConfiguration, int createDisplayWithId) {
            //.......
            // LoadedApk赋值 
            mPackageInfo = packageInfo;
            mResourcesManager = ResourcesManager.getInstance();
            // resources初始化:通过LoadedApk.getResources来创建一个Resources实例
            Resources resources = packageInfo.getResources(mainThread);
            mResources = resources;// 赋值
            //......
            mContentResolver = new ApplicationContentResolver(this, mainThread, user);
    }

LoadedApk.getResources()

    public Resources getResources(ActivityThread mainThread) {
        if (mResources == null) {
            // ActivityThread.getTopLevelResources()
            mResources = mainThread.getTopLevelResources(mResDir/*APK文件位置*/, mSplitResDirs, mOverlayDirs,
                    mApplicationInfo.sharedLibraryFiles, Display.DEFAULT_DISPLAY, null, this);
        }
        return mResources;
    }

ActivityThread.getTopLevelResources()

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

ResourcesManager.getTopLevelResources

/**
     * Creates the top level Resources for applications with the given compatibility info.
     *
     * @param resDir the resource directory.
     * @param splitResDirs split resource directories.
     * @param overlayDirs the resource overlay directories.
     * @param libDirs the shared library resource dirs this app references.
     * @param displayId display Id.
     * @param overrideConfiguration override configurations.
     * @param compatInfo the compatibility info. Must not be null.
     */
    Resources getTopLevelResources(String resDir, String[] splitResDirs,
            String[] overlayDirs, String[] libDirs, int displayId,
            Configuration overrideConfiguration, CompatibilityInfo compatInfo) {
        final float scale = compatInfo.applicationScale;
        Configuration overrideConfigCopy = (overrideConfiguration != null)
                ? new Configuration(overrideConfiguration) : null;
        ResourcesKey key = new ResourcesKey(resDir, displayId, overrideConfigCopy, scale);
        Resources r;
        synchronized (this) {
            // Resources is app scale dependent.
            if (DEBUG) Slog.w(TAG, "getTopLevelResources: " + resDir + " / " + scale);
            // Resources是以ResourcesKey为key以弱应用的方式保存在mActiveResources这个Map中
            WeakReference<Resources> wr = mActiveResources.get(key);
            r = wr != null ? wr.get() : null;
            //if (r != null) Log.i(TAG, "isUpToDate " + resDir + ": " + r.getAssets().isUpToDate());
            if (r != null && r.getAssets().isUpToDate()) {/
                // 缓存里面有，并且是最新的
                if (DEBUG) Slog.w(TAG, "Returning cached resources " + r + " " + resDir
                        + ": appScale=" + r.getCompatibilityInfo().applicationScale
                        + " key=" + key + " overrideConfig=" + overrideConfiguration);
                return r;
            }
        }

        //if (r != null) {
        //    Log.w(TAG, "Throwing away out-of-date resources!!!! "
        //            + r + " " + resDir);
        //}
        // AssetManager创建
        AssetManager assets = new AssetManager();
        // resDir can be null if the 'android' package is creating a new Resources object.
        // This is fine, since each AssetManager automatically loads the 'android' package
        // already.
        if (resDir != null) {
            if (assets.addAssetPath(resDir) == 0) {
                return null;
            }
        }

        if (splitResDirs != null) {
            for (String splitResDir : splitResDirs) {
                if (assets.addAssetPath(splitResDir) == 0) {
                    return null;
                }
            }
        }

        if (overlayDirs != null) {
            for (String idmapPath : overlayDirs) {
                assets.addOverlayPath(idmapPath);
            }
        }

        if (libDirs != null) {
            for (String libDir : libDirs) {
                if (libDir.endsWith(".apk")) {
                    // Avoid opening files we know do not have resources,
                    // like code-only .jar files.
                    if (assets.addAssetPath(libDir) == 0) {
                        Log.w(TAG, "Asset path '" + libDir +
                                "' does not exist or contains no resources.");
                    }
                }
            }
        }

        //Log.i(TAG, "Resource: key=" + key + ", display metrics=" + metrics);
        DisplayMetrics dm = getDisplayMetricsLocked(displayId);
        Configuration config;
        final 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);
                if (DEBUG) Slog.v(TAG, "Applied overrideConfig=" + key.mOverrideConfiguration);
            }
        } else {
            config = getConfiguration();
        }
        // 创建Resources
        r = new Resources(assets, dm, config, compatInfo);
        if (DEBUG) Slog.i(TAG, "Created app resources " + resDir + " " + r + ": "
                + r.getConfiguration() + " appScale=" + r.getCompatibilityInfo().applicationScale);

        synchronized (this) {
            // 可能其他线程已经创建好了，则直接返回
            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
            // 把最新的对象保存到缓存中
            mActiveResources.put(key, new WeakReference<>(r));
            if (DEBUG) Slog.v(TAG, "mActiveResources.size()=" + mActiveResources.size());
            return r;
        }
    }

过程是：先从缓存中取，如果缓存中有且没有过时，则直接返回，否则依次创建AssetManager 和Resources
AssetManager构造函数

/**
     * Create a new AssetManager containing only the basic system assets.
     * Applications will not generally use this method, instead retrieving the
     * appropriate asset manager with {@link Resources#getAssets}.    Not for
     * use by applications.
     * {@hide}
     */
    public AssetManager() {
        synchronized (this) {
            //......
            init(false);
            // 确保有能够访问系统资源的AssetManager对象
            ensureSystemAssets();
        }
    }

android_util_AssetManager.android_content_AssetManager_init()

static void android_content_AssetManager_init(JNIEnv* env, jobject clazz, jboolean isSystem)
{
    if (isSystem) {// false
        verifySystemIdmaps();
    }
    AssetManager* am = new AssetManager();
    if (am == NULL) {
        jniThrowException(env, "java/lang/OutOfMemoryError", "");
        return;
    }

    am->addDefaultAssets();

    ALOGV("Created AssetManager %p for Java object %p\n", am, clazz);
    env->SetLongField(clazz, gAssetManagerOffsets.mObject, reinterpret_cast<jlong>(am));
}

AssetManager.cpp:addDefaultAssets()

bool AssetManager::addDefaultAssets()
{
    // root = /system/
    const char* root = getenv("ANDROID_ROOT");
    LOG_ALWAYS_FATAL_IF(root == NULL, "ANDROID_ROOT not set");

    String8 path(root);
    // path = /system/framework/framework-res.apk
    path.appendPath(kSystemAssets);

    return addAssetPath(path, NULL);
}

bool AssetManager::addAssetPath(const String8& path, int32_t* cookie)
{
    AutoMutex _l(mLock);

    asset_path ap;

    String8 realPath(path);
    if (kAppZipName) {
        // 如果kAppZipName不为NULL(classes.jar),这里这个值是为NULL的
        realPath.appendPath(kAppZipName);
    }
    ap.type = ::getFileType(realPath.string());
    if (ap.type == kFileTypeRegular) {// kAppZipName不为NULL
        ap.path = realPath;
    } else {
    // kAppZipName为NULL
        ap.path = path;//ap.path指向APK文件
        ap.type = ::getFileType(path.string());
        if (ap.type != kFileTypeDirectory && ap.type != kFileTypeRegular) {
            ALOGW("Asset path %s is neither a directory nor file (type=%d).",
                 path.string(), (int)ap.type);
            return false;
        }
    }

    // Skip if we have it already.
    for (size_t i=0; i<mAssetPaths.size(); i++) {
        if (mAssetPaths[i].path == ap.path) {
            if (cookie) {
                *cookie = static_cast<int32_t>(i+1);
            }
            return true;
        }
    }

    ALOGV("In %p Asset %s path: %s", this,
         ap.type == kFileTypeDirectory ? "dir" : "zip", ap.path.string());

    // Check that the path has an AndroidManifest.xml
    Asset* manifestAsset = const_cast<AssetManager*>(this)->openNonAssetInPathLocked(
            kAndroidManifest, Asset::ACCESS_BUFFER, ap);
    if (manifestAsset == NULL) {
        // This asset path does not contain any resources.
        delete manifestAsset;
        return false;
    }
    delete manifestAsset;

    mAssetPaths.add(ap);

    // new paths are always added at the end
    if (cookie) {
        *cookie = static_cast<int32_t>(mAssetPaths.size());
    }

#ifdef __ANDROID__
    // Load overlays, if any
    asset_path oap;
    for (size_t idx = 0; mZipSet.getOverlay(ap.path, idx, &oap); idx++) {
        mAssetPaths.add(oap);
    }
#endif

    if (mResources != NULL) {
        appendPathToResTable(ap);
    }

    return true;
}

以上，是AssetManager.java构造函数的第一步：init(false)，其主要工作是加载系统资源（framework-res.apk）,以供后续应用程序使用。而其第二部：ensureSystemAssets()也是为了创建系统资源使用对象AssetManager

private static void ensureSystemAssets() {
        synchronized (sSync) {
            if (sSystem == null) {
                AssetManager system = new AssetManager(true);
                system.makeStringBlocks(null);
                sSystem = system;
            }
        }
    }

初始化了AssetManager之后，就可以通过AssetManager.addAssetPath()加载本身资源了.
在AssetManager加载完相关资源后，就可以创建Resources了：

   /**
     * Creates a new Resources object with CompatibilityInfo.
     *
     * @param assets Previously created AssetManager.
     * @param metrics Current display metrics to consider when
     *                selecting/computing resource values.
     *              设备分辨率相关信息:屏幕分辨率，density,font scaling
     * @param config Desired device configuration to consider when
     *               selecting/computing resource values (optional).
     *              该配置信息用来决定使用哪套资源
     * @param compatInfo this resource's compatibility info. Must not be null.
     *              资源兼容性信息
     * @hide
     */
    public Resources(AssetManager assets, DisplayMetrics metrics, Configuration config,
            CompatibilityInfo compatInfo) {
        mAssets = assets;
        mMetrics.setToDefaults();
        if (compatInfo != null) {
            mCompatibilityInfo = compatInfo;
        }
        // 设备相关配置信息更新处理
        updateConfiguration(config, metrics);
        // 创建字符串资源池
        assets.ensureStringBlocks();
    }