【Android原始碼】資源載入AssetManager原始碼分析 app是如何載入資源以及我們是如何從記憶體中獲取

指間沙似流年發表於2017-12-23

通常情況下,當我們需要使用資源的時候,都是通過api直接呼叫:

getResources().getDrawable(R.mipmap.ic_launcher);
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通過getResources()的眾多方法可以獲取到整個apk包裡面的資源,那麼我們是如何獲取到資源的?這些資源又是如何被載入到記憶體中的? 今天我們來一起分析一下app是如何載入資原始檔的。

資源載入過程

首先我們通過getResources()來看一下Resource是在哪裡被建立出來的。

// AppCompatActivity.java
@Override
public Resources getResources() {
   if (mResources == null && VectorEnabledTintResources.shouldBeUsed()) {
       mResources = new VectorEnabledTintResources(this, super.getResources());
   }
   return mResources == null ? super.getResources() : mResources;
}

// Context.java
public abstract Resources getResources();
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一路點選過去發現,最終又到了Context這個上下文中,而Context是一個抽象類,所以我們需要找到其實現類ContextImpl.java

// ContextImpl.java
@Override
public Resources getResources() {
   return mResources;
}

private ContextImpl(ContextImpl container, ActivityThread mainThread,
       LoadedApk packageInfo, IBinder activityToken, UserHandle user, int flags,
       Display display, Configuration overrideConfiguration, int createDisplayWithId) {
            
	Resources resources = packageInfo.getResources(mainThread);
	mResources = resources;	
}

// LoadedApk.java
public Resources getResources(ActivityThread mainThread) {
   if (mResources == null) {
       mResources = mainThread.getTopLevelResources(mResDir, mSplitResDirs, mOverlayDirs,
               mApplicationInfo.sharedLibraryFiles, Display.DEFAULT_DISPLAY, this);
   }
   return mResources;
}

// ResourcesManager.java
Resources getTopLevelResources(String resDir, String[] splitResDirs, String[] overlayDirs,
       String[] libDirs, int displayId, LoadedApk pkgInfo) {
   return mResourcesManager.getResources(null, resDir, splitResDirs, overlayDirs, libDirs,
           displayId, null, pkgInfo.getCompatibilityInfo(), pkgInfo.getClassLoader());
}
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發現在ContextImpl.java中Resources是被ResourcesManager的getTopLevelResources方法返回的。ResourcesManager是資源的管理類,我們著重看一下它。

public @NonNull Resources getResources(@Nullable IBinder activityToken,
       @Nullable String resDir,
       @Nullable String[] splitResDirs,
       @Nullable String[] overlayDirs,
       @Nullable String[] libDirs,
       int displayId,
       @Nullable Configuration overrideConfig,
       @NonNull CompatibilityInfo compatInfo,
       @Nullable ClassLoader classLoader) {
   try {
       Trace.traceBegin(Trace.TRACE_TAG_RESOURCES, "ResourcesManager#getResources");
       // 生成資源key物件
       final ResourcesKey key = new ResourcesKey(
               resDir,
               splitResDirs,
               overlayDirs,
               libDirs,
               displayId,
               overrideConfig != null ? new Configuration(overrideConfig) : null, // Copy
               compatInfo);
       classLoader = classLoader != null ? classLoader : ClassLoader.getSystemClassLoader();
       return getOrCreateResources(activityToken, key, classLoader);
   } finally {
       Trace.traceEnd(Trace.TRACE_TAG_RESOURCES);
   }
}

private @NonNull Resources getOrCreateResources(@Nullable IBinder activityToken,
       @NonNull ResourcesKey key, @NonNull ClassLoader classLoader) {
   synchronized (this) {

			// 從快取中獲取Resources
       if (activityToken != null) {
           final ActivityResources activityResources =
                   getOrCreateActivityResourcesStructLocked(activityToken);
				// 通過資源key物件獲取快取Resources
           ResourcesImpl resourcesImpl = findResourcesImplForKeyLocked(key);
           if (resourcesImpl != null) {
               if (DEBUG) {
                   Slog.d(TAG, "- using existing impl=" + resourcesImpl);
               }
               return getOrCreateResourcesForActivityLocked(activityToken, classLoader,
                       resourcesImpl);
           }
       } else {
       		// 通過資源key物件獲取快取Resources
           ResourcesImpl resourcesImpl = findResourcesImplForKeyLocked(key);
           if (resourcesImpl != null) {
               if (DEBUG) {
                   Slog.d(TAG, "- using existing impl=" + resourcesImpl);
               }
               return getOrCreateResourcesLocked(classLoader, resourcesImpl);
           }
       }
   }

   // 沒有快取建立ResourcesImpl
   ResourcesImpl resourcesImpl = createResourcesImpl(key);

   synchronized (this) {
   		// 通過資源key物件獲取快取Resources
       ResourcesImpl existingResourcesImpl = findResourcesImplForKeyLocked(key);
       if (existingResourcesImpl != null) {
           resourcesImpl.getAssets().close();
           resourcesImpl = existingResourcesImpl;
       } else {
           // 如果快取中沒有,將建立好的ResourceImpl加入到快取中
           mResourceImpls.put(key, new WeakReference<>(resourcesImpl));
       }

       final Resources resources;
       if (activityToken != null) {
       		// 通過資源key物件獲取快取Resources
           resources = getOrCreateResourcesForActivityLocked(activityToken, classLoader,
                   resourcesImpl);
       } else {
       		// 通過資源key物件獲取快取Resources
           resources = getOrCreateResourcesLocked(classLoader, resourcesImpl);
       }
       return resources;
   }
}
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生成資源Resources物件的步驟:

  1. 首先生成資源Key物件,用於將資源物件新增到快取map中或者從快取map中查詢資源物件
  2. 通過activityToken判斷是否關聯Activity,並從快取中通過key獲取快取的ResourcesImpl物件,如果有直接返回
  3. 如果沒有快取的ResourcesImpl物件,則直接建立新的快取ResourcesImpl物件,通過createResourcesImpl(key)來建立。
  4. 建立完之後在將其加入快取Map中,並最終返回Resources.setImpl(impl)方法,建立出Resources物件並返回。

所以我們需要著重看下createResourcesImpl方法,這個方法才是建立ResourcesImpl的核心。

private @NonNull ResourcesImpl createResourcesImpl(@NonNull ResourcesKey key) {
   final DisplayAdjustments daj = new DisplayAdjustments(key.mOverrideConfiguration);
   daj.setCompatibilityInfo(key.mCompatInfo);

   final AssetManager assets = createAssetManager(key);
   final DisplayMetrics dm = getDisplayMetrics(key.mDisplayId, daj);
   final Configuration config = generateConfig(key, dm);
   final ResourcesImpl impl = new ResourcesImpl(assets, dm, config, daj);
   if (DEBUG) {
       Slog.d(TAG, "- creating impl=" + impl + " with key: " + key);
   }
   return impl;
}
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可以發現這兒方法建立了AssetManager資產管理類:

@VisibleForTesting
protected @NonNull AssetManager createAssetManager(@NonNull final ResourcesKey key) {
   AssetManager assets = new AssetManager();
   if (key.mResDir != null) {
       if (assets.addAssetPath(key.mResDir) == 0) {
           throw new Resources.NotFoundException("failed to add asset path " + key.mResDir);
       }
   }
   return assets;
}

// LoadedApk.java
private void setApplicationInfo(ApplicationInfo aInfo) {
   final int myUid = Process.myUid();
   aInfo = adjustNativeLibraryPaths(aInfo);
   mApplicationInfo = aInfo;
   mAppDir = aInfo.sourceDir;
   mResDir = aInfo.uid == myUid ? aInfo.sourceDir : aInfo.publicSourceDir;
}
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AssetManager是用來載入制定路徑下的資原始檔的,所以我們想要獲取的資原始檔都是在這個類中找到的。而key.mResDir就是app的資源路徑。

所以AssetManager才是整個資源載入的關鍵所在。

public AssetManager() {
   synchronized (this) {
       if (DEBUG_REFS) {
           mNumRefs = 0;
           incRefsLocked(this.hashCode());
       }
       init(false);
       if (localLOGV) Log.v(TAG, "New asset manager: " + this);
       ensureSystemAssets();
   }
}

private native final void init(boolean isSystem);

private native final int loadResourceValue(int ident, short density, TypedValue outValue,
            boolean resolve);
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在建立AssetManager的時候,它首先會去呼叫native層中的init方法

static void android_content_AssetManager_init(JNIEnv* env, jobject clazz, jboolean isSystem)
{
    if (isSystem) {
        verifySystemIdmaps();
    }
    //  AssetManager.cpp
    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));
}

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

    String8 path(root);
    // kSystemAssets -> framework/framework-res.apk  
    // 載入系統的資源如顏色、圖片、文字
    path.appendPath(kSystemAssets);

    return addAssetPath(path, NULL);
}

bool AssetManager::addAssetPath(const String8& path, int32_t* cookie)
{
    asset_path ap;

    // 判斷是否已經載入過了
    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;
        }
    }

    // 檢查路徑是否有一個androidmanifest.xml
    Asset* manifestAsset = const_cast<AssetManager*>(this)->openNonAssetInPathLocked(
            kAndroidManifest, Asset::ACCESS_BUFFER, ap);
    if (manifestAsset == NULL) {
        // 如果不包含任何資源
        delete manifestAsset;
        return false;
    }
    delete manifestAsset;
    // 新增 
    mAssetPaths.add(ap);
    if (mResources != NULL) {
        appendPathToResTable(ap);
    }

    return true;
}

bool AssetManager::appendPathToResTable(const asset_path& ap) const {

    Asset* ass = NULL;
    ResTable* sharedRes = NULL;
    bool shared = true;
    bool onlyEmptyResources = true;
    MY_TRACE_BEGIN(ap.path.string());

    Asset* idmap = openIdmapLocked(ap);
    size_t nextEntryIdx = mResources->getTableCount();
    ALOGV("Looking for resource asset in '%s'\n", ap.path.string());
    // 資源包路徑不是一個資料夾
    if (ap.type != kFileTypeDirectory) {
        if (nextEntryIdx == 0) {
            // mAssetPaths中儲存的第一個資源包路徑是系統資源的路徑,
            // 即framework-res.apk的路徑,它在zygote啟動時已經載入了
            // 可以通過mZipSet.getZipResourceTable獲得其ResTable物件
            sharedRes = const_cast<AssetManager*>(this)->
                mZipSet.getZipResourceTable(ap.path);
            // 對於APP來說,肯定不為NULL
            if (sharedRes != NULL) {
                // 得到系統資源包路徑中resources.arsc個數
                nextEntryIdx = sharedRes->getTableCount();
            }
        }
        // 當引數是mAssetPaths中除第一個以外的其他資源資源包路徑,
        // 比如app自己的資源包路徑時,走下面的邏輯
        if (sharedRes == NULL) {
            // 檢查該資源包是否被其他程式載入了,這與ZipSet資料結構有關,後面在詳細介紹
            ass = const_cast<AssetManager*>(this)->
                mZipSet.getZipResourceTableAsset(ap.path);
            if (ass == NULL) {
                ALOGV("loading resource table %s\n", ap.path.string());
                // 建立Asset物件,就是開啟resources.arsc
                ass = const_cast<AssetManager*>(this)->
                    openNonAssetInPathLocked("resources.arsc",
                                             Asset::ACCESS_BUFFER,
                                             ap);
                if (ass != NULL && ass != kExcludedAsset) {
                    ass = const_cast<AssetManager*>(this)->
                        mZipSet.setZipResourceTableAsset(ap.path, ass);
                }
            }
            // 只有在zygote啟動時,才會執行下面的邏輯
            // 為系統資源建立 ResTable,並加入到mZipSet裡。
            if (nextEntryIdx == 0 && ass != NULL) {
                ALOGV("Creating shared resources for %s", ap.path.string());
                // 建立ResTable物件,並把前面與resources.arsc關聯的Asset物件,加入到這個ResTabl中
                sharedRes = new ResTable();
                sharedRes->add(ass, idmap, nextEntryIdx + 1, false);
                sharedRes = const_cast<AssetManager*>(this)->
                    mZipSet.setZipResourceTable(ap.path, sharedRes);
            }
        }
    } else {
        ALOGV("loading resource table %s\n", ap.path.string());
        ass = const_cast<AssetManager*>(this)->
            openNonAssetInPathLocked("resources.arsc",
                                     Asset::ACCESS_BUFFER,
                                     ap);
        shared = false;
    }

    if ((ass != NULL || sharedRes != NULL) && ass != kExcludedAsset) {
        ALOGV("Installing resource asset %p in to table %p\n", ass, mResources);
        // 系統資源包時
        if (sharedRes != NULL) {
            ALOGV("Copying existing resources for %s", ap.path.string());
            mResources->add(sharedRes);
        } else {
            // 非系統資源包時,將與resources.arsc關聯的Asset物件加入到Restable中
            // 此過程會解析resources.arsc檔案。
            ALOGV("Parsing resources for %s", ap.path.string());
            mResources->add(ass, idmap, nextEntryIdx + 1, !shared);
        }
        onlyEmptyResources = false;

        if (!shared) {
            delete ass;
        }
    } else {
        mResources->addEmpty(nextEntryIdx + 1);
    }

    if (idmap != NULL) {
        delete idmap;
    }
    MY_TRACE_END();

    return onlyEmptyResources;
}
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獲取系統路徑並解析resources.arsc這個檔案對映表,其中存放的就是資源的索引。

至此,系統資源載入就完成了。

獲取資源

現在所有資源的索引已經載入到記憶體中了,我們就需要獲取資源並載入了。

<ImageView
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"
   android:src="@drawable/image"/>
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我們看一下ImageView的src屬性圖片是如何載入的,通常情況下,src屬性和我們自定義屬性很想,都是自定義的屬性,只不過一個是系統定義的,一個是我們定義的,所有我們直接看ImageView的建構函式:

public ImageView(Context context, @Nullable AttributeSet attrs, int defStyleAttr,
       int defStyleRes) {
   super(context, attrs, defStyleAttr, defStyleRes);
   final TypedArray a = context.obtainStyledAttributes(
                attrs, R.styleable.ImageView, defStyleAttr, defStyleRes);

   final Drawable d = a.getDrawable(R.styleable.ImageView_src);
	if (d != null) {
       setImageDrawable(d);
   }   
}

@Nullable
public Drawable getDrawable(@StyleableRes int index) {
   final TypedValue value = mValue;
   if (getValueAt(index*AssetManager.STYLE_NUM_ENTRIES, value)) {
       if (value.type == TypedValue.TYPE_ATTRIBUTE) {
           throw new UnsupportedOperationException(
                   "Failed to resolve attribute at index " + index + ": " + value);
       }
       return mResources.loadDrawable(value, value.resourceId, mTheme);
   }
   return null;
}
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最後也是通過Resources的loadDrawable(value, value.resourceId, mTheme)方法來獲取到資源:

// Resources.java
@NonNull
Drawable loadDrawable(@NonNull TypedValue value, int id, @Nullable Theme theme)
       throws NotFoundException {
   return mResourcesImpl.loadDrawable(this, value, id, theme, true);
}

// ResourcesImpl.java
@Nullable
Drawable loadDrawable(Resources wrapper, TypedValue value, int id, Resources.Theme theme,
       boolean useCache) throws NotFoundException {
   try {
       Drawable dr;
       if (cs != null) {
           dr = cs.newDrawable(wrapper);
       } else if (isColorDrawable) {
           dr = new ColorDrawable(value.data);
       } else {
           dr = loadDrawableForCookie(wrapper, value, id, null);
       }

       return dr;
   } catch (Exception e) {
       throw nfe;
   }
}
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載入drawable的時候,首先會現在快取中獲取,如果有就直接返回,如果沒有的話,說明沒有載入過這個資原始檔,就需要先載入並快取到caches中,所有我們看loadDrawableForCookie方法:

private Drawable loadDrawableForCookie(Resources wrapper, TypedValue value, int id,
       Resources.Theme theme) {
   final String file = value.string.toString();

   final Drawable dr;

   Trace.traceBegin(Trace.TRACE_TAG_RESOURCES, file);
   try {
       if (file.endsWith(".xml")) {
       	// 類似於shape的xml檔案
           final XmlResourceParser rp = loadXmlResourceParser(
                   file, id, value.assetCookie, "drawable");
           dr = Drawable.createFromXml(wrapper, rp, theme);
           rp.close();
       } else {
       	// 不是xml檔案,就是圖片檔案,就將其載入到記憶體中
       	// value.assetCookie這個cookie就是圖片在native層中的mAssetPaths中的索引
           final InputStream is = mAssets.openNonAsset(
                   value.assetCookie, file, AssetManager.ACCESS_STREAMING);
           dr = Drawable.createFromResourceStream(wrapper, value, is, file, null);
           is.close();
       }
   } catch (Exception e) {
       Trace.traceEnd(Trace.TRACE_TAG_RESOURCES);
       final NotFoundException rnf = new NotFoundException(
               "File " + file + " from drawable resource ID #0x" + Integer.toHexString(id));
       rnf.initCause(e);
       throw rnf;
   }
   Trace.traceEnd(Trace.TRACE_TAG_RESOURCES);

   return dr;
}
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loadDrawableForCookie方法會首先判斷是否是.xml檔案,如果是xml檔案,則直接使用xml解析並建立Drawable物件返回,如果不是xml檔案,則認為是圖片的資原始檔,使用流的方式將資源讀取到快取中。

至此為止,資源是如何載入的和如何從記憶體中獲取資源都完成了。

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