在上一篇 Android 項目中資源文件 -- asset 目錄和 res 目錄 中，我們簡單記錄了一下assets和res目錄下的文件的異同。這篇就簡單介紹一下Android 項目中資源文件 -- assets目錄文件的訪問。

assets簡介

Android項目中的assets目錄結構圖

如圖所示，在Android Studio項目中的根目錄下，就會有一個assets的文件目錄（若不存在，可以自己新建）。它里面創(chuàng)建存儲：圖像、音視頻、字體、數(shù)據(jù)庫等等。之所以說它適合用來管理這些文件，是因為程序在編譯時不會去處理這個目錄下的文件，但是會將它們打包進apk中。而其他你隨便創(chuàng)建的目錄在編譯時就會被直接忽略掉。還可以在assets文件中創(chuàng)建子目錄，方便分類管理。

在assets目錄的使用上有一點是要注意的：（這點之前沒有說到過）
assets目錄內的文件在程序打包發(fā)布以后是只讀的。只可讀取不可修改。

assets使用

assets目錄文件的訪問，不同res文件的訪問。Android中提供了一個專門的 android.content.res.AssetManager 類用來對assets文件的訪問。

AssetManager類結構局部截圖

從上面AssetManager類的結構圖來看，他提供的api方法很多。但是我們需要重點關注的就以下幾個方法：

• 構造方法
• open() 方法
• openFd() 方法
• openNonAssetFd() 方法
• openXmlResourceParser() 方法

構造方法

通常情況下，我們要獲取AssetManager對象的實例時，不需要使用new的方式。通過下面的方式獲?。?/p>

• 通過Context實例的getAssets()方法：mActivity.getAssets();

• 通過Resource實例的getAssets()方法：mActivity.getResources().getAssets();

open(string) & open(string,int) 方法

open(String fileName) 方法

    /**
     * Open an asset using ACCESS_STREAMING mode.  This provides access to
     * files that have been bundled with an application as assets -- that is,
     * files placed in to the "assets" directory.
     * 
     * @param fileName The name of the asset to open.  This name can be hierarchical.
     * 
     * @see #open(String, int)
     * @see #list
     */
    public @NonNull InputStream open(@NonNull String fileName) throws IOException {
        return open(fileName, ACCESS_STREAMING);
    }

open(@NonNull String fileName, int accessMode) 方法

/**
     * Open an asset using an explicit access mode, returning an InputStream to
     * read its contents.  This provides access to files that have been bundled
     * with an application as assets -- that is, files placed in to the
     * "assets" directory.
     * 
     * @param fileName The name of the asset to open.  This name can be hierarchical.
     * @param accessMode Desired access mode for retrieving the data.
     * 
     * @see #ACCESS_UNKNOWN
     * @see #ACCESS_STREAMING
     * @see #ACCESS_RANDOM
     * @see #ACCESS_BUFFER
     * @see #open(String)
     * @see #list
     */
    public @NonNull InputStream open(@NonNull String fileName, int accessMode) throws IOException {
        Preconditions.checkNotNull(fileName, "fileName");
        synchronized (this) {
            ensureOpenLocked();
            final long asset = nativeOpenAsset(mObject, fileName, accessMode);
            if (asset == 0) {
                throw new FileNotFoundException("Asset file: " + fileName);
            }
            final AssetInputStream assetInputStream = new AssetInputStream(asset);
            incRefsLocked(assetInputStream.hashCode());
            return assetInputStream;
        }
    }

這兩個方法的作用是一樣的。都是將assets 目錄下的某個文件封裝成 InputStream 的形式供使用。

參數(shù)

• String fileName 指某個文件在assets目錄下的相對路徑。如：aaa.mp3 或者 dir/file.avi

• int accessMode 訪問模式

在 open(string) 中，它使用的文件讀取模式是 ACCESS_STREAMING 模式。

openFd(string) 方法

    /**
     * Open an uncompressed asset by mmapping it and returning an {@link AssetFileDescriptor}.
     * This provides access to files that have been bundled with an application as assets -- that
     * is, files placed in to the "assets" directory.
     *
     * The asset must be uncompressed, or an exception will be thrown.
     *
     * @param fileName The name of the asset to open.  This name can be hierarchical.
     * @return An open AssetFileDescriptor.
     */
    public @NonNull AssetFileDescriptor openFd(@NonNull String fileName) throws IOException {
        Preconditions.checkNotNull(fileName, "fileName");
        synchronized (this) {
            ensureOpenLocked();
            final ParcelFileDescriptor pfd = nativeOpenAssetFd(mObject, fileName, mOffsets);
            if (pfd == null) {
                throw new FileNotFoundException("Asset file: " + fileName);
            }
            return new AssetFileDescriptor(pfd, mOffsets[0], mOffsets[1]);
        }
    }

將 assets 目錄中的文件以 FileDescriptor 的形式打開，返回一個 AssetFileDescriptor 實例。

openNonAssetFd(string) & openNonAssetFd(int,string) 方法

    /**
     * Open a non-asset as an asset by mmapping it and returning an {@link AssetFileDescriptor}.
     * This provides direct access to all of the files included in an application
     * package (not only its assets).  Applications should not normally use this.
     *
     * The asset must not be compressed, or an exception will be thrown.
     *
     * @param fileName Name of the asset to retrieve.
     */
    public @NonNull AssetFileDescriptor openNonAssetFd(@NonNull String fileName)
            throws IOException {
        return openNonAssetFd(0, fileName);
    }

    /**
     * Open a non-asset as an asset by mmapping it and returning an {@link AssetFileDescriptor}.
     * This provides direct access to all of the files included in an application
     * package (not only its assets).  Applications should not normally use this.
     *
     * The asset must not be compressed, or an exception will be thrown.
     *
     * @param cookie Identifier of the package to be opened.
     * @param fileName Name of the asset to retrieve.
     */
    public @NonNull AssetFileDescriptor openNonAssetFd(int cookie, @NonNull String fileName)
            throws IOException {
        Preconditions.checkNotNull(fileName, "fileName");
        synchronized (this) {
            ensureOpenLocked();
            final ParcelFileDescriptor pfd =
                    nativeOpenNonAssetFd(mObject, cookie, fileName, mOffsets);
            if (pfd == null) {
                throw new FileNotFoundException("Asset absolute file: " + fileName);
            }
            return new AssetFileDescriptor(pfd, mOffsets[0], mOffsets[1]);
        }
    }

這個方法其實和上面的openFd() 是一樣的。只不過它是跳出了 assets 目錄的范圍限定，是站在工程根目錄的視角來打開文件的 FileDescriptor 的。換句話說，它允許打開 APK中任意位置文件的 AssetFileDescriptor 實例。

openXmlResourceParser(int,string) 方法

    /**
     * Retrieve a parser for a compiled XML file.
     * 
     * @param cookie Identifier of the package to be opened.
     * @param fileName The name of the file to retrieve.
     */
    public @NonNull XmlResourceParser openXmlResourceParser(int cookie, @NonNull String fileName)
            throws IOException {
        try (XmlBlock block = openXmlBlockAsset(cookie, fileName)) {
            XmlResourceParser parser = block.newParser();
            // If openXmlBlockAsset doesn't throw, it will always return an XmlBlock object with
            // a valid native pointer, which makes newParser always return non-null. But let's
            // be paranoid.
            if (parser == null) {
                throw new AssertionError("block.newParser() returned a null parser");
            }
            return parser;
        }
    }

打開 assets 目錄下的 xml 文件，直接返回 XmlResourceParser 實例。就是官方替我們做了從 InputStream 到 XML 解析器之間的轉換。

assets使用舉例

最普通的讀取 assets 目錄的文件

try {
    InputStream is = this.getAssets().open("aaa.png");
    Log.d("type1", "File available:" + is.available());

    InputStream is2 = this.getResources().getAssets().open("bbb.png");
    Log.d("type1", "File available2:" + is2.available());
} catch (IOException e) {
    e.printStackTrace();
}

注意：最后一定不要忘記將用完了的流關閉

讀取自定義目錄層級的文件的方法。

try {
    InputStream is = this.getAssets().open("a/b/c.png");
    Log.d("type2", "File available:" + is.available());

} catch (IOException e) {
    e.printStackTrace();
}

以文件描述符形式讀取

//這里將一張圖片以 AssetFileDescriptor 的形式讀取出來，并轉換成 Bitmap 顯示在 ImageView 上
try {
    AssetFileDescriptor afd = this.getAssets().openFd("river.png");
    Log.d("type3", "File available:" + afd.getLength());

    InputStream is = afd.createInputStream();
    Bitmap bm = BitmapFactory.decodeStream(is);

    is.close();
    afd.close();

    iv.setImageBitmap(bm);
} catch (IOException e) {
    e.printStackTrace();
}