基本上有两种方法。

首先，您可以使用AssetManager。打开和，如Rohith Nandakumar所述，并在输入流上迭代。

其次，您可以使用AssetManager。openFd，它允许你使用一个FileChannel(它有transferTo和transferFrom方法)，所以你不必自己循环输入流。

我将在这里描述openFd方法。

压缩

首先，您需要确保文件以未压缩的方式存储。打包系统可能会选择压缩任何扩展名未标记为noCompress的文件，并且压缩后的文件不能映射到内存中，因此您必须依赖于AssetManager。在那个箱子里打开。

你可以给你的文件添加一个“。mp3”扩展名来阻止它被压缩，但正确的解决方案是修改你的app/build。gradle文件并添加以下行(禁用PDF文件压缩)

aaptOptions {
    noCompress 'pdf'
}

包装文件

请注意，打包器仍然可以将多个文件打包成一个文件，因此您不能只读取AssetManager提供的整个文件。您需要询问AssetFileDescriptor需要哪些部分。

找到已打包文件的正确部分

一旦您确保您的文件存储为未压缩的，您就可以使用AssetManager。openFd方法来获取一个AssetFileDescriptor，它可以用来获取FileInputStream(不像AssetManager. openFd方法)。open，返回一个InputStream)，其中包含一个FileChannel。它还包含起始偏移量(getStartOffset)和大小(getLength)，用于获取文件的正确部分。

实现

下面给出了一个实现示例:

private void copyFileFromAssets(String in_filename, File out_file){
    Log.d("copyFileFromAssets", "Copying file '"+in_filename+"' to '"+out_file.toString()+"'");
    AssetManager assetManager = getApplicationContext().getAssets();
    FileChannel in_chan = null, out_chan = null;
    try {
        AssetFileDescriptor in_afd = assetManager.openFd(in_filename);
        FileInputStream in_stream = in_afd.createInputStream();
        in_chan = in_stream.getChannel();
        Log.d("copyFileFromAssets", "Asset space in file: start = "+in_afd.getStartOffset()+", length = "+in_afd.getLength());
        FileOutputStream out_stream = new FileOutputStream(out_file);
        out_chan = out_stream.getChannel();
        in_chan.transferTo(in_afd.getStartOffset(), in_afd.getLength(), out_chan);
    } catch (IOException ioe){
        Log.w("copyFileFromAssets", "Failed to copy file '"+in_filename+"' to external storage:"+ioe.toString());
    } finally {
        try {
            if (in_chan != null) {
                in_chan.close();
            }
            if (out_chan != null) {
                out_chan.close();
            }
        } catch (IOException ioe){}
    }
}

这个答案是基于摩根大通的答案。

由于一些错误，上述解决方案无法工作:

目录创建失败 Android返回的资产也包含三个文件夹:图像，声音和webkit 增加了处理大文件的方法:在项目的资产文件夹中添加扩展名.mp3到文件，在复制目标文件时将没有.mp3扩展名

下面是代码(我留下了Log语句，但你现在可以删除它们):

final static String TARGET_BASE_PATH = "/sdcard/appname/voices/";

private void copyFilesToSdCard() {
    copyFileOrDir(""); // copy all files in assets folder in my project
}

private void copyFileOrDir(String path) {
    AssetManager assetManager = this.getAssets();
    String assets[] = null;
    try {
        Log.i("tag", "copyFileOrDir() "+path);
        assets = assetManager.list(path);
        if (assets.length == 0) {
            copyFile(path);
        } else {
            String fullPath =  TARGET_BASE_PATH + path;
            Log.i("tag", "path="+fullPath);
            File dir = new File(fullPath);
            if (!dir.exists() && !path.startsWith("images") && !path.startsWith("sounds") && !path.startsWith("webkit"))
                if (!dir.mkdirs())
                    Log.i("tag", "could not create dir "+fullPath);
            for (int i = 0; i < assets.length; ++i) {
                String p;
                if (path.equals(""))
                    p = "";
                else 
                    p = path + "/";

                if (!path.startsWith("images") && !path.startsWith("sounds") && !path.startsWith("webkit"))
                    copyFileOrDir( p + assets[i]);
            }
        }
    } catch (IOException ex) {
        Log.e("tag", "I/O Exception", ex);
    }
}

private void copyFile(String filename) {
    AssetManager assetManager = this.getAssets();

    InputStream in = null;
    OutputStream out = null;
    String newFileName = null;
    try {
        Log.i("tag", "copyFile() "+filename);
        in = assetManager.open(filename);
        if (filename.endsWith(".jpg")) // extension was added to avoid compression on APK file
            newFileName = TARGET_BASE_PATH + filename.substring(0, filename.length()-4);
        else
            newFileName = TARGET_BASE_PATH + filename;
        out = new FileOutputStream(newFileName);

        byte[] buffer = new byte[1024];
        int read;
        while ((read = in.read(buffer)) != -1) {
            out.write(buffer, 0, read);
        }
        in.close();
        in = null;
        out.flush();
        out.close();
        out = null;
    } catch (Exception e) {
        Log.e("tag", "Exception in copyFile() of "+newFileName);
        Log.e("tag", "Exception in copyFile() "+e.toString());
    }

}

编辑:更正了一个错位的“;”，这是抛出一个系统的“无法创建目录”错误。

这在Kotlin中是一种简洁的方式。

    fun AssetManager.copyRecursively(assetPath: String, targetFile: File) {
        val list = list(assetPath)
        if (list.isEmpty()) { // assetPath is file
            open(assetPath).use { input ->
                FileOutputStream(targetFile.absolutePath).use { output ->
                    input.copyTo(output)
                    output.flush()
                }
            }

        } else { // assetPath is folder
            targetFile.delete()
            targetFile.mkdir()

            list.forEach {
                copyRecursively("$assetPath/$it", File(targetFile, it))
            }
        }
    }

如果其他人也有同样的问题，我就是这么做的

private void copyAssets() {
    AssetManager assetManager = getAssets();
    String[] files = null;
    try {
        files = assetManager.list("");
    } catch (IOException e) {
        Log.e("tag", "Failed to get asset file list.", e);
    }
    if (files != null) for (String filename : files) {
        InputStream in = null;
        OutputStream out = null;
        try {
          in = assetManager.open(filename);
          File outFile = new File(getExternalFilesDir(null), filename);
          out = new FileOutputStream(outFile);
          copyFile(in, out);
        } catch(IOException e) {
            Log.e("tag", "Failed to copy asset file: " + filename, e);
        }     
        finally {
            if (in != null) {
                try {
                    in.close();
                } catch (IOException e) {
                    // NOOP
                }
            }
            if (out != null) {
                try {
                    out.close();
                } catch (IOException e) {
                    // NOOP
                }
            }
        }  
    }
}
private void copyFile(InputStream in, OutputStream out) throws IOException {
    byte[] buffer = new byte[1024];
    int read;
    while((read = in.read(buffer)) != -1){
      out.write(buffer, 0, read);
    }
}

参考:使用Java移动文件

我知道这已经回答，但我有一个稍微更优雅的方式从资产目录复制到sdcard上的文件。它不需要“for”循环，而是使用文件流和通道来完成工作。

(注)如果使用任何类型的压缩文件，APK, PDF，…你可能想重命名文件扩展名之前插入到资产，然后重命名一旦你复制到SDcard)

AssetManager am = context.getAssets();
AssetFileDescriptor afd = null;
try {
    afd = am.openFd( "MyFile.dat");

    // Create new file to copy into.
    File file = new File(Environment.getExternalStorageDirectory() + java.io.File.separator + "NewFile.dat");
    file.createNewFile();

    copyFdToFile(afd.getFileDescriptor(), file);

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

一种复制文件而不必循环遍历它的方法。

public static void copyFdToFile(FileDescriptor src, File dst) throws IOException {
    FileChannel inChannel = new FileInputStream(src).getChannel();
    FileChannel outChannel = new FileOutputStream(dst).getChannel();
    try {
        inChannel.transferTo(0, inChannel.size(), outChannel);
    } finally {
        if (inChannel != null)
            inChannel.close();
        if (outChannel != null)
            outChannel.close();
    }
}

这是目前为止我能在网上找到的最好的解决方案。 我使用了以下链接https://gist.github.com/mhasby/026f02b33fcc4207b302a60645f6e217，但它有一个错误，我修复了，然后它的工作就像一个魅力。 这是我的代码。您可以轻松使用它，因为它是一个独立的java类。

public class CopyAssets {
public static void copyAssets(Context context) {
    AssetManager assetManager = context.getAssets();
    String[] files = null;
    try {
        files = assetManager.list("");
    } catch (IOException e) {
        Log.e("tag", "Failed to get asset file list.", e);
    }
    if (files != null) for (String filename : files) {
        InputStream in = null;
        OutputStream out = null;
        try {
            in = assetManager.open(filename);

            out = new FileOutputStream(Environment.getExternalStorageDirectory()+"/www/resources/" + filename);
            copyFile(in, out);
        } catch(IOException e) {
            Log.e("tag", "Failed to copy asset file: " + filename, e);
        }
        finally {
            if (in != null) {
                try {
                    in.close();
                    in = null;
                } catch (IOException e) {

                }
            }
            if (out != null) {
                try {
                    out.flush();
                    out.close();
                    out = null;
                } catch (IOException e) {

                }
            }
        }
    }
}

public static void copyFile(InputStream in, OutputStream out) throws IOException {
    byte[] buffer = new byte[1024];
    int read;
    while((read = in.read(buffer)) != -1){
        out.write(buffer, 0, read);
    }
}}

As you can see, just create an instance of CopyAssets in your java class which has an activity. Now this part is important, as far as my testing and researching on the internet, You cannot use AssetManager if the class has no activity . It has something to do with the context of the java class. Now, the c.copyAssets(getApplicationContext()) is an easy way to access the method, where c is and instance of CopyAssets class. As per my requirement, I allowed the program to copy all my resource files inside the asset folder to the /www/resources/ of my internal directory. You can easily find out the part where you need to make changes to the directory as per your use. Feel free to ping me if you need any help.