问题

Prior to Android 5.0 I was able to load DEX files dynamically using DexClassLoader and calling loadClass() method but with the latest Android version I get a ClassNotFoundException.

Here is what I am doing:

Generate DEX file.

../android-sdk/android-sdk-linux_86/build-tools/21.1.1/dx --dex --output=bin/output.dex  bin/output.jar

Create a DexClassLoader.

DexClassLoader cl = new DexClassLoader(
dexFile.getAbsolutePath(),
odexFile.getAbsolutePath(),
null,
mContext.getClassLoader());

Call cl.loadClass("myMethod");

I am aware that ART uses dex2oat to generate an ELF file that is the loaded by ART but in step 2 I am generating an ODEX file so I am not what needs to be done in ART to load a DEX file at runtime, can anyone help me ?

回答1:

Update

This works both on Dalvik and ART: new DexClassLoader(jarredDex.getAbsolutePath(), context.getDir("outdex", Context.MODE_PRIVATE).getAbsolutePath(), null, context.getClassLoader()); where jarredDex is a jar-file with classes.dex. Jar can be obtained by running dx --dex --output=filename.jar your/classes/dir.

Original answer

I've took a code sample from this article. But ART uses PathClassLoader instead of Dalvik's DexClassLoader. This code is tested on emulator with Android 6 and on Xiaomi with Android 5.1 and works fine:

// Before the secondary dex file can be processed by the DexClassLoader,
// it has to be first copied from asset resource to a storage location.
File dexInternalStoragePath = new File(getDir("dex", Context.MODE_PRIVATE), SECONDARY_DEX_NAME);
try (BufferedInputStream bis = new BufferedInputStream(getAssets().open(SECONDARY_DEX_NAME));
     OutputStream dexWriter = new BufferedOutputStream(new FileOutputStream(dexInternalStoragePath))) {

    byte[] buf = new byte[BUF_SIZE];
    int len;
    while((len = bis.read(buf, 0, BUF_SIZE)) > 0) {
        dexWriter.write(buf, 0, len);
    }
} catch (IOException e) {
    throw new RuntimeException(e);
}

try {
    PathClassLoader loader = new PathClassLoader(dexInternalStoragePath.getAbsolutePath(), getClassLoader());
    Class<?> toasterClass = loader.loadClass("my.package.ToasterImpl");
    Toaster toaster = (Toaster) toasterClass.newInstance();
    toaster.show(this, "Success!");
} catch (ReflectiveOperationException e) {
    throw new RuntimeException(e);
}

回答2:

It is the problem with making dex file. Change the command as below then will work in all versions of android.

dx.bat --dex --output path/output.jar  path/input.jar

The above method in windows will generate a .jar file with classes.dex file inside that jar file. The DexClassLoader is searching for classes.dex file inside, that is why it is throwing ClassNotFoundException

来源：https://stackoverflow.com/questions/27349097/load-dex-file-dynamically-on-android-5-0

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android