Android类由DexClassLoader加载

public class DexClassLoader extends BaseDexClassLoader {
    public DexClassLoader(String dexPath, String optimizedDirectory,
            String libraryPath, ClassLoader parent) {
        super(dexPath, new File(optimizedDirectory), libraryPath, parent);
    }
}

转向BaseDexClassLoader

public BaseDexClassLoader(String dexPath, File optimizedDirectory,
        String libraryPath, ClassLoader parent) {
    super(parent);
    this.originalPath = dexPath;
    this.pathList =
        new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
}

追踪下参数 dexPath，这个鬼通常是"/data/../*.apk"，现用作创建pathList

public DexPathList(ClassLoader definingContext, String dexPath,
        String libraryPath, File optimizedDirectory) {
    ...
    this.dexElements =
        makeDexElements(splitDexPath(dexPath), optimizedDirectory);
    ...
}

再创建dexElements，注意下面代码里的 Archive File Loading Block，压缩包加载代码块

private static final String DEX\_SUFFIX = ".dex";
private static final String JAR\_SUFFIX = ".jar";
private static final String ZIP\_SUFFIX = ".zip";
private static final String APK\_SUFFIX = ".apk";
...
private static Element[] makeDexElements(ArrayList\<File\> files,
        File optimizedDirectory) {
    ArrayList\<Element\> elements = new ArrayList\<Element\>();
    /\*
 \* Open all files and load the (direct or contained) dex files
 \* up front.
 \*/
    for (File file : files) {
        ZipFile zip = null;
        DexFile dex = null;
        String name = file.getName();
        if (name.endsWith(DEX\_SUFFIX)) {
            // Raw dex file (not inside a zip/jar).
            ...
        } else if (name.endsWith(APK\_SUFFIX) || name.endsWith(JAR\_SUFFIX)
                || name.endsWith(ZIP\_SUFFIX)) {
            //---------------------------------------------------
            // Archive File Loading Block
            // if (name.endsWith(".so") doFollowingWithReflect();
            //---------------------------------------------------
            try {
                zip = new ZipFile(file);
            } catch (IOException ex) {
                /\*
 \* Note: ZipException (a subclass of IOException)
 \* might get thrown by the ZipFile constructor
 \* (e.g. if the file isn't actually a zip/jar
 \* file).
 \*/
                System.logE("Unable to open zip file: " + file, ex);
            }
            try {
                dex = loadDexFile(file, optimizedDirectory);
            } catch (IOException ignored) {
                /\*
 \* IOException might get thrown "legitimately" by
 \* the DexFile constructor if the zip file turns
 \* out to be resource-only (that is, no
 \* classes.dex file in it). Safe to just ignore
 \* the exception here, and let dex == null.
 \*/
            }
        } else {
            System.logW("Unknown file type for: " + file);
        }
        if ((zip != null) || (dex != null)) {
            elements.add(new Element(file, zip, dex));
        }
    }
    return elements.toArray(new Element[elements.size()]);
}

可以看到，DexClassLoader不支持".so"后缀

为了让应用启动时能自动复制插件包到应用存储目录，需要支持".so"后缀。做法就是模拟 压缩包加载代码块，创建一个dex元素，再反射添加到宿主class loader里的dexPathList。

伪代码：

Context context = getApplicationContext();
File plugin = new File(context.getApplicationInfo().dataDir, "lib/\*\*.so");
Element element = makeDexElement(plugin); // dalvik.system.DexPathList$Element
context.getClassLoader() // dalvik.system.DexClassLoader
        .@dexPathList // dalvik.system.DexPathList
        .@dexElements // dalvik.system.DexPathList$Element []
        .insert(element, 0);