Android So动态加载原理分析与优雅实现

背景:漫品Android客户端集成适配转换功能(基于目标识别(So库35M)和人脸识别库(5M)),导致apk体积50M左右,为优化客户端体验,决定实现So文件动态加载.

开源地址: https://github.com/AnyMarvel/ManPinAPP

本文默认认为大家对JNI开发有一定的了解。在 Android 开发中调用动态链接库文件*.so都是通过 jni 的加载方式,一般的开发方式往往是在 apk 或 jar 包中调用so文件时,都要将对应 so 文件打包进 apk 或 jar 包。

基于以上的常见的方式,我们一般使用java提供的so加载api

  • System.loadLibrary(String libName):参数为so库名称,位于 apk 压缩文件中的 libs 目录,最后复制到 apk 安装目录下;

  • System.load(String pathName) :参数为 so 库在磁盘中完整的路径,可以加载自定义外部 so 库文件;

  • 使用第三方库ReLinker,有so加载成功、失败的回调,安全加载不崩溃。(relinker也是对上面两种api的封装,但更推荐大家使用这种加载方式)

看到上述API其实大家可以看出来System.load(String pathName)这个api其实已经可以实现So的动态加载,那为什么我们还要搞一些所谓的黑科技呢?

如果项目native库很多,还支持各种平台,为了减少apk size,so库动态下发,按需加载是不错的选择。比如x86库服务器下发,动态加载,瘦身效果将非常可观。但是采取常规load方式,改动有点大,底层jar包,第三库不好改加载路径。

在应用启动的时,一次注入本地so路径path,待程序使用过程中so准备后安全加载。(原因后面分析,我们先看下实践)

  • 一. 下载So文件到sdk卡

  • 二. copy So文件到app缓存空间

  • 三. 修改nativeLibraryDirectories(6.0以下)或nativeLibraryPathElements(6.0及以上)(关键:把自定义的native库path插入native数组最前面,即使安装包libs目录里面有同名的so,也优先加载指定路径的外部so。)

  • 四. 正常加载so使用

有兴趣可以直接查看开源代码:https://github.com/AnyMarvel/ManPinAPP

核心类,修改nativeLibraryDirectories列表,加载需要的so文件映射

/**
 * com.google.android.apps.photolab.storyboard.soloader.LoadLibraryUtil
 * Description:动态加载so文件的核心,注入so路径到nativeLibraryDirectories数组第一个位置,会优先从这个位置查找so
 * 更多姿势,请参考开源库动态更新so的黑科技,仅供学习交流
 *
 */
public class LoadLibraryUtil {
    private static final String TAG = LoadLibraryUtil.class.getSimpleName() + "-duqian";
    private static File lastSoDir = null;


    public static synchronized boolean installNativeLibraryPath(ClassLoader classLoader, File folder)
            throws Throwable {
        if (classLoader == null || folder == null || !folder.exists()) {
            Log.e(TAG, "classLoader or folder is illegal " + folder);
            return false;
        }
        final int sdkInt = Build.VERSION.SDK_INT;
        final boolean aboveM = (sdkInt == 25 && getPreviousSdkInt() != 0) || sdkInt > 25;
        if (aboveM) {
            try {
                V25.install(classLoader, folder);
            } catch (Throwable throwable) {
                try {
                    V23.install(classLoader, folder);
                } catch (Throwable throwable1) {
                    V14.install(classLoader, folder);
                }
            }
        } else if (sdkInt >= 23) {
            try {
                V23.install(classLoader, folder);
            } catch (Throwable throwable) {
                V14.install(classLoader, folder);
            }
        } else if (sdkInt >= 14) {
            V14.install(classLoader, folder);
        }
        lastSoDir = folder;
        return true;
    }

    private static final class V23 {
        private static void install(ClassLoader classLoader, File folder) throws Throwable {
            Field pathListField = ReflectUtil.findField(classLoader, "pathList");
            Object dexPathList = pathListField.get(classLoader);

            Field nativeLibraryDirectories = ReflectUtil.findField(dexPathList, "nativeLibraryDirectories");
            List<File> libDirs = (List<File>) nativeLibraryDirectories.get(dexPathList);

            //去重
            if (libDirs == null) {
                libDirs = new ArrayList<>(2);
            }
            final Iterator<File> libDirIt = libDirs.iterator();
            while (libDirIt.hasNext()) {
                final File libDir = libDirIt.next();
                if (folder.equals(libDir) || folder.equals(lastSoDir)) {
                    libDirIt.remove();
                    Log.d(TAG, "dq libDirIt.remove() " + folder.getAbsolutePath());
                    break;
                }
            }

            libDirs.add(0, folder);
            Field systemNativeLibraryDirectories =
                    ReflectUtil.findField(dexPathList, "systemNativeLibraryDirectories");
            List<File> systemLibDirs = (List<File>) systemNativeLibraryDirectories.get(dexPathList);

            //判空
            if (systemLibDirs == null) {
                systemLibDirs = new ArrayList<>(2);
            }
            Log.d(TAG, "dq systemLibDirs,size=" + systemLibDirs.size());

            Method makePathElements = ReflectUtil.findMethod(dexPathList, "makePathElements", List.class, File.class, List.class);
            ArrayList<IOException> suppressedExceptions = new ArrayList<>();
            libDirs.addAll(systemLibDirs);

            Object[] elements = (Object[]) makePathElements.invoke(dexPathList, libDirs, null, suppressedExceptions);
            Field nativeLibraryPathElements = ReflectUtil.findField(dexPathList, "nativeLibraryPathElements");
            nativeLibraryPathElements.setAccessible(true);
            nativeLibraryPathElements.set(dexPathList, elements);
        }
    }

    /**
     * 把自定义的native库path插入nativeLibraryDirectories最前面,即使安装包libs目录里面有同名的so,也优先加载指定路径的外部so
     */
    private static final class V25 {
        private static void install(ClassLoader classLoader, File folder) throws Throwable {
            Field pathListField = ReflectUtil.findField(classLoader, "pathList");
            Object dexPathList = pathListField.get(classLoader);
            Field nativeLibraryDirectories = ReflectUtil.findField(dexPathList, "nativeLibraryDirectories");

            List<File> libDirs = (List<File>) nativeLibraryDirectories.get(dexPathList);
            //去重
            if (libDirs == null) {
                libDirs = new ArrayList<>(2);
            }
            final Iterator<File> libDirIt = libDirs.iterator();
            while (libDirIt.hasNext()) {
                final File libDir = libDirIt.next();
                if (folder.equals(libDir) || folder.equals(lastSoDir)) {
                    libDirIt.remove();
                    Log.d(TAG, "dq libDirIt.remove()" + folder.getAbsolutePath());
                    break;
                }
            }

            libDirs.add(0, folder);
            //system/lib
            Field systemNativeLibraryDirectories = ReflectUtil.findField(dexPathList, "systemNativeLibraryDirectories");
            List<File> systemLibDirs = (List<File>) systemNativeLibraryDirectories.get(dexPathList);

            //判空
            if (systemLibDirs == null) {
                systemLibDirs = new ArrayList<>(2);
            }
            Log.d(TAG, "dq systemLibDirs,size=" + systemLibDirs.size());

            Method makePathElements = ReflectUtil.findMethod(dexPathList, "makePathElements", List.class);
            libDirs.addAll(systemLibDirs);

            Object[] elements = (Object[]) makePathElements.invoke(dexPathList, libDirs);
            Field nativeLibraryPathElements = ReflectUtil.findField(dexPathList, "nativeLibraryPathElements");
            nativeLibraryPathElements.setAccessible(true);
            nativeLibraryPathElements.set(dexPathList, elements);
        }
    }


    private static final class V14 {
        private static void install(ClassLoader classLoader, File folder) throws Throwable {
            Field pathListField = ReflectUtil.findField(classLoader, "pathList");
            Object dexPathList = pathListField.get(classLoader);

            ReflectUtil.expandFieldArray(dexPathList, "nativeLibraryDirectories", new File[]{folder});
        }
    }

    /**
     * fuck部分机型删了该成员属性,兼容
     *
     * @return 被厂家删了返回1,否则正常读取
     */
    @TargetApi(Build.VERSION_CODES.M)
    private static int getPreviousSdkInt() {
        try {
            return Build.VERSION.PREVIEW_SDK_INT;
        } catch (Throwable ignore) {
        }
        return 1;
    }

}

上面说了把自定义的native库path插入nativeLibraryDirectories最前面,那为什么这么做会生效呢

  1. 首先了解以下So的加载流程

  2. So热修复技术介绍及如何动态加载So

这里先做下准备工作

So文件加载流程 不同的同学请戳这里 Android 的 so 文件加载机制

System.loadlibrary() 方法分析so文件的加载流程,如下图所示:

System.loadLibrary()

//System.java
public static void loadLibrary(String libname) {

    Runtime.getRuntime().loadLibrary0(VMStack.getCallingClassLoader(), libname);
}

此处VMStack.getCallingClassLoader()拿到的是调用者的ClassLoader,一般情况下是PathClassLoader。我们进入Runtime类的loadLibrary0()方法看看。

//Runtime.java
synchronized void loadLibrary0(ClassLoader loader, String libname) {
    if (libname.indexOf((int)File.separatorChar) != -1) {
        throw new UnsatisfiedLinkError("Directory separator should not appear in library name: " + libname);
    }
    String libraryName = libname;
    // 1. 如果classloder存在,通过loader.findLibrary()查找到so路径
    if (loader != null) {
        String filename = loader.findLibrary(libraryName);
        if (filename == null) {
            // It's not necessarily true that the ClassLoader used
            // System.mapLibraryName, but the default setup does, and it's
            // misleading to say we didn't find "libMyLibrary.so" when we
            // actually searched for "liblibMyLibrary.so.so".
            throw new UnsatisfiedLinkError(loader + " couldn't find \"" +
                                           System.mapLibraryName(libraryName) + "\"");
        }
        String error = doLoad(filename, loader);
        if (error != null) {
            throw new UnsatisfiedLinkError(error);
        }
        return;
    }

    // 2. 如果classloder不存在,通过loader.findLibrary()查找到so路径
    String filename = System.mapLibraryName(libraryName);
    List<String> candidates = new ArrayList<String>();
    String lastError = null;
    for (String directory : getLibPaths()) {    // getLibPaths()代码在最下方
        String candidate = directory + filename;
        candidates.add(candidate);

        if (IoUtils.canOpenReadOnly(candidate)) {
            String error = doLoad(candidate, loader);
            if (error == null) {
                return; // We successfully loaded the library. Job done.
            }
            lastError = error;
        }
    }

    // 3. 都没找到,抛出 UnsatisfiedLinkError 异常
    if (lastError != null) {
        throw new UnsatisfiedLinkError(lastError);
    }
    throw new UnsatisfiedLinkError("Library " + libraryName + " not found; tried " + candidates);
}

这里根据ClassLoader是否存在分了两种情况:

  • 当ClasssLoader存在的时候通过loader的 findLibrary()查看目标库所在路径;

  • 当ClassLoader不存在的时候通过getLibPaths()查找加载路径。

    最终他们都会调用doLoad()加载动态库。

这里我们只查看ClasssLoader存在时的情况:

前面知道了这个ClassLoader其实是PathClassLoader,但是findLibrary位于PathClassLoader的父类BaseDexClassLoader中:

//BaseDexClassLoader.java
public String findLibrary(String name) {
    return pathList.findLibrary(name);
}

其中pathList的类型为DexPathList,我们看看它的findLibrary()方法.关键点在DexPathList.findLibrary(String libraryName) 方法中

http://androidxref.com/5.0.0_r2/xref/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java

       /**
371     * Finds the named native code library on any of the library
372     * directories pointed at by this instance. This will find the
373     * one in the earliest listed directory, ignoring any that are not
374     * readable regular files.
375     *
376     * @return the complete path to the library or {@code null} if no
377     * library was found
378     */
379    public String findLibrary(String libraryName) {
380        String fileName = System.mapLibraryName(libraryName);
381        for (File directory : nativeLibraryDirectories) {
382            String path = new File(directory, fileName).getPath();
383            if (IoUtils.canOpenReadOnly(path)) {
384                return path;
385            }
386        }
387        return null;
388    }

可以看到首先会调用System.mapLibraryName函数获得so的名字,比如我传入的参数是Test(这个Test就是在调用System.loadLibrary(“Test”)时传入的),则这个函数的作用就是将其转换为类似libTest.so这样的名字,然后遍历nativeLibraryDirectories数组,这是一个File文件夹数组,看其文件夹下是否存在对应的so,并且是否可读,如果满足条件,则直接返回。

那么我们就可以将我们的patch的so所在目录插入到这个数组最前面即可完成so的修复。具体代码就不贴了,实践后得出的结论是这种方式是完全可行的,只不过Android 6.0以后版本中这部分代码逻辑发生了改变。

在Android 4.0-5.1中,只需要将文件夹目录插入到nativeLibraryDirectories数组最前面即可,这个过程直接使用反射插入patch的so所在目录到数组最前面。(nativeLibraryDirectories存储了so文件加载的映射表,这里相当于修改了应用加载so的列表)

61    /** List of native library directories. */
62    private final File[] nativeLibraryDirectories;

但是在Android 6.0以后,查找逻辑转为了Elements查找

http://androidxref.com/9.0.0_r3/xref/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java

       /**
536     * Finds the named native code library on any of the library
537     * directories pointed at by this instance. This will find the
538     * one in the earliest listed directory, ignoring any that are not
539     * readable regular files.
540     *
541     * @return the complete path to the library or {@code null} if no
542     * library was found
543     */
544    public String findLibrary(String libraryName) {
545        String fileName = System.mapLibraryName(libraryName);
546
547        for (NativeLibraryElement element : nativeLibraryPathElements) {
548            String path = element.findNativeLibrary(fileName);
549
550            if (path != null) {
551                return path;
552            }
553        }
554
555        return null;
556    }

所以在6.0中需要将so的patch目录转换为Element对象,插入到nativeLibraryPathElements最前面,Element的对象可以直接用反射去实现下面的代码进行构造即可。

66    /** List of native library path elements. */
67    // Some applications rely on this field being an array or we'd use a final list here
68    /* package visible for testing */ NativeLibraryElement[] nativeLibraryPathElements;

最后的难点就是如何将对应cpu类型的so拿到,这个过程还是十分复杂的,比如说一个so同时存在x86,armeabi-v7a,armeabi的patch,而手机cpu是armeabi-v7a的,这时候就应该加载armeabi-v7a的so。总之这种情况组合起来会十分复杂了。但一般的厂商都是只兼容armeabi架构,其他的架构就可以向上兼容了(除x86架构外)

手机的cpu结构类型可以通过Build.CPU_ABI和Build.CPU_ABI2拿到,后面做的事就是根据这两个值去加载对应目录下的so,其实把这两个目录都插进去就没问题了。

总结:

到此处,那么so文件的动态加载(也可以叫做So文件的热修复)已经介绍完了,起始还是比较简单的,只是修改了so文件列表的数组映射,加载了需要使用的真实的so文件.

经过动态加载,漫品客户端也由原来50M的体积缩小到了15M的输出包.

安利 欢迎大家的start

开源地址: https://github.com/AnyMarvel/ManPinAPP

热修复so代码包位置: com.google.android.apps.photolab.storyboard.soloader.LoadLibraryUtil

如有问题请留言,或github issure

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