android 加载so的流程

Posted 2020-09-19 寻步

源码版本：android-4.4.4_r1

以 [java.lang.Runtime -> load()] 为例来说明（loadLiabrary() 最后和 load() 殊途同归，有兴趣的可以自行分析），对应的 Android 源码在 [srcAndroid/libcore/luni/src/main/java/java/lang/Runtime.java/home/shanyu/srcAndroid/libcore/luni/src/main/java/java/lang/Runtime.java]，

从 320 行开始。

    /**
     * Loads and links the dynamic library that is identified through the
     * specified path. This method is similar to {@link #loadLibrary(String)},
     * but it accepts a full path specification whereas {@code loadLibrary} just
     * accepts the name of the library to load.
     *
     * @param pathName
     *            the absolute (platform dependent) path to the library to load.
     * @throws UnsatisfiedLinkError
     *             if the library can not be loaded.
     */
    public void load(String pathName) {
        load(pathName, VMStack.getCallingClassLoader());
    }

    /*
     * Loads and links the given library without security checks.
     */
    void load(String pathName, ClassLoader loader) {
        if (pathName == null) {
            throw new NullPointerException("pathName == null");
        }
        String error = doLoad(pathName, loader);
        if (error != null) {
            throw new UnsatisfiedLinkError(error);
        }
    }

最终调用了doLoad（String name, ClassLoader loader）函数，这个函数仍然在Runtime.java文件中：

    private String doLoad(String name, ClassLoader loader) {
        // Android apps are forked from the zygote, so they can‘t have a custom LD_LIBRARY_PATH,
        // which means that by default an app‘s shared library directory isn‘t on LD_LIBRARY_PATH.

        // The PathClassLoader set up by frameworks/base knows the appropriate path, so we can load
        // libraries with no dependencies just fine, but an app that has multiple libraries that
        // depend on each other needed to load them in most-dependent-first order.

        // We added API to Android‘s dynamic linker so we can update the library path used for
        // the currently-running process. We pull the desired path out of the ClassLoader here
        // and pass it to nativeLoad so that it can call the private dynamic linker API.

        // We didn‘t just change frameworks/base to update the LD_LIBRARY_PATH once at the
        // beginning because multiple apks can run in the same process and third party code can
        // use its own BaseDexClassLoader.

        // We didn‘t just add a dlopen_with_custom_LD_LIBRARY_PATH call because we wanted any
        // dlopen(3) calls made from a .so‘s JNI_OnLoad to work too.

        // So, find out what the native library search path is for the ClassLoader in question...
        String ldLibraryPath = null;
        if (loader != null && loader instanceof BaseDexClassLoader) {
            ldLibraryPath = ((BaseDexClassLoader) loader).getLdLibraryPath();
        }
        // nativeLoad should be synchronized so there‘s only one LD_LIBRARY_PATH in use regardless
        // of how many ClassLoaders are in the system, but dalvik doesn‘t support synchronized
        // internal natives.
        synchronized (this) {
            return nativeLoad(name, loader, ldLibraryPath);
        }
    }

    // TODO: should be synchronized, but dalvik doesn‘t support synchronized internal natives.
    private static native String nativeLoad(String filename, ClassLoader loader, String ldLibraryPath);

最终调用到了"String nativeLoad(String filename, ClassLoader loader, String ldLibraryPath)"函数，这个一个native函数，定义位于[srcAndroid/dalvik/vm/native/java_lang_Runtime.cpp]文件中。从64行开始：

/*
 * static String nativeLoad(String filename, ClassLoader loader, String ldLibraryPath)
 *
 * Load the specified full path as a dynamic library filled with
 * JNI-compatible methods. Returns null on success, or a failure
 * message on failure.
 */
static void Dalvik_java_lang_Runtime_nativeLoad(const u4* args,
    JValue* pResult)
{
    StringObject* fileNameObj = (StringObject*) args[0];
    Object* classLoader = (Object*) args[1];
    StringObject* ldLibraryPathObj = (StringObject*) args[2];

    assert(fileNameObj != NULL);
    char* fileName = dvmCreateCstrFromString(fileNameObj);

    if (ldLibraryPathObj != NULL) {
        char* ldLibraryPath = dvmCreateCstrFromString(ldLibraryPathObj);
        void* sym = dlsym(RTLD_DEFAULT, "android_update_LD_LIBRARY_PATH");
        if (sym != NULL) {
            typedef void (*Fn)(const char*);
            Fn android_update_LD_LIBRARY_PATH = reinterpret_cast<Fn>(sym);
            (*android_update_LD_LIBRARY_PATH)(ldLibraryPath);
        } else {
            ALOGE("android_update_LD_LIBRARY_PATH not found; .so dependencies will not work!");
        }
        free(ldLibraryPath);
    }

    StringObject* result = NULL;
    char* reason = NULL;
    bool success = dvmLoadNativeCode(fileName, classLoader, &reason);
    if (!success) {
        const char* msg = (reason != NULL) ? reason : "unknown failure";
        result = dvmCreateStringFromCstr(msg);
        dvmReleaseTrackedAlloc((Object*) result, NULL);
    }

    free(reason);
    free(fileName);
    RETURN_PTR(result);
}

还是传值 + 检查，然后执行 [bool success = dvmLoadNativeCode(fileName, classLoader, &reason);] ，看下 dvmLoadNativeCode(...) 的代码，位于 vm/Native.cpp # 301 行。