理论基础

我们在android开发项目过程中都必然会更so加载打交道，那么so加载在系统中的顺序和流程是怎样的，我们就有必要对这个加载过程进行熟悉了解掌握。
so的加载是一种解析式装载，这与dex有一定区别，dex是先加载进行优化验证生成odex，再去解析odex文件，而so更像边解析边装载，在加载过程中主要解析是load段。
下面主要是以java层的so加载进行从源码上进行解析加载流程。

java层的so加载流程分析

System.loadLibrary入口点

在java层我们知道加载so文件是通过System.loadLibrary函数其实现的，下面就以其作为入口点进行分析它的调用关系和实现。 System.loadLibrary在的函数定义系统source\libcore\luni\src\main\java\java\lang\system.java的文件中。

下面是其函数定义实现。

//参数就是要加载的so文件名称
 public static void loadLibrary(String libName) {
         //通过调用Runtime下面的loadLibrary函数实现
         //函数有两个参数，参数1是加载的so文件名，参数2 类加载器。
        Runtime.getRuntime().loadLibrary(libName, VMStack.getCallingClassLoader());
    }

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Runtime的loadLibray

通过上面的System.java的loadLibrary函数我们需要继续分析Runtime.java文件中的loadLibray函数的定义实现。 Runtime的loadLibrary函数在android系统中的位置是 source\libcore\luni\src\main\java\java\lang\Runtime.java文件。

下面是Runtime的 loadLibrary函数的定义实现源码。

    /*
     * Searches for and loads the given shared library using the given ClassLoader.
     */
    void loadLibrary(String libraryName, ClassLoader loader) {
        if (loader != null) {
            //通过加载器去查找要加载的so文件名
            String filename = loader.findLibrary(libraryName);
            //查找失败
            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) + "\"");
            }
            //加载so文件名
            String error = doLoad(filename, loader);
            if (error != null) {
                throw new UnsatisfiedLinkError(error);
            }
            return;
        }
        
        String filename = System.mapLibraryName(libraryName);
        List<String> candidates = new ArrayList<String>();
        String lastError = null;
        //循环遍历文件路径
        for (String directory : mLibPaths) {
            //文件路径和文件名进行拼接
            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;
            }
        }

        if (lastError != null) {
            throw new UnsatisfiedLinkError(lastError);
        }
        throw new UnsatisfiedLinkError("Library " + libraryName + " not found; tried " + candidates);
    }
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Runtime的doLoad

通过上面的Runtime的loadLibrary函数，我们看到加载so的函数是走到doLoad函数，那么我们就需要继续分析Runtime下的doload函数的定义实现。 Rutime下的doload函数在系统中的 source\libcore\luni\src\main\java\java\lang\Runtime.java文件中。

下面的代码是Runtime的doload函数的定义实现。

 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);
        }
    }
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总结

从以上的源码实现流程分析，我们可以看出Android在java层加载so的接口是System.loadLibrary()，通过层层递进关系从而实现java层的加载so。