System.loadLibrary调用SO源码解析

最新推荐文章于 2023-03-21 15:54:08 发布

原创最新推荐文章于 2023-03-21 15:54:08 发布 · 3.9k 阅读

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#源码

本文详细解析了Java调用So库的具体流程，从System.loadLibrary方法开始，深入到Runtime类中的loadLibrary0方法，解释了ClassLoader如何查找并加载本地库文件。并通过Android平台上的BaseDexClassLoader类说明了库文件查找的具体实现。

首先跟着源码走

System.loadLibrary(“native-lib”)

在loadLibrary里面会调用Runtime.getRuntime().loadLibrary0()方法，里面有两个参数VMStack.getCallingClassLoader()和libname

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

继续在Runtime类中会找到loadLibrary0方法

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;
        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;
        }

        String filename = System.mapLibraryName(libraryName);
        List<String> candidates = new ArrayList<String>();
        String lastError = null;
        for (String directory : getLibPaths()) {
            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);
    }

这里面有个判断,判断ClassLoader在不在，一般都是有的所以会进入，这里面有个重要的方法 loader.findLibrary(libraryName);当我们点进去的时候发现返回的是NULL

protected String findLibrary(String libname) {
        return null;
    }

这样我们就想到了，他还会有子类于是我们通过LOG打印出当前的ClassLoader

Log.d(TAG,"ClassLoader ++"+this.getClassLoader().toString());

//   这是打印的子类dalvik.system.PathClassLoader

于是我们翻看Android源码7.1.1（有5.23个G已下载好）PathClassLoader类，发现没有实现太多。于是我们找到了他的父类BaseDexClassLoader的findLibrary()方法

他这里面返回了pathList.findLibrary(name)
@Override
public String findLisbrary(String name){
    return pathList.findLibrary(name);
}