达尔维克如何找到图书馆？

Question

first, in System.java, it calls Runtime to loadLibrary.

public static void loadLibrary(String libName) {
    SecurityManager smngr = System.getSecurityManager();
    if (smngr != null) {
        smngr.checkLink(libName);
    }
    Runtime.getRuntime().loadLibrary(libName, VMStack.getCallingClassLoader());
}

second, it calles VMStack.getCallingClassLoader() to findLibrary.

void loadLibrary(String libraryName, ClassLoader loader) {
    if (loader != null) {
        String filename = loader.findLibrary(libraryName);
        if (filename == null) {
            throw new UnsatisfiedLinkError("Couldn't load " + libraryName + ": " +
                    "findLibrary returned null");
        }
        //....
    }
}

So, i think VMStack.getCallingClassLoader() is the most meaningful. But in its jni file dalvik_system_VMStack.cpp , the Dalvik_dalvik_system_VMStack_getCallingClassLoader function is hard to learn. And at last, how dalvik findLibrary?

static void Dalvik_dalvik_system_VMStack_getCallingClassLoader(const u4* args,
    JValue* pResult){
    ClassObject* clazz =
        dvmGetCaller2Class(dvmThreadSelf()->interpSave.curFrame);

    UNUSED_PARAMETER(args);

    if (clazz == NULL)
        RETURN_PTR(NULL);
    RETURN_PTR(clazz->classLoader);
}

Answer 1

VMStack.getCallingClassLoader() returns the class loader of the class that declared the method that called the current method. In other words, if my function foo() called Runtime.loadLibrary() , it would return foo 's class loader.

The point of all that is to make sure the library is loaded in the context of whoever called findLibrary , rather than in the context of java.lang.Runtime .

There's a good chance findLibrary() is implemented by BaseDexClassLoader , which calls into DexPathList.findLibrary() , which does the actual work. The interesting bit there is the walk through nativeLibraryDirectories , which is initialized from the libraryPath argument to the BaseDexClassLoader constructor (which gets it from PathClassLoader or DexClassLoader ).

For Android apps, take a look at android.app.ApplicationLoaders , which uses a PathClassLoader . If you trace that back far enough you'll see the directory being retrieved from ApplicationInfo.nativeLibraryDir .

EDIT: going deeper to address comment...

/system/lib comes from the java.library.path property, which the core libraries extract from the LD_LIBRARY_PATH environment variable. The app-specific library dir is configured by the framework.

The PackageManagerService constructor sets the lib path in mAppLibInstallDir , and setInternalAppNativeLibraryPath() configures nativeLibraryDir .

DexPathList.splitLibraryPath() combines the java.library.path path with the APK-specific path. See the comments there for notes on ordering.