【Android】动态链接库so的加载原理

川长思鸟来 2022-05-21 09:20 455阅读 0赞

## 前言 ##

最近开发的组件时常出现了运行时加载so库失败问题，每天都会有`java.lang.UnsatisfiedLinkError`的错误爆出来，而且线上总是偶然复现，很疑惑。所以本文将从AOSP源码简单跟踪Android中的动态链接库so的加载原理，试图找出一丝线索。

## 加载入口 ##

首先我们知道在Android(Java)中加载一个动态链接库非常简单。就是我们日常调用的 `System.load(Sring filename)` 或者`System.loadLibrary(String libname)`开始。 看过[《理解JNI技术》][JNI]的应该知道上述代码执行过程中会调用native层的`JNI_OnLoad()`方法，一般用于动态注册native方法。

## \# 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。

我们下面分别看下这三个步骤。

### ClasssLoader存在时 ###

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：

\[DexPathList.java\]

public String findLibrary(String libraryName) {
        String fileName = System.mapLibraryName(libraryName);
    
        for (NativeLibraryElement element : nativeLibraryPathElements) {
            String path = element.findNativeLibrary(fileName);
    
            if (path != null) {
                return path;
            }
        }
    
        return null;
    }

可以看到，就是在`nativeLibraryPathElements` 变量中遍历查找对应的so文件。那么这个`nativeLibraryPathElements`变量从何而来呢？可以很快查到是在DexPathList的构造方法中赋值的，它的构造方法如下：

\[DexPathList.java\]

public DexPathList(ClassLoader definingContext, String dexPath,
            String librarySearchPath, File optimizedDirectory) {
    
        if (definingContext == null) {
            throw new NullPointerException("definingContext == null");
        }
    
        if (dexPath == null) {
            throw new NullPointerException("dexPath == null");
        }
    
        if (optimizedDirectory != null) {
            if (!optimizedDirectory.exists())  {
                throw new IllegalArgumentException(
                        "optimizedDirectory doesn't exist: "
                        + optimizedDirectory);
            }
    
            if (!(optimizedDirectory.canRead()
                            && optimizedDirectory.canWrite())) {
                throw new IllegalArgumentException(
                        "optimizedDirectory not readable/writable: "
                        + optimizedDirectory);
            }
        }
    
        this.definingContext = definingContext;
    
        ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
        // save dexPath for BaseDexClassLoader
        this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                                           suppressedExceptions, definingContext);
    
        // Native libraries may exist in both the system and
        // application library paths, and we use this search order:
        //
        //   1. This class loader's library path for application libraries (librarySearchPath):
        //   1.1. Native library directories
        //   1.2. Path to libraries in apk-files
        //   2. The VM's library path from the system property for system libraries
        //      also known as java.library.path
        //
        // This order was reversed prior to Gingerbread; see http://b/2933456.
        this.nativeLibraryDirectories = splitPaths(librarySearchPath, false);
        this.systemNativeLibraryDirectories =
                splitPaths(System.getProperty("java.library.path"), true);
        List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
        allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);
    
        // 这里赋值
        this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories);
    
        if (suppressedExceptions.size() > 0) {
            this.dexElementsSuppressedExceptions =
                suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
        } else {
            dexElementsSuppressedExceptions = null;
        }
    }

这里`nativeLibraryPathElements`收集了apk的so目录，一般位于：`/data/app/${package-name}/lib/arm/` 还有系统的so目录：System.getProperty(“java.library.path”)，可以打印看一下它的值：`/vendor/lib:/system/lib`，其实就是前后两个目录，事实上64位系统是`/vendor/lib64:/system/lib64`。 最终查找so文件的时候就会在这三个路径中查找，优先查找apk目录。

可以看到，PathClassLoader中传入了apk的so目录，然后我们来看没有ClassLoader的情况。

### ClassLoader不存在 ###

当ClassLoader不存在时，通过`getLibPaths()`查找加载路径。

\[Runtime.java\]

// 返回mLibPaths
    private String[] getLibPaths() {
        if (mLibPaths == null) {
            synchronized(this) {
                if (mLibPaths == null) {
                    mLibPaths = initLibPaths();
                }
            }
        }
        return mLibPaths;
    }
    
    // 其实就是环境变量 java.library.path 中的路径
    private static String[] initLibPaths() {
        String javaLibraryPath = System.getProperty("java.library.path");
        if (javaLibraryPath == null) {
            return EmptyArray.STRING;
        }
        String[] paths = javaLibraryPath.split(":");
        // Add a '/' to the end of each directory so we don't have to do it every time.
        for (int i = 0; i < paths.length; ++i) {
            if (!paths[i].endsWith("/")) {
                paths[i] += "/";
            }
        }
        return paths;
    }

可以看到其实很简单，返回的结果就是拆分环境变量 java.library.path 中的路径。

也就是说，ClassLoader为空时使用系统目录，否则使用ClassLoader提供的目录，ClassLoader提供的目录中包括apk目录和系统目录。在这两步各自得到路径之后，最后我们来看看so文件是如何加载的。

### \# doLoad() ###

\[Runtime.java\]

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 librarySearchPath = null;
        if (loader != null && loader instanceof BaseDexClassLoader) {
            BaseDexClassLoader dexClassLoader = (BaseDexClassLoader) loader;
            librarySearchPath = dexClassLoader.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, librarySearchPath);
        }
    }
    
    
    private static native String nativeLoad(String filename, ClassLoader loader,
                                                String librarySearchPath);

这里最后调用了native方法`nativeLoad()`的代码：

\[libcore/ojluni/src/main/native/Runtime.c\]

JNIEXPORT jstring JNICALL
    Runtime_nativeLoad(JNIEnv* env, jclass ignored, jstring javaFilename,
                       jobject javaLoader, jstring javaLibrarySearchPath)
    {
        return JVM_NativeLoad(env, javaFilename, javaLoader, javaLibrarySearchPath);
    }

继续跟进`JVM_NativeLoad()`方法：

\[art/runtime/openjdkjvm/OpenjdkJvm.cc\]

JNIEXPORT jstring JVM_NativeLoad(JNIEnv* env,
                                     jstring javaFilename,
                                     jobject javaLoader,
                                     jstring javaLibrarySearchPath) {
      ScopedUtfChars filename(env, javaFilename);
      if (filename.c_str() == NULL) {
        return NULL;
      }
    
      std::string error_msg;
      {
        art::JavaVMExt* vm = art::Runtime::Current()->GetJavaVM();
        // 实际加载
        bool success = vm->LoadNativeLibrary(env,
                                             filename.c_str(),
                                             javaLoader,
                                             javaLibrarySearchPath,
                                             &error_msg);
        if (success) {
          return nullptr;
        }
      }
    
      // Don't let a pending exception from JNI_OnLoad cause a CheckJNI issue with NewStringUTF.
      env->ExceptionClear();
      return env->NewStringUTF(error_msg.c_str());
    }

接着通过jvm的`LoadNativeLibary()`执行实际工作。具体实现在`java_vm_ext.cc`中：

\[art/runtime/java\_vm\_ext.cc\]

bool JavaVMExt::LoadNativeLibrary(JNIEnv* env,
                                      const std::string& path,
                                      jobject class_loader,
                                      jstring library_path,
                                      std::string* error_msg) {
      error_msg->clear();
    
      // See if we've already loaded this library.  If we have, and the class loader
      // matches, return successfully without doing anything.
      // TODO: for better results we should canonicalize the pathname (or even compare
      // inodes). This implementation is fine if everybody is using System.loadLibrary.
      SharedLibrary* library;
      Thread* self = Thread::Current();
      {
        // TODO: move the locking (and more of this logic) into Libraries.
        MutexLock mu(self, *Locks::jni_libraries_lock_);
        // 1. 判断是否已经加载过这个library
        library = libraries_->Get(path);
      }
      void* class_loader_allocator = nullptr;
      {
        ScopedObjectAccess soa(env);
        // As the incoming class loader is reachable/alive during the call of this function,
        // it's okay to decode it without worrying about unexpectedly marking it alive.
        ObjPtr<mirror::ClassLoader> loader = soa.Decode<mirror::ClassLoader>(class_loader);
    
        ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
        if (class_linker->IsBootClassLoader(soa, loader.Ptr())) {
          loader = nullptr;
          class_loader = nullptr;
        }
    
        class_loader_allocator = class_linker->GetAllocatorForClassLoader(loader.Ptr());
        CHECK(class_loader_allocator != nullptr);
      }
      if (library != nullptr) {
        // Use the allocator pointers for class loader equality to avoid unnecessary weak root decode.
        if (library->GetClassLoaderAllocator() != class_loader_allocator) {
          // The library will be associated with class_loader. The JNI
          // spec says we can't load the same library into more than one
          // class loader.
          StringAppendF(error_msg, "Shared library \"%s\" already opened by "
              "ClassLoader %p; can't open in ClassLoader %p",
              path.c_str(), library->GetClassLoader(), class_loader);
          LOG(WARNING) << error_msg;
          return false;
        }
        VLOG(jni) << "[Shared library \"" << path << "\" already loaded in "
                  << " ClassLoader " << class_loader << "]";
        if (!library->CheckOnLoadResult()) {
          StringAppendF(error_msg, "JNI_OnLoad failed on a previous attempt "
              "to load \"%s\"", path.c_str());
          return false;
        }
        return true;
      }
    
      // Open the shared library.  Because we're using a full path, the system
      // doesn't have to search through LD_LIBRARY_PATH.  (It may do so to
      // resolve this library's dependencies though.)
    
      // Failures here are expected when java.library.path has several entries
      // and we have to hunt for the lib.
    
      // Below we dlopen but there is no paired dlclose, this would be necessary if we supported
      // class unloading. Libraries will only be unloaded when the reference count (incremented by
      // dlopen) becomes zero from dlclose.
    
      Locks::mutator_lock_->AssertNotHeld(self);
      const char* path_str = path.empty() ? nullptr : path.c_str();
      bool needs_native_bridge = false;
      // 2. 加载so
      void* handle = android::OpenNativeLibrary(env,
                                                runtime_->GetTargetSdkVersion(),
                                                path_str,
                                                class_loader,
                                                library_path,
                                                &needs_native_bridge,
                                                error_msg);
    
      VLOG(jni) << "[Call to dlopen(\"" << path << "\", RTLD_NOW) returned " << handle << "]";
    
      // 3. 如果handle为空指针，说明上面OpenNativeLibrary失败了。
      if (handle == nullptr) {
        VLOG(jni) << "dlopen(\"" << path << "\", RTLD_NOW) failed: " << *error_msg;
        return false;
      }
    
      if (env->ExceptionCheck() == JNI_TRUE) {
        LOG(ERROR) << "Unexpected exception:";
        env->ExceptionDescribe();
        env->ExceptionClear();
      }
      // Create a new entry.
      // TODO: move the locking (and more of this logic) into Libraries.
      bool created_library = false;
      {
        // Create SharedLibrary ahead of taking the libraries lock to maintain lock ordering.
        std::unique_ptr<SharedLibrary> new_library(
            new SharedLibrary(env,
                              self,
                              path,
                              handle,
                              needs_native_bridge,
                              class_loader,
                              class_loader_allocator));
    
        MutexLock mu(self, *Locks::jni_libraries_lock_);
        library = libraries_->Get(path);
        if (library == nullptr) {  // We won race to get libraries_lock.
          library = new_library.release();
          // 4. 加载成功的library需要记录下来
          libraries_->Put(path, library);
          created_library = true;
        }
      }
      if (!created_library) {
        LOG(INFO) << "WOW: we lost a race to add shared library: "
            << "\"" << path << "\" ClassLoader=" << class_loader;
        return library->CheckOnLoadResult();
      }
      VLOG(jni) << "[Added shared library \"" << path << "\" for ClassLoader " << class_loader << "]";
    
      // 查找并调用执行 JNI_OnLoad 方法回调
      bool was_successful = false;
      void* sym = library->FindSymbol("JNI_OnLoad", nullptr);
      if (sym == nullptr) {
        VLOG(jni) << "[No JNI_OnLoad found in \"" << path << "\"]";
        was_successful = true;
      } else {
        // Call JNI_OnLoad.  We have to override the current class
        // loader, which will always be "null" since the stuff at the
        // top of the stack is around Runtime.loadLibrary().  (See
        // the comments in the JNI FindClass function.)
        ScopedLocalRef<jobject> old_class_loader(env, env->NewLocalRef(self->GetClassLoaderOverride()));
        self->SetClassLoaderOverride(class_loader);
    
        VLOG(jni) << "[Calling JNI_OnLoad in \"" << path << "\"]";
        typedef int (*JNI_OnLoadFn)(JavaVM*, void*);
        JNI_OnLoadFn jni_on_load = reinterpret_cast<JNI_OnLoadFn>(sym);
        int version = (*jni_on_load)(this, nullptr);
    
        if (runtime_->GetTargetSdkVersion() != 0 && runtime_->GetTargetSdkVersion() <= 21) {
          // Make sure that sigchain owns SIGSEGV.
          EnsureFrontOfChain(SIGSEGV);
        }
    
        self->SetClassLoaderOverride(old_class_loader.get());
    
        if (version == JNI_ERR) {
          StringAppendF(error_msg, "JNI_ERR returned from JNI_OnLoad in \"%s\"", path.c_str());
        } else if (JavaVMExt::IsBadJniVersion(version)) {
          StringAppendF(error_msg, "Bad JNI version returned from JNI_OnLoad in \"%s\": %d",
                        path.c_str(), version);
          // It's unwise to call dlclose() here, but we can mark it
          // as bad and ensure that future load attempts will fail.
          // We don't know how far JNI_OnLoad got, so there could
          // be some partially-initialized stuff accessible through
          // newly-registered native method calls.  We could try to
          // unregister them, but that doesn't seem worthwhile.
        } else {
          was_successful = true;
        }
        VLOG(jni) << "[Returned " << (was_successful ? "successfully" : "failure")
                  << " from JNI_OnLoad in \"" << path << "\"]";
      }
    
      library->SetResult(was_successful);
      return was_successful;
    }

LoadNativeLibrary方法开始的时候会去缓存查看是否已经加载过动态库，如果已经加载过会判断上次加载的ClassLoader和这次加载的ClassLoader是否一致，如果不一致则加载失败，如果一致则返回上次加载的结果，换句话说就是**不允许不同的ClassLoader加载同一个动态库**。为什么这么做我们这里不进行分析。 上面的整体操作步骤如下：

1.  判断缓存中是否已经加载过这个library，如果加载过就检查下ClassLoader，直接返回；
2.  调用`android::OpenNativeLibrary()`方法加载library；
3.  如果上一步的加载动作的返回值handle为空指针，说明上面OpenNativeLibrary失败了，返回；
4.  记录加载成功的library，然后查找并调用library中的`JNI_OnLoad`回调方法。

总之这个LoadNativeLibrary方法目的就是利用的是OpenNativeLibrary这个函数去加载动态链接库，然后执行其中的`JNI_OnLoad`接口（这个函数是jni库的首选入口，可以利用它完成一些初始化工作，或者动态注册JNI方法）。

\[system/core/libnativeloader/native\_loader.cpp\]

void* OpenNativeLibrary(JNIEnv* env,
                            int32_t target_sdk_version,
                            const char* path,
                            jobject class_loader,
                            jstring library_path,
                            bool* needs_native_bridge,
                            std::string* error_msg) {
    #if defined(__ANDROID__)
      UNUSED(target_sdk_version);
      if (class_loader == nullptr) {
        *needs_native_bridge = false;
        return dlopen(path, RTLD_NOW);
      }
    
      std::lock_guard<std::mutex> guard(g_namespaces_mutex);
      NativeLoaderNamespace ns;
    
      if (!g_namespaces->FindNamespaceByClassLoader(env, class_loader, &ns)) {
        // This is the case where the classloader was not created by ApplicationLoaders
        // In this case we create an isolated not-shared namespace for it.
        if (!g_namespaces->Create(env,
                                  target_sdk_version,
                                  class_loader,
                                  false /* is_shared */,
                                  false /* is_for_vendor */,
                                  library_path,
                                  nullptr,
                                  &ns,
                                  error_msg)) {
          return nullptr;
        }
      }
    
      if (ns.is_android_namespace()) {
        android_dlextinfo extinfo;
        extinfo.flags = ANDROID_DLEXT_USE_NAMESPACE;
        extinfo.library_namespace = ns.get_android_ns();
    
        void* handle = android_dlopen_ext(path, RTLD_NOW, &extinfo);
        if (handle == nullptr) {
          *error_msg = dlerror();
        }
        *needs_native_bridge = false;
        return handle;
      } else {
        void* handle = NativeBridgeLoadLibraryExt(path, RTLD_NOW, ns.get_native_bridge_ns());
        if (handle == nullptr) {
          *error_msg = NativeBridgeGetError();
        }
        *needs_native_bridge = true;
        return handle;
      }
    #else
      UNUSED(env, target_sdk_version, class_loader, library_path);
      *needs_native_bridge = false;
      void* handle = dlopen(path, RTLD_NOW);
      if (handle == nullptr) {
        if (NativeBridgeIsSupported(path)) {
          *needs_native_bridge = true;
          handle = NativeBridgeLoadLibrary(path, RTLD_NOW);
          if (handle == nullptr) {
            *error_msg = NativeBridgeGetError();
          }
        } else {
          *needs_native_bridge = false;
          *error_msg = dlerror();
        }
      }
      return handle;
    #endif
    }

先利用`FindNamespaceByClassLoader`查找当前的ClassLoader是否有相关的Namespace，如果没有直接跳转到`android_dlopen_ext`；如果有调用其Create方法创建一个Namespace。

`android_dlopen_ext`跟`dlopen`类似，第一个参数是要打开的动态库的名称，第二个参数RTLD\_NOW，表示动态库中所有未定义的符号在`dlopen`返回前都会被解析。

接下来的实现，是调用find\_libary来查找动态库，找到后，调用dlsym来查找加载的动态库中是否包含JNI\_OnLoader入口函数。

## Linux 加载动态库的系统调用 ##

Android是基于Linux系统的，那么在Linux系统下是如何加载动态链接库的呢？Linux环境下加载动态库主要包括如下函数，位于头文件\#include

void *dlopen(const char *filename, int flag);   //打开动态链接库
    char *dlerror(void);                            //获取错误信息
    void *dlsym(void *handle, const char *symbol);  //获取方法指针
    int dlclose(void *handle);                      //关闭动态链接库

大家感兴趣的可以进一步Google，这里就不再深入到系统调用了。

看完这篇文章我们明确了几点：

1.  System.loadLibrary会优先查找apk中的so目录，再查找系统目录，系统目录包括：/vendor/lib(64)，/system/lib(64)
2.  不能使用不同的ClassLoader加载同一个动态库
3.  System.loadLibrary加载过程中会调用目标库的`JNI_OnLoad`方法，我们可以在动态库中加一个`JNI_OnLoad`方法用于动态注册
4.  如果加了`JNI_OnLoad`方法，其的返回值为JNI\_VERSION\_1\_2 ，JNI\_VERSION\_1\_4， JNI\_VERSION\_1\_6其一。我们一般使用JNI\_VERSION\_1\_4即可
5.  Android动态库的加载与Linux一致使用dlopen系列函数，通过动态库的句柄和函数名称来调用动态库的函数

## 参考资料 ##

*  [Android 动态链接库加载原理及 HotFix 方案介绍][Android _ HotFix]
 *  [深入理解 System.loadLibrary][System.loadLibrary]

[JNI]: http://47.98.205.211/2017/05/17/jni/
[Android _ HotFix]: https://cloud.tencent.com/developer/article/1071447
[System.loadLibrary]: https://pqpo.me/2017/05/31/system-loadlibrary/