[移动开发]Android9.0动态链接库加载链接完全解读(1)--System.loadLibrary到dlopen

什么是Android Linker

Android 的加载/链接器linker 主要用于实现共享库的加载与链接。

从 System.loadLibrary 追溯

jni 开发的 so load

平时我们进行JNI开发 要链接我们的so库只需要执行System.loadLibrary函数即可

static {
        try {
            CLogUtils.e("开始加载 Test so文件 ");
            System.loadLibrary("Test");
            init();
        } catch (Throwable e) {
            CLogUtils.e("加载So出现异常 " + e.toString());
            e.printStackTrace();
        }
    }

或许你没有想过 为什么这样就可以将so加载进来了。

我们继续跟进代码

本次采用SDK28的代码 也就是安卓9.0的代码分析

System.loadLibrary("Test");->Runtime loadLibrary0

loadLibrary0方法跟进

...
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 = nativeLoad(candidate, loader);
                if (error == null) {
                    return; // We successfully loaded the library. Job done.
                }
                lastError = error;
            }
        }
 ...

走到了 nativeLoad 方法

nativeLoad

private static native String nativeLoad(String filename, ClassLoader loader);显然是一个Native方法

我们要找到他对应的cpp代码。通过查找资料得知他的文件路径在

libcore/ojluni/src/main/native/Runtime.c

打开源码阅读网站分析:
runtime.c

核心的代码部分

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

转到了 JVM_NativeLoad(env, javaFilename, javaLoader);方法中

JVM_NativeLoad

这个方法在libcore/ojluni/src/main/native/jvm.h中

jvm.h

方法申明

JNIEXPORT jstring JVM_NativeLoad(JNIEnv* env, jstring javaFilename,
                                 jobject javaLoader);

具体的方法实现要看谁实现这个方法申明

art/openjdkjvm/OpenjdkJvm.cc

art/openjdkjvm/OpenjdkJvm.cc

JNIEXPORT jstring JVM_NativeLoad(JNIEnv* env,
                                 jstring javaFilename,
                                 jobject javaLoader) {
  ScopedUtfChars filename(env, javaFilename);
  if (filename.c_str() == NULL) {
    return NULL;
  }

  std::string error_msg;
  {
    art::JavaVMExt* vm = art::Runtime::Current()->GetJavaVM();
    bool success = vm->LoadNativeLibrary(env,
                                         filename.c_str(),
                                         javaLoader,
                                         &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());
}

转到了LoadNativeLibrary(env, filename.c_str(),javaLoader,&error_msg);方法里面

这个方法来自 JavaVMExt

LoadNativeLibrary

源码文件路径 art/runtime/java_vm_ext.cc

这个方法很长 我们具体来分析 根据注释来看

bool JavaVMExt::LoadNativeLibrary(JNIEnv* env,
                                  const std::string& path,
                                  jobject class_loader,
                                  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_);
    library = libraries_->Get(path);
  }
  
  // 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.

  // Retrieve the library path from the classloader, if necessary.
  ScopedLocalRef<jstring> library_path(env, GetLibrarySearchPath(env, class_loader));

  Locks::mutator_lock_->AssertNotHeld(self);
  const char* path_str = path.empty() ? nullptr : path.c_str();
  bool needs_native_bridge = false;
  void* handle = android::OpenNativeLibrary(env,
                                            runtime_->GetTargetSdkVersion(),
                                            path_str,
                                            class_loader,
                                            library_path.get(),
                                            &needs_native_bridge,
                                            error_msg);

  VLOG(jni) << "[Call to dlopen(\"" << path << "\", RTLD_NOW) returned " << handle << "]";

  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();
      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 << "]";

  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;
}

整个流程分为判断 是否已经加载-> 寻找-> 加载-> 检查

最核心的部分就是 OpenNativeLibrary部分的代码 也就是dlopen 打开so

检查 部分 对so路径找到的so文件进行检查 JNI_load 函数检查

FindSymbol("JNI_OnLoad", nullptr);

如果不存在直接返回成功
如果存在判断返回的version版本

bool JavaVMExt::IsBadJniVersion(int version) {
  // We don't support JNI_VERSION_1_1. These are the only other valid versions.
  return version != JNI_VERSION_1_2 && version != JNI_VERSION_1_4 && version != JNI_VERSION_1_6;
}

只有这几个版本是正确的版本

总结

这是第一部分的so加载链接部分 随后进入到linker部分开始解析elf文件。