System.loadLibrary源码分析

简介

现在越来越多的项目都会使用第三方so库，提交so库，那么自然想到的就是so库的导入，很多在使用so库的时候莫名会出现很多问题，不管是导库的过程，还是使用so库方法时。如果你对so使用一无所知，那么接下来的几篇文章将对你很有帮助，我会从浅到深将整个过程讲解清楚。

系统源码

Android8.0

System.loadLibrary

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

• 类加载器分类

  1. BootClassLoader，Android系统启动时加载系统相关的类文件。
  2. PathClassLoader，加载已经安装应用类，也可以加载/vendor/lib, /system/lib下的so库。
  3. DexClassLoader，加载dex文件。

• VMStack.getCallingClassLoader()
  这里返回的是应用的类加载器。我们在Activity里面打印：

   Log.e("loadLibrary"," class_loader = ${this.classLoader}" )
  

  打印结果：

   class_loader = dalvik.system.PathClassLoader[DexPathList[[zip file"/data/app/blog.pds.com.socket-koKxbY-cMvLU6BAkp-notg==/base.apk"],
   nativeLibraryDirectories=[/data/app/blog.pds.com.socket-koKxbY-cMvLU6BAkp-notg==/lib/arm64, /system/lib64, /vendor/lib64]]]
  

  so库目录

   nativeLibraryDirectories=[/data/app/blog.pds.com.socket-koKxbY-cMvLU6BAkp-notg==/lib/arm64, /system/lib64, /vendor/lib64]]]
  

这里可以看出VMStack.getCallingClassLoader()返回的是PathClassLoader加载器对象。

• loadLibrary0方法

      synchronized void loadLibrary0(ClassLoader loader, String libname) {
        ...
        String libraryName = libname;
        if (loader != null) {
            String filename = loader.findLibrary(libraryName);
            ...
            String error = nativeLoad(filename, loader);
            if (error != null) {
                throw new UnsatisfiedLinkError(error);
            }
            return;
        }
  
        String filename = System.mapLibraryName(libraryName);
        List candidates = new ArrayList();
        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;
            }
        }
        ...
    }
  

  loader一般都不会null，所以这里只分析loader != null的情况。来看一下loader.findLibrary方法。定位到dalvik.system.BaseDexClassLoader类(请下载Android源码查看该类)。

    @Override
    public String findLibrary(String name) {
        return pathList.findLibrary(name);
    }
  

  我们来看一下pathList的值是什么，查看BaseDexClassLoader的toString方法。

  @Override public String toString() {
      return getClass().getName() + "[" + pathList + "]";
    }
  

  可以看出pathList就是上面打印日志[]里面的内容。pathList是一个DexPathList对象。
  看一下DexPathList中的findLibrary方法

      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;
  }
  

  获取so库完整名字

  String fileName = System.mapLibraryName(libraryName)
  

  定位到/libcore/ojluni/src/main/native/System.c文件

  System_mapLibraryName(JNIEnv *env, jclass ign, jstring libname)
  {
      int len;
      int prefix_len = (int) strlen(JNI_LIB_PREFIX);
      int suffix_len = (int) strlen(JNI_LIB_SUFFIX);
  
      jchar chars[256];
      if (libname == NULL) {
          JNU_ThrowNullPointerException(env, 0);
          return NULL;
      }
      len = (*env)->GetStringLength(env, libname);
      if (len > 240) {
          JNU_ThrowIllegalArgumentException(env, "name too long");
          return NULL;
      }
      cpchars(chars, JNI_LIB_PREFIX, prefix_len);
      (*env)->GetStringRegion(env, libname, 0, len, chars + prefix_len);
      len += prefix_len;
      cpchars(chars + len, JNI_LIB_SUFFIX, suffix_len);
      len += suffix_len;
      return (*env)->NewString(env, chars, len);
  }
  

  可以看出 库的名字不能超过240个字符。看一下常量字符定义

  #define JNI_LIB_PREFIX "lib"
  #ifdef __APPLE__
  #define JNI_LIB_SUFFIX ".dylib"
  #define VERSIONED_JNI_LIB_NAME(NAME, VERSION) JNI_LIB_PREFIX     NAME "." VERSION JNI_LIB_SUFFIX
  #else
  #define JNI_LIB_SUFFIX ".so"
  

  APPLE是苹果平台的全集，所以JNI_LIB_SUFFIX定位为.so。cpchars(chars, JNI_LIB_PREFIX, prefix_len)和cpchars(chars + len, JNI_LIB_SUFFIX, suffix_len)分别给给我们传入的库名字加上前缀"lib"和后缀".so"形成了完整的so库名字。

  看一下nativeLibraryPathElements变量初始化

  // 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 allNativeLibraryDirectories = new ArrayList<(nativeLibraryDirectories);
  allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);
  

  从上面代码可以看出nativeLibraryPathElements包含了应用程序库库路径和系统库路径。
  打印一下系统库路径：

  Log.e("loadLibrary"," system lib path =${System.getProperty("java.library.path")}" )
  

  打印结果：

  system lib path = /system/lib64:/vendor/lib64
  

  找so库路径就分析到这里，如果找不到或者文件不能打开，者会报异常。

• nativeLoad(filename, loader);
  加载so库，我们想要调用so库里的方法，那么必须先加载so库文件。
  定位到"/libcore/ojluni/src/main/native/Runtime.c"文件.

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

  定位到/art/runtime/openjdkjvm/OpenjdkJvm.cc文件，找到JVM_NativeLoad方法：

  JNIEXPORT jstring JVM_NativeLoad(JNIEnv* env,
                                jstring javaFilename,
                                jobject javaLoader,
                                jstring javaLibrarySearchPath) {
   ...
   std::string error_msg;
   {
     art::JavaVMExt* vm = art::Runtime::Current()->GetJavaVM();
     // 加载so库
     bool success = vm->LoadNativeLibrary(env,
                                        filename.c_str(),
                                        javaLoader,
                                        javaLibrarySearchPath,
                                        &error_msg);
     if (success) {
       return nullptr;
     }
   }
  
   ...
  
  }
  

  定位到/art/runtime/java_vm_ext.cc文件找到LoadNativeLibrary方法

  bool JavaVMExt::LoadNativeLibrary(JNIEnv* env,
                                  const std::string& path,
                                  jobject class_loader,
                                  jstring library_path,
                                  std::string* error_msg) {
    ...
    void* handle = android::OpenNativeLibrary(env,
                                            runtime_->GetTargetSdkVersion(),
                                            path_str,
                                            class_loader,
                                            library_path,
                                            &needs_native_bridge,
                                            error_msg);
    ...
    // 找到so库里面的JNI_OnLoad方法指针
    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 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*);
      // 强转
      JNI_OnLoadFn jni_on_load = reinterpret_cast(sym);
     // 调用so库里面的JNI_OnLoad方法，所以动态注册可以在这里面完成
      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());
      // 版本检测，一会在JNI_OnLoad里面进行动态注册的时候需要返回一个int数字，返回的依据就是根据这里来的
      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);
      } else {
        was_successful = true;
      }
      ...
    }
  }
  

结束

到此System.loadLibrary就分析到这里，想要了解JNI动态注册的可以直接看下一篇文章。动态注册JNI方法