System.loadLibrary源码分析

2019-07-13 本文已影响1人拔萝卜占坑

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<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;
          }
      }
      ...
  }

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<File> 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<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*);
    // 强转
    JNI_OnLoadFn jni_on_load = reinterpret_cast<JNI_OnLoadFn>(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。

System.loadLibrary源码分析

简介

系统源码

System.loadLibrary

结束

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