Android ART虚拟机_jniinvocation.getjniinvocation-优快云博客

本文对比了Android系统中ART和Dalvik两种虚拟机的特点。ART在应用安装时进行预编译，提高运行效率；Dalvik则在运行时进行即时编译。文章详细介绍了这两种虚拟机的工作原理及其对字节码的处理方式。

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Android ART虚拟机

2014-01-18 0 个评论来源：yangwen123的专栏

Android 4.4提供了一种与Dalvik截然不同的运行环境ART(Android runtime)支持,ART源于google收购的Flexycore的公司。ART模式与Dalvik模式最大的不同在于，启用ART模式后，系统在安装应用的时候会进行一次预编译，将字节码转换为机器语言存储在本地，这样在运行程序时就不会每次都进行一次编译了，执行效率也大大提升。

虚拟机切换设置

Settings> Developer Options> Select Runtime
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JniInvocation提供统一了接口：
libnativehelper\JniInvocation.cpp

jint JniInvocation::JNI_GetDefaultJavaVMInitArgs(void* vmargs) {
  return JNI_GetDefaultJavaVMInitArgs_(vmargs);
}

jint JniInvocation::JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
  return JNI_CreateJavaVM_(p_vm, p_env, vm_args);
}

jint JniInvocation::JNI_GetCreatedJavaVMs(JavaVM** vms, jsize size, jsize* vm_count) {
  return JNI_GetCreatedJavaVMs_(vms, size, vm_count);
}

该函数间接调用以下函数来实现：

extern "C" jint JNI_GetDefaultJavaVMInitArgs(void* vm_args) {
  return JniInvocation::GetJniInvocation().JNI_GetDefaultJavaVMInitArgs(vm_args);
}

extern "C" jint JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
  return JniInvocation::GetJniInvocation().JNI_CreateJavaVM(p_vm, p_env, vm_args);
}

extern "C" jint JNI_GetCreatedJavaVMs(JavaVM** vms, jsize size, jsize* vm_count) {
  return JniInvocation::GetJniInvocation().JNI_GetCreatedJavaVMs(vms, size, vm_count);
}

libnativehelper\JniInvocation.h

static JniInvocation* jni_invocation_;
void* handle_;//保存so库的首地址
jint (*JNI_GetDefaultJavaVMInitArgs_)(void*);
jint (*JNI_CreateJavaVM_)(JavaVM**, JNIEnv**, void*);
jint (*JNI_GetCreatedJavaVMs_)(JavaVM**, jsize, jsize*);

因此JniInvocation对外提供的接口函数是由JniInvocation中定义的函数指针来实现。JniInvocation中定义的函数指针是在哪里初始化的呢？我们知道 Android虚拟机是在Zygote进程中通过调用AndroidRuntime::start()函数启动的： frameworks\base\core\jni\AndroidRuntime.cpp

void AndroidRuntime::start(const char* className, const char* options)
{
   ...
    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env) != 0) {
        return;
    }
    onVmCreated(env);
	...
}

在启动虚拟机前通过调用JniInvocation::Init()函数来初始化JniInvocation中声明的函数指针

bool JniInvocation::Init(const char* library) {
#ifdef HAVE_ANDROID_OS
  char default_library[PROPERTY_VALUE_MAX];
  property_get("persist.sys.dalvik.vm.lib", default_library, "libdvm.so");
#else
  const char* default_library = "libdvm.so";
#endif
  if (library == NULL) {
    library = default_library;
  }

  handle_ = dlopen(library, RTLD_NOW);
  if (handle_ == NULL) {
    ALOGE("Failed to dlopen %s: %s", library, dlerror());
    return false;
  }
  //查找JNI_GetDefaultJavaVMInitArgs函数指针
  if (!FindSymbol(reinterpret_cast(&JNI_GetDefaultJavaVMInitArgs_),
                  "JNI_GetDefaultJavaVMInitArgs")) {
    return false;
  }
  //查找JNI_CreateJavaVM函数指针
  if (!FindSymbol(reinterpret_cast(&JNI_CreateJavaVM_),
                  "JNI_CreateJavaVM")) {
    return false;
  }
  //查找JNI_GetCreatedJavaVMs函数指针
  if (!FindSymbol(reinterpret_cast(&JNI_GetCreatedJavaVMs_),
                  "JNI_GetCreatedJavaVMs")) {
    return false;
  }
  return true;
}

该函数就是根据persist.sys.dalvik.vm.lib属性值来加载不同的虚拟机动态库，并从动态库中查找JNI_GetDefaultJavaVMInitArgs，JNI_CreateJavaVM，JNI_GetCreatedJavaVMs函数首地址

bool JniInvocation::FindSymbol(void** pointer, const char* symbol) {
  *pointer = dlsym(handle_, symbol);
  if (*pointer == NULL) {
    ALOGE("Failed to find symbol %s: %s\n", symbol, dlerror());
    dlclose(handle_);
    handle_ = NULL;
    return false;
  }
  return true;
}