Android Chromium：不成功的尝试，无法从Typeface.java类获得C++ SkTypeface对象

这也就是说，Java Activity层的用户自定义默认字体通过标准framework/JNI的途径是无法传递应用到Chromium内核的。

本来的基本想法是：通过Typeface.DEFAULT获得Java层的系统当前默认字体设置，通过Java动态反射调用获得native_instance句柄，然后尝试用JNI C++代码获得SkTypeface*指针，调用SkTypeface::serialize序列化到一个临时字体文件，然后将此路径回传Java，再重新传到Chromium内核代码。

帖2段核心demo代码：

AwSettings.java:

//#if ENABLE_USER_OVERRIDES_FONTFALLBACK
    //hacker techniques to get current system's default typeface and apply it to chromium/skia kernel code:
    @CalledByNative
    private static long getDefaultTypefaceNativePointer() {
        try{
            android.graphics.Typeface defaultTypeface = android.graphics.Typeface.DEFAULT;
            java.lang.reflect.Field field = null;
            field = android.graphics.Typeface.class.getDeclaredField("native_instance");
            long ret = 0;
            if (android.os.Build.VERSION.SDK_INT>=21) {
                //5.0+
                ret = field.getLong(defaultTypeface);
            }else{
                //4.4-
                ret = field.getInt(defaultTypeface);//boost to long
            }
            Log.i(TAG, "android.graphics.Typeface.DEFAULT.native_instance="+ret);
            return ret;
        }catch(Exception e){
            Log.e(TAG, e.getMessage(), e);
            return 0;
        }
    }
    //private boolean sUserOverridesFontFallbackEnabled = false;
    private static native String nativeGetTempUserOverridesFontFallbackPath();
    public void setUserOverridesFontFallbackEnabled(boolean enable) {
        if (enable) {
            String path = AwSettings.nativeGetTempUserOverridesFontFallbackPath();
            setUserOverridesFontFallback(true, path);
        }else{
            setUserOverridesFontFallback(false, "");
        }
    }
 //#if

aw_settings.cc:

#if ENABLE_USER_OVERRIDES_FONTFALLBACK
static jstring GetTempUserOverridesFontFallbackPath(JNIEnv* env, jclass clazz) {
    VLOG(0)<<"GetTempUserOverridesFontFallbackPath enter";
    jlong native_instance_pointer = Java_AwSettings_getDefaultTypefaceNativePointer(env);
    if(native_instance_pointer==0){
      VLOG(0)<<"GetTempUserOverridesFontFallbackPath getDefaultTypefaceNativePointer failed!";
      return 0;
    }
    //SkTypeface* typeface = (SkTypeface*)(int)native_instance_pointer;
    //  //ALERT: this C++ SkTypeface is got from framework layer, it may not have the same ABI compatibility with chromioum's internal skia library
    TypefaceImpl* typefaceImpl = (TypefaceImpl*)native_instance_pointer;
    SkTypeface* typeface = 0; //TODO: cannot get SkTypeface* from TypefaceImpl*

    //i need a writable path!
    FilePath cache_dir_filepath;
    /*bool ret = */base::android::GetCacheDirectory(&cache_dir_filepath);
    FilePath temp_filepath = cache_dir_filepath.Append( FilePath("test.ttf.ser") ); //<-- TODO: generate this
    std::string gen_temp_path = temp_filepath.AsUTF8Unsafe();

    const char* temp_path = gen_temp_path.c_str(); //"/data/local/test.ttf.ser";
    VLOG(0)<<"GetTempUserOverridesFontFallbackPath gen_temp_path="<<temp_path;
    SkFILEWStream wstream(temp_path);
    typeface->serialize(&wstream);//may fail, but we don't know!

    ScopedJavaLocalRef<jstring> jpath = ConvertUTF8ToJavaString(env, temp_path);
    return jpath.Release();
  }
#endif

遗憾的是，native_instance实际上对应于不透明的TypefaceImpl*，在JNI下可能也是无法访问的。即使能访问，从TypefaceImpl*也得不到SkTypeface*。