调用移植库的本机函数

Question

我一直在关注如何使用Android NDK移植LAME库. http://blog.libertadtech.com/2011/02/porting-lame-encoder-to-android-arm.html

每个工作正常,我得到了libmp3lame.so文件.

现在进行第一次测试我想调用简单的函数:

 void get_lame_version(char *strbuf, size_t buflen, const char *prefix);

我为LAME库编写了一个包装器:

package de.goddchen.android.youtubeconverter;

public class LameWrapper {

public static native void get_1lame_1version(char[] buf, int len,
        char[] prefix);
}

我这样称呼它:

public class MainActivity extends Activity {

static {
    System.loadLibrary("mp3lame");
}

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        char[] buffer = new char[100];
        LameWrapper.get_1lame_1version(buffer, 100, "".toCharArray());
        ((TextView) findViewById(R.id.text)).setText(buffer.toString());
    }

}

但是我收到以下错误:

05-05 10:36:11.973: DEBUG/dalvikvm(199): Trying to load lib /data/data/de.goddchen.android.youtubeconverter/lib/libmp3lame.so 0x43757f78
05-05 10:36:12.053: DEBUG/dalvikvm(199): Added shared lib /data/data/de.goddchen.android.youtubeconverter/lib/libmp3lame.so 0x43757f78
05-05 10:36:12.123: DEBUG/dalvikvm(199): No JNI_OnLoad found in /data/data/de.goddchen.android.youtubeconverter/lib/libmp3lame.so 0x43757f78
05-05 10:36:12.573: DEBUG/dalvikvm(199): +++ not scanning '/system/lib/libwebcore.so' for 'get_1lame_1version' (wrong CL)
05-05 10:36:12.693: DEBUG/dalvikvm(199): +++ not scanning '/system/lib/libexif.so' for 'get_1lame_1version' (wrong CL)
05-05 10:36:12.693: DEBUG/dalvikvm(199): +++ not scanning '/system/lib/libFFTEm.so' for 'get_1lame_1version' (wrong CL)
05-05 10:36:12.703: DEBUG/dalvikvm(199): +++ not scanning '/system/lib/libmedia_jni.so' for 'get_1lame_1version' (wrong CL)
05-05 10:36:12.703: DEBUG/dalvikvm(199): +++ not scanning '/system/lib/libsrec_jni.so' for 'get_1lame_1version' (wrong CL)
05-05 10:36:12.703: WARN/dalvikvm(199): No implementation found for native Lde/goddchen/android/youtubeconverter/LameWrapper;.get_1lame_1version ([CI[C)V
05-05 10:36:12.703: DEBUG/AndroidRuntime(199): Shutting down VM
05-05 10:36:12.753: INFO/ActivityManager(52): Start proc com.android.alarmclock for broadcast com.android.alarmclock/.AlarmInitReceiver: pid=213 uid=10017 gids={}
05-05 10:36:12.773: WARN/dalvikvm(199): threadid=3: thread exiting with uncaught exception (group=0x4001aa28)
05-05 10:36:12.783: ERROR/AndroidRuntime(199): Uncaught handler: thread main exiting due to uncaught exception
05-05 10:36:12.883: ERROR/AndroidRuntime(199): java.lang.UnsatisfiedLinkError: get_1lame_1version
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at de.goddchen.android.youtubeconverter.LameWrapper.get_1lame_1version(Native Method)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at de.goddchen.android.youtubeconverter.MainActivity.onCreate(MainActivity.java:19)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2364)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2417)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at android.app.ActivityThread.access$2100(ActivityThread.java:116)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at android.os.Looper.loop(Looper.java:123)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at android.app.ActivityThread.main(ActivityThread.java:4203)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at java.lang.reflect.Method.invokeNative(Native Method)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at java.lang.reflect.Method.invoke(Method.java:521)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:549)
05-05 10:36:12.883: ERROR/AndroidRuntime(199):     at dalvik.system.NativeStart.main(Native Method)

有人可以指出我正确的方式来调用libmp3lame.so的功能吗？

Answer 1

遗憾的是,你不能只用Java调用任何旧的C函数.您必须使用具有"损坏"名称的函数或使用JNI_OnLoad机制.无论哪种方式,Dalvik VM都会为常规lame get_version_number函数不期望的函数调用添加额外的参数.

这个使用JNI的例子解释了名称修改.对于Java类,您需要具有此原型的函数:

void Java_de_goddchen_android_youtubeconverter_LameWrapper_get_11lame_11version
  (JNIEnv *, jclass, jcharArray, jint, jcharArray);

JNIEnv和jclass参数是调用接口的一部分.请记住指定extern "C"您是否使用C++.javah工具将为您生成正确的头文件.

如果我这样做,我可能会为我需要的蹩脚电话编写JNI C包装函数.在这种特殊情况下,不是使用char *带有长度参数的a ,而是将空char *作为最后一个参数传递,我将创建一个更简单的Java to C接口,它只返回一个Java String.在Java中:

package de.goddchen.android.youtubeconverter;

public class LameWrapper {
    public static native String getVersion();
}

然后在C中调用实际的lame API:

JNIEXPORT jstring JNICALL Java_de_goddchen_android_youtubeconverter_LameWrapper_getVersion
  (JNIEnv * env, jclass c)
{
    char buffer[80];
    get_lame_version(buffer, sizeof(buffer) - 1, "");
    return (*env)->NewStringUTF(env, buffer));
}

然后,在Java/Dalvik端控制此处创建的新String的生命周期.