如何通过NDK使用c代码检查运行API级别的设备？

Question

我开始使用NDK在Android上工作,我想检查设备在我的c代码上运行的Android API级别.我怎样才能做到这一点？

起初我以为我可以使用__ANDROID_API__/android/api-level.h下的定义但这是一个错误的假设.

**注意:我不是问如何通过java检查API级别.

Answer 1

我刚刚研究了一些JNI代码,并希望查询Jona所描述的正在运行的操作系统构建版本.我希望尽早(即在JNI_OnLoad中)这样做,所以不要像FUBU所描述的那样从Java手中接收它.从API Level 4开始,这个信息已作为android.os.Build.VERSION中的int字段SDK_INT提供,这是我在这个片段中查找的内容:

static const char* TAG = "testjnjni";

static bool _disableHttpKeepAliveOnBuggyPlatforms(JNIEnv *env)
{
    // Based on article here:
    //   http://android-developers.blogspot.co.uk/2011/09/androids-http-clients.html
    // Which references the issue documented here:
    //   http://code.google.com/p/android/issues/detail?id=2939
    // We need to set "http.keepAlive" to "false" if running an OS version earlier than Froyo (API Level 8)

    if ((*env)->ExceptionCheck(env))
        return false; // already got an exception pending

    bool success = true;

    // VERSION is a nested class within android.os.Build (hence "$" rather than "/")
    jclass versionClass = (*env)->FindClass(env, "android/os/Build$VERSION");
    if (NULL == versionClass)
        success = false;

    jfieldID sdkIntFieldID = NULL;
    if (success)
        success = (NULL != (sdkIntFieldID = (*env)->GetStaticFieldID(env, versionClass, "SDK_INT", "I")));

    jint sdkInt = 0;
    if (success)
    {
        sdkInt = (*env)->GetStaticIntField(env, versionClass, sdkIntFieldID);
        __android_log_print(ANDROID_LOG_VERBOSE, TAG, "sdkInt = %d", sdkInt);
    }

    if (success && sdkInt < 8)
    {
        jclass systemClass = (*env)->FindClass(env, "java/lang/System");
        if (NULL == systemClass)
            success = false;

        jmethodID setPropertyMethodID = NULL;
        if (success)
            success = (NULL != (setPropertyMethodID = (*env)->GetStaticMethodID(env, systemClass, "setProperty", "(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;")));

        jstring propString = NULL;
        if (success)
            success = (NULL != (propString = (*env)->NewStringUTF(env, "http.keepAlive")));

        jstring valueString = NULL;
        if (success)
            success = (NULL != (valueString = (*env)->NewStringUTF(env, "false")));

        jobject oldValueString = NULL;
        if (success)
        {
            __android_log_print(ANDROID_LOG_VERBOSE, TAG, "Disabling http.keepAlive");
             oldValueString = (*env)->CallStaticObjectMethod(env, systemClass, setPropertyMethodID, propString, valueString);
        }

        // cleanup
        (*env)->DeleteLocalRef(env, propString);
        (*env)->DeleteLocalRef(env, valueString);
        (*env)->DeleteLocalRef(env, oldValueString);
        (*env)->DeleteLocalRef(env, systemClass);
    }

    // cleanup
    (*env)->DeleteLocalRef(env, versionClass);

    return success;
}

编写此代码所需的所有信息都清楚地记录在盛亮的PDF文档"The Java Native Interface:Programmer's Guide and Specification"中,以前可以在Oracle网站上找到,但也可以作为书籍购买(例如这里).JNI是一种非常强大的技术,我强烈建议任何想要掌握它的开发人员阅读PDF以及Android开发者的JNI技巧.

哦,最后,不能强调理解本地和全球参考是多么重要.Android的开发者博客在这里有一篇很好的文章,涵盖了ICS的变化(没有什么能够偏离JNI规范,但重点要重申!).