Google的Ndk-Sample学习笔记之一(hello-jniCallback)

发布时间：2024年01月11日

前言:? 近段时间因为项目的需求,需要使用JNI,所以下载了Google的Ndk-Sample学习下,准备记录下来,留给后期自己查看

问题点一:JNI_OnLoad方法必须返回JNI的版本

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {
  JNIEnv *env;
  memset(&g_ctx, 0, sizeof(g_ctx));

  g_ctx.javaVM = vm;
  if ((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) != JNI_OK) {
    return JNI_ERR;  // JNI version not supported.
  }

  jclass clz =
      (*env)->FindClass(env, "com/example/hellojnicallback/JniHandler");
  g_ctx.jniHelperClz = (*env)->NewGlobalRef(env, clz);

  jmethodID jniHelperCtor =
      (*env)->GetMethodID(env, g_ctx.jniHelperClz, "<init>", "()V");
  jobject handler = (*env)->NewObject(env, g_ctx.jniHelperClz, jniHelperCtor);
  g_ctx.jniHelperObj = (*env)->NewGlobalRef(env, handler);
  queryRuntimeInfo(env, g_ctx.jniHelperObj);

  g_ctx.done = 0;
  g_ctx.mainActivityObj = NULL;
  return JNI_VERSION_1_6;
}

上面是hello-jniCallback项目中的原始文件hello-jnicallback.c中相关代码,为了慢慢学习知识点,我自己没有使用hello-jnicallback.c文件,而是自己创建了自己的学习文件,hello_jni.h 跟hello_jni.c.

根据原始sample的中的文件,大概代码可以简化为如下所示,返回0

jint JNI_OnLoad(JavaVM *vm, void *reserved)
{
    LOGI("enter JNI_OnLoad");
    JNIEnv* env;

    if ((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) != JNI_OK)
    {
        LOGE("Failed to get GetEnv");
        return JNI_ERR;
    }

    LOGI("leave JNI_OnLoad");
    return 0;
}

但是运行程序之后,程序发生了错误,通过LOG信息可以发现,错误提示是错误的Jni 版本

估计是这里返回值是要返回一个JNI的对应版本,尝试了下返回JNI_VERSIOn_1_2,JNI_VERSIOn_1_4,?JNI_VERSIOn_1_6都没有问题

jint JNI_OnLoad(JavaVM *vm, void *reserved)
{
    LOGI("enter JNI_OnLoad");
    JNIEnv* env;

    ......

    LOGI("leave JNI_OnLoad");
    return JNI_VERSION_1_2;
}

问题点二:JNI如何调用java中的方法

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {
  JNIEnv *env;
  memset(&g_ctx, 0, sizeof(g_ctx));

  g_ctx.javaVM = vm;
  if ((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) != JNI_OK) {
    return JNI_ERR;  // JNI version not supported.
  }

  jclass clz =
      (*env)->FindClass(env, "com/example/hellojnicallback/JniHandler");
  g_ctx.jniHelperClz = (*env)->NewGlobalRef(env, clz);

  jmethodID jniHelperCtor =
      (*env)->GetMethodID(env, g_ctx.jniHelperClz, "<init>", "()V");
  jobject handler = (*env)->NewObject(env, g_ctx.jniHelperClz, jniHelperCtor);
  g_ctx.jniHelperObj = (*env)->NewGlobalRef(env, handler);
  queryRuntimeInfo(env, g_ctx.jniHelperObj);

  g_ctx.done = 0;
  g_ctx.mainActivityObj = NULL;
  return JNI_VERSION_1_6;
}

void queryRuntimeInfo(JNIEnv *env, jobject instance) {
  // Find out which OS we are running on. It does not matter for this app
  // just to demo how to call static functions.
  // Our java JniHelper class id and instance are initialized when this
  // shared lib got loaded, we just directly use them
  //    static function does not need instance, so we just need to feed
  //    class and method id to JNI
  jmethodID versionFunc = (*env)->GetStaticMethodID(
      env, g_ctx.jniHelperClz, "getBuildVersion", "()Ljava/lang/String;");
  if (!versionFunc) {
    LOGE("Failed to retrieve getBuildVersion() methodID @ line %d", __LINE__);
    return;
  }
  jstring buildVersion =
      (*env)->CallStaticObjectMethod(env, g_ctx.jniHelperClz, versionFunc);
  const char *version = (*env)->GetStringUTFChars(env, buildVersion, NULL);
  if (!version) {
    LOGE("Unable to get version string @ line %d", __LINE__);
    return;
  }
  LOGI("Android Version - %s", version);
  (*env)->ReleaseStringUTFChars(env, buildVersion, version);

  // we are called from JNI_OnLoad, so got to release LocalRef to avoid leaking
  (*env)->DeleteLocalRef(env, buildVersion);

  // Query available memory size from a non-static public function
  // we need use an instance of JniHelper class to call JNI
  jmethodID memFunc = (*env)->GetMethodID(env, g_ctx.jniHelperClz,
                                          "getRuntimeMemorySize", "()J");
  if (!memFunc) {
    LOGE("Failed to retrieve getRuntimeMemorySize() methodID @ line %d",
         __LINE__);
    return;
  }
  jlong result = (*env)->CallLongMethod(env, instance, memFunc);
  LOGI("Runtime free memory size: %" PRId64, result);
  (void)result;  // silence the compiler warning
}

?java中的JniHandler的相关方法如下

public class JniHandler {

    private static final String TAG = "hello-jniCallback";

    /*
     * Print out status to logcat
     */
    @Keep
    private void updateStatus(String msg) {
        if (msg.toLowerCase().contains("error")) {
            Log.e("JniHandler", "Native Err: " + msg);
        } else {
            Log.i("JniHandler", "Native Msg: " + msg);
        }
    }

    /*
     * Return OS build version: a static function
     */
    @Keep
    static public String getBuildVersion() {
        return Build.VERSION.RELEASE;
    }

    /*
     * Return Java memory info
     */
    @Keep
    public long getRuntimeMemorySize() {
        return Runtime.getRuntime().freeMemory();
    }
}

Log信息如下:

通过上面的代码分析:这里有两种方法的调用,一种是CallStaicObjectMethod方法,另外一种是CalllongMethod方法,以为我之前接触过一点JNI的知识,了解JNI跟java的交互分为对象的调用跟类的调用

1.JNI如何调用java静态方法

经过代码分析,大概的调用流程如下所示

findClass
NewGlobalRef
GetMethodId
CallMeehod

jclass clz =
      (*env)->FindClass(env, "com/example/hellojnicallback/JniHandler");
  g_ctx.jniHelperClz = (*env)->NewGlobalRef(env, clz);

  jmethodID jniHelperCtor =
      (*env)->GetMethodID(env, g_ctx.jniHelperClz, "<init>", "()V");
  jobject handler = (*env)->NewObject(env, g_ctx.jniHelperClz, jniHelperCtor);
  g_ctx.jniHelperObj = (*env)->NewGlobalRef(env, handler);


jstring buildVersion =
      (*env)->CallStaticObjectMethod(env, g_ctx.jniHelperClz, versionFunc);
  const char *version = (*env)->GetStringUTFChars(env, buildVersion, NULL);
  if (!version) {
    LOGE("Unable to get version string @ line %d", __LINE__);
    return;
  }
  LOGI("Android Version - %s", version);
  (*env)->ReleaseStringUTFChars(env, buildVersion, version);

  // we are called from JNI_OnLoad, so got to release LocalRef to avoid leaking
  (*env)->DeleteLocalRef(env, buildVersion);

代码修改为如下

jint JNI_OnLoad(JavaVM *vm, void *reserved)
{
    LOGI("enter JNI_OnLoad");
    JNIEnv* env;

    if ((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) != JNI_OK)
    {
        LOGE("Failed to get GetEnv");
        return JNI_ERR;
    }

    LOGI("leave JNI_OnLoad");

    jclass clz = (*env)->FindClass(env, "com/example/hellojnicallback/JniHandler");
    jobject objRef = (*env)->NewGlobalRef(env, clz);

    jmethodID versionFun = (*env)->GetStaticMethodID(env, objRef, "getBuildVersion", "()Ljava/lang/String;");
    jstring buildVersion = (*env)->CallStaticObjectMethod(env, objRef, versionFun);
    const char* version = (*env)->GetStringUTFChars(env, buildVersion, NULL);
    if (version == NULL)
    {
        LOGE("Failed to get version");
        return JNI_ERR;
    }

    LOGI("java build version - %s", version);

    return JNI_VERSION_1_6;
}

经过测试,这样是没有问题的,

但是上面有一个NewGlobalRef的调用通过jni.h的查询发现,jclass跟jobject其实是同一个类型void*

准备测试下,如果不调用这个这个NewGlobalRef能不能正常运行,通过这个方法描述,这个好像只是一个全局的引用,修改为如下所示:

jint JNI_OnLoad(JavaVM *vm, void *reserved)
{
    LOGI("enter JNI_OnLoad");
    JNIEnv* env;

    if ((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) != JNI_OK)
    {
        LOGE("Failed to get GetEnv");
        return JNI_ERR;
    }

    LOGI("leave JNI_OnLoad");

    jclass clz = (*env)->FindClass(env, "com/example/hellojnicallback/JniHandler");
    jmethodID versionFun = (*env)->GetStaticMethodID(env, clz, "getBuildVersion", "()Ljava/lang/String;");
    jstring buildVersion = (*env)->CallStaticObjectMethod(env, clz, versionFun);
    const char* version = (*env)->GetStringUTFChars(env, buildVersion, NULL);
    if (version == NULL)
    {
        LOGE("Failed to get version");
        return JNI_ERR;
    }

    LOGI("java build version - %s", version);

    return JNI_VERSION_1_6;
}

经过测试发现,没有任何问题

总结:JNI调用java中的静态方法的流程是

FindClass
GetStaticMethodID
CallStaticobjectMethod

2.JNI如何调用java的对象方法

通过跟上面调用JNI调用静态方法的对比,猜测调用对象方法,应该是一样的

在?java中的JniHandler中添加一个新方法,如下所示

@Keep
    public String getBuildVersion2() {
        return "BuildVersion2";
    }

新的代码如下所示

但是发现报错了,错误提示信息如下

JVM object referenced by 'clz' is of type 'Class<JniHandler>' and it does not have access to method 'getBuildVersion2()' declared in class 'JniHandler'.?

猜测是类型不匹配,这里应该是一个jobject类型,而不是jclass类型?,在上面的步骤中,不是可以通过调用NewGlobalR来获取jobject类型吗,修改如下,

信心满满,这下应该没有问题吧,现实是残酷的,啪啪打脸.c代码发生crash了

?再对比原始的方法,发现有一段这样的代码调用

jmethodID jniHelperCtor =
      (*env)->GetMethodID(env, g_ctx.jniHelperClz, "<init>", "()V");
  jobject handler = (*env)->NewObject(env, g_ctx.jniHelperClz, jniHelperCtor);

应该需要通过先调用java的非静态方法,然后再根据生成的jmethodID去调用NewObject.开始编写代码.大概是如下所示

在编写过程中, 在传入参数,versionFun的时候, Android Studio报错了,提示

Not a constructor.??

不是一个构造期,难道需要跟java反射一样吗.需要搞一个构造器,再次去看了下原来的代码,发现有如下几行代码

jmethodID jniHelperCtor =
    (*env)->GetMethodID(env, g_ctx.jniHelperClz, "<init>", "()V");
jobject handler = (*env)->NewObject(env, g_ctx.jniHelperClz, jniHelperCtor);

根据名字猜测JnihelperCtor,这个难道是需要调用构造器吗,立马行动起来,修改如下

jint JNI_OnLoad(JavaVM *vm, void *reserved)
{
    LOGI("enter JNI_OnLoad");
    JNIEnv* env;

    if ((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) != JNI_OK)
    {
        LOGE("Failed to get GetEnv");
        return JNI_ERR;
    }

    LOGI("leave JNI_OnLoad");

    jclass clz = (*env)->FindClass(env, "com/example/hellojnicallback/JniHandler");
    jmethodID ctor = (*env)->GetMethodID(env, clz,  "<init>", "()V");
    jobject obj = (*env)->NewObject(env, clz, ctor);

    jmethodID versionFun = (*env)->GetMethodID(env, clz, "getBuildVersion2", "()Ljava/lang/String;");
    jstring buildVersion = (*env)->CallObjectMethod(env, obj, versionFun);
    const char* version = (*env)->GetStringUTFChars(env, buildVersion, NULL);
    if (version == NULL)
    {
        LOGE("Failed to get version");
        return JNI_ERR;
    }

    LOGI("java build version - %s", version);

    return JNI_VERSION_1_6;
}

?运行测试,发现可以了

考虑了下,调用流程应该跟java的反射一样,需要调用构造器函数,调用流程如下

FindClass
GeMethodID(必须是构造器方法)
NewObject
GetMethodID(你想调用的java非静态方法)?
CallObjectMethod