Android JNI - 字符串作为随机数返回

Question

我是 JNI 和 Android 的新手。我想从本机代码返回一个字符串值，但它在执行时返回一些随机数。我无法理解我做错了什么。

我的“ndk.c”文件：

int Java_com_example_mayankvijh_test_1app_MainActivity_main(){
return 3;
}

jstring JNICALL Java_com_example_mayankvijh_test_1app_MainActivity_getMessage(JNIEnv *env, jobject thiz) {
  return (*env)->NewStringUTF(env,"HELLO");
  }

我的 MainActivity.java

 protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    try{
        System.loadLibrary("wrapper");

    }catch(Exception e)
    {
        e.printStackTrace();
    }

    System.out.println("main return =" + main());
    System.out.println("Hi " + getMessage());
}

public native int main();
public native int getMessage();

我已成功加载库（“.so”），我能够返回类型 jint 并显示在应用程序中，但不知何故 jstring 返回随机数。

我的原生 main 函数正在返回 int 并且正在工作，但是对于我的 getMessage() 函数，字符串没有返回

getMessage() 的输出：

Hi -429916127

我的应用程序.mk

APP_ABI := armeabi-v7a
APP_PLATFORM := android-9

我的 Android.mk

LOCAL_PATH:=$(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE := wrapper

LOCAL_C_INCLUDES := $(LOCAL_PATH)

LOCAL_SRC_FILES := ndk.c

LOCAL_LDLIBS := -landroid
#--llog -lz

include $(BUILD_SHARED_LIBRARY)

感谢您的帮助。

Answer 1

您有冲突的返回类型：

jstring JNICALL Java_com_example_mayankvijh_test_1app_MainActivity_getMessage(JNIEnv *env, jobject thiz) {
  return (*env)->NewStringUTF(env,"HELLO");
  }

和

public native int getMessage();

指针，在本机代码中返回，被视为整数。您应该将 java 中的声明修复为：

public native String getMessage();

Answer 2

Java 端：

  public class HelloJni extends Activity{

 @Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);  
    TextView  tv = new TextView(this);
    tv.setText( stringFromJNI() );
    setContentView(tv);
}

public native String  stringFromJNI();
public native String  unimplementedStringFromJNI();  
static {
    System.loadLibrary("hello-jni");
  }
 }

Native Side：文件名hello-jni.c

#include <string.h>
#include <jni.h>

/* This is a trivial JNI example where we use a native method
 * to return a new VM String. See the corresponding Java source
 * file located at:
 *
 *   apps/samples/hello-jni/project/src/com/example/hellojni/HelloJni.java
 */
jstring
Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,
                                                  jobject thiz )
{
#if defined(__arm__)
  #if defined(__ARM_ARCH_7A__)
    #if defined(__ARM_NEON__)
      #if defined(__ARM_PCS_VFP)
        #define ABI "armeabi-v7a/NEON (hard-float)"
      #else
        #define ABI "armeabi-v7a/NEON"
      #endif
    #else
      #if defined(__ARM_PCS_VFP)
        #define ABI "armeabi-v7a (hard-float)"
      #else
        #define ABI "armeabi-v7a"
      #endif
    #endif
  #else
   #define ABI "armeabi"
  #endif
#elif defined(__i386__)
   #define ABI "x86"
#elif defined(__x86_64__)
   #define ABI "x86_64"
#elif defined(__mips64)  /* mips64el-* toolchain defines __mips__ too */
   #define ABI "mips64"
#elif defined(__mips__)
   #define ABI "mips"
#elif defined(__aarch64__)
   #define ABI "arm64-v8a"
#else
   #define ABI "unknown"
#endif

    return (*env)->NewStringUTF(env, "Hello from JNI !  Compiled with ABI " ABI ".");
}

更新：

jstring JNICALL Java_com_example_mayankvijh_test_1app_MainActivity_getMessage(JNIEnv *env, jobject thiz) {
return  (*env)->NewStringUTF(env,"HELLO");
}