Question

我是android JNI的新手并尝试使用以下代码：

套餐名称： com.example.jnitest

Java类：

public class MainActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    MainActivity mainActivity = new MainActivity();


    System.out.println("Sum of two variables : "+ sumOfTwovariable((int)mainActivity.multiply(10, 25), 10));
}

public native long sumOfTwovariable(int v1, int v2);

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

public native long multiply(int v1, int v2);
static{
    System.loadLibrary("com_example_pdemo_MainActivity");
}

}

Android.mk文件

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := JNITest
LOCAL_SRC_FILES := JNITest.cpp
LOCAL_SHARED_LIBRARIES := Prebuild_com_example_pdemo_MainActivity
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/includes
LOCAL_STATIC_LIBRARIES := Prebuild_com_example_pdemo_MainActivity
include $(BUILD_SHARED_LIBRARY)


include $(CLEAR_VARS)
LOCAL_MODULE    := Prebuild_com_example_pdemo_MainActivity
LOCAL_SRC_FILES :=    $(TARGET_ARCH_ABI)/libcom_example_pdemo_MainActivity.so

include $(PREBUILT_SHARED_LIBRARY)

我已预先建立了com_example_pdemo_MainActivity.so lib，它位于JNI-＆gt;内。 armeabi（写多个函数）文件夹包含在Android.mk文件中

JNI内部：

使用以下命令创建头文件：javah -jni -classpath ....

Header file created using javah : com_example_jnitest_MainActivity.h

  /* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_example_jnitest_MainActivity */

#ifndef _Included_com_example_jnitest_MainActivity
#define _Included_com_example_jnitest_MainActivity
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_example_jnitest_MainActivity
  * Method:    sumOfTwovariable
  * Signature: (II)J
*/
JNIEXPORT jlong JNICALL    Java_com_example_jnitest_MainActivity_sumOfTwovariable(JNIEnv *, jobject, jint, jint);

 #ifdef __cplusplus
 }
#endif

#endif

本机函数的定义：JNITest.cpp

  #include "com_example_jnitest_MainActivity.h"

extern "C" {
long multiply(int val1,int val2);
}

   JNIEXPORT jlong JNICALL      Java_com_example_jnitest_MainActivity_sumOfTwovariable(
    JNIEnv *env, jobject obj, jint val1, jint val2) {
int val=multiply(val1,val2);
return (val+val1);

}

当我编译它时，我得到了'multiply'的未定义引用。让我知道我做了什么错误。

提前致谢..

Answer 1

multiply也是本机方法。它应该定义为与sumOfTwo变量相同：

JNIEXPORT jlong JNICALL    Java_com_example_jnitest_MainActivity_multiply(JNIEnv *, jobject, jint, jint);

Answer 2

在com_example_pdemo_MainActivity.so的本机代码中，定义乘法如下：

// this can be called from native code, including in different .so files:
long multiply(int val1,int val2)
{
    long result;
    // ...compute result...
    return result;
}

// this can be called from Java via JNI:
JNIEXPORT jlong JNICALL Java_com_example_pdemo_MainActivity_multiply(JNIEnv *env, jobject obj, jint a, jint b)
{
     return multiply(a, b);
}

这将允许您从本机代码或通过JNI调用它。

将您的Android.mk文件更改为此文件，首先放置预先构建的.so文件：

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := Prebuild_com_example_pdemo_MainActivity
LOCAL_SRC_FILES :=    $(TARGET_ARCH_ABI)/libcom_example_pdemo_MainActivity.so
include $(PREBUILT_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE    := JNITest
LOCAL_SRC_FILES := JNITest.cpp
LOCAL_SHARED_LIBRARIES := Prebuild_com_example_pdemo_MainActivity
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/includes
include $(BUILD_SHARED_LIBRARY)

最后，在Java代码中，按顺序将两个库加载到单个静态块中：

static{
    System.loadLibrary("com_example_pdemo_MainActivity");
    System.loadLibrary("JNITest");
}

JNITest需要com_example_pdemo_MainActivity所以它需要先行，否则你会遇到运行时链接错误。

此外，在JNITest.cpp中，删除围绕multiply（）声明的extern "C"。如果您使用C ++链接编译了com_example_pdemo_MainActivity.so文件，则在尝试与JNITest链接时，使用C extern将导致链接错误：

// extern "C"{ <-- remove
    long multiply(int v1,int v2);
//} <-- remove

如何在字节缓冲区中将数据传入和传出JNI：

JNIEXPORT int JNICALL Java_com_example_jnitest_MainActivity_compressImage(JNIEnv *pEnv, jobject this, jobject inputByteBuffer, jobject outputByteBuffer)
{
    jbyte* pInBuf = (jbyte*)(*pEnv)->GetDirectBufferAddress(pEnv, inputByteBuffer);
    jbyte* pOutBuf = (jbyte*)(*pEnv)->GetDirectBufferAddress(pEnv, outputByteBuffer);

    int lenOutImage = 0; char *pOutputImage;
    pOutputImage = CompressImage((char *)pInBuf,&lenOutImage);

    // copy to byte buffer
    memcpy((char)pOutBuf, pOutputImage, lenOutImage);        
    free(pOutputImage); // free pOutputImage?

    return lenOutImage;
}

在android中使用JNI的'multiply'未定义引用

#endif

2 个答案: