调用 C++ 流程

1. 创建JNI.java文件，实现方法

public class JNI {

    static {
        System.loadLibrary("demo_03");
    }

    //传递 int 类型的数据
    public native int add(int x,int y);
    //传递String
    public native String sayHelloInc(String str);
    //传递int类型的数组
    public native  int[] arrElementsIncrease(int[] intArray);
}

2. 初始化，调用Native方法，并提示返回值

  2.1 Demo布局文件 activity_main.xml

  2.2 Java调用类实现 MainActivity.java

public class MainActivity extends AppCompatActivity {
    private ActivityMainBinding binding;
    private JNI jni;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());
        jni = new JNI();
        initView();
    }

    //初始化View
    private void initView() {
        binding.btnPassInt.setOnClickListener(view -> {
            int result = jni.add(3, 5);
            Toast.makeText(getApplicationContext(), String.valueOf(result), Toast.LENGTH_SHORT).show();
        });
        binding.btnPassString.setOnClickListener(view -> {
            String result = jni.sayHelloInc("abcedf");
            Toast.makeText(getApplicationContext(), result, Toast.LENGTH_SHORT).show();
        });
        binding.btnPassIntArray.setOnClickListener(view -> {
            int[] arr = {1, 2, 3, 4, 5, 6};
            // 直接修改数组中的值
            jni.arrElementsIncrease(arr);
            String str = "";
            for (int i : arr)
                str += i;
            Toast.makeText(getApplicationContext(), str, Toast.LENGTH_SHORT).show();
        });
    }
}

  2.3 C++ 实现接口文件

    2.3.1 CMakeLists.txt文件

# Sets the minimum version of CMake required to build the native library.
cmake_minimum_required(VERSION 3.18.1)

# Declares and names the project.
project("demo_03")

add_library( # Sets the name of the library.
        demo_03
        # Sets the library as a shared library.
        SHARED
        # Provides a relative path to your source file(s).
        Java_JNI.cpp)

find_library( # Sets the name of the path variable.
        log-lib
        # Specifies the name of the NDK library that
        # you want CMake to locate.
        log)

target_link_libraries( # Specifies the target library.
        demo_03
        # Links the target library to the log library
        # included in the NDK.
        ${log-lib})

    2.3.2 Java_JNI.cpp文件

#include 
#include 
#include 
#include 

#define LOG_TAG "System.out"
#define LOGD(...)  __android_log_print(ANDROID_LOG_DEBUG,LOG_TAG,__VA_ARGS__)
#define LOGI(...)  __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)

/* Header for class com_example_demo_03_javac_JNI */
#ifndef _Included_com_example_demo_03_javac_JNI
#define _Included_com_example_demo_03_javac_JNI
#ifdef __cplusplus

extern "C" {
#endif
/*
 * Class:     com_example_demo_03_javac_JNI
 * Method:    add
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_com_example_demo_103_1javac_JNI_add
        (JNIEnv *env, jobject obj, jint a, jint b) {
    return a + b;
}

char *jStringToChar(JNIEnv *env, jstring jstr) {
    char *rtn = NULL;
    jclass clsstring = env->FindClass("java/lang/String");
    jstring strencode = env->NewStringUTF("GB2312");
    jmethodID mid = env->GetMethodID(clsstring, "getBytes", "(Ljava/lang/String;)[B");
    jbyteArray barr = (jbyteArray) env->CallObjectMethod(jstr, mid, strencode);
    jsize alen = env->GetArrayLength(barr);
    jbyte *ba = env->GetByteArrayElements(barr, JNI_FALSE);
    if (alen > 0) {
        rtn = (char *) malloc(alen + 1);
        memcpy(rtn, ba, alen);
        rtn[alen] = 0;
    }
    env->ReleaseByteArrayElements(barr, ba, 0);
    return rtn;
}


/*
 * Class:     com_example_demo_03_javac_JNI
 * Method:    sayHelloInc
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_example_demo_103_1javac_JNI_sayHelloInc
        (JNIEnv *env, jobject obj, jstring str) {
    // char *cstr = jStringToChar(env, str);
    char *cstr = (char *) env->GetStringUTFChars(str, nullptr);
    //获取C字符串的长度
    int len = strlen(cstr);
    //for 循环执行完 cstr 每一个元素都 +1 了
    for (int i = 0; i < len; ++i) {
        *(cstr + i) += 1;
    }
    return env->NewStringUTF(cstr);
}

/*
 * Class:     com_example_demo_03_javac_JNI
 * Method:    arrElementsIncrease
 * Signature: ([I)[I
 */
JNIEXPORT jintArray JNICALL Java_com_example_demo_103_1javac_JNI_arrElementsIncrease
        (JNIEnv *env, jobject obj, jintArray intArray) {
    //获取数组的长度
    jsize len = env->GetArrayLength(intArray);

    LOGI("length = %d",len);
    //获取数组首地址
    jint *intArr = env->GetIntArrayElements(intArray, JNI_FALSE);
    for (int i = 0; i < len; ++i) {
        //intArr[i] += 2;
        *(intArr + i) += 2;
    }
    //jintArray  newIntArray = env->NewIntArray(len);
    //把计算的值赋值到原有的数组中
    env->SetIntArrayRegion(intArray, 0, len, intArr);
    //返回数组
    return intArray;
}

#ifdef __cplusplus
}
#endif
#endif

3. 向android logcat 输出日志

#include 

#define LOG_TAG "System.out"
#define LOGD(...)  __android_log_print(ANDROID_LOG_DEBUG,LOG_TAG,__VA_ARGS__)
#define LOGI(...)  __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)

//实现打印
 LOGI("length = %d",len);