JNI开发探索之旅

cmake_minimum_required(VERSION 3.4.1)
add_library( # 生成的so库名称，此处生成的so文件名称是libnative-lib.so
             native-lib
             # SHARED是动态库，会被动态链接，在运行时被加载
             # STATIC：静态库，是目标文件的归档文件，在链接其它目标的时候使用
             # MODULE：模块库，是不会被链接到其它目标中的插件
             SHARED
             # 资源路径是相对路径，相对于本CMakeLists.txt所在目录
             src/main/cpp/native-lib.cpp )
# 从系统查找依赖库
find_library( # android系统每个类型的库会存放一个特定的位置，而log库存放在log-lib中
              log-lib
              # android系统在c环境下打log到logcat的库
              log )
# 配置库的链接（依赖关系）
target_link_libraries( # 目标库
                       native-lib
                       # 依赖于
                       ${log-lib} )

#include <jni.h>
#include <string>
extern "C" JNIEXPORT jstring
JNICALL
Java_com_ndk_lingxiao_ndkproject_MainActivity_stringFromJNI(
        JNIEnv *env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

public class MainActivity extends AppCompatActivity {
    static {
        System.loadLibrary("native-lib");
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Example of a call to a native method
        TextView tv = (TextView) findViewById(R.id.sample_text);
        tv.setText(stringFromJNI());
    }
    public static native String stringFromJNI();
}

defaultConfig {
		ndk{
             ldLibs "gomp"
           }
       }

#include <android/log.h>
#define LOG_TAG "NATIVE_LIB"

#define DEBUG
#define ANDROID_PLATFORM

#ifdef DEBUG
#ifdef ANDROID_PLATFORM
#define LOGD(...) ((void)__android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__))
#define LOGI(...) ((void)__android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__))
#define LOGW(...) ((void)__android_log_print(ANDROID_LOG_WARN, LOG_TAG, __VA_ARGS__))
#define LOGE(...) ((void)__android_log_print(ANDROID_LOG_ERROR, LOG_TAG, __VA_ARGS__))
#else
#define LOGD(fmt, ...) printf(fmt"\n", ##__VA_ARGS__)
#define LOGI(fmt, ...) printf(fmt"\n", ##__VA_ARGS__)
#define LOGW(fmt, ...) printf(fmt"\n", ##__VA_ARGS__)
#define LOGE(fmt, ...) printf(fmt"\n", ##__VA_ARGS__)
#endif
#else
#define LOGD(...)
#define LOGI(...)
#define LOGW(...)
#define LOGE(...)
#endif

extern "C"
JNIEXPORT void JNICALL
Java_com_ndk_lingxiao_ndkproject_Hello_callStaticMethod(JNIEnv *env, jclass type, jint i) {
    LOGD("im from static moethod C++ , value is : %d",i);
}

extern "C"
JNIEXPORT void JNICALL
Java_com_ndk_lingxiao_ndkproject_Hello_callInstanceMethod
        (JNIEnv *, jobject, jint i){
    LOGD("im from instance moethod C++ , value is : %d",i);
}

javap -s 类的.class路径

public static void staticMethod(String data){
        logMessage(data);
    }
public static void logMessage(String data){
        Log.d("hello", data);
    }

extern "C"
JNIEXPORT void JNICALL
Java_com_ndk_lingxiao_ndkproject_Hello_callJavaStaticMethod(JNIEnv *env, jclass type) {
    jclass clazz = NULL;
    jmethodID method_id = NULL;
    jstring str_log = NULL;

    clazz = env->FindClass("com/ndk/lingxiao/ndkproject/Hello");
    if (clazz == NULL){
        LOGD("没有发现该类");
        return;
    }
    method_id = env->GetStaticMethodID(clazz,"staticMethod","(Ljava/lang/String;)V");
    if (method_id == NULL){
        LOGD("没有发现该方法名");
        return;
    }
    str_log = env->NewStringUTF("c++ 调用java的静态方法");
    env->CallStaticVoidMethod(clazz,method_id,str_log);

    env->DeleteLocalRef(clazz);
    env->DeleteLocalRef(str_log);
    return ;
}

extern "C"
JNIEXPORT void JNICALL
Java_com_ndk_lingxiao_ndkproject_Hello_callJavaInstanceMethod(JNIEnv *env, jobject instance) {
    jclass clazz = NULL;
    jmethodID method_id = NULL;
    jstring str_log = NULL;

    clazz = env->FindClass("com/ndk/lingxiao/ndkproject/Hello");
    if (clazz == NULL){
        LOGD("没有发现该类");
        return;
    }
    method_id = env->GetMethodID(clazz,"instanceMethod","(Ljava/lang/String;)V");
    if (method_id == NULL){
        LOGD("没有发现该方法名");
        return;
    }
    str_log = env->NewStringUTF("c++ 调用java的实例方法");
    env->CallVoidMethod(instance,method_id,str_log); //clazz 改为instance

    env->DeleteLocalRef(clazz);
    env->DeleteLocalRef(str_log);
    return ;
}

public String name = "im is java";

extern "C"
JNIEXPORT void JNICALL
Java_com_ndk_lingxiao_ndkproject_Hello_changeField(JNIEnv *env, jobject instance) {
    jclass clazz = env->GetObjectClass(instance);
    if (clazz == NULL){
        return;
    }
    jfieldID jfieldID = env->GetFieldID(clazz,"name","Ljava/lang/String;");
    if (jfieldID == NULL){
        return;
    }
    jstring obj_str = (jstring) env->GetObjectField(instance,jfieldID);
    if (obj_str == NULL){
        return;
    }
    char* c_str = (char*) env->GetStringUTFChars(obj_str,JNI_FALSE);

    const char new_char[40] = "changed from c";
    //复制new_char的内容到c_str
    strcpy(c_str,new_char);

    jstring new_str = env->NewStringUTF(c_str);
    LOGD("%s",new_char);
    env->SetObjectField(instance,jfieldID,new_str);

    env->DeleteLocalRef(clazz);
    env->DeleteLocalRef(obj_str);
    env->DeleteLocalRef(new_str);
    return;
}

extern "C"
JNIEXPORT void JNICALL
Java_com_ndk_lingxiao_ndkproject_Hello_changeStaticField(JNIEnv *env, jclass type) {
    jclass clazz = env->FindClass("com/ndk/lingxiao/ndkproject/Hello");
    if (clazz == NULL){
        return;
    }
    jfieldID jfieldID = env->GetStaticFieldID(clazz,"age","I");
    if (jfieldID == NULL){
        return;
    }
    int age = env->GetStaticIntField(clazz,jfieldID);
    LOGD("%d",age);
    jint change_int = 12;
    env->SetStaticIntField(clazz,jfieldID,change_int);

    env->DeleteLocalRef(clazz);
}