Android Studio平台下JNI开发：Java传参给C

之前道长和小伙伴们分享了一下Android Studio平台下JNI开发：入门使用及常见错误。这里在和小伙伴们分享一下Java传参给C语言。如果对JNI开发还不熟悉可以看一下道长之前分享的。

1.在MainActivity中声明本地方法

 // 声明一个本地方法，用native关键字修饰 public native String getStringFromNative(); // 传int值向C public native int sendInt2C(int x ,int y); // 传string向C public native String sendString2C(String s); // 传int[]向C public native int[] sendIntArray2C(int[] iNum);

2.向.h文件中添加代码

/* * Class:     com_yushan_jnidemo_MainActivity * Method:    getStringFromNative * Signature: ()Ljava/lang/String; */JNIEXPORT jstring JNICALL Java_com_yushan_jnidemo_MainActivity_getStringFromNative        (JNIEnv *, jobject);/* * Class:     com_yushan_jnidemo_MainActivity * Method:    add * Signature: (II)I */JNIEXPORT jint JNICALL Java_com_yushan_jnidemo_MainActivity_sendInt2C        (JNIEnv *, jobject, jint, jint);/* * Class:     com_yushan_jnidemo_MainActivity * Method:    sayHelloInC * Signature: (Ljava/lang/String;)Ljava/lang/String; */JNIEXPORT jstring JNICALL Java_com_yushan_jnidemo_MainActivity_sendString2C        (JNIEnv *, jobject, jstring);/* * Class:     com_yushan_jnidemo_MainActivity * Method:    intMethod * Signature: ([I)[I */JNIEXPORT jintArray JNICALL Java_com_yushan_jnidemo_MainActivity_sendIntArray2C        (JNIEnv *, jobject, jintArray);

3.向.c文件中添加代码

JNIEXPORT jstring JNICALL Java_com_yushan_jnidemo_MainActivity_getStringFromNative        (JNIEnv *env, jobject obj){    char *str = "String from native C";    return (*env)->NewStringUTF(env, str);}// java中的jstring, 转化为c的一个字符数组char *Jstring2CStr(JNIEnv *env, jstring jstr) {    char *rtn = NULL;    jclass clsstring = (*env)->FindClass(env, "java/lang/String");    jstring strencode = (*env)->NewStringUTF(env, "GB2312");    jmethodID mid = (*env)->GetMethodID(env, clsstring, "getBytes", "(Ljava/lang/String;)[B");    jbyteArray barr = (jbyteArray)(*env)->CallObjectMethod(env, jstr, mid, strencode);    jsize alen = (*env)->GetArrayLength(env, barr);    jbyte *ba = (*env)->GetByteArrayElements(env, barr, JNI_FALSE);    if (alen > 0) {        rtn = (char *) malloc(alen + 1);        memcpy(rtn, ba, alen);        rtn[alen] = 0;    }    // 释放内存    (*env)->ReleaseByteArrayElements(env, barr, ba, 0);    return rtn;}// 处理整形相加JNIEXPORT jint JNICALL Java_com_yushan_jnidemo_MainActivity_sendInt2C        (JNIEnv *env, jobject obj, jint x, jint y) {    return x + y;}// 处理字符串追加JNIEXPORT jstring JNICALL Java_com_yushan_jnidemo_MainActivity_sendString2C        (JNIEnv *env, jobject obj, jstring str) {    char *p = Jstring2CStr(env, str);    char *newstr = "append string";    // strcat(dest, sorce) 把sorce字符串添加到dest字符串的后面    return (*env)->NewStringUTF(env, strcat(p, newstr));}// 处理数组中的每一个元素JNIEXPORT jintArray JNICALL Java_com_yushan_jnidemo_MainActivity_sendIntArray2C        (JNIEnv *env, jobject obj, jintArray arr) {    // 获取arr的长度    int len = (*env)->GetArrayLength(env, arr);    if (len == 0) {        return arr;    }    // 取出数组中第一个元素的内存地址    jint * p = (*env)->GetIntArrayElements(env, arr, 0);    int i = 0;    for (; i < len; i++) {        *(p + i) += 5;    }    return arr;}

4.在MainActivity中调用

    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        TextView textView = (TextView) findViewById(R.id.tv);        int[] arr = {2,3,5};        String str = "hello";        String fromNative = getStringFromNative();        int fromInt2C = sendInt2C(5,11);        String fromString2C = sendString2C(str);        int[] fromIntArray2C = sendIntArray2C(arr);        textView.setText(fromNative + ":" +fromInt2C+ ":" + fromString2C + ":" + fromIntArray2C[2]);    }

好了，关于Java传参给C就先说这三种类型的数据，希望能给小伙伴们一些帮助。

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