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JNI常用类型转换

std::string hello = "Hello from C++";
jstring jHello=env->NewStringUTF(hello.c_str());
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char*转jstring

jstring util_char_to_jstring(JNIEnv *env, const char *pat) {
    //定义java String类 strClass
    jclass strClass = (env)->FindClass("java/lang/String");
    //获取String(byte[],String)的构造器,用于将本地byte[]数组转换为一个新String
    jmethodID ctorID = (env)->GetMethodID(strClass, "<init>", "([BLjava/lang/String;)V");
    //建立byte数组
    jbyteArray bytes = (env)->NewByteArray(strlen(pat));
    //将char* 转换为byte数组
    (env)->SetByteArrayRegion(bytes, 0, strlen(pat), (jbyte *) pat);
    // 设置String, 保存语言类型,用于byte数组转换至String时的参数
    jstring encoding = (env)->NewStringUTF("utf-8");
    //将byte数组转换为java String,并输出
    return (jstring) (env)->NewObject(strClass, ctorID, bytes, encoding);
}
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jstring转char*：

void util_jstring_to_char(JNIEnv *env, jstring jstr, char *result) {
    if (env == NULL || jstr == NULL) {
        return;
    }
    jclass clsstring = env->FindClass("java/lang/String");
    jstring strencode = env->NewStringUTF("utf-8");
    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) {
        memcpy(result, ba, alen);
        result[alen] = 0;
    }
    env->ReleaseByteArrayElements(barr, ba, 0);
}

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而 char* 是可以直接赋值给 string 的：

string s;
 char *p = "hello";//直接赋值
 s = p;
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int转string

string util_int_to_string(unsigned int n) {
    string sResult;
    stringstream ssTmp;
    ssTmp << n;
    ssTmp >> sResult;
    return sResult;
}
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JByteArray转char*

char *convertJByteArrayToChar(JNIEnv *env, jbyteArray byYUV1_) {

    jbyte *bytes = env->GetByteArrayElements(byYUV1_, 0);
    int arrayLength = env->GetArrayLength(byYUV1_);
    char *chars = new char[arrayLength + 1];
    memset(chars, 0x0, arrayLength + 1);
    memcpy(chars, bytes, arrayLength);
    chars[arrayLength] = 0;
    env->ReleaseByteArrayElements(byYUV1_, bytes, 0);

    return chars;
}
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char*转JByteArray

jbyteArray util_char_to_jbyteArray(JNIEnv *env, char *data, int length) {
    jbyte *by = (jbyte *) data;
    jbyteArray jarray = env->NewByteArray(length);
    env->SetByteArrayRegion(jarray, 0, length, by);
    return jarray;
}
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原文 https://juejin.im/post/5afd694ef265da0b96727210

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