Android------JNI 数据类型转换

java的基本类型可以强转。例如jint 和 int (c++中的int ) 。强转包含：基本类型的变量，或者指向基本类型变量的指针

ndk可以用代码1调用 ， 但例子中是用代码2。代码2可能是c++中的标准语法，代码1是ndk的。引用时要注意

代码1：env->GetStringUTFChars(str)

代码2：(*env)->NewStringUTF(env, str);

#include   
#include "com_test_Test.h"  
#include   
#include   
#include   
  
//获取字符串  
JNIEXPORT void JNICALL Java_com_test_Test_sayHello(JNIEnv * env, jobject obj,  
  jstring s) {  
  
 char * str = (char *) (*env)->GetStringUTFChars(env, s, 0);  
 printf("\n c-string: hello - %s", str);  
  
}  
  
//获取，返回int  
JNIEXPORT jint JNICALL Java_com_test_Test_add(JNIEnv * env, jobject obj,  
  jint a, jint b) {  
 int i = a;  
 int j = b;  
 printf("\n c-int: %d - %d", i, j);  
 char str[256];  
 sprintf(str, "%d", i + j);  
 return (jint) i + j;  
}  
  
//获取，返回float  
JNIEXPORT jfloat JNICALL Java_com_test_Test_getFloat(JNIEnv * env, jobject obj,  
  jfloat f) {  
 float fl = f;  
 printf("\n c-float: %3f", fl);  
  
 fl = 200.555;  
 return (jfloat) fl;  
}  
  
//获取，返回 double  
JNIEXPORT jdouble JNICALL Java_com_test_Test_getDouble(JNIEnv * env,  
  jobject obj, jdouble dou) {  
 double d = dou;  
 printf("\n c-double: %3f", d);  
  
 d = 800.88;  
 return (jdouble) d;  
}  
  
//获取，返回boolean  
JNIEXPORT jboolean JNICALL Java_com_test_Test_getBoolean(JNIEnv * env,  
  jobject obj, jboolean bool) {  
  
 unsigned char b = bool;  
 printf("\n c-boolean: %lu  ", b);  
 if (b) {  
  printf("true");  
 } else {  
  printf("false");  
 }  
  
 b = 1;  
 return (jboolean) b;  
}  
  
//获取，返回 string  
JNIEXPORT jstring JNICALL Java_com_test_Test_getString(JNIEnv * env,  
  jobject obj, jstring s) {  
 char * st = (char *) (*env)->GetStringUTFChars(env, s, 0);  
 printf("\n c-string:  %s", st);  
  
 char * str = "hello wang wu!";  
 jstring rtn;  
 rtn = (*env)->NewStringUTF(env, str);  
 return rtn;  
}  
  
//获取，返回 string []  
JNIEXPORT jobjectArray JNICALL Java_com_test_Test_getStringArray(JNIEnv * env,  
  jobject obj, jobjectArray arr) {  
 int len = (*env)->GetArrayLength(env, arr);  
 printf("\n c-stringArray :");  
 int i = 0;  
 for (i = 0; i < len; i++) {  
  jobject obj = (*env)->GetObjectArrayElement(env, arr, i);  
  jstring str = (jstring) obj;  
  const char * szStr = (*env)->GetStringUTFChars(env, str, 0);  
  printf(" %d-%s ", i, szStr);  
  (*env)->ReleaseStringChars(env, str, szStr);  
//env->ReleaseStringUTFChars(str, szStr); 应该是这句
 }  
 //-----返回----  
  
 jstring str;  
 jobjectArray args = 0;  
 jsize size = 5;  
 char* sa[] = { "Hello,", "world!", "zhang", "san", "yuang" };  
 int j = 0;  
 jclass objClass = (*env)->FindClass(env, "java/lang/String");  
 args = (*env)->NewObjectArray(env, size, objClass, 0);  
 for (j = 0; j < size; j++) {  
  str = (*env)->NewStringUTF(env, sa[j]);  
  (*env)->SetObjectArrayElement(env, args, j, str);  
 }  
 return args;  
}  

 

package com.test;  
  
public class Test {  
 private native void sayHello(String str);  
  
 private native int add(int a, int b);  
  
 private native float getFloat(float f);  
  
 private native double getDouble(double d);  
  
 private native boolean getBoolean(boolean b);  
  
 private native String getString(String str);  
  
 private native String[] getStringArray(String[] sa);  
  
 static {  
  System.loadLibrary("Test");  
 }  
  
 public static void main(String[] args) {  
  Test test = new Test();  
  test.sayHello("zhang san");  
  System.out.println("int     --> " + test.add(10, 20));  
  System.out.println("float   --> " + test.getFloat((float) 20.123));  
  System.out.println("double  --> " + test.getDouble(100.369));  
  System.out.println("boolean --> " + test.getBoolean(true));  
  System.out.println("string  --> " + test.getString("wang Wu"));  
  
  String[] ss = { "hello", "-", "zhang", "san" };  
  Object[] obj = test.getStringArray(ss);  
  System.out.print("string[]-->  ");  
  for (Object object : obj) {  
   System.out.print(object + " ");  
  }  
  
 }  
  

 

我用的是jni动态注册，所以方法有点不一样。

注意变量的动态释放，现在自己还有点拿不准

jstring tan_string_From_JNI(JNIEnv *env, jobject obj,jstring jstr,jint age) {

    /*, jint a, jint b
    int rtn = (int)(a + b);
    return (jint)rtn;*/

    int temp_age=(int)age;
    std::string age_str;
    int2str((temp_age+1),age_str);

    const char *my_name = env->GetStringUTFChars(jstr, NULL);
    std::string str_name(my_name);
    str_name=str_name+"123,age="+age_str;

    jstring result_str=env->NewStringUTF(str_name.c_str());

    jclass cmake_cls = env->GetObjectClass(obj);
    if(cmake_cls != NULL)
    {
        jmethodID mid = env->GetMethodID(cmake_cls, "showResult", "(Ljava/lang/String;)V");
        env->CallVoidMethod(obj , mid , result_str);
    }
    return result_str;
}

jbyteArray tan_test_byte(JNIEnv *env, jobject obj, jbyteArray byarray) {
    jsize len = env->GetArrayLength(byarray);
    jbyte *jbarray = (jbyte *)malloc(len * sizeof(jbyte));
    env->GetByteArrayRegion(byarray,0,len,jbarray);
    BYTE *dData=(BYTE*)jbarray;

    BYTE aByte[4]={0x10,0x01};
    aByte[2]=dData[0];
    aByte[3]=dData[1];
    jbyte *jy=(jbyte*)aByte;

    free(jbarray);
    jbarray=NULL;

    jbyteArray resultArray = env->NewByteArray(4);//建立jbarray数组
    env->SetByteArrayRegion(resultArray, 0, 4, jy);
    return  resultArray;
}



jobjectArray get_string_array(JNIEnv * env, jobject obj, jobjectArray arr) {
    int len = env->GetArrayLength(arr);
    printf("\n c-stringArray :");
    int i = 0;
    for (i = 0; i < len; i++) {
        jobject obj = env->GetObjectArrayElement(arr, i);
        jstring str = (jstring) obj;
        const char * szStr = env->GetStringUTFChars( str, 0);
        printf(" %d-%s ", i, szStr);
        env->ReleaseStringUTFChars(str, szStr);
    }
    //-----返回----  

    jstring str;
    jobjectArray args = 0;
    jsize size = 5;
    char* sa[] = { "Hello,", "world!", "zhang", "san", "yuang" };
    int j = 0;
    jclass objClass = env->FindClass("java/lang/String");
    args = env->NewObjectArray(size, objClass, 0);
    for (j = 0; j < size; j++) {
        str = env->NewStringUTF(sa[j]);
        env->SetObjectArrayElement(args, j, str);
    }
    return args;
}