JNI 数据类型转换

Java代码

#include <jni.h>
 #include "com_test_Test.h"
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>

 //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);
 }
  //-----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;
}

-----------------------------------------------------------------

下面是Test.java

 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 + " ");
  }

 }

}