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JNI高级教程之数据类型转换

分类： C/C++

2012-05-19 14:41:04


最近做Android开发的人越来越多，Android开发难免会遇到调用本地库，这就需要采用JNI技术，JNI本身并不复杂，但大多数开发者在类型转换上遇到麻烦，今天特地将几种常用类型转换写成一个实例来告诉大家如何转换，尤其是Java的类和C的结构的转换，结构体中嵌套结构体如何处理，这部分网上的资料也比较少。

1. 编写Java类

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  1. package com.jinhill.util;
  2. public class NativeModule
  3. {
  4.     public native int testArg(int i, boolean b, char c, double d);
  5.     public native byte[] testByte(byte[] b);
  6.     public native String[] testString(String s, String[] sarr);
  7.     public native int setInfo(MyInfo info);
  8.     public native MyInfo getInfo();

  10.     static
  11.     {
  12.         System.loadLibrary("NativeModule");
  13.     }
  14. }

其中MyInfo类定义如下：

点击(此处)折叠或打开

  1. public class Record
  2. {
  3.     int id;
  4.     String name;
  5.     byte[] data;
  6. }

  8. public class MyInfo
  9. {
  10.     public boolean b;
  11.     public char c;
  12.     public double d;
  13.     public int i;
  14.     public byte[] array;
  15.     public String s;
  16.     public Record rec;
  17. }

C自定义结构体 

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  1. typedef struct
  2. {
  3.     int id;
  4.     char name[255];
  5.     char data[255];
  6. } Record;

  8. typedef struct
  9. {
  10.     BOOL b;
  11.     char c;
  12.     double d;
  13.     int i;
  14.     char arr[255];
  15.     char sz[255];
  16.     Record rec;
  17. }MyInfo;

2. 生成jni头文件

1) 编译javac com/jinhill/util/NativeModule.java

2) javah –jni com.jinhill.util.NativeModule

这样com_jinhill_util_NativeModule.h文件就生成好了。

3. 编写C库

1) Java与C不同类型参数转换实例

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  1. //不同类型参数处理

  3. JNIEXPORT jintJNICALL Java_com_jinhill_util_NativeModule_testArg
  4. (JNIEnv *env, jobject jo, jint ji, jbooleanjb, jchar jc, jdouble jd)
  5. {
  6.     //获取jint型值
  7.     int i = ji;

  9.     //获取jboolean型值
  10.     BOOL b = jb;

  12.     //获取jdouble型值
  13.     double d = jd;

  15.     //获取jchar型值,Java的char两字节
  16.     char ch[5] = {0};

  18.     int size = 0;

  20.     size = WideCharToMultiByte(CP_ACP,NULL, (LPCWSTR)&jc, -1, ch, 5, NULL, FALSE);

  22.     if(size <= 0)
  23.     {
  24.         return -1;
  25.     }

  27.     Trace("ji=%d,jb=%d,jc=%s,jd=%lf",i, b, ch, d);
  28.     return 0;
  29. }

2) Java byte与C char数组类型数组转换实例

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  1. //btye数组处理，形参作为输入或输出，返回btye数组

  3. JNIEXPORTjbyteArray JNICALL Java_com_jinhill_util_NativeModule_testByte
  4. (JNIEnv *env, jobject jo, jbyteArray jbArr)
  5. {
  6.     char chTmp[] = "Hello JNI!";
  7.     int nTmpLen = strlen(chTmp);

  9.     //获取jbyteArray
  10.     char *chArr = (char*)env->GetByteArrayElements(jbArr,0);

  12.     //获取jbyteArray长度
  13.     int nArrLen = env->GetArrayLength(jbArr);
  14.     char *szStrBuf =(char*)malloc(nArrLen*2+10);
  15.     memset(szStrBuf, 0, nArrLen*2+10);
  16.     Bytes2String(chArr, nArrLen, szStrBuf,nArrLen*2+10);
  17.     Trace("jbArr=%s", szStrBuf);
  18.     //将jbArr作为输出形参
  19.     memset(chArr, 0, nArrLen);
  20.     memcpy(chArr, chTmp, nTmpLen);

  22.     //返回jbyteArray
  23.     jbyteArray jarrRV =env->NewByteArray(nTmpLen);
  24.     jbyte *jby =env->GetByteArrayElements(jarrRV, 0);
  25.     memcpy(jby, chTmp, strlen(chTmp));
  26.     env->SetByteArrayRegion(jarrRV, 0,nTmpLen, jby);
  27.     return jarrRV;
  28. }
3) Java String与C char数组类型转换实例

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  1. //String 和String[]处理
  2. JNIEXPORTjobjectArray JNICALL Java_com_jinhill_util_NativeModule_testString
  3. (JNIEnv *env, jobject jo, jstring jstr,jobjectArray joarr)
  4. {
  5.     int i = 0;
  6.     char chTmp[50] = {0};

  8.     //获取jstring值
  9.     const char* pszStr = (char*)env->GetStringUTFChars(jstr, 0);
  10.     Trace("jstr=%s", pszStr);

  12.     //获取jobjectArray值
  13.     int nArrLen =env->GetArrayLength(joarr);
  14.     Trace("joarr len=%d",nArrLen);
  15.     for(i=0; i<nArrLen; i++)
  16.     {
  17.         jstring js =(jstring)env->GetObjectArrayElement(joarr, i);
  18.         const char* psz = (char*)env->GetStringUTFChars(js, 0);
  19.         Trace("joarr[%d]=%s",i, psz);
  20.     }
  21.     
  22.     //将joarr作为输出形参
  23.     jstring jstrTmp = NULL;
  24.     for(i=0; i<nArrLen; i++)
  25.     {
  26.         sprintf(chTmp, "No.%dHello JNI!", i);
  27.         jstrTmp =env->NewStringUTF(chTmp);
  28.         env->SetObjectArrayElement(joarr,i, jstrTmp);
  29.         env->DeleteLocalRef(jstrTmp);
  30.     }

  32.     //返回jobjectArray
  33.     jclass jstrCls =env->FindClass("Ljava/lang/String;");
  34.     jobjectArray jstrArray =env->NewObjectArray(2, jstrCls, NULL);

  36.     for(i=0; i<2; i++)
  37.     {
  38.         sprintf(chTmp, "No. %dReturn JNI!", i);
  39.         jstrTmp =env->NewStringUTF(chTmp);
  40.         env->SetObjectArrayElement(jstrArray,i, jstrTmp);
  41.         env->DeleteLocalRef(jstrTmp);
  42.     }
  43.     return jstrArray;
  44. }

4) Java 类与C结构体类型转换实例

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  1. JNIEXPORT jint JNICALL Java_com_jinhill_util_NativeModule_setInfo
  2. (JNIEnv *env, jobject jo, jobject jobj)
  3. {
  4.     char chHexTmp[512] = {0};
  5.     //将Java类转换成C结构体
  6.     MyInfo mi;

  8.     //获取Java中的实例类Record
  9.     jclass jcRec = env->FindClass("com/jinhill/util/Record");
  10.     //int id

  12.     jfieldID jfid = env->GetFieldID(jcRec, "id", "I");
  13.     //String name

  15.     jfieldID jfname = env->GetFieldID(jcRec, "name", "Ljava/lang/String;");
  16.     //byte[] data;

  18.     jfieldID jfdata = env->GetFieldID(jcRec, "data", "[B");
  19.     //获取Java中的实例类MyInfo

  21.     jclass jcInfo = env->FindClass("com/jinhill/util/MyInfo");
  22.     //获取类中每一个变量的定义

  24.     //boolean b
  25.     jfieldID jfb = env->GetFieldID(jcInfo, "b", "Z");

  27.     //char c
  28.     jfieldID jfc = env->GetFieldID(jcInfo, "c", "C");

  30.     //double d
  31.     jfieldID jfd = env->GetFieldID(jcInfo, "d", "D");

  33.     //int i
  34.     jfieldID jfi = env->GetFieldID(jcInfo, "i", "I");

  36.     //byte[] array
  37.     jfieldID jfa = env->GetFieldID(jcInfo, "array", "[B");

  39.     //String s
  40.     jfieldID jfs = env->GetFieldID(jcInfo, "s", "Ljava/lang/String;");

  42.     //Record rec;
  43.     jfieldID jfrec = env->GetFieldID(jcInfo, "rec", "Lcom/jinhill/util/Record;");
  44.     //获取实例的变量b的值
  45.     mi.b = env->GetBooleanField(jobj, jfb);
  46.     
  47.     //获取实例的变量c的值
  48.     jchar jc = env->GetCharField(jobj, jfc);
  49.     char ch[5] = {0};
  50.     int size = 0;
  51.     size = WideCharToMultiByte(CP_ACP, NULL, (LPCWSTR)&jc, -1, ch, 5, NULL, FALSE);
  52.     mi.c = ch[0];
  53.     
  54.     //获取实例的变量d的值
  55.     mi.d = env->GetDoubleField(jobj, jfd);
  56.     
  57.     //获取实例的变量i的值
  58.     mi.i = env->GetIntField(jobj, jfi);
  59.     
  60.     //获取实例的变量array的值
  61.     jbyteArray ja = (jbyteArray)env->GetObjectField(jobj, jfa);
  62.     int nArrLen = env->GetArrayLength(ja);
  63.     char *chArr = (char*)env->GetByteArrayElements(ja, 0);
  64.     memcpy(mi.arr, chArr, nArrLen);
  65.     
  66.     //获取实例的变量s的值
  67.     jstring jstr = (jstring)env->GetObjectField(jobj, jfs);
  68.     const char* pszStr = (char*)env->GetStringUTFChars(jstr, 0);
  69.     strcpy(mi.sz, pszStr);

  71.     //获取Record对象
  72.     jobject joRec = env->GetObjectField(jobj, jfrec);
  73.     
  74.     //获取Record对象id值
  75.     mi.rec.id = env->GetIntField(joRec, jfid);
  76.     Trace("mi.rec.id=%d",mi.rec.id);
  77.     
  78.     //获取Record对象name值
  79.     jstring jstrn = (jstring)env->GetObjectField(joRec, jfname);
  80.     pszStr = (char*)env->GetStringUTFChars(jstrn, 0);
  81.     strcpy(mi.rec.name, pszStr);
  82.     
  83.     //获取Record对象data值
  84.     jbyteArray jbd = (jbyteArray)env->GetObjectField(joRec, jfdata);
  85.     nArrLen = env->GetArrayLength(jbd);
  86.     chArr = (char*)env->GetByteArrayElements(jbd, 0);
  87.     memcpy(mi.rec.data, chArr, nArrLen);
  88.     
  89.     //日志输出
  90.     Bytes2String(mi.arr, nArrLen, chHexTmp, sizeof(chHexTmp));
  91.     Trace("mi.arr=%s, mi.b=%d, mi.c=%c, mi.d=%lf, mi.i=%d, \n mi.sz=%s\n mi.rec.id=%d, mi.rec.name=%s", chHexTmp, mi.b, mi.c, mi.d, mi.i, mi.sz, mi.rec.id, mi.rec.name);
  92.     return 0;
  93. }

5) C结构体类型与Java 类转换实例

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  1. JNIEXPORT jobject JNICALL Java_com_jinhill_util_NativeModule_getInfo
  2.   (JNIEnv *env, jobject jo)
  3. {
  4.     wchar_t wStr[255] = {0};
  5.     char chTmp[] = "Hello JNI";
  6.     int nTmpLen = strlen(chTmp);
  7.     //将C结构体转换成Java类

  9.     MyInfo mi;
  10.     memcpy(mi.arr, chTmp, strlen(chTmp));
  11.     mi.b = TRUE;
  12.     mi.c = 'B';
  13.     mi.d = 2000.9;
  14.     mi.i = 8;
  15.     strcpy(mi.sz, "Hello World!");
  16.     mi.rec.id = 2011;
  17.     memcpy(mi.rec.data, "\x01\x02\x03\x04\x05\x06", 6);
  18.     strcpy(mi.rec.name, "My JNI");

  20.     //获取Java中的实例类Record
  21.     jclass jcRec = env->FindClass("com/jinhill/util/Record");

  23.     //int id
  24.     jfieldID jfid = env->GetFieldID(jcRec, "id", "I");

  26.     //String name
  27.     jfieldID jfname = env->GetFieldID(jcRec, "name", "Ljava/lang/String;");

  29.     //byte[] data;
  30.     jfieldID jfdata = env->GetFieldID(jcRec, "data", "[B");


  33.     //获取Java中的实例类
  34.     jclass jcInfo = env->FindClass("com/jinhill/util/MyInfo");

  36.     //获取类中每一个变量的定义
  37.     //boolean b
  38.     jfieldID jfb = env->GetFieldID(jcInfo, "b", "Z");

  40.     //char c
  41.     jfieldID jfc = env->GetFieldID(jcInfo, "c", "C");

  43.     //double d
  44.     jfieldID jfd = env->GetFieldID(jcInfo, "d", "D");

  46.     //int i
  47.     jfieldID jfi = env->GetFieldID(jcInfo, "i", "I");

  49.     //byte[] array
  50.     jfieldID jfa = env->GetFieldID(jcInfo, "array", "[B");

  52.     //String s
  53.     jfieldID jfs = env->GetFieldID(jcInfo, "s", "Ljava/lang/String;");

  55.     //Record rec;
  56.     jfieldID jfrec = env->GetFieldID(jcInfo, "rec", "Lcom/jinhill/util/Record;");

  58.     //创建新的对象
  59.     jobject joRec = env->AllocObject(jcRec);
  60.     env->SetIntField(joRec, jfid, mi.rec.id);
  61.     jstring jstrn = env->NewStringUTF(mi.rec.name);
  62.     env->SetObjectField(joRec, jfname, jstrn);
  63.     jbyteArray jbarr = env->NewByteArray(6);
  64.     jbyte *jb = env->GetByteArrayElements(jbarr, 0);
  65.     memcpy(jb, mi.rec.data, 6);
  66.     env->SetByteArrayRegion(jbarr, 0, 6, jb);
  67.     env->SetObjectField(joRec, jfdata, jbarr);
  68.     //创建新的对象
  69.     jobject joInfo = env->AllocObject(jcInfo);
  70.     //给类成员赋值
  71.     env->SetBooleanField(joInfo, jfb, mi.b);
  72.     // MultiByteToWideChar (CP_ACP, 0, mi.c, -1, wStr, 255);

  74.     // env->SetCharField(joInfo, jfc, (jchar)wStr);
  75.     env->SetCharField(joInfo, jfc, (jchar)mi.c);
  76.     env->SetDoubleField(joInfo, jfd, mi.d);
  77.     env->SetIntField(joInfo, jfi, mi.i);
  78.     jbyteArray jarr = env->NewByteArray(nTmpLen);
  79.     jbyte *jby = env->GetByteArrayElements(jarr, 0);
  80.     memcpy(jby, mi.arr, nTmpLen);
  81.     env->SetByteArrayRegion(jarr, 0, nTmpLen, jby);
  82.     env->SetObjectField(joInfo, jfa, jarr);
  83.     jstring jstrTmp = env->NewStringUTF(chTmp);
  84.     env->SetObjectField(joInfo, jfs, jstrTmp);
  85.     env->SetObjectField(joInfo, jfrec, joRec);
  86.     return joInfo;
  87. }
4. 编写Java测试代码

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  1. public class TestInfo {

  3.     public static void main(String[] args)
  4.     {
  5.         int i =0;
  6.         String[] sArr = new String[2];
  7.         for(i=0; i<2; i++)
  8.         {
  9.             sArr[i] = "ID=" + i;
  10.         }

  12.         byte[] b = new byte[10];
  13.         for(i=0; i<10; i++)
  14.         {
  15.             b[i] = (byte)i;
  16.         }

  18.         MyInfo mi = new MyInfo();
  19.         mi.array = b;
  20.         mi.b = false;
  21.         mi.c = 'C';
  22.         mi.d = 2011.11;
  23.         mi.i = 1752;
  24.         mi.s = "Hello World!";
  25.         mi.rec = new Record();
  26.         mi.rec.id = 2012;
  27.         mi.rec.name = "Record";
  28.         mi.rec.data = b;
  29.         NativeModule nm = new NativeModule();
  30.         nm.testArg(mi.i, mi.b, mi.c, mi.d);
  31.         byte[] b2 = nm.testByte(mi.array);
  32.         String[] s = nm.testString(mi.s, sArr);
  33.         nm.setInfo(mi);
  34.         MyInfo mi2 = nm.getInfo();
  35.         System.out.println("finish");
  36.     }
  37. }
5. Java String与 C char数组转换时的中文问题

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  1. //将jstring类型转换成windows类型
  2. char* jstringToWindows( JNIEnv *env, jstring jstr )
  3. {
  4.     int length = (env)->GetStringLength(jstr );
  5.     const jchar* jcstr = (env)->GetStringChars(jstr, 0 );
  6.     char* rtn = (char*)malloc( length*2+1 );
  7.     int size = 0;
  8.     size = WideCharToMultiByte( CP_ACP, 0, (LPCWSTR)jcstr, length,rtn,(length*2+1), NULL, NULL );
  9.     if( size <= 0 )
  10.         return NULL;
  11.     (env)->ReleaseStringChars(jstr, jcstr );
  12.     rtn[size] = 0;
  13.     return rtn;
  14. }

  16. //将windows类型转换成jstring类型
  17. jstring WindowsTojstring( JNIEnv* env, char* str )
  18. {
  19.     jstring rtn = 0;
  20.     int slen = strlen(str);
  21.     unsigned short * buffer = 0;
  22.     if( slen == 0 )
  23.         rtn = (env)->NewStringUTF(str );
  24.     else
  25.     {
  26.         int length = MultiByteToWideChar( CP_ACP, 0, (LPCSTR)str, slen, NULL, 0 );
  27.         buffer = (unsigned short *)malloc( length*2 + 1 );
  28.         if( MultiByteToWideChar( CP_ACP, 0, (LPCSTR)str, slen, (LPWSTR)buffer, length )>0 )
  29.         rtn = (env)->NewString( (jchar*)buffer, length );
  30.     }
  31.     if( buffer )
  32.         free( buffer );
  33.     return rtn;
  34. }

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