JNI 对象处理和数据类型转换

====================================对象处理====================================================

JNI 的基本问题就是解决 Java 和 C++ 代码互相调用的通信问题，在 C++ 代码编写过程中最大的问题莫过于适应其中的代码编写规则，C++调用或是返回的内容必须遵守 JVM 和 C++ 代码的通信规则。

C++ 调用 Java 的一般步骤如下：

获得类：

jclass cls = env->FindClass("com/ldq/Student");
cls 可认为是类的句柄
"com/ldq/Student" 就是类文件，注意不能用 "com.ldq.Student"

获得方法：

jmethodID mid = env->GetMethodID(cls,"<init>","()V");
以上为构造函数，参数是 "<init>" "()V"
jmethodID mid = env->GetMethodID(cls,"getAge","()I");
以上为类的方法，第一个参数是类句柄，第二个参数是方法名字，第三个参数是签名标识

Java类型	符号
boolean	Z
byte	B
char	C
short	S
int	I
long	L
float	F
double	D
void	V
objects对象	Lfully-qualified-class-name; L类名;
Arrays数组	[array-type [数组类型
methods方法	(argument-types)return-type(参数类型)返回类型

获得对象：

jobject obj=env->NewObject(cls,mid);
以上便获得了一个对象的句柄

获得对象成员变量：

jfieldID fid=env->GetFieldID(cls,"age","I");
以上和获得类方法差不多

操作成员变量：

jint a=env->GetIntField(obj,mid);
age=age+10;
env->SetIntField(obj,fid,a);

返回：

return obj;

下面是本人练习的例子

ExList.java

Java代码






package com.ldq.list;

import java.util.List;

public class ExList {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub

System.out.println("-------WifiManager.test()");
System.out.println(WifiManager.test());

System.out.println("-------WifiManager.testArray()");
String[] s1 = WifiManager.testArray();
for (int i = 0; i < s1.length; i++) {
System.out.println(s1[i]);
}

System.out.println("-------WifiManager.testObject()");
System.out.println(WifiManager.testObject().ssid);
System.out.println(WifiManager.testObject().mac);
System.out.println(WifiManager.testObject().level);

System.out.println("-------WifiManager.getScanResultsA()");
ScanResult[] s2 = WifiManager.getScanResultsA();
for (int i = 0; i < s2.length; i++) {
System.out.println(s2[i].ssid);
System.out.println(s2[i].mac);
System.out.println(s2[i].level);
}

System.out.println("-------WifiManager.getScanResults()");
List<ScanResult> list = WifiManager.getScanResults();
System.out.println(list.get(0).ssid);
System.out.println(list.get(0).mac);
System.out.println(list.get(0).level);

}
}

package com.ldq.list;import java.util.List;public class ExList {	/**	 * @param args	 */	public static void main(String[] args) {		// TODO Auto-generated method stub		System.out.println("-------WifiManager.test()");		System.out.println(WifiManager.test());		System.out.println("-------WifiManager.testArray()");		String[] s1 = WifiManager.testArray();		for (int i = 0; i < s1.length; i++) {			System.out.println(s1[i]);		}		System.out.println("-------WifiManager.testObject()");		System.out.println(WifiManager.testObject().ssid);		System.out.println(WifiManager.testObject().mac);		System.out.println(WifiManager.testObject().level);		System.out.println("-------WifiManager.getScanResultsA()");		ScanResult[] s2 = WifiManager.getScanResultsA();		for (int i = 0; i < s2.length; i++) {			System.out.println(s2[i].ssid);			System.out.println(s2[i].mac);			System.out.println(s2[i].level);		}		System.out.println("-------WifiManager.getScanResults()");		List<ScanResult> list = WifiManager.getScanResults();		System.out.println(list.get(0).ssid);		System.out.println(list.get(0).mac);		System.out.println(list.get(0).level);	}}

ScanResult.java

Java代码






package com.ldq.list;

public class ScanResult {
String ssid;
String mac;
int level;

public ScanResult() {
}

public ScanResult(String ssid, String mac, int level) {
this.ssid = ssid;
this.mac = mac;
this.level = level;
}
}

package com.ldq.list;public class ScanResult {	String ssid;	String mac;	int level;	public ScanResult() {	}	public ScanResult(String ssid, String mac, int level) {		this.ssid = ssid;		this.mac = mac;		this.level = level;	}}

WifiManager.java

Java代码






package com.ldq.list;

import java.util.List;

public class WifiManager {

static {
System.loadLibrary("wifi");
}

public native static String test();

public native static String[] testArray();

public native static ScanResult testObject();

public native static ScanResult[] getScanResultsA();

public native static List<ScanResult> getScanResults();
}

package com.ldq.list;import java.util.List;public class WifiManager {	static {		System.loadLibrary("wifi");	}	public native static String test();	public native static String[] testArray();	public native static ScanResult testObject();	public native static ScanResult[] getScanResultsA();	public native static List<ScanResult> getScanResults();}

wifi.cpp

Cpp代码






#include <jni.h>
#include "com_ldq_list_WifiManager.h"

/*
* Class: com_ldq_list_WifiManager
* Method: test
* Signature: ()Ljava/lang/String;
*/
JNIEXPORT jstring JNICALL Java_com_ldq_list_WifiManager_test (JNIEnv *env, jclass cls)
{
return env->NewStringUTF("hello");
}

/*
* Class: com_ldq_list_WifiManager
* Method: testArray
* Signature: ()[Ljava/lang/String;
*/
JNIEXPORT jobjectArray JNICALL Java_com_ldq_list_WifiManager_testArray (JNIEnv *env, jclass cls)
{
jobjectArray ret;
int i;

char *message[5]= {"first",
"second",
"third",
"fourth",
"fifth"};

ret= (jobjectArray)env->NewObjectArray(5,
env->FindClass("java/lang/String"),
env->NewStringUTF(""));

for(i=0;i<5;i++) {
env->SetObjectArrayElement(
ret,i,env->NewStringUTF(message[i]));
}
return(ret);

}

/*
* Class: com_ldq_list_WifiManager
* Method: testObject
* Signature: ()Lcom/ldq/list/ScanResult;

*/
JNIEXPORT jobject JNICALL Java_com_ldq_list_WifiManager_testObject (JNIEnv *env, jclass cls)
{
jclass m_cls = env->FindClass("com/ldq/list/ScanResult");
jmethodID mid = env->GetMethodID(m_cls,"<init>","()V");
jobject obj = env->NewObject(m_cls,mid);

jfieldID fid_ssid = env->GetFieldID(m_cls,"ssid","Ljava/lang/String;");
jfieldID fid_mac = env->GetFieldID(m_cls,"mac","Ljava/lang/String;");
jfieldID fid_level = env->GetFieldID(m_cls,"level","I");

env->SetObjectField(obj,fid_ssid,env->NewStringUTF("AP1"));
env->SetObjectField(obj,fid_mac,env->NewStringUTF("00-11-22-33-44-55"));
env->SetIntField(obj,fid_level,-66);
return obj;
}

/*
* Class: com_ldq_list_WifiManager
* Method: getScanResultsA
* Signature: ()[Lcom/ldq/list/ScanResult;

*/
JNIEXPORT jobjectArray JNICALL Java_com_ldq_list_WifiManager_getScanResultsA (JNIEnv *env, jclass cls)
{
jclass cls_array=env->FindClass("java/lang/Object");
jobjectArray obj_array=env->NewObjectArray(2,cls_array,0);

jclass cls_obj = env->FindClass("com/ldq/list/ScanResult");
jmethodID m = env->GetMethodID(cls_obj,"<init>","()V");

jfieldID fid_ssid = env->GetFieldID(cls_obj,"ssid","Ljava/lang/String;");
jfieldID fid_mac = env->GetFieldID(cls_obj,"mac","Ljava/lang/String;");
jfieldID fid_level = env->GetFieldID(cls_obj,"level","I");

for(int i=0;i<2;i++)
{
jobject obj=env->NewObject(cls_obj,m);

jobject o1=env->NewStringUTF("AP2");
env->SetObjectField(obj,fid_ssid,o1);

jobject o2=env->NewStringUTF("22-22-22-22-22-22");
env->SetObjectField(obj,fid_mac,o2);

env->SetIntField(obj,fid_level,-66);

env->SetObjectArrayElement(obj_array,i,obj);
}
return obj_array;
}

/*
* Class: com_ldq_list_WifiManager
* Method: getScanResults
* Signature: ()Ljava/util/List;
*/
JNIEXPORT jobject JNICALL Java_com_ldq_list_WifiManager_getScanResults (JNIEnv *env, jclass cls)
{
jclass m_cls_list = env->FindClass("java/util/ArrayList");
jmethodID m_mid_list = env->GetMethodID(m_cls_list,"<init>","()V");
jobject m_obj_list = env->NewObject(m_cls_list,m_mid_list);

jmethodID m_mid_add = env->GetMethodID(m_cls_list,"add","(Ljava/lang/Object;)Z");

jclass m_cls_result = env->FindClass("com/ldq/list/ScanResult");
jmethodID m_mid_result = env->GetMethodID(m_cls_result,"<init>","()V");
jobject m_obj_result = env->NewObject(m_cls_result,m_mid_result);

jfieldID m_fid_1 = env->GetFieldID(m_cls_result,"ssid","Ljava/lang/String;");
jfieldID m_fid_2 = env->GetFieldID(m_cls_result,"mac","Ljava/lang/String;");
jfieldID m_fid_3 = env->GetFieldID(m_cls_result,"level","I");

env->SetObjectField(m_obj_result,m_fid_1,env->NewStringUTF("AP6"));
env->SetObjectField(m_obj_result,m_fid_2,env->NewStringUTF("66-66-66-66-66-66"));
env->SetIntField(m_obj_result,m_fid_3,-66);

env->CallBooleanMethod(m_obj_list,m_mid_add,m_obj_result);

return m_obj_list;
}

===============================================数据类型转换===================================================
作者：陈波 2011/10/30（转载请注明出处,From:http://blog.csdn.net/jinhill/article/details/6918821）
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

package com.jinhill.util;
public class NativeModule {
public native int testArg(int i, boolean b, char c, double d);
public native byte[] testByte(byte[] b);
public native String[] testString(String s, String[] sarr);
public native int setInfo(MyInfo info);
public native MyInfo getInfo();
static {
System.loadLibrary("NativeModule");
}
}

其中MyInfo类定义如下：

public class Record {
int id;
String name;
byte[] data;
}

public class MyInfo {
public boolean b;
public char c;
public double d;
public int i;
public byte[] array;
public String s;
public Record rec;
}

C自定义结构体
typedef struct{

int id;

char name[255];

char data[255];

}Record;

typedef struct{

BOOL b;

char c;

double d;

int i;

char arr[255];

char sz[255];

Record rec;

}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

//不同类型参数处理

JNIEXPORT jintJNICALL Java_com_jinhill_util_NativeModule_testArg

(JNIEnv *env, jobject jo, jint ji, jbooleanjb, jchar jc, jdouble jd)

{

//获取jint型值

int i = ji;

//获取jboolean型值

BOOL b = jb;

//获取jdouble型值

double d = jd;

//获取jchar型值,Java的char两字节

char ch[5] = {0};

int size = 0;

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

if(size <= 0)

{

return -1;

}

Trace("ji=%d,jb=%d,jc=%s,jd=%lf",i, b, ch, d);

return 0;

}

2) Java byte与C char数组类型数组转换实例

//btye数组处理，形参作为输入或输出，返回btye数组

JNIEXPORTjbyteArray JNICALL Java_com_jinhill_util_NativeModule_testByte

(JNIEnv *env, jobject jo, jbyteArray jbArr)

{

char chTmp[] = "Hello JNI!";

int nTmpLen = strlen(chTmp);

//获取jbyteArray

char *chArr = (char*)env->GetByteArrayElements(jbArr,0);

//获取jbyteArray长度

int nArrLen = env->GetArrayLength(jbArr);

char *szStrBuf =(char*)malloc(nArrLen*2+10);

memset(szStrBuf, 0, nArrLen*2+10);

Bytes2String(chArr, nArrLen, szStrBuf,nArrLen*2+10);

Trace("jbArr=%s", szStrBuf);

//将jbArr作为输出形参

memset(chArr, 0, nArrLen);

memcpy(chArr, chTmp, nTmpLen);

//返回jbyteArray

jbyteArray jarrRV =env->NewByteArray(nTmpLen);

jbyte *jby =env->GetByteArrayElements(jarrRV, 0);

memcpy(jby, chTmp, strlen(chTmp));

env->SetByteArrayRegion(jarrRV, 0,nTmpLen, jby);

return jarrRV;

}

3) Java String与C char数组类型转换实例

//String 和String[]处理

JNIEXPORTjobjectArray JNICALL Java_com_jinhill_util_NativeModule_testString

(JNIEnv *env, jobject jo, jstring jstr,jobjectArray joarr)

{

int i = 0;

char chTmp[50] = {0};

//获取jstring值

const char* pszStr = (char*)env->GetStringUTFChars(jstr, 0);

Trace("jstr=%s", pszStr);

//获取jobjectArray值

int nArrLen =env->GetArrayLength(joarr);

Trace("joarr len=%d",nArrLen);

for(i=0; i<nArrLen; i++)

{

jstring js =(jstring)env->GetObjectArrayElement(joarr, i);

const char* psz = (char*)env->GetStringUTFChars(js, 0);

Trace("joarr[%d]=%s",i, psz);

}

//将joarr作为输出形参

jstring jstrTmp = NULL;

for(i=0; i<nArrLen; i++)

{

sprintf(chTmp, "No.%dHello JNI!", i);

jstrTmp =env->NewStringUTF(chTmp);

env->SetObjectArrayElement(joarr,i, jstrTmp);

env->DeleteLocalRef(jstrTmp);

}

//返回jobjectArray

jclass jstrCls =env->FindClass("Ljava/lang/String;");

jobjectArray jstrArray =env->NewObjectArray(2, jstrCls, NULL);

for(i=0; i<2; i++)

{

sprintf(chTmp, "No. %dReturn JNI!", i);

jstrTmp =env->NewStringUTF(chTmp);

env->SetObjectArrayElement(jstrArray,i, jstrTmp);

env->DeleteLocalRef(jstrTmp);

}

return jstrArray;

}

4) Java 类与C结构体类型转换实例

JNIEXPORT jint JNICALL Java_com_jinhill_util_NativeModule_setInfo

(JNIEnv *env, jobject jo, jobject jobj)

{

char chHexTmp[512] = {0};

//将Java类转换成C结构体

MyInfo mi;

//获取Java中的实例类Record

jclass jcRec = env->FindClass("com/jinhill/util/Record");

//int id

jfieldID jfid = env->GetFieldID(jcRec, "id", "I");

//String name

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

//byte[] data;

jfieldID jfdata = env->GetFieldID(jcRec, "data", "[B");

//获取Java中的实例类MyInfo

jclass jcInfo = env->FindClass("com/jinhill/util/MyInfo");

//获取类中每一个变量的定义

//boolean b

jfieldID jfb = env->GetFieldID(jcInfo, "b", "Z");

//char c

jfieldID jfc = env->GetFieldID(jcInfo, "c", "C");

//double d

jfieldID jfd = env->GetFieldID(jcInfo, "d", "D");

//int i

jfieldID jfi = env->GetFieldID(jcInfo, "i", "I");

//byte[] array

jfieldID jfa = env->GetFieldID(jcInfo, "array", "[B");

//String s

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

//Record rec;

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

//获取实例的变量b的值

mi.b = env->GetBooleanField(jobj, jfb);

//获取实例的变量c的值

jchar jc = env->GetCharField(jobj, jfc);

char ch[5] = {0};

int size = 0;

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

mi.c = ch[0];

//获取实例的变量d的值

mi.d = env->GetDoubleField(jobj, jfd);

//获取实例的变量i的值

mi.i = env->GetIntField(jobj, jfi);

//获取实例的变量array的值

jbyteArray ja = (jbyteArray)env->GetObjectField(jobj, jfa);

int nArrLen = env->GetArrayLength(ja);

char *chArr = (char*)env->GetByteArrayElements(ja, 0);

memcpy(mi.arr, chArr, nArrLen);

//获取实例的变量s的值

jstring jstr = (jstring)env->GetObjectField(jobj, jfs);

const char* pszStr = (char*)env->GetStringUTFChars(jstr, 0);

strcpy(mi.sz, pszStr);

//获取Record对象

jobject joRec = env->GetObjectField(jobj, jfrec);

//获取Record对象id值

mi.rec.id = env->GetIntField(joRec, jfid);

Trace("mi.rec.id=%d",mi.rec.id);

//获取Record对象name值

jstring jstrn = (jstring)env->GetObjectField(joRec, jfname);

pszStr = (char*)env->GetStringUTFChars(jstrn, 0);

strcpy(mi.rec.name, pszStr);

//获取Record对象data值

jbyteArray jbd = (jbyteArray)env->GetObjectField(joRec, jfdata);

nArrLen = env->GetArrayLength(jbd);

chArr = (char*)env->GetByteArrayElements(jbd, 0);

memcpy(mi.rec.data, chArr, nArrLen);

//日志输出

Bytes2String(mi.arr, nArrLen, chHexTmp, sizeof(chHexTmp));

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);

return 0;

}

5) C结构体类型与Java 类转换实例

JNIEXPORT jobject JNICALL Java_com_jinhill_util_NativeModule_getInfo

(JNIEnv *env, jobject jo)

{

wchar_t wStr[255] = {0};

char chTmp[] = "Hello JNI";

int nTmpLen = strlen(chTmp);

//将C结构体转换成Java类

MyInfo mi;

memcpy(mi.arr, chTmp, strlen(chTmp));

mi.b = TRUE;

mi.c = 'B';

mi.d = 2000.9;

mi.i = 8;

strcpy(mi.sz, "Hello World!");

mi.rec.id = 2011;

memcpy(mi.rec.data, "\x01\x02\x03\x04\x05\x06", 6);

strcpy(mi.rec.name, "My JNI");

//获取Java中的实例类Record

jclass jcRec = env->FindClass("com/jinhill/util/Record");

//int id

jfieldID jfid = env->GetFieldID(jcRec, "id", "I");

//String name

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

//byte[] data;

jfieldID jfdata = env->GetFieldID(jcRec, "data", "[B");

//获取Java中的实例类

jclass jcInfo = env->FindClass("com/jinhill/util/MyInfo");

//获取类中每一个变量的定义

//boolean b

jfieldID jfb = env->GetFieldID(jcInfo, "b", "Z");

//char c

jfieldID jfc = env->GetFieldID(jcInfo, "c", "C");

//double d

jfieldID jfd = env->GetFieldID(jcInfo, "d", "D");

//int i

jfieldID jfi = env->GetFieldID(jcInfo, "i", "I");

//byte[] array

jfieldID jfa = env->GetFieldID(jcInfo, "array", "[B");

//String s

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

//Record rec;

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

//创建新的对象

jobject joRec = env->AllocObject(jcRec);

env->SetIntField(joRec, jfid, mi.rec.id);

jstring jstrn = env->NewStringUTF(mi.rec.name);

env->SetObjectField(joRec, jfname, jstrn);

jbyteArray jbarr = env->NewByteArray(6);

jbyte *jb = env->GetByteArrayElements(jbarr, 0);

memcpy(jb, mi.rec.data, 6);

env->SetByteArrayRegion(jbarr, 0, 6, jb);

env->SetObjectField(joRec, jfdata, jbarr);

//创建新的对象

jobject joInfo = env->AllocObject(jcInfo);

//给类成员赋值

env->SetBooleanField(joInfo, jfb, mi.b);

// MultiByteToWideChar (CP_ACP, 0, mi.c, -1, wStr, 255);

// env->SetCharField(joInfo, jfc, (jchar)wStr);

env->SetCharField(joInfo, jfc, (jchar)mi.c);

env->SetDoubleField(joInfo, jfd, mi.d);

env->SetIntField(joInfo, jfi, mi.i);

jbyteArray jarr = env->NewByteArray(nTmpLen);

jbyte *jby = env->GetByteArrayElements(jarr, 0);

memcpy(jby, mi.arr, nTmpLen);

env->SetByteArrayRegion(jarr, 0, nTmpLen, jby);

env->SetObjectField(joInfo, jfa, jarr);

jstring jstrTmp = env->NewStringUTF(chTmp);

env->SetObjectField(joInfo, jfs, jstrTmp);

env->SetObjectField(joInfo, jfrec, joRec);

return joInfo;

}

4. 编写Java测试代码

public class TestInfo {

/**
* @param args
*/
public static void main(String[] args) {
int i =0;
String[] sArr = new String[2];
for(i=0; i<2; i++)
{
sArr[i] = "ID=" + i;
}
byte[] b = new byte[10];
for(i=0; i<10; i++)
{
b[i] = (byte)i;
}
MyInfo mi = new MyInfo();
mi.array = b;
mi.b = false;
mi.c = 'C';
mi.d = 2011.11;
mi.i = 1752;
mi.s = "Hello World!";

mi.rec = new Record();
mi.rec.id = 2012;
mi.rec.name = "Record";
mi.rec.data = b;
NativeModule nm = new NativeModule();
nm.testArg(mi.i, mi.b, mi.c, mi.d);
byte[] b2 = nm.testByte(mi.array);
String[] s = nm.testString(mi.s, sArr);
nm.setInfo(mi);
MyInfo mi2 = nm.getInfo();
System.out.println("finish");
}

}

5. Java String与 C char数组转换时的中文问题
//将jstring类型转换成windows类型

char* jstringToWindows( JNIEnv *env, jstring jstr )

{

int length = (env)->GetStringLength(jstr );

const jchar* jcstr = (env)->GetStringChars(jstr, 0 );

char* rtn = (char*)malloc( length*2+1 );

int size = 0;

size = WideCharToMultiByte( CP_ACP, 0, (LPCWSTR)jcstr, length,rtn,(length*2+1), NULL, NULL );

if( size <= 0 )

return NULL;

(env)->ReleaseStringChars(jstr, jcstr );

rtn[size] = 0;

return rtn;

}

//将windows类型转换成jstring类型

jstring WindowsTojstring( JNIEnv* env, char* str )

{

jstring rtn = 0;

int slen = strlen(str);

unsigned short * buffer = 0;

if( slen == 0 )

rtn = (env)->NewStringUTF(str );

else

{

int length = MultiByteToWideChar( CP_ACP, 0, (LPCSTR)str, slen, NULL, 0 );

buffer = (unsigned short *)malloc( length*2 + 1 );

if( MultiByteToWideChar( CP_ACP, 0, (LPCSTR)str, slen, (LPWSTR)buffer, length )>0 )

rtn = (env)->NewString( (jchar*)buffer, length );

}

if( buffer )

free( buffer );

return rtn;

}

6.
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