JNI和Qt通信 (Part 2)

Part2

JNI数据转换成C数据

e.g. jstring - GetStringUTFChars(), NewStringUTF(), ReleaseStringUTFChars()

JNIEXPORT  void  JNICALL Java_JNISample_sampleFunction(JNIEnv* env, jobject obj, jstring name)
{
    const  char * pname = env->GetStringUTFChars(name, NULL);
    env->ReleaseStringUTFChars(name, pname);
}

e.g. Array

JNIEXPORT jint JNICALL Java_IntArray_sumArray
        (JNIEnv *env, jobject obj, jintArray arr) {
    jint buf[10];
    jint i, sum = 0;
    // This line is necessary, since Java arrays are not guaranteed
    // to have a continuous memory layout like C arrays.
    env->GetIntArrayRegion(arr, 0, 10, buf);
    for  (i = 0; i < 10; i++) {
        sum += buf[i];
    }
    return  sum;
}

<Refer to> http://ironurbane.iteye.com/blog/425513

JNI的数据定义

// In "win\jni_mh.h" - machine header which is machine dependent
typedef  long             jint;
typedef  __int64          jlong;
typedef  signed  char      jbyte;

// In "jni.h"
typedef  unsigned  char    jboolean;
typedef  unsigned  short   jchar;
typedef  short            jshort;
typedef  float            jfloat;
typedef  double           jdouble;
typedef  jint            jsize;

C++ 调用Java方法

Read: http://stackoverflow.com/questions/819536/how-to-call-java-function-from-c

Windows http://public0821.iteye.com/blog/423941

Linux http://blog.sina.com.cn/s/blog_48eef8410100fjxr.html

JNI数据类型

Java Type	Native Type	Description
boolean	jboolean	8 bits, unsigned
byte	jbyte	8 bits, signed
char	jchar	16 bits, unsigned
double	jdouble	64 bits
float	jfloat	32 bits
int	jint	32 bits, signed
long	jlong	64 bits, signed
short	jshort	16 bits, signed
void	void	N/A

JNI的类型签名

Java Type	Signature
boolean	Z
byte	B
char	C
double	D
float	F
int	I
long	J
void	V
object	Lfully-qualified-class;
type[]	[type
method signature	( arg-types) ret-type

e.g.

Java side

class  JNISample
{
    public  native  void  launchSample();
    static
    {
        System.loadLibrary( "Sample" );
    }

    public  static  int  add( int  a, int  b) {
        return  a+b;
    }
    public  boolean  judge( boolean  bool) {
        return  !bool;
    }
}

C++side

JNIEnv *env = GetJNIEnv();  //Get env from JNI
jclass cls;
cls = env->FindClass( "JNISample" );
if (cls !=0)
{
    printf ( "find java class success\n" );
    // constructor
    mid = env->GetMethodID(cls, "<init>" , "()V" );
    if (mid !=0)
    {
        jobj=env->NewObject(cls,mid);
    }

    // static function
    mid = env->GetStaticMethodID( cls,  "add" ,  "(II)I" );
    if (mid !=0)
    {
        square = env->CallStaticIntMethod( cls, mid, 5,5);
    }

    // function returns boolean
    mid = env->GetMethodID( cls,  "judge" , "(Z)Z" );
    if (mid !=0){
        jnot = env->CallBooleanMethod(jobj, mid, 1);
    }
}

查看属性和方法的签名

Java版本 "java -version"

反编译工具 javap: 

javap -s -p -classpath R:\test.Demo

Check JNI version

#ifdef JNI_VERSION_1_4
printf ( "Version is 1.4 \n" );
#endif

使用API

jint GetVersion(JNIEnv *env);

返回值需要转换, Need convert the result from DEC to HEX;

JNI实现过程中的Issue

x86 or x64 "Can't load load IA 32-bit dll on a amd 64 bit platform" 

确定本机上的默认JVM的版本和动态库的版本一致(x86或x64), Make sure JAVA's default path; check with "java -version" in command line.

3rdParty can't find dependent libraries 保证所依赖的动态库都能被找到;

1) copy the dll into executable file's folder 2) System.load() the dlls by dependecy orders

JNI_CreateJavaVM failed 

C++创建JVM调用Java方法 

http://docs.oracle.com/javase/1.4.2/docs/guide/jni/jni-12.html#JNI_CreateJavaVM & http://blog.csdn.net/louka/article/details/7318656

[我机器上装了多个版本的Java, 测试的时候没有成功]

jvm.dll(C:\Program Files (x86)\Java\jdk1.7.0_17\jre\bin\client; C:\Program Files (x86)\Java\jdk1.7.0_17\jre\bin\server; need check); jvm.lib(C:\Program Files (x86)\Java\jdk1.7.0_17\lib)

<Refer to> http://home.pacifier.com/~mmead/jni/cs510ajp/ & http://www.ntu.edu.sg/home/ehchua/programming/java/JavaNativeInterface.html

Sample http://chnic.iteye.com/category/20179

JNI doc http://docs.oracle.com/javase/7/docs/technotes/guides/jni/ 

>JNA https://github.com/twall/jna/  XstartOnFirstThread

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