JNI和Qt通信 (Part 2)

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Part2

JNI数据转换成C数据

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

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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 TypeNative TypeDescriptionbooleanjboolean8 bits, unsignedbytejbyte8 bits, signedcharjchar16 bits, unsigneddoublejdouble64 bitsfloatjfloat32 bitsintjint32 bits, signedlongjlong64 bits, signedshortjshort16 bits, signedvoidvoidN/A

JNI的类型签名

Java TypeSignaturebooleanZbyteBcharCdoubleDfloatFintIlongJvoidVobjectLfully-qualified-class;type[][typemethod 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:

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

Check JNI version

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

使用API

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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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