JNI in C++ to read file to jbyteArray

问题

I am writing a C++ program in UNIX to generate a shared library which will be called in java using JNI. This C++ program has to read a file in UNIX box then it will have to be converted into jbyteArray (JNI data type) so that JAVA can use it.

I read the file in C++ into char* but could not convert into jbyteArray. Please help it.

Code is below::

#include <iostream>
#include <fstream>
#include <stdio.h>
#include "com_sp_dll_NativeMethods.h"   // this header file was generated by java

using namespace std;

JNIEXPORT void JNICALL Java_HelloWorld_displayMessage(JNIEnv *env, jobject obj)
{
    printf("Hello World!\n");
}

JNIEXPORT jbyteArray JNICALL Java_com_sp_dll_NativeMethods_getFile(JNIEnv *env, jobject obj)
{
    ifstream fl("/home/rkannan/myFile.txt");
    fl.seekg(0, ios::end );
    size_t len = fl.tellg();
    char *ret = new char[len];
    fl.seekg(0, ios::beg);
    fl.read(ret, len);
    fl.close();

    int i = 0;
    jbyte *a1 = null;

    while(ret[i] != '\0'){
        cout<<ret[i];
        al[i] = ret[i];
        i++;
    }

    jbyteArray result = env->NewByteArray( len);
    env->SetByteArrayRegion( result, 0, len, (const jbyte*) ret );
    delete[] ret;
    return result;
  }

The error is as follows

bash-3.00$ g++ -I /usr/jdk/instances/jdk1.5.0/include -I /usr/jdk/instances/jdk1.5.0/include/solaris -o libSample.so -shared com_sp_dll_NativeMethods.cpp
        Text relocation remains                         referenced
            against symbol                  offset      in file
        std::__ioinit                       0x550       /var/tmp//ccKCiEKq.o
        std::__ioinit                       0x554       /var/tmp//ccKCiEKq.o
        std::__ioinit                       0x588       /var/tmp//ccKCiEKq.o
        std::__ioinit                       0x58c       /var/tmp//ccKCiEKq.o
        .rodata (section)                   0x204       /var/tmp//ccKCiEKq.o
        .rodata (section)                   0x208       /var/tmp//ccKCiEKq.o
        .rodata (section)                   0x244       /var/tmp//ccKCiEKq.o
        .rodata (section)                   0x248       /var/tmp//ccKCiEKq.o
        .rodata (section)                   0x2f4       /var/tmp//ccKCiEKq.o
        .rodata (section)                   0x2f8       /var/tmp//ccKCiEKq.o
        std::fpos<__mbstate_t>::operator long long() const0x348         /var/tmp//ccKCiEKq.o
        std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char)0x400             /var/tmp//ccKCiEKq.o
        std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)0x2cc      /var/tmp//ccKCiEKq.o
        std::basic_ifstream<char, std::char_traits<char> >::is_open()0x260      /var/tmp//ccKCiEKq.o
        std::basic_istream<char, std::char_traits<char> >::read(char*, int)0x39c        /var/tmp//ccKCiEKq.o
        JNIEnv_::SetByteArrayRegion(_jbyteArray*, int, int, signed char const*)0x468            /var/tmp//ccKCiEKq.o
        operator new[](unsigned int)        0x364       /var/tmp//ccKCiEKq.o
        std::basic_istream<char, std::char_traits<char> >::seekg(long long, std::_Ios_Seekdir)0x31c             /var/tmp//ccKCiEKq.o
        std::basic_istream<char, std::char_traits<char> >::seekg(long long, std::_Ios_Seekdir)0x384             /var/tmp//ccKCiEKq.o
        std::basic_istream<char, std::char_traits<char> >& std::operator>><char, std::char_traits<char> >(std::basic_istream<char, std::char_traits<char> >&, char*)0x2b4       /var/tmp//ccKCiEKq.o
        printf                              0x20c       /var/tmp//ccKCiEKq.o
        JNIEnv_::NewByteArray(int)          0x444       /var/tmp//ccKCiEKq.o
        std::ios_base::Init::Init()         0x558       /var/tmp//ccKCiEKq.o
        std::basic_string<char, std::char_traits<char>, std::allocator<char> >::size() const0x14        /var/tmp//ccKCiEKq.o
        std::cout                           0x2c0       /var/tmp//ccKCiEKq.o
        std::cout                           0x2c4       /var/tmp//ccKCiEKq.o
        std::cout                           0x3f4       /var/tmp//ccKCiEKq.o
        std::cout                           0x3f8       /var/tmp//ccKCiEKq.o
        std::basic_ifstream<char, std::char_traits<char> >::basic_ifstream()0x234       /var/tmp//ccKCiEKq.o
        std::basic_string<char, std::char_traits<char>, std::allocator<char> >::operator[](unsigned int) const0x9c              /var/tmp//ccKCiEKq.o
        std::basic_string<char, std::char_traits<char>, std::allocator<char> >::operator[](unsigned int) const0x128             /var/tmp//ccKCiEKq.o
        std::basic_string<char, std::char_traits<char>, std::allocator<char> >::operator[](unsigned int) const0x184             /var/tmp//ccKCiEKq.o
        std::basic_ifstream<char, std::char_traits<char> >::open(char const*, std::_Ios_Openmode)0x250          /var/tmp//ccKCiEKq.o
        std::ios_base::Init::~Init()        0x590       /var/tmp//ccKCiEKq.o
        std::basic_ios<char, std::char_traits<char> >::eof() const0x288         /var/tmp//ccKCiEKq.o
        unsigned int const& std::min<unsigned int>(unsigned int const&, unsigned int const&)0x44        /var/tmp//ccKCiEKq.o
        std::basic_ifstream<char, std::char_traits<char> >::~basic_ifstream()0x49c      /var/tmp//ccKCiEKq.o
        std::basic_ifstream<char, std::char_traits<char> >::~basic_ifstream()0x4ac      /var/tmp//ccKCiEKq.o
        std::basic_ifstream<char, std::char_traits<char> >::~basic_ifstream()0x4d4      /var/tmp//ccKCiEKq.o
        std::basic_ifstream<char, std::char_traits<char> >::~basic_ifstream()0x4f8      /var/tmp//ccKCiEKq.o
        std::basic_ifstream<char, std::char_traits<char> >::close()0x2e4        /var/tmp//ccKCiEKq.o
        std::basic_ifstream<char, std::char_traits<char> >::close()0x3ac        /var/tmp//ccKCiEKq.o
           std::basic_ifstream<char, std::char_traits<char> >::basic_ifstream(char const*,     std::_Ios_Openmode)0x300        /var/tmp//ccKCiEKq.o
            operator delete[](void*)            0x484       /var/tmp//ccKCiEKq.o
           std::basic_istream<char, std::char_traits<char> >::tellg()       0x334         /var/tmp//ccKCiEKq.o
            _Unwind_Resume                      0x508       /var/tmp//ccKCiEKq.o
            ld: fatal: relocations remain against allocatable but non-writable sections
            collect2: ld returned 1 exit status

回答1:

You don't appear to be setting any byte data to the array. In fact as a jbyte is typedef'd to a char you can directly set your char array in setByteArrayRegion as follows:

env->SetByteArrayRegion( result, 0, 100, ret );

Edit: Also, assuming that the al[i] should be a1[i] you are doing something very dangerous as you are not allocating space for a1. Basically doing as I suggest above means you can get rid of that entire loop. Also don't forget to delete the C++ Array when you have finished with it (ie copied it into the jByteArray)!

JNIEXPORT jbyteArray JNICALL Java_com_sp_dll_NativeMethods_getFile
       (JNIEnv *env, jobject obj)
{
        ifstream fl("/home/rkannan/myFile.txt");
        fl.seekg(0, ios::end );
        size_t len = fl.tellg();
        char *ret = new char[len];
        fl.seekg(0, ios::beg);
        fl.read(ret, len);
        fl.close();

        jbyteArray result = env->NewByteArray( len);
        env->SetByteArrayRegion( result, 0, len, (const jbyte*)ret );

        delete[] ret;

        return result;
}

来源：https://stackoverflow.com/questions/12854333/jni-in-c-to-read-file-to-jbytearray

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java

c++

java-native-interface

jnienv

jniwrapper