I am writing a sendmail milter application in Java. The incoming mails will
usually have image file as attachments. My application is currently able to
extract the ImageFile and save it on the filesystem. This part is working
perfectly.

        ((MimeBodyPart)p).saveFile(new File(p.getFileName()));

However, I would like to pass this file as a byte array to a c++ library
instead and do the file saving and additional processing there instead. I
know very little c++ though. Can someone help please? I am also not sure if
the code I have written is doing what I think it is doing i.e. converting
the file into a byte array and passing it to the native library.

My current Java code is as following :

 private Boolean passToDLL(Part p, Boolean result) throws IOException {
  InputStream is = null;
  try {
   is = p.getInputStream();

   DataInputStream dis = new DataInputStream(is);
   String data = "";
   int size = dis.available();
   int offset = 0;
   do{
    byte [] byteArray = new byte[size];
    dis.read(byteArray, offset, size);
    data += new String(byteArray);
    offset = size + 1;
    size = dis.available();
    }while(size > 0);
   byte [] fileData = data.getBytes();
      result = passAttachment(fileData);
  } catch (MessagingException e) {
   e.printStackTrace();
  }
  finally{
      try {
        if (is != null)
           is.close();
      } catch (IOException ex) {
         ex.printStackTrace();
      }
  }
  return result;
 }

 public native boolean passAttachment(byte[] buf);

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

My c++ code

#include <jni.h>
#include "MsgParser.h"
#include <stdio.h>

JNIEXPORT jboolean JNICALL Java_MsgParser_passAttachment
 (JNIEnv *env, jobject obj, jbyteArray array){

// not sure what to do here to save image file data in bytearray to
harddisk.

Perhaps this example will help in some way:

// A.java
class A
{
     static
     {
         System.loadLibrary("A");
     }
     private native static void writeToFile(byte[] bytes);

     public static final void main(String[] args)
     {
         writeToFile("Hello, world!".getBytes());
     }

// A wrapper class to ensure that GetByteArrayElements is always properly
// paired with a ReleaseByteArrayElements.
class jniByteArray
{
private:
     // Purposely not implemented to prevent copying.
     jniByteArray(const jniByteArray &);
     jniByteArray & operator=(const jniByteArray &);

     JNIEnv * m_env;
     jbyteArray m_byteArray;
     jbyte * m_bytes;
     jboolean m_isCopy;
public:
     jniByteArray(JNIEnv * env, jbyteArray bytes)
         : m_env(env)
         , m_byteArray(bytes)
         , m_bytes(env->GetByteArrayElements(bytes, &m_isCopy))
     {
         if (!m_bytes)
             throw std::bad_alloc();
     }

     ~jniByteArray()
     {
         m_env->ReleaseByteArrayElements(m_byteArray, m_bytes, 0);
     }

     char * getBytes()
     {
         return reinterpret_cast<char *>(m_bytes);
     }

     std::size_t getSize() const
     {
         return std::size_t(m_env->GetArrayLength(m_byteArray));
     }

     bool isCopy() const
     {
         return bool(m_isCopy) ;
     }

extern "C"
JNIEXPORT void JNICALL Java_A_writeToFile(JNIEnv * env,
     jclass, jbyteArray bytes)
{
     jniByteArray b(env, bytes);

     std::ofstream out("data.bin", std::ios::binary);
     out.write(b.getBytes(), b.getSize());