JNI Invocation: open file in Java, write file in CPP

Question

Hello,
Warning: I am a dunce, bad CPP code ahead!
Using JNI invocation, I am trying to read a binary file in Java and write it in CPP.
Everything compiles and runs without error, but the input and output jpg files are of course different.
TIA to any help,
kirst
(begin ByteMe.java)
import java.io.*;
public class ByteMe
    // Returns the contents of the file in a byte array.
    public byte[] getBytesFromFile(String strInfo) throws IOException
        System.out.println("Testing:" + strInfo);
        File file = new File("1.jpg");
        InputStream is = new FileInputStream(file);
        // Get the size of the file
        long length = file.length();
        // Create the byte array to hold the data
        byte[] bytes = new byte[(int)length];
        // Read in the bytes
        int offset = 0;
        int numRead = 0;
        while (offset < bytes.length
               && (numRead=is.read(bytes, offset, bytes.length-offset)) >= 0) {
            offset += numRead;
        // Ensure all the bytes have been read in
        if (offset < bytes.length)
            throw new IOException("Could not completely read file "+file.getName());
        // Close the input stream and return bytes
        is.close();
        return bytes;
    public ByteMe()
          //System.out.println("in constructor");
}(end ByteMe.java)
(begin ByteMe.cpp)
#include <stdlib.h>
#include <string.h>
#include <jni.h>
#include <windows.h>
// for file operations:
#include <fstream>
int main( int argc, char *argv[] )
     JNIEnv *env;
     JavaVM *jvm;
     JavaVMInitArgs vm_args;
     JavaVMOption options[5];
     jint res;
     jclass cls;
     jmethodID mid;
     jstring jstr;
     jobject obj_print;
     options[0].optionString = "-Xms4M";
     options[1].optionString = "-Xmx64M";
     options[2].optionString = "-Xss512K";
     options[3].optionString = "-Xoss400K";
     options[4].optionString = "-Djava.class.path=.";
     vm_args.version = JNI_VERSION_1_4;
     vm_args.options = options;
     vm_args.nOptions = 5;
     vm_args.ignoreUnrecognized = JNI_FALSE;
     // Create the Java VM
     res = JNI_CreateJavaVM(&jvm,(void**)&env,&vm_args);
     if (res < 0)
          printf("Can't create Java VM");
          goto destroy;
     cls = env->FindClass("ByteMe");
     if (cls == 0)
          printf("Can't find ByteMe class");
          goto destroy;
     jmethodID id_construct = env->GetMethodID(cls,"<init>","()V");
     jstr = env->NewStringUTF(" from C++\n");
     obj_print = env->NewObject(cls,id_construct);
     // signature obtained using javap -s -p ByteMe
     mid = env->GetMethodID(cls, "getBytesFromFile", "(Ljava/lang/String;)[B");
     if (mid == 0)
          printf("Can't find ByteMe.getBytesFromFile\n");
          goto destroy;
     else
          jbyteArray jbuf = (jbyteArray) env->CallObjectMethod(obj_print,mid,jstr);
          jlong size = jsize(jbuf);
          printf("size is: %d\n", size); // size shown in output is
          std::ofstream out("data.jpg", std::ios::binary);
          out.write ((const char *)jbuf, 100000);     
     destroy:
         if (env->ExceptionOccurred())
            env->ExceptionDescribe();
    jvm->DestroyJavaVM();
}(end ByteMe.cpp)

Answer 1

Hello,
Me again. Well, not such a dunce after all. Here is code that works correctly, and compiles with no errors and no warnings.
Will much appreciate help with clean-up code.
TIA,
kirst
(begin ByteMe.java)
import java.io.*;
public class ByteMe
    public long getFilezize(String strInfo) throws IOException
          // demonstrates String parameter passed from CPP to Java:
          System.out.println("(getFilesize) Hello world" + strInfo);
          File file = new File("1.bmp");
          InputStream is = new FileInputStream(file);
          // Get the size of the file
          long length = file.length();
          is.close();
          return length;
    // Returns the contents of the file in a byte array.
    public byte[] getBytesFromFile(String strInfo) throws IOException
        System.out.println("(getBytesFromFile) Hello world" + strInfo);
        File file = new File("1.bmp");
        InputStream is = new FileInputStream(file);
        // Get the size of the file
        long length = file.length();
        System.out.println("length is: " + String.valueOf(length));
        // Create the byte array to hold the data
        byte[] bytes = new byte[(int)length];
        // Read in the bytes
        int offset = 0;
        int numRead = 0;
        while (offset < bytes.length && (numRead=is.read(bytes, offset, bytes.length-offset)) >= 0)
            offset += numRead;
        // Ensure all the bytes have been read in
        if (offset < bytes.length)
            throw new IOException("Could not completely read file "+ file.getName());
        // Close the input stream and return bytes
        is.close();
        return bytes;
    public ByteMe()
          //System.out.println("in constructor");
}(end ByteMe.java)
(begin ByteMe.cpp)
          Signature obtained with command:
               "C:\Program Files\Java\jdk1.5.0_15\bin\javap.exe" -s -p ByteMe
               Compiled from "ByteMe.java"
               public class ByteMe extends java.lang.Object{
               public long getFilezize(java.lang.String)   throws java.io.IOException;
                 Signature: (Ljava/lang/String;)J
               public byte[] getBytesFromFile(java.lang.String)   throws java.io.IOException;
                 Signature: (Ljava/lang/String;)[B
               public ByteMe();
                 Signature: ()V
     Compiled VC++ 2005 Express, run on Vista
#include <stdlib.h>
#include <string.h>
#include <jni.h>
// file operations
#include <fstream>
int main( int argc, char *argv[] )
     JNIEnv *env;
     JavaVM *jvm;
     JavaVMInitArgs vm_args;
     JavaVMOption options[5];
     jint res;
     jclass cls;
     jmethodID mid;
     jmethodID sizeid;
     jstring jstr;
     jobject obj_print;
     jlong filesize;
     options[0].optionString = "-Xms4M";
     options[1].optionString = "-Xmx64M";
     options[2].optionString = "-Xss512K";
     options[3].optionString = "-Xoss400K";
     options[4].optionString = "-Djava.class.path=.";
     vm_args.version = JNI_VERSION_1_4;
     vm_args.options = options;
     vm_args.nOptions = 5;
     vm_args.ignoreUnrecognized = JNI_FALSE;
     // Create the Java VM
     res = JNI_CreateJavaVM(&jvm,(void**)&env,&vm_args);
     if (res < 0)
          printf("Can't create Java VM");
          goto destroy;
     cls = env->FindClass("ByteMe");
     if (cls == 0)
          printf("Can't find ByteMe class");
          goto destroy;
     jmethodID id_construct = env->GetMethodID(cls,"<init>","()V");
     printf("%s\n",id_construct);
     jstr = env->NewStringUTF(" from C++!\n");
     if (jstr == 0)
          // Normally not useful
          printf("Out of memory (could not instantiate new string jstr)\n");
          goto destroy;
     obj_print = env->NewObject(cls,id_construct);
     //BEGIN BLOCK to get file size
     sizeid = env->GetMethodID(cls, "getFilezize", "(Ljava/lang/String;)J");
     if (sizeid == 0)
          printf("Can't find ByteMe.getFilezize\n");
          goto destroy;
     else
          printf("got here\n");
          filesize =(jlong) env->CallObjectMethod(obj_print,sizeid,jstr);
          printf("got filesize\n");
     // END BLOCK to get file size
     // BEGIN BLOCK to write file
     mid = env->GetMethodID(cls, "getBytesFromFile", "(Ljava/lang/String;)[B");
     if (mid == 0)
          printf("Can't find ByteMe.getBytesFromFile\n");
          goto destroy;
     else
          jbyteArray ret =(jbyteArray) env->CallObjectMethod(obj_print,mid,jstr);
          // Access the bytes:
          jbyte *retdata = env->GetByteArrayElements(ret, NULL);
          // write the file
          std::ofstream out("data.bmp", std::ios::binary);
          //out.write ((const char *)retdata, 921654);
          out.write ((const char *)retdata, (long)filesize);
     // END BLOCK to write file
     destroy:
         if (env->ExceptionOccurred())
            env->ExceptionDescribe();
    jvm->DestroyJavaVM();
}(end ByteMe.cpp)