JNI GetByteArrayElements () Error

问题

I'm new in JNI so I'm not familiar in JNI and also English.

My JNI project is a simple File reading and writing. File reading in Java and passing the byte array to C API the write it into file using C.

My source code:

Java code is :

public class FileIO {
   static {
      System.loadLibrary("FileIO");         
   }

   private native void writeFile(byte[] msg);

   public static void main(String[] args) throws IOException { 

      byte[] array = Files.readAllBytes(new File("PtFBWCTn.mp3").toPath());

      // System.out.println(array.length);
      new FileIO(). writeFile(array); 
   }
}

C code :

#include <jni.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#include "HelloJNI.h"

JNIEXPORT void JNICALL Java_FileIO_ writeFile (JNIEnv *env, jobject job, jbyteArray array ){

    jsize num_bytes = (*env)->GetArrayLength(env, array);
    printf("Byte length : %d\n" , num_bytes);

    unsigned char * buffer;
    jbyte  *lib ;
    lib =(jbyte *) malloc( ( num_bytes +1 ) * sizeof(jbyte));

    (*env)->GetByteArrayRegion(env , array, 0 , num_bytes , lib);
    // lib = (*env)->GetByteArrayElements(env , array, 0);
    buffer =(char *) lib ;
    FILE *fp;
    fp=fopen("test.mp3", "wb");
    printf("size : %d , length : %d  ,contant : %s\n" ,(int)sizeof(buffer), (int)strlen(buffer) ,  buffer );

    fwrite(buffer, sizeof(buffer[0]), sizeof(buffer)/sizeof(buffer[0]), fp);
    return;
}

I got problem while I am passing (.zip , .mp3 , .mp4 , .jpg and .png) file byte array. I try some text file formats like .txt, .java , .c files are create what I expect.

What is the reason?

I tried (reading my java file(FileIO.java) in Java and the output is):

Byte length : 238653
size : 8 , length : 4  ,contant : import java.nio.file.Files;
import java.io.File;

public class FileIO {
   static {
      System.loadLibrary("FileIO");         
   }

   private native void writeFile(byte[] msg);

   public static void main(String[] args) throws IOException { 

      byte[] array = Files.readAllBytes(new File("PtFBWCTn.mp3").toPath());

      // System.out.println(array.length);
      new FileIO(). writeFile(array); 
   }
}

And I tried (some test.png and the output is):

Byte length : 381729
size : 8 , length : 8  ,contant : ?PNG

GetByteArrayElements () returns only 8 bytes while reading media and other some format. But in case of text file it returns correct amount of bytes. Please provide the reason for why the length of the 'png' and other some format length is not equal to Byte length. And provide way to how to solve this problem.

回答1:

    jsize num_bytes = (*env)->GetArrayLength(env, array);
    char * buffer;

    buffer = (char *) malloc (num_bytes);

    jbyte  *lib = (*env)->GetByteArrayElements(env , array, 0);

    memcpy ( buffer , lib , num_bytes ) ;

    FILE *fp;
    fp=fopen("test.png", "wb");
    fwrite(buffer, sizeof(char) , num_bytes , fp);

回答2:

First of all you shouldn't allocate extra byte for destination buffer:

lib =(jbyte *) malloc( ( num_bytes +1 ) * sizeof(jbyte));

it is unnecessary. Also you may omit sizeof(jbyte) since it is always 1:

lib = (jbyte *) malloc(num_bytes);

Then:

printf("size : %d , length : %d  ,contant : %s\n" ,(int)sizeof(buffer), (int)strlen(buffer) ,  buffer );

(int)sizeof(buffer) is a size of a pointer variable, not a size of addressed data, and program fairly tells you that pointer is 8-byte length object (you are on 64-bit host). Next, (int)strlen(buffer) doesn't return actual size of pointed data, instead it returns the number of bytes that precede the first zero ('\0') byte in your data.

And because of this strlen() result matches with length of data that were read from text files - they almost always don't contain zero bytes. And binary files, like PNG, MP3, so on contain zero bytes frequently, and actual length doesn't match with strlen() result.

To top it off: the only true data length - is the result of GetArrayLength().

EDIT

Actual arguments for fwrite() are not OK:

fwrite(buffer, sizeof(buffer[0]), sizeof(buffer)/sizeof(buffer[0]), fp);

sizeof(buffer)/sizeof(buffer[0] is the same as 8/1, since buffer is a 8-byte length pointer and buffer[0] is unsigned char i.e. one byte length entity. You should rewrite it like this:

fwrite(buffer, 1, num_bytes, fp);

And there is no standard function that can help you to print all data including zeros. I guess you should manually loop over each byte, format and print.

回答3:

As you can see the man says:

DESCRIPTION

The strlen() function calculates the length of the string pointed to by s, excluding the terminating null byte ('\0').

RETURN VALUE

The strlen() function returns the number of characters in the string pointed to by s.

That means that you cannot open a pure binary files and pretend to have the length of the content using that function, because of it stops counting chars at the first null terminator. In other words it stops counting at the first byte set to 0.

Obviously it works with text file that are encoded to ensure no null terminator between words.

The second error is (int)sizeof(buffer) it returns size in bytes of the object representation of type, that in your case is a pointer to unsigned char. On your platform (64 bits..) pointers are 8 bytes long.

Side note: you can avoid to cast return value of function that return size_t type using "%zu" format specifier.

printf("size : %zu , length : %zu  ,contant : %s\n" , sizeof(buffer), strlen(buffer) ,  buffer );

EDIT

You can't retrieve the size of read bytes that way.

There isn't a standard function that can retrieve size of a dynamically allocated array.

You must write your own code. For example for png file you can retrieve the length through the header of file. Take a look at PNG specifications

来源：https://stackoverflow.com/questions/38786093/jni-getbytearrayelements-error

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java

java-native-interface

c-strings