Making jar files using API

Question

hello!
I m trying to make a jar file using the API. When the jar tool is used for the same manifest file, it is correctly written and an executable jar is created. When I use the API, the jar file is made but it is nor executable. Here is that part of code:
jar = new File(parent, jarName);
System.out.println("Executing jar command...");
//create jar file here
Manifest mf = new Manifest(new FileInputStream(manifest));
fos = new FileOutputStream(jar);
jos = new JarOutputStream(new BufferedOutputStream(fos), mf);
BufferedInputStream reader = null;
byte[] data = new byte[BUFFER];
int byteCount = 0;
for(int i = 0; i < fileNames.length; i++)
System.out.println("Adding " + fileNames);
FileInputStream fis = new FileInputStream(fileNames);
reader = new BufferedInputStream(fis, BUFFER);
JarEntry entry = new JarEntry(new ZipEntry(fileNames));
jos.putNextEntry(entry);
while((byteCount = reader.read(data,0,BUFFER)) != -1)
jos.write(data, 0, byteCount);
}//end while
reader.close();
}//end for
jos.close();//close jar output stream
fos.close();
I m sure someone will be kind and intelligent enough to solve the problem. Thank you!
Umer

Answer 1

A jar file is simply a Zip file. So the two API's are quite similar. First, you create a CRC - that is a way to make sure that the file isn't corrupted and that all the bytes are there (it's a polynominal algorithm that returns a number which is written in the Zip(Jar) file as well). Each entry is a file or directory. Before writing any data to the Zip(Jar) file, you have to write info on the actual file (like name, path, length etc). That's why you use putNextEntry(). Each file can have it's storing methods, although the most common is to use the "default" methods (built in the ZipOutputStream/JarOutputStream)
But in this example you set the storing methods on the entry. You use the setMethod() method to switch between either storing or compression (don't know why, but in this example you don't compress the file - you should). I myself don't use the setSize() and setCompressedSize() methods at all (and it works fine). Don't know exactly what are the implications of using them. Neither do I use the setCrc() method (If I think a little, I've only used this API once - I don't use it that much), but it's quite straightforward. It's used to check that the data is ok, and since the CRC number is the result of an algorithm, it needs to know the bytes of UNCOMPRESSED data: that's why you use crc.update() on the bytes. After that you actually write all the info set before to the Zip (Jar) file using putNextEntry(). But remember that a ZipEntry doesn't have any data. You'll have to write it yourself: thus the need for the write method: Here are two small programs I used for Zipping and Unzipping. Changing them to work for Jar files is extremely simple:
//Zip.java
import java.io.*;
import java.util.zip.*;
public class Zip
     public static void main(String[] arg)
          try
               if (arg.length < 3)
                    System.out.println("Usage: java Zip <0-9> <zip file> <file 1> [file 2] [file 3] ...");
                    System.exit(0);
               System.out.println("Compressing files into file " + arg[1]);
               ZipOutputStream out = null;
               try
                    out = new ZipOutputStream(new FileOutputStream(arg[1]));
               catch (FileNotFoundException ex)
                    System.out.println("Cannot create file " + arg[1]);
                    System.exit(1);
               try
                    out.setLevel(Integer.parseInt(arg[0]));
               catch (IllegalArgumentException ex)
                    System.out.println("Illegal compression level");
                    new File(arg[1]).delete();
                    System.exit(1);
               for (int i=2; i<arg.length; i++)
                    System.out.println("\tCompressing file " + (i-1));
                    FileInputStream read = null;
                    try
                         read = new FileInputStream(arg);
                    catch (FileNotFoundException ex)
                         System.out.println("\tCannot find file " + arg[i]);
                         continue;
                    out.putNextEntry(new ZipEntry(arg[i]));
                    int av = 0;
                    while ((av = read.available()) != 0)
                         byte[] b = new byte[av < 64 ? av : 64];
                         read.read(b);
                         out.write(b);
                    out.closeEntry();
                    read.close();
                    System.out.println("\tDone compressing file " + i);
               System.out.println("Done compressing");
               out.finish();
               out.close();
          catch (Exception e)
               new File(arg[1]).delete();
               e.printStackTrace(System.err);
And the unzipping app:
//UnZip.java
import java.io.*;
import java.util.*;
import java.util.zip.*;
public class UnZip
     public static void main(String[] arg)
          try
               if (arg.length < 1)
                    System.out.println("Usage: java UnZip <zip file> [to dir]");
                    System.exit(0);
               String dir = "";
               if (arg.length > 1)
                    dir = arg[1] + System.getProperty("file.separator");
                    File f = new File(arg[1]);
                    if (!f.exists()) f.mkdirs();
               System.out.println("Decompressing files from file " + arg[0]);
               ZipFile read = null;
               try
                    read = new ZipFile(arg[0]);
               catch (ZipException ex)
                    System.err.println("Zip error when reading file " + arg[0]);
                    System.exit(1);
               catch (IOException ex)
                    System.err.println("I/O Exception when reading file " + arg[0]);
                    System.exit(1);
               Enumeration en = read.entries();
               while (en.hasMoreElements())
                    ZipEntry entry = (ZipEntry) en.nextElement();
                    System.out.println("\tDecompressing file " + entry.getName());
                    FileOutputStream out;
                    try
                         out = new FileOutputStream(dir + entry.getName());
                    catch (FileNotFoundException ex)
                         System.err.println("\tCannot write down to file " + dir + entry.getName());
                         continue;
                    InputStream re = read.getInputStream(entry);
                    int nRead = 0;
                    for (byte[] buffer = new byte[1024]; (nRead = re.read(buffer)) != -1; out.write(buffer, 0, nRead));
                    out.close();
                    System.out.println("\tDone decompressing file " + entry.getName());
               read.close();
               System.out.println("Done decompressing files");
          catch (Exception e)
               new File(arg[1]).delete();
               e.printStackTrace(System.err);