Appending files to a zip file with Java

As others mentioned, it's not possible to append content to an existing zip (or war). However, it's possible to create a new zip on the fly without temporarily writing extracted content to disk. It's hard to guess how much faster this will be, but it's the fastest you can get (at least as far as I know) with standard Java. As mentioned by Carlos Tasada, SevenZipJBindings might squeeze out you some extra seconds, but porting this approach to SevenZipJBindings will still be faster than using temporary files with the same library.

Here's some code that writes the contents of an existing zip (war.zip) and appends an extra file (answer.txt) to a new zip (append.zip). All it takes is Java 5 or later, no extra libraries needed.

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Enumeration;
import java.util.zip.ZipEntry;
import java.util.zip.ZipFile;
import java.util.zip.ZipOutputStream;

public class Main {

    // 4MB buffer
    private static final byte[] BUFFER = new byte[4096 * 1024];

    /**
     * copy input to output stream - available in several StreamUtils or Streams classes 
     */    
    public static void copy(InputStream input, OutputStream output) throws IOException {
        int bytesRead;
        while ((bytesRead = input.read(BUFFER))!= -1) {
            output.write(BUFFER, 0, bytesRead);
        }
    }

    public static void main(String[] args) throws Exception {
        // read war.zip and write to append.zip
        ZipFile war = new ZipFile("war.zip");
        ZipOutputStream append = new ZipOutputStream(new FileOutputStream("append.zip"));

        // first, copy contents from existing war
        Enumeration<? extends ZipEntry> entries = war.entries();
        while (entries.hasMoreElements()) {
            ZipEntry e = entries.nextElement();
            System.out.println("copy: " + e.getName());
            append.putNextEntry(e);
            if (!e.isDirectory()) {
                copy(war.getInputStream(e), append);
            }
            append.closeEntry();
        }

        // now append some extra content
        ZipEntry e = new ZipEntry("answer.txt");
        System.out.println("append: " + e.getName());
        append.putNextEntry(e);
        append.write("42\n".getBytes());
        append.closeEntry();

        // close
        war.close();
        append.close();
    }
}

I had a similar requirement sometime back - but it was for reading and writing zip archives (.war format should be similar). I tried doing it with the existing Java Zip streams but found the writing part cumbersome - especially when directories where involved.

I'll recommend you to try out the TrueZIP (open source - apache style licensed) library that exposes any archive as a virtual file system into which you can read and write like a normal filesystem. It worked like a charm for me and greatly simplified my development.

I don't know of a Java library that does what you describe. But what you described is practical. You can do it in .NET, using DotNetZip.

Michael Krauklis is correct that you cannot simply "append" data to a war file or zip file, but it is not because there is an "end of file" indication, strictly speaking, in a war file. It is because the war (zip) format includes a directory, which is normally present at the end of the file, that contains metadata for the various entries in the war file. Naively appending to a war file results in no update to the directory, and so you just have a war file with junk appended to it.

What's necessary is an intelligent class that understands the format, and can read+update a war file or zip file, including the directory as appropriate. DotNetZip does this, without uncompressing/recompressing the unchanged entries, just as you described or desired.

As Cheeso says, there's no way of doing it. AFAIK the zip front-ends are doing exactly the same as you internally.

Anyway if you're worried about the speed of extracting/compressing everything, you may want to try the SevenZipJBindings library.

I covered this library in my blog some months ago (sorry for the auto-promotion). Just as an example, extracting a 104MB zip file using the java.util.zip took me 12 seconds, while using this library took 4 seconds.

In both links you can find examples about how to use it.

Hope it helps.

See this bug report.

Using append mode on any kind of structured data like zip files or tar files is not something you can really expect to work. These file formats have an intrinsic "end of file" indication built into the data format.

If you really want to skip the intermediate step of un-waring/re-waring, you could read the war file file, get all the zip entries, then write to a new war file "appending" the new entries you wanted to add. Not perfect, but at least a more automated solution.

Based on the answer given by @sfussenegger above, following code is used to append to a jar file and download it:

public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    Resource resourceFile = resourceLoader.getResource("WEB-INF/lib/custom.jar");
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    try (ZipOutputStream zos = new ZipOutputStream(baos, StandardCharsets.ISO_8859_1);) {
        try (ZipFile zin = new ZipFile(resourceFile.getFile(), StandardCharsets.ISO_8859_1);) {
            zin.stream().forEach((entry) -> {
                try {
                    zos.putNextEntry(entry);
                    if (!entry.isDirectory()) {
                        zin.getInputStream(entry).transferTo(zos);
                    }
                    zos.closeEntry();
                } catch (Exception ex) {
                    ex.printStackTrace();
                }
            });
        }
        /* build file records to be appended */
        ....
        for (FileContents record : records) {
            zos.putNextEntry(new ZipEntry(record.getFileName()));
            zos.write(record.getBytes());
            zos.closeEntry();
        }
        zos.flush();
    }

    response.setContentType("application/java-archive");
    response.setContentLength(baos.size());
    response.setHeader(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"custom.jar\"");
    try (BufferedOutputStream out = new BufferedOutputStream(response.getOutputStream())) {
        baos.writeTo(out);
    }
}