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    <p>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.</p> <p>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.</p> <pre><code>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&lt;? extends ZipEntry&gt; 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(); } } </code></pre>
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    1. POAppending files to a zip file with Java
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    1. COMy war file is 30Mb compressed - not sure this approach will be the best way as it will require a lot of memory - I am already caching a lot of database queries in memory and this might make the memory footprint too big.
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    2. CO@Grouchal Actually you won't ever need more memory than `BUFFER` (I've chosen 4MB, but you're free to tailor it to your needs - it shouldn't hurt to reduce it to a few KB only). The file is never stored entirely in memory.
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    3. COthe idea is to decompress contents of the existing war into `BUFFER` and compress it into a new archive - entry after entry. After that, you end up with the same archive that's ready to take some more entries. I've chosen to write "42" into answer.txt. That's where you should place your code to append more entries.
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