"After all, the engineers only needed to refuse to fix anything, and modern industry would grind to a halt." -Michael Lewis

For Doers

Java IO: Zip Archive Manipulation

2018-11-03

The sample code associated with this post can be found on Github.

A ZIP file format is a compressed, lossless archive of files of files and/or directories. While the content being compressed will matter as to the algorithm's effectiveness, it is a common way to transfer files among peers, particularly on many windows machines.

Java provides an extension to the FileOutputStream called ZipOutputStream, which allows you to write data to a zip directory in the format described above. Here, we will write any text passed in to a single file (called "some-single-file.txt") in a zip directory at fullPath. The zip directory will be overwritten if it already exists:

private void writeAndZipText(String fullPath, String textToWrite) throws Exception {
    try (FileOutputStream fileOutputStream = new FileOutputStream(fullPath)) {
        try (ZipOutputStream zipOutputStream = new ZipOutputStream(fileOutputStream)) {
            ZipEntry zipEntry = new ZipEntry("some-single-file.txt");
            zipOutputStream.putNextEntry(zipEntry);

            for (int charIndex = 0; charIndex < textToWrite.length(); charIndex++) {
                zipOutputStream.write(textToWrite.charAt(charIndex));
            }

            zipOutputStream.closeEntry();
        }
    }
}

We can easily read anything from a zip directory using ZipFile. We can read all of the content as a String (by casting each byte value to a character) in sequential order from an entire zip directory like so:

private String readAllTextFromZip(String fullPath) throws Exception {
    StringBuilder builder = new StringBuilder();
    try (ZipFile zipFile = new ZipFile(fullPath)) {
        Enumeration<? extends ZipEntry> what = zipFile.entries();
        while (what.hasMoreElements()) {
            ZipEntry zipEntry = what.nextElement();
            try (InputStream inputStream = zipFile.getInputStream(zipEntry)) {
                while (inputStream.available() != 0) {
                    builder.append((char) inputStream.read());
                }
            }
        }
    }
    return builder.toString();
}

We can validate that this works as expected with a simple test:

@Test
public void writeZip() throws Exception {
    String pathToFile = Utils.pathToResources + "zip-output-ex.zip";
    String textToWrite = "something to zip up";
    writeAndZipText(pathToFile, textToWrite);

    String allTextWritten = readAllTextFromZip(pathToFile);

    assertEquals(textToWrite, allTextWritten);
}

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