How to Create and Update Zip File in Java

In this tutorial, we’ll explore how to easily create ZIP files, including multiple files, multiple directories, and updating an existing ZIP file in Java.

Table of Contents:

• 1. Import Necessary Classes
• 2. Creating a ZIP with Multiple Files and Directories
• 3. Adding Files or Directories to an Existing ZIP File
• 4. References

1. Import Necessary Classes

We use the built-in Java utility classes for creating ZIP files:

import java.io.*;
import java.nio.file.*;
import java.util.zip.*;
import java.util.stream.Stream;

2. Creating a ZIP with Multiple Files and Directories

Here’s how we can compress multiple files and directories:

package com.mkyong.zip;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.stream.Stream;
import java.util.zip.ZipEntry;
import java.util.zip.ZipOutputStream;

public class JavaZipExample {

    public static void main(String[] args) {
        String[] files = {"file1.txt", "file2.txt"};
        String[] directories = {"folder1", "folder2"};
        String zipFile = "files.zip";

        try (FileOutputStream fos = new FileOutputStream(zipFile);
             ZipOutputStream zos = new ZipOutputStream(fos)) {

            // Adding files
            for (String file : files) {
                Path filePath = Paths.get(file);
                if (!Files.exists(filePath)) {
                    throw new FileNotFoundException("File does not exists: " + filePath);
                }
                addToZipFile(filePath, zos);
            }

            // Adding directories
            for (String dir : directories) {
                Path dirPath = Paths.get(dir);
                if (!Files.exists(dirPath) || !Files.isDirectory(dirPath)) {
                    throw new IOException("Directory does not exist: " + dirPath);
                }
                try (Stream<Path> paths = Files.walk(dirPath)) {
                    paths.filter(path -> !Files.isDirectory(path))
                            .forEach(path -> addToZipFile(path, zos));
                }
            }

            System.out.println("ZIP file created successfully!");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static void addToZipFile(Path file, ZipOutputStream zos) {
        try (FileInputStream fis = new FileInputStream(file.toFile())) {
            // Replace backslashes with forward slashes for compatibility
            String zipEntryName = file.toString().replace("\\", "/");
            zos.putNextEntry(new ZipEntry(zipEntryName));

            byte[] buffer = new byte[1024];
            int len;
            while ((len = fis.read(buffer)) > 0) {
                zos.write(buffer, 0, len);
            }
            zos.closeEntry();
        } catch (IOException e) {
            System.err.println("Failed to zip file: " + file);
            e.printStackTrace();
        }
    }

}

The line file.toString().replace("\\", "/") ensures compatibility with ZIP file standards, which use forward slashes (/) as path separators. Java typically returns file paths with backslashes (\) on Windows. Replacing backslashes prevents issues when the ZIP file is extracted on other operating systems such as Linux or macOS.

3. Adding Files or Directories to an Existing ZIP File

To add new files or directories into an existing ZIP file, use this source code:

package com.mkyong.zip;

import java.io.FileInputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardCopyOption;
import java.util.stream.Stream;
import java.util.zip.ZipEntry;
import java.util.zip.ZipInputStream;
import java.util.zip.ZipOutputStream;

public class JavaAddToExistingZip {

    private static void addToZipFile(Path file, ZipOutputStream zos) {
        try (FileInputStream fis = new FileInputStream(file.toFile())) {
            // Replace backslashes with forward slashes for compatibility
            String zipEntryName = file.toString().replace("\\", "/");
            zos.putNextEntry(new ZipEntry(zipEntryName));

            byte[] buffer = new byte[1024];
            int len;
            while ((len = fis.read(buffer)) > 0) {
                zos.write(buffer, 0, len);
            }
            zos.closeEntry();
        } catch (IOException e) {
            System.err.println("Failed to zip file: " + file);
            e.printStackTrace();
        }
    }

    public static void addToExistingZip(String existingZipPath, String newPath) throws IOException {
        Path zipPath = Paths.get(existingZipPath);
        if (!Files.exists(zipPath)) {
            throw new IOException("ZIP file does not exist: " + existingZipPath);
        }

        Path tempZip = Files.createTempFile(null, null);

        try (ZipOutputStream zos = new ZipOutputStream(Files.newOutputStream(tempZip));
             ZipInputStream zis = new ZipInputStream(new FileInputStream(existingZipPath))) {

            ZipEntry entry;
            byte[] buffer = new byte[1024];

            // Copy existing ZIP entries to tempZip 
            while ((entry = zis.getNextEntry()) != null) {
                zos.putNextEntry(new ZipEntry(entry.getName()));
                int len;
                while ((len = zis.read(buffer)) > 0) {
                    zos.write(buffer, 0, len);
                }
                zos.closeEntry();
            }

            // Check if path is file or directory
            Path newPathObj = Paths.get(newPath);
            if (Files.isDirectory(newPathObj)) {
                try (Stream<Path> paths = Files.walk(newPathObj)) {
                    paths.filter(path -> !Files.isDirectory(path))
                            .forEach(path -> addToZipFile(path, zos));
                }
            } else {
                addToZipFile(newPathObj, zos);
            }
        }

        // Replace original ZIP with updated ZIP (tempZip)
        Files.move(tempZip, Paths.get(existingZipPath), StandardCopyOption.REPLACE_EXISTING);
    }

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

        String file = "newfile.txt";
        //String file = "newfolder";
        String zipFile = "existingfiles.zip";

        addToExistingZip(zipFile, file);

    }
}

4. References

• ZipOutputStream JavaDoc
• ZipEntry Documentation
• ZIP File Format Specification
• zip4j GitHub
• How to compress and decompress zip file in Kotlin – Mkyong.com
• How to Unzip a ZIP File in Java – Mkyong.com