How do I decompress a GZip file in Java?

In the previous example we have learn how to compress a file in GZIP format. To get the file back to its original version we will now learn how to decompress the gzip file. Just like the previous example, we are also going to use the FileInputStream and FileOutputStream class to read the compressed source file and to write out the decompressed file. While GZipOutputStream was used to to create the gzip file, the GZipInputStream is the class that handles the decompression.

Here is your code snippet:

package org.kodejava.example.zip;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.zip.GZIPInputStream;

public class GZipDecompressExample {
    public static void main(String[] args) {
        // GZip input and output file.
        String sourceFile = "output.gz";
        String targetFile = "data-1.txt";

        try (
            // Create a file input stream to read the source file.
            FileInputStream fis = new FileInputStream(sourceFile);

            // Create a gzip input stream to decompress the source
            // file defined by the file input stream.
            GZIPInputStream gzis = new GZIPInputStream(fis);

            // Create file output stream where the decompress result
            // will be stored.
            FileOutputStream fos = new FileOutputStream(targetFile)) {

            // Create a buffer and temporary variable used during the
            // file decompress process.
            byte[] buffer = new byte[1024];
            int length;

            // Read from the compressed source file and write the
            // decompress file.
            while ((length = gzis.read(buffer)) > 0) {
                fos.write(buffer, 0, length);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

How do I decompress Java objects?

In the previous example How do I compress Java objects? we have manage to compress Java objects and stored them in file. In this example we will read the file and reconstruct the compressed objects. For the User class you can see in the previous example mentioned above.

package org.kodejava.example.zip;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.util.zip.GZIPInputStream;

public class UnzipObjectDemo {
    public static void main(String[] args) {
        File file = new File("user.dat");
        try (FileInputStream fis = new FileInputStream(file);
             GZIPInputStream gis = new GZIPInputStream(fis);
             ObjectInputStream ois = new ObjectInputStream(gis)) {

            User admin = (User) ois.readObject();
            User foo = (User) ois.readObject();

            System.out.println("Admin = [" + admin + "]");
            System.out.println("Foo = [" + foo + "]");
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

Running the code snippet above will give us the following output:

Admin = [User{id=1, username='admin', password='secret', firstName='System', lastName='Administrator'}]
Foo = [User{id=2, username='foo', password='secret', firstName='Foo', lastName='Bar'}]

How do I decompress a zip file using ZipFile?

In this example we use the java.util.zip.ZipFile class to decompress and extract a zip file.

package org.kodejava.example.zip;

import java.util.zip.ZipFile;
import java.util.zip.ZipEntry;
import java.util.Enumeration;
import java.io.*;

public class ZipFileUnzipDemo {
    public static void main(String[] args) throws Exception {
        String zipName = "data.zip";

        ZipFile zip = new ZipFile(zipName);
        Enumeration enumeration = zip.entries();

        while (enumeration.hasMoreElements()) {
            ZipEntry zipEntry = (ZipEntry) enumeration.nextElement();
            System.out.println("Unzipping: " + zipEntry.getName());

            int size;
            byte[] buffer = new byte[2048];

            try (BufferedInputStream bis =
                     new BufferedInputStream(zip.getInputStream(zipEntry));
                 FileOutputStream fos =
                     new FileOutputStream(zipEntry.getName());
                 BufferedOutputStream bos =
                     new BufferedOutputStream(fos, buffer.length)) {

                while ((size = bis.read(buffer, 0, buffer.length)) != -1) {
                    bos.write(buffer, 0, size);
                }
                bos.flush();
            }
        }
    }
}

How do I decompress a zip file using ZipInputStream?

The code below shows how to decompress and extract files from a zip archive. In the example we use the java.util.zip.ZipInputStream class.

package org.kodejava.example.zip;

import java.io.*;
import java.util.zip.ZipInputStream;
import java.util.zip.ZipEntry;

public class UnzipDemo {
    public static void main(String[] args) {
        String zipName = "data.zip";

        try (FileInputStream fis = new FileInputStream(zipName);
             ZipInputStream zis =
                 new ZipInputStream(new BufferedInputStream(fis))) {

            ZipEntry entry;

            // Read each entry from the ZipInputStream until no
            // more entry found indicated by a null return value
            // of the getNextEntry() method.
            while ((entry = zis.getNextEntry()) != null) {
                System.out.println("Unzipping: " + entry.getName());

                int size;
                byte[] buffer = new byte[2048];

                try (FileOutputStream fos =
                         new FileOutputStream(entry.getName());
                     BufferedOutputStream bos =
                         new BufferedOutputStream(fos, buffer.length)) {

                    while ((size = zis.read(buffer, 0, buffer.length)) != -1) {
                        bos.write(buffer, 0, size);
                    }
                    bos.flush();
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}