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 = "core-sample/target/classes/output.gz";
        String targetFile = "core-sample/target/classes/input-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 a buffer and temporary variable used during the
            // file decompress process.
            //
            byte[] buffer = new byte[1024];
            int length;

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

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

            //
            // Close the resources.
            //
            fos.close();
            gzis.close();
            fis.close();
        } 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.util;

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) {
        User admin = null;
        User foo = null;

        try {
            FileInputStream fis = new FileInputStream(new File("user.dat"));
            GZIPInputStream gis = new GZIPInputStream(fis);
            ObjectInputStream ois = new ObjectInputStream(gis);

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

            ois.close();
            gis.close();
            fis.close();
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }

        System.out.println("Admin = [" + admin + "]");
        System.out.println("Foo = [" + foo + "]");
    }
}

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'}]