Opening Jars with Visual Basic

Introduction

This article shall describe an approach that may be used to extract Jar files.  Jars are merely files compressed using zip compression; the primary difference between a standard trash zip file and a jar file is that the jar file includes a manifest; also a true jar files can be run as an executable on a machine equipped with the java runtime.  Jars typically contain java class files; if you have some need to extract a jar and view its contents, it is no more difficult to do that than it is to unzip a normal zip file.

Figure 1:  Extracting a Jar File

Since the task of extracting a jar is really the same as it is to extract a zip file, the demo application will rely upon the free SharpZipLib libraries to perform the extraction; these libraries may be downloaded from this location:  http://www.icsharpcode.net/OpenSource/SharpZipLib/

In addition to handling jar and zip files, the library also handles tar, gzip, and bzip2 compression.

The Solution

The solution contains a single project (Jars).  The example is provided in the form of a single Windows Forms project; the project contains a single main form; the references are all in the default configuration with the exception being that the SharpZipLib DLL has been added (first item in the reference list).  Having downloaded the DLL from the www.icsharpcode.net web, the download was installed into the local file system and was adding by using the "Add Reference" dialog's Browse option.  All of the code necessary to drive the application is included in the main form's code file.

Figure 2:  The Solution Explorer Showing the Project

The Code:  Jars - Form1

The Jars project is a Windows Forms project containing a single form.  All UI and code required by the project are contained in the single main form.

The only references added to the project were the SharpZipLib, System.IO, and System.Text.   The imports and class declaration are as follows:

Imports ICSharpCode.SharpZipLib.Zip

Imports System.IO

Imports System.Text

 

Public Class Form1

After the class declaration, two private variables are declared;  these variables are used to retain the path to the jar file and the path to the destination folder (where the jar will be unpacked).

    ' set local variables to hold

    ' the paths to the jar file and

    ' to destination folder

    Private mSourceJar As String

    Private mDestinationFolder As String

The next block of code is the default constructor; nothing added there:

    Public Sub New()

 

        ' This call is required by the Windows Form Designer.

        InitializeComponent()

 

        ' Add any initialization after the InitializeComponent() call.

 

    End Sub

The next block of code is used to perform the actual extraction.  The ExtractJar function accepts two arguments, a string value defining the path to the jar file, and a string value defining the path to the destination folder (the location at which the jar will be unpacked).  The method uses the SharpZipLibrary to extract the jar file.  The code in annotated to describe the activity.

    ''' <summary>

    ''' Extracts the jar file to a specified

    ''' destination folder. 

    ''' </summary>

    Private Sub ExtractJar(ByVal pathToJar As String, _

                           ByVal saveFolderPath As String)

 

        Dim jarPath As String = pathToJar

        Dim savePath As String = saveFolderPath

 

        Try

 

            ' verify the paths are set

            If (String.IsNullOrEmpty(jarPath) = False And _

                String.IsNullOrEmpty(saveFolderPath) = False) Then

 

                Try

 

                    ' use the SharpZip library FastZip

                    ' utilities ExtractZip method to

                    ' extract the jar file

                    Dim fz As New FastZip()

                    fz.ExtractZip(jarPath, saveFolderPath, "")

 

                    ' Success at this point, tell the user

                    ' we are done.

                    MessageBox.Show("Jar File: " + jarPath + " was extracted

                        to " + saveFolderPath, "Finished")

 

                    ' open the destination folder in explorer

                    System.Diagnostics.Process.Start("explorer",

                    saveFolderPath)

 

                Catch ex As Exception

 

                    ' something went wrong

                    MessageBox.Show(ex.Message, "Extraction Error")

 

                End Try 

 

            Else

 

                ' the paths were not, tell the user to

                ' get with the program

 

                Dim sb As New StringBuilder()

                sb.Append("Set the paths to both the jar file and " +

                Environment.NewLine)

                sb.Append("destination folder before attempting to " +

                Environment.NewLine)

                sb.Append("to extract a jar file.")

 

                MessageBox.Show(sb.ToString(), "Unable to Extract")

 

            End If

 

        Catch ex As Exception

 

            ' something else went wrong

            MessageBox.Show(ex.Message, "Extraction Error")

 

        End Try 

    End Sub       

The next block of code is used to set the path to the jar file.  This button click event handler is used to display an open file dialog box; the user can use the dialog box to navigate to the location of the jar file and select it.  The selected file name is used to set the mSourceJar string variable to the file path of the jar file; the file path is also displayed in the text box adjacent to the button.

    ''' <summary>

    ''' Set the path to the Jar file

    ''' </summary>

    ''' <param name="sender"></param>

    ''' <param name="e"></param>

    ''' <remarks></remarks>

    Private Sub btnSetJar_Click(ByVal sender As System.Object, _

                                ByVal e As System.EventArgs) _

                                Handles btnSetJar.Click

 

        Try

 

            OpenFileDialog1.Title = "Extract JAR"

            OpenFileDialog1.DefaultExt = "jar"

            OpenFileDialog1.Filter = "Jar Files|*.jar"

            OpenFileDialog1.FileName = String.Empty

            OpenFileDialog1.Multiselect = False

 

            If (OpenFileDialog1.ShowDialog() = DialogResult.OK) Then

 

                If (OpenFileDialog1.FileName = String.Empty) Then

                    Return

                End If

 

                Dim strExt As String

                strExt =

                System.IO.Path.GetExtension(OpenFileDialog1.FileName)

                strExt = strExt.ToUpper()

 

                If (strExt = ".JAR") Then

                    mSourceJar = OpenFileDialog1.FileName

                    txtJarPath.Text = mSourceJar

                Else

                    MessageBox.Show("The selected file is not a jar", "File

                    Error")

                End If

 

            Else

                MessageBox.Show("File request cancelled by user.",

                "Cancelled")

            End If

 

        Catch ex As Exception

 

            MessageBox.Show(ex.Message.ToString(), "Error")

 

        End Try 

    End Sub

The next section of code is used to set the path to the destination folder.  The destination folder defines the location where the selected jar file will be unpacked.  The button click event handler opens a folder browser dialog which will permit the user to navigate to and/or to create a destination folder.  Once set, the folder browser dialog's selected path property is used to the mDestinationFolder variable and it is also used to display the selected destination folder path using the selected path's value.

    ''' <summary>

    ''' Set the path to the folder where

    ''' the jar file is to be extracted

    ''' </summary>

    ''' <param name="sender"></param>

    ''' <param name="e"></param>

    ''' <remarks></remarks>

    Private Sub btnDestinationFolder_Click(ByVal sender As System.Object, _

                                           ByVal e As System.EventArgs) _

                                           Handles btnDestinationFolder.Click

 

        Try

 

            folderBrowserDialog1.ShowNewFolderButton = True

 

            Dim result As DialogResult = folderBrowserDialog1.ShowDialog()

            If (result = DialogResult.OK) Then

 

                mDestinationFolder = folderBrowserDialog1.SelectedPath

                txtDestinationFolder.Text = mDestinationFolder

 

            End If

 

        Catch ex As Exception

 

            MessageBox.Show(ex.Message, "Error Setting Destination Folder")

 

        End Try 

    End Sub

The last button click event handler is used to call the ExtractJar method.  The click event handler merely evokes the ExtractJar method and passes it the two variables used to contain the source jar file and the destination folder.

    ''' <summary>

    ''' Use the ExtractJar function to

    ''' extract the source jar file into the

    ''' destination folder

    ''' </summary>

    ''' <param name="sender"></param>

    ''' <param name="e"></param>

    Private Sub btnExtract_Click(ByVal sender As System.Object, ByVal e As

    System.EventArgs) Handles btnExtract.Click

 

        ExtractJar(mSourceJar, mDestinationFolder) 

    End Sub

That wraps up the description of the code used in this project.

Summary.

This example demonstrates extracting a jar file using SharpZipLib; Jar files are nothing more than zip compressed archives containing some meta data along with (typically) java class files; extracting them is no more difficult than extracting a normal zipped archive; particularly with the assistance of the SharpZipLib.