Tag Archives: Java

Getting the Manifest of a Jar file.


JAR files support a wide range of functionality, including electronic signing, version control, package sealing, and others. What gives a JAR file this versatility? The answer is the JAR file’s manifest.

The manifest is a special file that can contain information about the files packaged in a JAR file. By tailoring this “meta” information that the manifest contains, you enable the JAR file to serve a variety of purposes.

This example  will explain the contents of the manifest file and show you how to work with it, with examples for the basic features.

For more information about the MANIFEST See https://docs.oracle.com/javase/tutorial/deployment/jar/manifestindex.html

try {
    // Open the JAR file
    JarFile jarfile = new JarFile("filename.jar");

    // Get the manifest
    Manifest manifest = jarfile.getManifest();

    // Get the manifest entries
    Map map = manifest.getEntries();

    // Enumerate each entry
    for (Iterator it=map.keySet().iterator(); it.hasNext(); ) {
        // Get entry name
        String entryName = (String)it.next();

        // Get all attributes for the entry
        Attributes attrs = (Attributes)map.get(entryName);

        // Enumerate each attribute
        for (Iterator it2=attrs.keySet().iterator(); it2.hasNext(); ) {
            // Get attribute name
            Attributes.Name attrName = (Attributes.Name)it2.next();

            // Get attribute value
            String attrValue = attrs.getValue(attrName);
} catch (IOException e) {


Getting the http Request Parameters in a Servlet


The implicit HttpServlet interface comes with two methods which must be implemented.  These are doGet(HttpServletRequest req, HttpServletResponse res) and doPost(HttpServletRequest req, HttpServletResponse res). The example below shows how to get both.

// See also The HttpServlet

// This method is called by the servlet container to process a GET request.
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
    doGetOrPost(req, resp);

// This method is called by the servlet container to process a POST request.
public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException {
    doGetOrPost(req, resp);

// This method handles both GET and POST requests.
private void doGetOrPost(HttpServletRequest req, HttpServletResponse resp) throws IOException {
    // Get the value of a request parameter; the name is case-sensitive
    String name = "param";
    String value = req.getParameter(name);
    if (value == null) {
        // The request parameter 'param' was not present in the query string
        // e.g. http://hostname.com?a=b
    } else if ("".equals(value)) {
        // The request parameter 'param' was
       // present in the query string but has no
       // value
       // e.g. http://hostname.com?param=&a=b

    // The following generates a page showing all
    // the request parameters
    PrintWriter out = resp.getWriter();

    // Get the values of all request parameters
    Enumeration enum = req.getParameterNames();
    for (; enum.hasMoreElements(); ) {
        // Get the name of the request parameter
        name = (String)enum.nextElement();

        // Get the value of the request parameter
        value = req.getParameter(name);

        // If the request parameter can appear
        // more than once in the query string,
        // get all values
        String[] values =

        for (int i=0; i<values.length; i++) {
            out.println("    "+values[i]);


Creating a Temp File


Every Operating System has it’s own location for storing temporary files. In UNIX/Linux this is /tmp. In Windows it’s C:\temp.

The example below shows how it can be done in the (platform independent way with Java:

try {
    // Create temp file.
    File temp = File.createTempFile("pattern", ".suffix");

    // Delete temp file when program exits.

    // Write to temp file
    BufferedWriter out = new BufferedWriter(new FileWriter(temp));
} catch (IOException e) {


Centering the Screen


This example shows how a Swing Window Object  can be centered on the screen. Note that the Window Object doesn’t have a CENTER method of its own so you need to implement the logic yourself.

// Get the size of the screen
Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();

// Determine the new location of the window
int w = window.getSize().width;
int h = window.getSize().height;
int x = (dim.width-w)/2;
int y = (dim.height-h)/2;

// Move the window
window.setLocation(x, y);


HTTP Post from a socket


Earlier on we already saw how to read from a Socket. This example shows how to do a HTTP POST. The HTTP post can take more data. Such as binary data.

This example can therefore be useful to post multiple documents to another program such as Apache Solr. Note that you also can do this with the UNIX / Linux cURL command

try {
    // Construct data
    String data = URLEncoder.encode("key1", "UTF-8") + "=" + URLEncoder.encode("value1", "UTF-8");
    data += "&" + URLEncoder.encode("key2", "UTF-8") + "=" + URLEncoder.encode("value2", "UTF-8");

    // Create a socket to the host
    String hostname = "hostname.com";
    int port = 80;
    InetAddress addr = InetAddress.getByName(hostname);
    Socket socket = new Socket(addr, port);

    // Send header
    String path = "/servlet/SomeServlet";
    BufferedWriter wr = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream(), "UTF8"));
    wr.write("POST "+path+" HTTP/1.0rn");
    wr.write("Content-Length: "+data.length()+"rn");
    wr.write("Content-Type: application/x-www-form-urlencodedrn");

    // Send data

    // Get response
    BufferedReader rd = new BufferedReader(new InputStreamReader(socket.getInputStream()));
    String line;
    while ((line = rd.readLine()) != null) {
        // Process line...
} catch (Exception e) {


Determining the selected file in the JFileChooser


The JFileChooser is a Swing component to choose files from the FileSystem. After you chose a file (or Directory you need to get the selected file back in your code. This example shows how it’s done.

JFileChooser chooser = new JFileChooser();

// Set the current directory to the 
// application's current directory
try {
    // Create a File object containing the
    // canonical path of the
    // desired file
    File f = new File(new File("filename.txt").getCanonicalPath());

    // Set the selected file
} catch (IOException e) {

// Show the dialog; wait until dialog is closed

// Get the currently selected file
File curFile = chooser.getSelectedFile();


Determining if a Group is in the Path with Regular expressions


Note: See this article about working with Regular Expressions.

// Compile regular expression with a back reference to group 1
String patternStr = "<(\S+?).*?>(.*?)</\1>";
Pattern pattern = Pattern.compile(patternStr);
Matcher matcher = pattern.matcher("");

// Set the input
matcher.reset("xx <tag a=b> yy </tag> zz");j

// Get tagname and contents of tag
boolean matchFound = matcher.find();   // true
String tagname = matcher.group(1);     // tag
String contents = matcher.group(2);    //  yy

matcher.reset("xx <tag> yy </tag0>");
matchFound = matcher.find();           // false


Showing a Print Dialog


Showing a print dialog is a common task for every print job. This example shows how to do it:

PrinterJob pjob = PrinterJob.getPrinterJob();
PageFormat pf = pjob.defaultPage();
pjob.setPrintable(new PrintableClass(), pf);
try {
    if (pjob.printDialog()) {
} catch (PrinterException e) {