• Download Active Log - 428 Kb
• Download demo project - 5 Kb

Introduction

Many web applications use the Applet client. With this type of client, the user accesses an Application Server through the Java Applet in the HTML document. Because Applets restrict using files for trace output all tracing and logging for Java applets are available through the Java console window on the browser. An alternative way to log messages from Java applets is using Active Log application. This aticle gives some ideas and provides online demo how to trace and log Java applets using Active Log.

How to use the Active Log

Active Log is a Windows ActiveX application. It can be launched in Internet Explorer browser through JavaScript by calling the ActiveXObject object constructor:

var ActiveLog = 
        new ActiveXObject("VAALOG.ActiveLog");

Use the LogData method to write a message to Active Log:

ActiveLog.LogData(message,title);

Parameters

Returns

Internet Explorer allows communication between Java Applet and JavaScript through the Java wrapper class netscape.javascript.JSObject. In general, applets access JSObject through the static method getWindow which returns a JSObject representing the Window object in the JavaScript for the window containing the given applet. This method takes java.awt.Applet as parameter. Once this Window JSObject is obtained, the applet can call JavaScript functions on an HTML page using call method. The Java code below demonstrates this approach.

JSObject win = (JSObject)JSObject.getWindow(this);
win.call(String methodName, Object args[]);

Live Demo

This live demo demonstrates how Active Log can be used to log messages from Java Applet.
Note    This demo works in IE browser only.

Attach Active Log to Java Applet

Please follow these steps to implement this demo example:

1. Install Active Log.
2. Include the jaws.jar (or plugin.jar if there's no jaws.jar) package from the lib of the JRE in the CLASSPATH to compile demo Applet.

   This is the completed Java Applet code :

   import java.lang.*; import java.applet.*; import java.awt.*; import java.awt.event.*; import netscape.javascript.*; public class ActiveLogEx extends Applet implements ActionListener { TextField txtTitle; TextArea txtMessage; Button cmdLog; Button cmdXml; public void init() { // Create controls Label lblTitle = new Label("Message Title"); Label lblMessage = new Label("Message Body"); txtTitle = new TextField("<Enter title>", 40); txtMessage = new TextArea( "<Enter message in text/xml format>", 8, 40); cmdLog = new Button("Log"); cmdXml = new Button("Generate XML"); // Register events cmdLog.addActionListener(this); cmdXml.addActionListener(this); // Build applet layout Panel p1 = new Panel(new BorderLayout()); p1.add(BorderLayout.NORTH, lblTitle); p1.add(BorderLayout.CENTER, lblMessage); p1.add(BorderLayout.SOUTH, cmdXml); BorderLayout layout2 = new BorderLayout(); layout2.setVgap(10); Panel p2 = new Panel(layout2); p2.add(BorderLayout.NORTH, txtTitle); p2.add(BorderLayout.CENTER, txtMessage); FlowLayout layout3 = new FlowLayout(); layout3.setAlignment(FlowLayout.RIGHT); Panel p3 = new Panel(layout3); p3.add(cmdLog); BorderLayout layout4 = new BorderLayout(); layout4.setHgap(10); Panel p4 = new Panel(layout4); p4.add(BorderLayout.WEST, p1); p4.add(BorderLayout.CENTER, p2); p4.add(BorderLayout.SOUTH, p3); FlowLayout layout = new FlowLayout(); layout.setHgap(10); layout.setVgap(10); this.setLayout(layout); this.setBackground(SystemColor.control); this.add(p4); } public void actionPerformed(ActionEvent e) { if (e.getSource() == cmdLog) { // Writes a message to the Active Log JSObject win = (JSObject)JSObject.getWindow(this); String methodName = "Trace"; String[] argArray = { txtMessage.getText(), txtTitle.getText() }; win.call(methodName, argArray); } if (e.getSource() == cmdXml) { // Generates XML and populate txtMessage textarea txtMessage.setText(generateXml()); } } // Creates sample XML // @return XML as String private String generateXml() { StringBuffer sb = new StringBuffer(); sb.append("<ADDRESS>\n"); sb.append("<HOUSENUMBER>525</HOUSENUMBER>\n"); sb.append("<STREET>Chaplin Crescent, SW</STREET>\n"); sb.append("<CITY>Toronto</CITY>\n"); sb.append("<PROVINCE>ON</PROVINCE>\n"); sb.append("<POSTALCODE>M5N2N2</POSTALCODE>\n"); sb.append("<COUNTRY>Canada</COUNTRY>\n"); sb.append("</ADDRESS>\n"); return sb.toString(); } }
   
3. Add the mayscript attribute to the applet tag to enable JSObject support in Java Plug-in. HTML will look like this:
   <applet codebase="SiteApplets" code="ActiveLogEx.class" width="350" height="220" mayscript> </applet>
   
4. Within the head tag of the HTML, insert the JavaScript jsoActiveLog function used to interact with the Java Applet and pass messages to the Active Log.

   This is the completed JavaScript placed in the HTML head:

   function jsoActiveLog(bActiveLogMode) { // Constructor // @param bActiveLogMode - Boolean value(true or false) // whether to turn Active Log on. function jsoActiveLog(bActiveLogMode) { this.ActiveLogMode = bActiveLogMode; this.ActiveLog = null; try{ this.ActiveLog = new ActiveXObject("VAALOG.ActiveLog"); } catch (e){ this.ActiveLogMode = false; } } // Logs message to Active Log. // @param message - Text message to be logged. // @param title - Message title that is displayed // in the message list pane. jsoActiveLog.prototype.Trace = function Trace(message, title) { if(this.ActiveLogMode) this.ActiveLog.LogData(message, title); } return new jsoActiveLog(bActiveLogMode); } var ActiveLog = new jsoActiveLog(true); function Trace(message, title) { ActiveLog.Trace(message, title); }

Conclusion

The Java console window already provides support for applets tracing and logging. However, several factors make Active Log a suitable technology for logging messages in real-time. In this article, we've covered the basics of using Active Log, and we've seen the example of a plugging Active Log to Java Applet step by step. Many developers and system administrators can benefit from its use.