JavaScript code on a web page can interact with Java applets embedded on the page. JavaScript code can perform operations such as the following:
The LiveConnect Specification describes details about how JavaScript code communicates with Java code.
Security warnings are shown when JavaScript code makes calls to a Java applet. To suppress these warnings, add the Caller-Allowable-Codebase attribute to the JAR file manifest. Specify the location of the JavaScript code that is allowed to make calls to the applet. See
JAR File Manifest Attributes for Security for information about the Caller-Allowable-Codebase attribute.
This topic explores JavaScript code to Java applet communication using the Math applet example. The MathApplet class and supporting Calculator class provide a set of public methods and variables. The JavaScript code on the web page invokes and evaluates these public members to pass data and retrieve calculated results.
Here is the source code for the
MathAppletgetCalculator method returns a reference to the Calculator helper class.
package jstojava;
import java.applet.Applet;
public class MathApplet extends Applet{
public String userName = null;
public String getGreeting() {
return "Hello " + userName;
}
public Calculator getCalculator() {
return new Calculator();
}
public DateHelper getDateHelper() {
return new DateHelper();
}
public void printOut(String text) {
System.out.println(text);
}
}
The methods in the
Calculator
package jstojava;
public class Calculator {
private int a = 0;
private int b = 0; // assume b > a
public void setNums(int numA, int numB) {
a = numA;
b = numB;
}
public int add() {
return a + b;
}
public int [] getNumInRange() {
int x = a;
int len = (b - a) + 1;
int [] range = new int [len];
for (int i = 0; i < len; i++) {
range[i]= x++;
System.out.println("i: " + i + " ; range[i]: " + range[i]);
}
return range;
}
}
The getDate method of the
DateHelper
package jstojava;
import java.util.Date;
import java.text.SimpleDateFormat;
public class DateHelper {
public static String label = null;
public String getDate() {
return label + " " + new SimpleDateFormat().format(new Date());
}
}
Deploy the applet in a web page,
AppletPage.html
<script src=
"https://www.java.com/js/deployJava.js"></script>
<script>
<!-- applet id can be used to get a reference to
the applet object -->
var attributes = { id:'mathApplet',
code:'jstojava.MathApplet', width:1, height:1} ;
var parameters = { jnlp_href: 'math_applet.jnlp'} ;
deployJava.runApplet(attributes, parameters, '1.6');
</script>
Next, add some JavaScript code to the
AppletPage.htmlCalculator). The JavaScript code is able to handle primitive, array, and object return types.
<script language="javascript">
function enterNums(){
var numA = prompt('Enter number \'a\'?','0');
var numB = prompt(
'Enter number \'b\' (should be greater than number \'a\' ?','1');
// set applet's public variable
mathApplet.userName = "John Doe";
// invoke public applet method
var greeting = mathApplet.getGreeting();
// get another class referenced by applet and
// invoke its methods
var calculator = mathApplet.getCalculator();
calculator.setNums(numA, numB);
// primitive datatype returned by applet
var sum = calculator.add();
// array returned by applet
var numRange = calculator.getNumInRange();
// check Java console log for this message
mathApplet.printOut("Testing printing to System.out");
// get another class, set static field and invoke its methods
var dateHelper = mathApplet.getDateHelper();
dateHelper.label = "Today\'s date is: ";
var dateStr = dateHelper.getDate();
<!-- ... -->
</script>
The Math applet displays the following results on the web page when the number a = 0 and b = 5:
Results of JavaScript to Java Communication Hello John Doe a = 0 ; b = 5 Sum: 5 Numbers in range array: [ 0, 1, 2, 3, 4, 5 ] Today's date is: 5/28/13 4:12 PM //shows current date
Open
AppletPage.html
Check security restrictions placed on applets invoked by JavaScript code.
Download source code for the Invoking Applet Methods From JavaScript Code example to experiment further.