Since release 5.0, Java Management Extensions (JMX) technology is a standard part of JSE platform. JMX provides a simple, standard way of dynamically monitoring and managing application resources. In this tutorial we will learn what we can do with JMX with an example.
Writing Standard MBeans
We will write our application as usual, but by implementing a special interface (called MBean interface), we will expose certain attributes to be managed by a JMX console application (e.g. JConsole) externally.
public interface CalculatorMBean {
void setDecimalPlaces(int decimalPlaces);
int getDecimalPlaces();
}
Following is the implementation of MBean.
public class Calculator implements CalculatorMBean {
private int decimalPlaces = 2;
public double add(double d1, double d2) {
BigDecimal bd1 = new BigDecimal(d1);
BigDecimal bd2 = new BigDecimal(d2);
BigDecimal sum = bd1.add(bd2);
return sum.setScale(decimalPlaces, RoundingMode.HALF_UP)
.doubleValue();
}
public void setDecimalPlaces(int decimalPlaces) {
this.decimalPlaces = decimalPlaces;
}
public int getDecimalPlaces() {
return decimalPlaces;
}
}
By convention, a MBean interface takes the name of the Java class that implements it, with the suffix MBean added.
Application logic and JMX agent
Following main class writes a command line application which asks user to enter two number whose sum is returned after rounding the result according to the 'decimalPlaces' attribute.
Our main class also registers the Calculator MBean to MBeanServer. That means our main class exposes the application as a JMX agent so that an external JMX console (e.g. JConsole) can connect and manage the MBean attributes.
public class AppMain {
public static void main(String[] args) throws Exception {
Calculator calculator = new Calculator();
registerWithJmxAgent(calculator);
startConsoleApp(calculator);
}
private static void startConsoleApp(Calculator calculator) {
Scanner scanner = new Scanner(System.in);
while (true) {
System.out.println("-----------------");
String input1 = getUserInput(scanner, "enter first number");
double d1 = toDouble(input1);
String input2 = getUserInput(scanner, "enter second number");
double d2 = toDouble(input2);
double sum = calculator.add(d1, d2);
System.out.printf("sum = %s (rounded to %s decimal places)%n",
sum, calculator.getDecimalPlaces());
}
}
private static double toDouble(String input) {
try {
return Double.parseDouble(input);
} catch (NumberFormatException e) {
System.out.println("Not a valid number, defaulting to 0");
return 0;
}
}
private static void registerWithJmxAgent(Calculator calculator) throws Exception {
MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
ObjectName name = new ObjectName("com.logicbig.example:type=calculator");
mbs.registerMBean(calculator, name);
}
public static String getUserInput(Scanner scanner, String msg) {
System.out.print(msg + ">");
String s = scanner.nextLine();
if ("exit".equals(s)) {
System.exit(0);
}
return s;
}
}
