Generic Operation Invoker

A dynamic MBean must implement the invoke method so that operations in the management interface can be called. This method requires the same considerations as the generic getter and setter:

• Operations and their parameters must be correctly mapped to their internal representation and the result must be returned.

• Operation names and parameter types must be verified.

• These verifications are usually hard-coded, again making modifications to the management interface more difficult than in a standard MBean.

The implementation in the SimpleDynamic MBean is relatively simple due to the one operation with no parameters:

Example 2–6 Implementation of the invoke Method

public Object invoke(
        String operationName, Object params[], String signature[])
    throws MBeanException, ReflectionException {

    // Check operationName to avoid NullPointerException later on
    if (operationName == null) {
        throw new RuntimeOperationsException(
            new IllegalArgumentException(
                "Operation name cannot be null"),
            "Cannot invoke a null operation in " + dClassName);
    }

    // Call the corresponding operation for a recognized name
    if (operationName.equals("reset")){
        // this code is specific to the internal "reset" method:
        reset();     // no parameters to check
        return null; // and no return value
    } else { 
        // unrecognized operation name:
        throw new ReflectionException(
            new NoSuchMethodException(operationName), 
            "Cannot find the operation " + operationName +
                " in " + dClassName);
    }
}

// internal variable
private int         nbResets = 0;

// internal method for implementing the reset operation
public void reset() {
    state = "initial state";
    nbChanges = 0;
    nbResets++;
}

// Method not revealed in the MBean description and not accessible
// through "invoke" therefore it is only available for internal mgmt
public Integer getNbResets() {
    return new Integer(nbResets);
}

As it is written, the SimpleDynamic MBean correctly provides a description of its management interface and implements its attributes and operations. However, this example demonstrates the need for a strict coherence between what is exposed by the getMBeanInfo method and what can be accessed through the generic getters, setters, and invoker.

A dynamic MBean whose getMBeanInfo method describes an attribute or operation that cannot be accessed is not compliant with the JMX specification and is technically not a manageable resource. Similarly, a class could make attributes or operations accessible without describing them in the returned MBeanInfo object. Since MBeans should raise an exception when an undefined attribute or operation is accessed, this would, again, technically not be a compliant resource.