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Java Dynamic Management Kit 4.0 Tutorial

Implementing a Dynamic MBean

A dynamic MBean consists of a class that implements the DynamicMBean interface coherently. By this, we mean a class which exposes a management interface whose description matches the attributes and operations which are accessible through the generic getters, setters and invokers.

Note -

MBeans are not allowed to be both standard and dynamic. When a class is instantiated as an MBean, the agent checks the interfaces that it implements. If the class implements or inherits an implementation of both the corresponding MBean interface and the DynamicMBean interface, then an exception is raised and the MBean cannot be created.

Beyond this restriction, a dynamic MBean must also follow the same two rules as a standard MBean, namely:

Thereafter, a dynamic MBean class is free to declare any number of public or private methods and variables. None of these are visible to management applications, only the methods implementing the DynamicMBean interface are exposed for management. A dynamic MBean is also free to rely on other classes which may be a part of the manageable resource.

Dynamic Programming Issues

An MBean is a manageable resource that exposes a specific management interface. The name dynamic MBean refers to the fact that the interface is revealed at runtime, as opposed to through the introspection of static class names. The term dynamic is not meant to imply that the MBean can dynamically change its management interface. The management architecture defined by JMX and implemented in the Java Dynamic Management Kit does not support MBeans whose management interface is modified during runtime.

This is not an issue with standard MBeans which would need to be recompiled in order to change their interface. However, dynamic MBeans could be programmed so that their interface description and their generic getters, setters and the invoker have a different behavior at different times. In practice, this type of MBean could be created but it couldn't be managed after any change of interface.

As a rule, the value returned by the MBeanInfo method of a dynamic MBean, and the corresponding behavior of getters, setters and the invoker, must never change over the lifetime of a given instance of the MBean. However, it is permissible to have the same dynamic MBean class expose different management interfaces depending upon the instantiation conditions. This would be a valid MBean, since the agent architecture manages object instances, not class types. It would also be a very advanced MBean for a complex management solution, beyond the scope of this tutorial.

The getMBeanInfo Method

Since the MBean description should never change, it is usually created once at instantiation, and the getMBeanInfo method just returns its reference at every call. The MBean constructor should therefore build the MBeanInfo object from the MBean descriptor classes such that it accurately describes the management interface. And since most dynamic MBeans will always be instantiated with the same management interface, building the MBeanInfo object is fairly straightforward.

The following code shows how the SimpleDynamic MBean defines its management interface, as built at instantiation and returned by its getMBeanInfo method:

Example 2-2 Implemention of the getMBeanInfo Method

// class constructor
public SimpleDynamic() {

    buildDynamicMBeanInfo();
}

// internal variables describing the MBean
private String dClassName = this.getClass().getName();
private String dDescription = "Simple implementation of a dynamic MBean.";

// internal variables for describing MBean elements
private MBeanAttributeInfo[] dAttributes = new MBeanAttributeInfo[2];
private MBeanConstructorInfo[] dConstructors = new MBeanConstructorInfo[1];
private MBeanOperationInfo[] dOperations = new MBeanOperationInfo[1];
private MBeanInfo dMBeanInfo = null;

// internal method
private void buildDynamicMBeanInfo() {

    dAttributes[0] = new MBeanAttributeInfo(
        "State",                 // name
        "java.lang.String",      // type
        "State: state string.",  // description
        true,                    // readable
        true);                   // writable
    dAttributes[1] = new MBeanAttributeInfo(
        "NbChanges",
        "java.lang.Integer",
        "NbChanges: number of times the State string has been changed.",
        true,
        false);

    // use reflection to get constructor signatures
    Constructor[] constructors = this.getClass().getConstructors();
    dConstructors[0] = new MBeanConstructorInfo(
        "SimpleDynamic(): No-parameter constructor",  //description
        constructors[0]);                  // the contructor object

    MBeanParameterInfo[] params = null;
    dOperations[0] = new MBeanOperationInfo(
        "reset",                     // name
        "Resets State and NbChanges attributes to their initial values",
                                     // description
        params,                      // parameter types
        "void",                      // return type
        MBeanOperationInfo.ACTION);  // impact

    dMBeanInfo = new MBeanInfo(dClassName,
                               dDescription,
                               dAttributes,
                               dConstructors,
                               dOperations,
                               new MBeanNotificationInfo[0]);
}

// exposed method implementing the DynamicMBean.getMBeanInfo interface
public MBeanInfo getMBeanInfo() {

    // return the information we want to expose for management:
    // the dMBeanInfo private field has been built at instantiation time,
    return(dMBeanInfo);
}

Generic Attribute Getters and Setters

Generic getters and setters take a parameter that indicates the name of the attribute to read or write. There are two issues to keep in mind when implementing these methods:

The getAttribute method is the simplest, since only the attribute name must be verified:

Example 2-3 Implementation of the getAttribute Method

public Object getAttribute(String attribute_name) 
    throws AttributeNotFoundException,
           MBeanException,
           ReflectionException {

    // Check attribute_name to avoid NullPointerException later on
    if (attribute_name == null) {
        throw new RuntimeOperationsException(
            new IllegalArgumentException("Attribute name cannot be null"), 
            "Cannot invoke a getter of " + dClassName +
                " with null attribute name");
    }

    // Call the corresponding getter for a recognized attribute_name
    if (attribute_name.equals("State")) {
        return getState();
    } 
    if (attribute_name.equals("NbChanges")) {
        return getNbChanges();
    }

    // If attribute_name has not been recognized
    throw(new AttributeNotFoundException(
        "Cannot find " + attribute_name + " attribute in " + dClassName));
}

// internal methods for getting attributes
public String getState() {
    return state;
}

public Integer getNbChanges() {
    return new Integer(nbChanges);
}

// internal variables representing attributes
private String      state = "initial state";
private int         nbChanges = 0;

The setAttribute method is more complicated, since you must also insure that the given type can be assigned to the attribute and handle the special case for a null value:

Example 2-4 Implementation of the setAttribute Method

public void setAttribute(Attribute attribute)
    throws AttributeNotFoundException,
           InvalidAttributeValueException,
           MBeanException, 
           ReflectionException {

    // Check attribute to avoid NullPointerException later on
    if (attribute == null) {
        throw new RuntimeOperationsException(
            new IllegalArgumentException("Attribute cannot be null"),
            "Cannot invoke a setter of " + dClassName +
                " with null attribute");
    }
    // Note: Attribute class constructor ensures the name not null
    String name = attribute.getName();
    Object value = attribute.getValue();

    // Call the corresponding setter for a recognized attribute name
    if (name.equals("State")) {
        // if null value, try and see if the setter returns any exception
        if (value == null) {
            try {
                setState( null );
            } catch (Exception e) {
                throw(new InvalidAttributeValueException(
                    "Cannot set attribute "+ name +" to null")); 
            }
        }
        // if non null value, make sure it is assignable to the attribute
        else {
            try {
                if ((Class.forName("java.lang.String")).isAssignableFrom(
                        value.getClass())) {
                    setState((String) value);
                }
                else {
                    throw(new InvalidAttributeValueException(
                        "Cannot set attribute "+ name +
                            " to a " + value.getClass().getName() +
                            " object, String expected"));
                }
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        }
    }

    // optional: recognize an attempt to set a read-only attribute
    else if (name.equals("NbChanges")) {
        throw(new AttributeNotFoundException(
            "Cannot set attribute "+ name +
                " because it is read-only"));
    }

    // unrecognized attribute name
    else {
        throw(new AttributeNotFoundException(
            "Attribute " + name + " not found in " +
                this.getClass().getName()));
    }
}

// internal method for setting attribute
public void setState(String s) {
    state = s;
    nbChanges++;
}

Notice that the generic getter and setter methods usually hard-code information about the attributes. If a change in your management solution requires you to change your management interface, it will be harder to do with a dynamic MBean. In a standard MBean, each attribute and operation is a separate method, so unchanged attributes are unaffected. In a dynamic MBean, you must modify these generic methods that encode all attributes.

Bulk Getters and Setters

The DynamicMBean interface includes bulk getter and setter methods for reading or writing more than one attribute at once. These methods rely on the following classes:

Class Name 

Purpose 
Attribute A simple object which contains the name string and value object of any attribute.
AttributeListA dynamically extendable list of Attribute objects (extends java.util.ArrayList)

The AttributeList class extends the java.util.ArrayList class which is specific to Java 2. For this class and others that rely on similar sets and collection, the Java Dynamic Management Kit provides the collections.jar file for complete compatibility using any JDK version 1.1.x. See Directories and Classpath in the preface for more information.

The bulk getter and setter methods usually rely on the generic getter and setter, respectively. This makes them independent of the management interface, which can simplify certain modifications. In this case, their implementation consists mostly of error checking on the list of attributes. However, all bulk getters and setters must implement the following behavior: an error on any one attribute does not interrupt or invalidate the bulk operation on the other attributes.

If an attribute cannot be read, then its name-value pair does not figure in the list of results. If an attribute cannot be written, it will not be copied to the returned list of successful set operations. As a result, if there are any errors, the lists returned by bulk operators will not have the same length as the array or list passed to them. In any case, the bulk operators do not guarantee that their returned lists have the same ordering of attributes as the input array or list.

The SimpleDynamic MBean shows one way of implementing the bulk getter and setter methods:

Example 2-5 Implementation of the Bulk Getter and Setter

public AttributeList getAttributes(String[] attributeNames) {

    // Check attributeNames to avoid NullPointerException later on
    if (attributeNames == null) {
        throw new RuntimeOperationsException(
            new IllegalArgumentException(
                "attributeNames[] cannot be null"),
            "Cannot invoke a getter of " + dClassName);
    }
    AttributeList resultList = new AttributeList();

    // if attributeNames is empty, return an empty result list
    if (attributeNames.length == 0)
            return resultList;
        
    // build the result attribute list
    for (int i=0 ; i<attributeNames.length ; i++){
        try {        
            Object value = getAttribute((String) attributeNames[i]);     
            resultList.add(new Attribute(attributeNames[i],value));
        } catch (Exception e) {
            // print debug info but continue processing list
            e.printStackTrace();
        }
    }
    return(resultList);
}

public AttributeList setAttributes(AttributeList attributes) {

    // Check attributesto avoid NullPointerException later on
    if (attributes == null) {
        throw new RuntimeOperationsException(
            new IllegalArgumentException(
                "AttributeList attributes cannot be null"),
            "Cannot invoke a setter of " + dClassName);
    }
    AttributeList resultList = new AttributeList();

    // if attributeNames is empty, nothing more to do
    if (attributes.isEmpty())
        return resultList;

    // try to set each attribute and add to result list if successful
    for (Iterator i = attributes.iterator(); i.hasNext();) {
        Attribute attr = (Attribute) i.next();
        try {
            setAttribute(attr);
            String name = attr.getName();
            Object value = getAttribute(name); 
            resultList.add(new Attribute(name,value));
        } catch(Exception e) {
            // print debug info but keep processing list
            e.printStackTrace();
        }
    }
    return(resultList);
}

Generic Operation Invoker

Finally, 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:

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 which cannot be accessed is not compliant with the Java Management extensions 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.