The Table of Attributes

The second part of the MBean view is a table containing all attributes exposed by the MBean. For each attribute, this table lists its name, its Java type, its read-write access and a string representation of its current value.

While MBean attributes may be of any type, not all types can be displayed in the MBean view. The HTML adaptor is limited to basic data types that can be displayed and entered as strings. For the complete list, see the Javadoc API of the HtmlAdaptorServer class. The table of attributes contains only those which can be displayed or entered as a string. Read-only attributes whose type support the toString method are also displayed.

Attributes with array types are represented by a link to a page which displays the array values in a table. If the attribute is writeable, you may enter values for the array elements to set them. Attributes of type com.sun.jdmk.Enumerated or its sub-classes are displayed as a menu with a pop-up selection list.

The name of each attribute is a link that pops up a dialog box containing the description for this attribute. Like the MBean description, attribute descriptions can only be provided by dynamic MBeans. The MBean server inserts a generic message for standard MBean attributes. The following figure shows the attributes of the HTML adaptor with a description of the Active attribute:

Figure 4-3 MBean Attributes with a Description Dialog

Viewing Attribute Descriptions

1. Click on the attribute names of the HTML adaptor to read their description. Since the HTML adaptor is implemented as a dynamic MBean, its attribute descriptions are meaningful.

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   Note -

   Due to the use of JavaScript commands in the generated HTML, these pop-up windows might not be available on browsers that are not JavaScript-enabled.

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Writable attributes have a text field for entering new values. To set the value of a writable attribute, you would enter or replace its current value in the text field and click the "Apply" button at the bottom of the attributes table.

You should not try to modify the attributes of the HTML protocol adaptor here, we will see why in "Instantiating and Managing MBeans".

Because there is only one "Apply" button for all attributes, this button has a particular behavior: it systematically invokes the setter for all writeable attributes, whether or not their field has actually been modified. This may impact the MBean if setters have side effects, such as counting the number of modifications as in the SimpleStandard and SimpleDynamic examples given in Chapter 1, Standard MBeans and Chapter 2, Dynamic MBeans.

The HTML adaptor detects attributes which are ObjectName types and provides a link to the MBean view of the corresponding MBean. This link is labeled view, and it is located just under the displayed value of the object name. Since MBeans often need to reference other MBeans, this provides a quick way of navigating through MBean hierarchies.