Java Reference
In-Depth Information
building a condition across two MBeans is impossible to represent using this que-
rying service. For example, the following query is not supported:
If MBean A has an
attribute
MyAttribute
, then return MBean B with attribute
TheAttribute
value of
5
.
However, in chapter 11, we will discuss the relation service, which can provide for
MBean relationships.
8.3.2
Creating query expressions
Let's revisit the query we described earlier:
All MBeans from application A that have
an attribute
count
whose value is greater than 10 but less than 20, or that have an
attribute
owner
whose value equals
ABC
.
JMX
provides the capability to perform many types of query expressions,
including the ones in this query. In our query, we have attribute values that must
be either greater than, less than, or equal to some value. In addition, the query
combines these expressions by using an
AND
and an
OR
type expression.
JMX
provides the class
javax.management.Query
as a mechanism for objects to
build simple and complex queries. The
Query
class contains static methods to
build expressions and to relate expressions into more complex expressions. Each
method returns a type of expression, represented by one of the following classes:
■
QueryExp
■
ValueExp
■
AttributeValueExp
■
StringValueExp
The
ValueExp
and
StringValueExp
classes represent an attribute value. The
AttributeValueExp
represents the name of an attribute. The
QueryExp
class rep-
resents a query expression constructed of one or more other expressions. You
should never need to create instances of the classes yourself; rather, you should
use the methods of the
Query
class to build expressions.
As you will see in a moment, some of the methods of the
Query
class take
other expressions as inputs in order to build more complex queries. For exam-
ple, examine the following code that builds the query
MBeans with attribute
count
greater than
10
:
QueryExp exp = Query.gt( Query.attr( "count" ), Query.value( 10 ) );
The method
gt()
of the
Query
class indicates that a
greater-than
expression is being
constructed. The code uses the
attr()
method to indicate that the attribute
involved in the greater-than expression is called
count
and then uses the
value()
method to indicate the second half of the greater-than expression.
