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## Chapter 14. Filter Expressions

HornetQ provides a powerful filter language based on a subset of the SQL 92 expression syntax.

It is the same as the syntax used for JMS selectors, but the predefined identifiers are different. For documentation on JMS selector syntax please the JMS javadoc for javax.jms.Message.

Filter expressions are used in several places in HornetQ

- Predefined Queues. When pre-defining a queue, either in `hornetq-configuration.xml` or `hornetq-jms.xml` a filter expression can be defined for a queue. Only messages that match the filter expression will enter the queue.

- Core bridges can be defined with an optional filter expression, only matching messages will be bridged (see Chapter 36, Core Bridges).

- Diverts can be defined with an optional filter expression, only matching messages will be diverted (see Chapter 35, Diverting and Splitting Message Flows).

- Filter are also used programmatically when creating consumers, queues and in several places as described in Chapter 30, Management.

There are some differences between JMS selector expressions and HornetQ core filter expressions. Whereas JMS selector expressions operate on a JMS message, HornetQ core filter expressions operate on a core message.

The following identifiers can be used in a core filter expressions to refer to attributes of the core message in an expression:

- `HQPriority`. To refer to the priority of a message. Message priorities are integers with valid values from `0 - 9`. `0` is the lowest priority and `9` is the highest. E.g. `HQPriority = 3 AND animal = 'aardvark'`

- `HQExpiration`. To refer to the expiration time of a message. The value is a long integer.

- `HQDurable`. To refer to whether a message is durable or not. The value is a string with valid values: `DURABLE` or `NON_DURABLE`.

- `HQTimestamp`. The timestamp of when the message was created. The value is a long integer.

- `HQSize`. The size of a message in bytes. The value is an integer.

Any other identifiers used in core filter expressions will be assumed to be properties of the message.

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