= Filter Expressions
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Apache ActiveMQ Artemis 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 & Jakarta Messaging selectors, but the predefined identifiers are different.
For documentation on JMS selector syntax please the JavaDoc for https://docs.oracle.com/javaee/7/api/javax/jms/Message.html[`javax.jms.Message`].
For the corresponding Jakarta Messaging JavaDoc see https://jakarta.ee/specifications/messaging/3.0/apidocs/jakarta/jms/message[`jakarta.jms.Message`]

Filter expressions are used in several places in Apache ActiveMQ Artemis

* Predefined Queues.
When pre-defining a queue, in `broker.xml` in either the core or jms configuration 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 xref:core-bridges.adoc#core-bridges[Core Bridges]).
* Diverts can be defined with an optional filter expression, only matching messages will be diverted (see xref:diverts.adoc#diverting-and-splitting-message-flows[Diverts]).
* Filter are also used programmatically when creating consumers, queues and in several places as described in xref:management.adoc#management[management].

There are some differences between JMS selector expressions and Apache ActiveMQ Artemis core filter expressions.
Whereas JMS selector expressions operate on a JMS message, Apache ActiveMQ Artemis 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:

AMQUserID::
The ID set by the user when the message is sent.
This is analogous to the `JMSMessageID` for JMS-based clients.

AMQAddress::
The address to which the message was sent.

AMQGroupID::
The group ID used when sending the message.

AMQPriority::
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. `AMQPriority = 3 AND animal = 'aardvark'`

AMQExpiration::
To refer to the expiration time of a message.
The value is a long integer.

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

AMQTimestamp::
The timestamp of when the message was created.
The value is a long integer.

AMQSize::
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.

The JMS and Jakarta Messaging specs state that a String property should not get converted to a numeric when used in a selector.
So for example, if a message has the `age` property set to `String` `21` then the following selector should not match it: `age > 18`.
Since Apache ActiveMQ Artemis supports STOMP clients which can only send messages with string properties, that restriction is a bit limiting.
Therefore, if you want your filter expressions to auto-convert `String` properties to the appropriate number type, just prefix it with `convert_string_expressions:`.
If you changed the filter expression in the previous example to be `convert_string_expressions:age > 18`, then it would  match the aforementioned message.

The JMS and Jakarta Messaging specs also state that property identifiers (and therefore the identifiers which are valid for use in a filter expression) are an:

____
unlimited-length sequence of letters and digits, the first of which must be a letter.
A letter is any character for which the method  `Character.isJavaLetter` returns `true`.
This includes `_` and `$`.
A letter or digit is any character for which the method `Character.isJavaLetterOrDigit` returns `true`.
____

This constraint means that hyphens (i.e. `-`) cannot be used.
However, this constraint can be overcome by using the `hyphenated_props:` prefix.
For example, if a message had the `foo-bar` property set to `0` then the filter expression `hyphenated_props:foo-bar = 0` would match it.

== XPath

Apache ActiveMQ Artemis also supports special https://en.wikipedia.org/wiki/XPath[XPath] filters which operate on the _body_ of a message.
The body must be XML.
To use an XPath filter use this syntax:

----
XPATH '<xpath-expression>'
----

XPath filters are supported with and between producers and consumers using the following protocols:

* OpenWire JMS
* Core (and Core JMS)
* STOMP
* AMQP

Since XPath applies to the body of the message and requires parsing of XML *it may be significantly slower* than normal filters.

Large messages are *not* supported.

The XML parser used for XPath is configured with these default "features":

* `+http://xml.org/sax/features/external-general-entities+`: `false`
* `+http://xml.org/sax/features/external-parameter-entities+`: `false`
* `+http://apache.org/xml/features/disallow-doctype-decl+`: `true`

However, in order to deal with any implementation-specific issues the features can be customized by using system properties starting with the `org.apache.activemq.documentBuilderFactory.feature:` prefix, e.g.:

----
-Dorg.apache.activemq.documentBuilderFactory.feature:http://xml.org/sax/features/external-general-entities=true
----
