Predicates Attributes and Handlers


Predicates and Exchange attributes are an abstraction that allow handlers to read, write and make decisions based on certain attributes of a request without hard coding this into the handler. These form the basis of Undertow’s text based handler configuration format. Some examples are shown below:

Use the reverse proxy to send all requests to /reports to a different backend server:

path-prefix['/reports'] -> reverse-proxy[{'',''}]

Redirect all requests from /a to /b. The first example only redirects if there is an exact match, the later examples match all paths that start with /a:

path['/a'] -> redirect['/b']
path-prefix['/a'] -> redirect['/b${remaining}']
regex['/a(.*)'] -> set[attribute='%{o,Location}', value='/b${1}'] -> response-code[302]

Exchange Attributes

An exchange attribute represents the value of part of the exchange. For example the path attribute represents the request path, the method attribute represents the HTTP. Even though these attributes can be retrieved and modified directly this requires a handler to hard code the attribute that they wish to use. For example Undertow provides a handler that checks an attribute against an access control list. There are lots of different attributes we may wish to check against the ACL (e.g. username, User-Agent header, request path).


A predicate is a function that takes a value (in this case the HttpServerExchange) and returns a true or false value. This allows actions to be taken based on the return value of the predicate. In general any handler that needs to make a boolean decision based on the exchange should use a predicate to allow for maximum flexibility.

The provided predicate handler can be used to make a decision between which handler to invoke based on the value of a predicate.

Programmatic Representation of Exchange Attributes

An exchange attribute is represented by the io.undertow.attribute.ExchangeAttribute interface:

 * Representation of a string attribute from a HTTP server exchange.
public interface ExchangeAttribute {

     * Resolve the attribute from the HTTP server exchange. This may return null if the attribute is not present.
     * @param exchange The exchange
     * @return The attribute
    String readAttribute(final HttpServerExchange exchange);

     * Sets a new value for the attribute. Not all attributes are writable.
     * @param exchange The exchange
     * @param newValue The new value for the attribute
    void writeAttribute(final HttpServerExchange exchange, final String newValue) throws ReadOnlyAttributeException;

Undertow provides implementation of a lot of attributes out of the box, most of which can be accessed using the io.undertow.attribute.ExchangeAttributes utility class. Some of the attributes that are provided include request and response headers, cookies, path, query parameters, the current user and more.

Programmatic Representation of Predicates

Predicates are represented by the io.undertow.predicate.Predicate interface:

 * A predicate.
 * This is mainly uses by handlers as a way to decide if a request should have certain
 * processing applied, based on the given conditions.
public interface Predicate {

     * Attachment key that can be used to store additional predicate context that allows the predicates to store
     * additional information. For example a predicate that matches on a regular expression can place additional
     * information about match groups into the predicate context.
     * Predicates must not rely on this attachment being present, it will only be present if the predicate is being
     * used in a situation where this information may be required by later handlers.
    AttachmentKey<Map<String, Object>> PREDICATE_CONTEXT = AttachmentKey.create(Map.class);

    boolean resolve(final HttpServerExchange value);


Undertow provides built in predicates that can be created using the io.undertow.predicate.Predicates utility class. This includes basic boolean logic predicates (and, or and not), as well as other useful predicates such as path matching (including prefix and suffix based matches), regular expression matching, contains and exists. Many of these predicates operate on exchange attributes, so they can be used to match arbitrary parts of the exchange. The following example demonstrates a predicate that matches any exchange that has no Content-Type header where the method is POST:

Predicate predicate = Predicates.and(
        Predicates.equals("POST", ExchangeAttributes.requestMethod()));

Textual Representation

Undertows predicate language is still considered tech preview. Its syntax will likely change in a future version as the language is expanded.

All these attributes and predicates are all well and good, but unless there is a way for the end user to configure them without resorting to programmatic means they are not super useful. Fortunately Undertow provides a way to do just that.

Exchange Attributes

Exchange attributes may have up to two textual representations, a long one and a short one. The long version takes the form %{attribute}, while the short version is a percent sign followed by a single character. A list of the built in attributes provided by Undertow is below:

Attribute Short Form Long Form

Remote IP address



Local IP address



Bytes sent, excluding HTTP headers, or '-' if no bytes were sent


Bytes sent, excluding HTTP headers



Remote host name


Request protocol



Remote logical username from identd (always returns '-')


Request method



Local port



Query string (prepended with a '?' if it exists, otherwise an empty string)



First line of the request



HTTP status code of the response



Date and time, in Common Log Format format



Remote user that was authenticated



Requested URL path



Request relative path



Local server name



Time taken to process the request, in millis



Time taken to process the request, in seconds


Current request thread name



SSL cypher


SSL client certificate


SSL session id


Cookie value


Query parameter


Request header


Response header


Value from the predicate context


Any tokens that do not follow one of the above patterns are assumed to be literals. For example assuming a user name of 'Stuart' and a request method of 'GET' the attribute text Hello %u the request method is %m will give the value Hello Stuart the request method is GET.

These attributes are used anywhere that text based configuration is required, e.g. specifying the log pattern in the access log.

Some handlers may actually modify these attributes. In order for this to work the attribute must not be read only, and must consist of only a single token from the above table.

Textual Representation Predicates

Sometimes it is also useful to have a textual representation of a predicate. For examples when configuring a handler in Wildfly we may want it only to run if a certain condition is met, and when doing rewrite handling we generally do not want to re-write all requests, only a subset of them.

To this end Undertow provides a way to specify a textual representation of a predicate. In its simplest form, a predicate is represented as predicate-name[name1=value1,name2=value2].

For example, the following predicates all match POST requests:

equals[{%{METHOD}, POST}]
equals[%m, "POST"]
regex[pattern="POST", value="%m", full-match=true]

Lets examine these a bit more closely. The first one method[POST] uses the built in method predicate that matches based on the method. As this predicate takes only a single parameter (that is the default parameter) it is not necessary to explicitly specify the parameter name. Also note that POST is not quoted, quoting is only necessary if the token contains spaces, commas or square braces.

The second example method[value=POST] is the same as the first, except that the parameter name is explicitly specified.

The third and fourth examples demonstrates the 'equals' predicate. This predicate actually takes one parameter that is an array, and will return true if all items in the array are equal. Arrays are generally enclosed in curly braces, however in this case where there is a single parameter that is the default parameter the braces can be omitted.

The final examples shows the use of the regex predicate. This takes 3 parameters, the pattern to match, the value to match against and full-match, which determines if the pattern must match the whole value or simply part of it.

Some predicates may also capture additional information about the match and store it in the predicate context. For example the regex predicate will store the match under the key '0', and any match groups under the key '1', '2' etc.

These contextual values can then be retrieved by later predicates of handlers using the syntax ${0}, ${1} etc.

Predicates can be combined using the boolean operators 'and', 'or' and not. Some examples are shown below:

not method[POST]
method[POST] and path-prefix["/uploads"]
path-template[value="/user/{username}/*"] and equals[%u, ${username}]
regex[pattern="/user/(.*?)./.*", value=%U, full-match=true] and equals[%u, ${1}]

The first predicate will match everything except post requests. The second will match all post requests to /uploads. The third predicate will match all requests to URL’s of the form /user/{username}/* where the username is equal to the username of the currently logged in user. In this case the username part of the URL is captured, and the equals handler can retrieve it using the ${username} syntax shown above. The fourth example is the same as the third, however it uses a regex with a match group rather than a path template.

The complete list of built in predicates is shown below:

Name Parameters Parameter Types Default Parameter Optional Parameters Additional context


value, search

attribute, String[]



























String, attribute



Path template elements under the name


pattern, value, full-match

String, attribute, boolean



Match groups under number