How to activate gzip compression of selected content types in Tomcat or Wildfly

Another note for myself. To enable gzip compression in tomcat add an additional attribute to Connector tag in CATALINA_HOME/conf/server.xml:

<Connector port="8080" protocol="HTTP/1.1" connectionTimeout="20000" redirectPort="443" compressibleMimeType="application/javascript,text/css,application/json" compression="on"/>

Wildfly is not so well documented as Tomcat. So this note assembled from pieces of information saves time. Essentially one needs to enable and configure gzipFilter using Undertow predicates. Edit the default configuration file standalone.xml:

<subsystem xmlns="urn:jboss:domain:undertow:3.1">
    <buffer-cache name="default"/>
    <server name="default-server">
        <http-listener name="default" socket-binding="http" redirect-socket="https" enable-http2="true"/>
        <https-listener name="https" socket-binding="https" security-realm="ApplicationRealm" enable-http2="true"/>
        <host name="default-host" alias="localhost">
            <location name="/" handler="welcome-content"/>
            <access-log pattern="%h %t "%r" %s "%{i,User-Agent}"" prefix="myaccess."/>
            <filter-ref name="gzipfilter" predicate="regex[pattern='text/html|text/css|application/javascript|application/json',value=%{o,Content-Type}] and max-content-size[value=1024]"/>
        </host>
    </server>
    <servlet-container name="default">
        <jsp-config/>
        <persistent-sessions path="sessions" relative-to="jboss.server.temp.dir"/>
        <websockets/>
    </servlet-container>
    <handlers>
        <file name="welcome-content" path="${jboss.home.dir}/welcome-content"/>
    </handlers>
    <filters>
        <gzip name="gzipfilter"/>
    </filters>
</subsystem>

All the possible predicates are listed in Undertow documentation. Some people use url-based predicates like:

<filter-ref name="gzipFilter" predicate="path-suffix['.css'] or path-suffix['.js']" />

Alternatively, one can use a custom gzip compression servlet filter that can be more easily configured to target some specific output. A working example is in GitHub. I keed this sample only because it works well and its GIPOutputStream potentially could be replaced by some other stream to for example encrypt the output or produce hashes.