Custom Filters in WildFly

By James R. Perkins | January 04, 2020

What is a log filter?

A log filter is used to add fine grained control over a log message. In the case of WildFly this is a java.util.logging.Filter. As of WildFly 18 there is the ability to use custom log filters.

Creating a Filter

To create a filter you must implement the java.util.logging.Filter interface. The filter must be in a module and can be defined on a logger or a handler via the filter-spec attribute. A custom filter can also be combined on the filter-spec attribute with a filter expression. For example any(match(".*WELD.*"), myCustomFilter).

The below example will filter log messages based on the current thread’s context class loader. It takes advantage of WildFly’s use of JBoss Modules to get the module name from the class loader. The name is then checked to see if it matches the pattern configured on the filter.

Example Filter

public class ClassLoaderFilter implements Filter {

    @Override
    public boolean isLoggable(final LogRecord record) {
        final ClassLoader cl = getClassLoader();
        String value;
        if (cl instanceof ModuleClassLoader) {
            value = ((ModuleClassLoader) cl).getName();
        } else {
            value = cl.toString();
        }
        if (pattern == null || pattern.matcher(value).matches()) {
            MDC.put("moduleName", value);
            return true;
        }
        MDC.remove("moduleName");
        return false;
    }
 }

Adding a Filter

A filter can be added to WildFly by first creating a module based on the library the filter is in. We then need to create a filter resource on the logging subsystem based on this new module and filter. Finally the filter can be added to a logger or handler resource.

Example CLI Commands

module add --name=org.jboss.example.filter --resources=/path/to/log-filter.jar --dependencies=org.jboss.modules,java.logging,org.jboss.logging
/subsystem=logging/json-formatter=json:add(exception-output-type=formatted, date-format="yyyy-MM-dd'T'HH:mm:ss.SSSZZZZZ")
/subsystem=logging/filter=clFilter:add(module=org.jboss.example.filter, class=org.jboss.example.filter.ClassLoaderFilter, properties={pattern=".*deployment\.app.*"})
/subsystem=logging/file-handler=DEPLOYMENT:add(file={relative-to=jboss.server.log.dir, path=deployment.log}, level=TRACE, append=false, autoflush=true,named-formatter=json, filter-spec=clFilter)
/subsystem=logging/root-logger=ROOT:add-handler(name=DEPLOYMENT)

In the example above we create a filter which uses the pattern .*deployment\.app.*. This will match the module name from the current thread’s context class loader and only accept messages where the module name matches the pattern. In our case this will only log messages that are associated with our deployment.

We then add the filter created to the file handler created with a JSON formatter. Finally we add the file handler to the root logger.

Example Project

An example project can be found at https://github.com/jamezp/wildfly-examples/tree/master/custom-log-filter. To use the example project simply download the source and run mvn clean wildfly:run. Once started and the application is deployed you can access the example at http://localhost:8080/app.

You should initially see some log messages that were logged during the deployment process. You can then log a custom message or start a job which logs a message at the defined number of seconds.