Using custom Lookups for Virtual Properties

[wageeshagunasena]

I want to use a custom lookup extended from ValueResolver, for a virtual property to resolve the value. The virtual property is given as in the following configuration.

<JacksonJsonLayout>
   <VirtualProperty name="AdditionalField" value="UnresolvedValue" dynamic="true"/>
</JacksonJsonLayout>

I see the value resolver for JacksonJsonLayout is set for Log4j2Lookup. How can I set my custom value resolver to do this?

[rfoltyns]

You have to extend JacksonJsonLayout.Builder. There are a few ways you can do that:

1. You can extend JacksonJsonLayout and write your own Builder.createValueResolver() method. Sth like this maybe?

import com.fasterxml.jackson.databind.ObjectWriter;
import org.apache.logging.log4j.core.Layout;
import org.apache.logging.log4j.core.config.Configuration;
import org.apache.logging.log4j.core.config.Node;
import org.apache.logging.log4j.core.config.plugins.Plugin;
import org.apache.logging.log4j.core.config.plugins.PluginBuilderFactory;

@Plugin(name = CustomJacksonJsonLayout.PLUGIN_NAME, category = Node.CATEGORY, elementType = Layout.ELEMENT_TYPE, printObject = true)
public class CustomJacksonJsonLayout extends JacksonJsonLayout {

    public static final String PLUGIN_NAME = "CustomJacksonJsonLayout";

    protected CustomJacksonJsonLayout(Configuration config, ObjectWriter configuredWriter, ItemSourceFactory itemSourceFactory) {
        super(config, configuredWriter, itemSourceFactory);
    }

    @PluginBuilderFactory
    public static CustomJacksonJsonLayout.Builder newBuilder() {
        return new CustomJacksonJsonLayout.Builder();
    }

    public static class Builder extends JacksonJsonLayout.Builder {

        @Override
        protected ValueResolver createValueResolver() {
            return new ValueResolver() {
                @Override
                public String resolve(String unresolved) {
                    return "whatever_you_need";
                }

                @Override
                public String resolve(VirtualProperty property) {
                    return "and_more";
                }
            };
        }
    }
}

2. Or do the similar with programmatic config (not the nicest ever in this case, but very convenient):

JacksonJsonLayout.Builder layoutBuilder = new JacksonJsonLayout.Builder() {
            @Override
            protected ValueResolver createValueResolver() {
                return new ValueResolver() {
                    @Override
                    public String resolve(String unresolved) {
                        return "whatever_you_need";
                    }

                    @Override
                    public String resolve(VirtualProperty property) {
                        return "and_more";
                    }
                };
            }
        }
        .setConfiguration(ctx.getConfiguration())
        .withVirtualProperties(
            // see Jest and HC SmokeTests for examples
        );

3. Or (for file config) add new ValueResolver plugin, allow to set it on current JacksonJsonLayout.Builder and create a PR if you'd like to share it.

[wageeshagunasena]

Sure will do and create a PR for the 3rd option.

However, I managed to do this using a custom Lookup plugin in the category of StrLookup.

@Plugin( name="lookupliteral", category = StrLookup.CATEGORY)
public class CustomLookup implements StrLookup
{
    @Override public String lookup( String key )
    {
        return getResolvedValue(key);
    }

    @Override public String lookup( LogEvent event, String key )
    {
        return lookup( key );
    }

    /**
     * my way of resolving the value
     */
    private String getResolvedValue(key)
    {
        return "resolved_value"
    }   
}

[rfoltyns]

@wageeshagunasena Custom StrLookup plugin makes sense as well! Nice one!

[rfoltyns]

@wageeshagunasena would you like to create any PRs for this?

[manushakaru]

@rfoltyns is Custom StrLookup plugin solution still works with log4j 2.16.0 or 2.17.1 ? Do you have any idea ?

[rfoltyns]

@manushakaru It does. Have a look at this branch in log42-elasticsearch-examples