How can I configure log4j independently(Isolate 'LoggerContext')?

[youngledo]

I hope to dynamically configure log4j at some point and output logs without interfering with the core logging (log4j2-spring.xml). But after I did this, I also closed the core log. What should I do?

Here is my example code:

@RestController
public class SampleController {

    private static final Logger LOGGER = LogManager.getLogger();

    @RequestMapping("log")
    public String log() throws IOException {
        LOGGER.info("start");
        Set<Map<String, Object>> set = new HashSet<>();
        for (int i = 0; i < 1; i++) {
            HashMap<String, Object> map = new HashMap<>();
            map.put("name", i);
            map.put("age", i);
            set.add(map);
        }
        this.logMetadata(set);
        // There is no output because it is closed.
        LOGGER.info("finish!");
        return "ok";
    }

    public void logMetadata(Set<Map<String, Object>> set) throws IOException {
        try (LoggerContext loggerContext = this.buildMetadataLogContext()) {
            for (Map<String, Object> map : set) {
                String content = new ObjectMapper().writeValueAsString(map);
                Logger logger = loggerContext.getLogger(SampleController.class);
                logger.info(content);
            }
        }
    }

    private LoggerContext buildMetadataLogContext() {
        ConfigurationBuilder<BuiltConfiguration> builder = ConfigurationBuilderFactory.newConfigurationBuilder();
        // PatternLayout
        AppenderComponentBuilder console = builder.newAppender("console", "Console");
        LayoutComponentBuilder patternLayout = builder.newLayout("PatternLayout");
        console.add(patternLayout);
        builder.add(console);

        // Root Logger
        RootLoggerComponentBuilder rootLogger = builder.newRootLogger(Level.INFO);
        rootLogger.add(builder.newAppenderRef("console"));
        builder.add(rootLogger);

        Configuration configuration = builder.build(false);
        Configurator.reconfigure(configuration);
        return Configurator.initialize(configuration);
    }
}

[youngledo]

I understand, we need to create a new LoggerContext ourselves

[ppkarwasz]

I understand, we need to create a new LoggerContext ourselves

Yes, if you wish to have an entirely separate set of loggers, you need to create a new LoggerContext:

LoggerContext context = new LoggerContext("-myApp");
Configuration configuration = ...;
context.start(configuration);

The fact that LoggerContext can be used with try-with-resources is a bug: you should close the context at the appropriate moment, e.g. when your application closes.

Remarks

Hardcoded config

I would not recommend using a hardcoded configuration, since your users might want to modify that. Unless explicitly configured, each logger context looks for configuration files named log4j2-<contextName>.<extension> (see configuration file location), so you could use something like this:

private LoggerContext context;

@Value(${myApp.logging.config})
private String configLocation;

public void afterPropertiesSet() {
    context = new LoggerContext("-myApp", null, configLocation);
    context.start();
}

public void destroy() {
    context.stop();
}

You can put a log4j2-myApp.xml file in your application, while leaving users the option to specify a different one using the myApp.logging.config property.

Separate logger context

I am not entirely sure you need a separate logger context. You can also use the default one and specify filters to redirect your messages to the appropriate appender. For example a statement like:

logger.info(() ->
        new StringMapMessage().with("type", "test").with("name", i).with("age", i));

Can be filtered out using:

<Filters>
  <MapFilter onMatch="ACCEPT" onMismatch="NEUTRAL">
    <KeyValuePair key="type" value="test"/>
  </MapFilter/>
  <DenyAllFilter/>
</Filters>

[youngledo]

Thanks, this is what I want!