Implementation

Discord4J utilizes logging throughout their modules that will be forwarded to your implementation of choice, according to Reactor Loggers class which picks up common logging frameworks on the startup and configures logging appropriately.

• If you have any SLF4J implementation available, it will be picked up first.
• As a fallback, it will log to the console, using System.err for the WARN and ERROR log levels and System.out for the rest.
• If you prefer to log to JDK java.util.logging you must set the reactor.logging.fallback system property to JDK, For example, if running from the command line:

-Dreactor.logging.fallback=JDK

Logging a Stream

You have the ability to log events in a reactive sequence, like those coming from Discord4J. The log() operator is able to do that, peeking at every signal going through a sequence. You can learn more about this operator on the Reactor reference guide here.

Configuration

SLF4J Simple

SLF4J Simple is a basic implementation that outputs INFO and higher logging directly to System.err. It's easy to use and requires no additional files. Check it out here.

Logback

Logback is an SLF4J implementation you can use with Discord4J to further configure logging. The following is an example to use it. First add this dependency to your project. Then create a file under src/main/resources named logback.xml with the following content:

<?xml version="1.0" encoding="UTF-8"?>
<configuration scan="true">
    <!-- You can configure per-logger level at this point -->
    <!-- This set of preconfigured loggers is good if you want to have a DEBUG level as baseline -->
    <logger name="io.netty" level="INFO"/>
    <logger name="reactor" level="INFO"/>

    <!-- Display the logs in your console with the following format -->
    <!-- You can learn more about this here: https://logback.qos.ch/manual/layouts.html#ClassicPatternLayout -->
    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>

    <!-- Log to a file as well, including size and time based rolling -->
    <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
            <fileNamePattern>logs/d4j.%d{yyyy-MM-dd}.%i.log</fileNamePattern>
            <maxFileSize>100MB</maxFileSize>
            <maxHistory>90</maxHistory>
        </rollingPolicy>
        <encoder>
            <charset>UTF-8</charset>
            <Pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %-40.40logger{39} : %msg%n</Pattern>
        </encoder>
        <prudent>true</prudent>
    </appender>

    <!-- Avoid blocking while logging to file by wrapping our file appender with async capabilities -->
    <appender name="ASYNC" class="ch.qos.logback.classic.AsyncAppender">
        <queueSize>512</queueSize>
        <appender-ref ref="FILE"/>
    </appender>

    <!-- Here you can set the base logger level. If DEBUG is too chatty for you, you can use INFO -->
    <!-- Possible options are: ALL, TRACE, DEBUG, INFO, WARN, ERROR, OFF -->
    <root level="DEBUG">
        <appender-ref ref="CONSOLE"/>
        <appender-ref ref="ASYNC"/>
    </root>
</configuration>

Log4J2

Log4J2 can also work with Discord4J using an SLF4J adapter. To begin please add log4j-slf4j18-impl as dependency.

Available loggers

Discord4J has a logger structure that differs from v2, where you can tweak at the logger level the verbosity you prefer.

If you want to reduce the logging produced by websocket data you can use the following in your logback.xml file:

<logger name="discord4j.gateway" level="INFO"/>

The following table shows the levels you can set each logger to obtain your preferred details. They work on a hierarchy basis therefore setting a level to discord4j.rest affects every logger under it like discord4j.rest.traces.

v3.1 Loggers

Logger	Level	Description
io.netty	DEBUG	Low level details of underlying Netty implementation
reactor	DEBUG	Low level details for Reactor operations
reactor.netty	DEBUG	Details about Reactor Netty network operations
discord4j.core	INFO	Version information about Discord4J
discord4j.core.events	DEBUG	Event dispatcher subscription information
discord4j.core.events	TRACE	Event dispatcher event instances being published
discord4j.core.events.dispatch	DEBUG	Requests made by Discord4J while converting inbound payloads into events
discord4j.core.events.dispatch	TRACE	Details about caching while converting inbound payloads into events
discord4j.core.state	DEBUG	Details about entity cache configuration
discord4j.core.shard	DEBUG	Details about shard group bootstrapping
discord4j.rest.request	DEBUG	HTTP requests made by Discord4J, for example GET /gateway/bot
discord4j.rest.request	TRACE	Extra details about the lifecycle of an HTTP request
discord4j.limiter	TRACE	Lifecycle of the default rate limiter implementation
discord4j.rest.http.JacksonWriterStrategy	TRACE	HTTP request JSON body contents
discord4j.rest.http.JacksonReaderStrategy	TRACE	HTTP response JSON body contents
discord4j.rest.http.client.DiscordWebClient	TRACE	Lifecycle of the REST API client
discord4j.gateway	INFO	Main events of Discord Gateway connections (connected, reconnects, disconnects)
discord4j.gateway	DEBUG	Details of Discord Gateway connections (heartbeats, reconnect reasons)
discord4j.gateway.protocol.sender	TRACE	JSON payload sent to Discord Gateway
discord4j.gateway.protocol.receiver	TRACE	JSON payload received from Discord Gateway
discord4j.voice	INFO	Main events of Discord Voice Gateway connections (connecting, connected, reconnects, disconnects)
discord4j.voice	DEBUG	Details of Discord Gateway connections (heartbeats, reconnect reasons)
reactor.netty.udp	DEBUG	Details about Reactor Netty UDP connections (used by voice)
discord4j.voice.protocol.sender	TRACE	JSON payload sent to Discord Voice Gateway
discord4j.voice.protocol.receiver	TRACE	JSON payload received from Discord Voice Gateway
discord4j.voice.protocol.udp.sender	TRACE	JSON payload sent to a Discord Voice Server
discord4j.voice.protocol.udp.receiver	TRACE	JSON payload received from a Discord Voice Server

Advanced filtering using Logback

If you're looking to filter out certain gateway event types, you could copy this custom TurboFilter to your project or as a start point for your own.

Afterwards, apply it to your logback.xml, for example the following would only show created message events:

<configuration>
    <turboFilter class="discord4j.core.logback.GatewayEventFilter">
        <Include>MESSAGE_CREATE</Include>
    </turboFilter>
    ...

While this one would show all events except presence updates:

<configuration>
    <turboFilter class="discord4j.core.logback.GatewayEventFilter">
        <Exclude>PRESENCE_UPDATE</Exclude>
    </turboFilter>
    ...

The value like PRESENCE_UPDATE must match from the ones in this file.

For complete control over what you want to filter, use JaninoEvaluator. First add janino as a dependency and then apply the following to your logback.xml:

    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.core.filter.EvaluatorFilter">
            <evaluator>
                <expression>return formattedMessage.contains("PresenceUpdate");</expression>
            </evaluator>
            <OnMismatch>NEUTRAL</OnMismatch>
            <OnMatch>DENY</OnMatch>
        </filter>
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>

For more details of what you can put under the <expression> attribute, see this documentation page.