Log4j appender that logs event meta data such as the timestamp, the logger name, the exception stacktrace, ThreadContext (aka MDC) or the Java thread name to fields in systemd journal (aka "the Journal") .

Read Lennart Poettering's blog post systemd for Developers III if you are not familar with systemd journal.

Usage with Log4j 2.x

Add the following Maven dependency to your project:

<dependency>
	<groupId>de.bwaldvogel</groupId>
	<artifactId>log4j-systemd-journal-appender</artifactId>
	<version>2.2.2</version>
	<scope>runtime</scope>
</dependency>

Usage with Log4j 1.x

See the 1.x branch of this project.

Runtime dependencies

- Linux with systemd-journal
- Log4j 2.x

Configuration

The appender can be configured with the following properties

Property name         | Default           | Type    | Description
--------------------- | ----------------- | ------- | -----------
`logSource`           | false             | boolean | Determines whether the log locations are logged. Note that there is a performance overhead when switched on. The data is logged in standard systemd journal fields `CODE_FILE`, `CODE_LINE` and `CODE_FUNC`.
`logStacktrace`       | true              | boolean | Determines whether the full exception stack trace is logged. This data is logged in the user field `STACKTRACE`.
`logThreadName`       | true              | boolean | Determines whether the thread name is logged. This data is logged in the user field `THREAD_NAME`.
`logLoggerName`       | true              | boolean | Determines whether the logger name is logged. This data is logged in the user field `LOG4J_LOGGER`.
`logAppenderName`     | true              | boolean | Determines whether the appender name is logged. This data is logged in the user field `LOG4J_APPENDER`.
`logThreadContext`    | true              | boolean | Determines whether the [thread context][thread-context] is logged. Each key/value pair is logged as user field with the `threadContextPrefix` prefix.
`threadContextPrefix` | `THREAD_CONTEXT_` | String  | Determines how [thread context][thread-context] keys should be prefixed when `logThreadContext` is set to true. Note that keys need to match the regex pattern `[A-Z0-9_]+` and are normalized otherwise.
`syslogIdentifier`    | null              | String  | This data is logged in the user field `SYSLOG_IDENTIFIER`.  If this is not set, the underlying system will use the command name (usually `java`) instead.

Example

log4j2.xml

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="INFO" packages="de.bwaldvogel.log4j">
    <Appenders>
        <Console name="console" target="SYSTEM_OUT">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n" />
        </Console>
        <SystemdJournal name="journal" logStacktrace="true" logSource="false" />
    </Appenders>
    <Loggers>
        <Root level="INFO">
            <AppenderRef ref="console" />
            <AppenderRef ref="journal" />
        </Root>
    </Loggers>
</Configuration>

This will tell Log4j to log to systemd journal as well as to stdout (console). Note that a layout is optional for SystemdJournal. This is because meta data of a log event such as the timestamp, the logger name or the Java thread name are mapped to systemd-journal fields and need not be rendered into a string that loses all the semantic information.

YourExample.java

import org.apache.logging.log4j.*;

class YourExample {

    private static Logger logger = LogManager.getLogger(YourExample.class);

    public static void main(String[] args) {
        ThreadContext.put("MY_KEY", "some value");
        logger.info("this is an example");
    }
}

Running this sample class will log a message to journald:

Systemd Journal

# journalctl -n
Okt 13 21:26:00 myhost java[2370]: this is an example

Use journalctl -o verbose to show all fields:

# journalctl -o verbose -n
Di 2015-09-29 21:07:05.850017 CEST [s=45e0…;i=984;b=c257…;m=1833…;t=520e…;x=3e1e…]
    PRIORITY=6
    _TRANSPORT=journal
    _UID=1000
    _GID=1000
    _CAP_EFFECTIVE=0
    _SYSTEMD_OWNER_UID=1000
    _SYSTEMD_SLICE=user-1000.slice
    _MACHINE_ID=4abc6d…
    _HOSTNAME=myhost
    _SYSTEMD_CGROUP=/user.slice/user-1000.slice/session-2.scope
    _SYSTEMD_SESSION=2
    _SYSTEMD_UNIT=session-2.scope
    _BOOT_ID=c257f8…
    THREAD_NAME=main
    LOG4J_LOGGER=de.bwaldvogel.log4j.SystemdJournalAppenderIntegrationTest
    _COMM=java
    _EXE=/opt/oracle-jdk-bin-1.7.0.80/bin/java
    MESSAGE=this is a test message with a MDC
    CODE_FILE=SystemdJournalAppenderIntegrationTest.java
    CODE_FUNC=testMessageWithMDC
    CODE_LINE=36
    THREAD_CONTEXT_SOME_KEY1=some value %d
    THREAD_CONTEXT_SOME_KEY2=some other value with unicode: →←üöß
    SYSLOG_IDENTIFIER=log4j2-test
    LOG4J_APPENDER=Journal
    _PID=8224
    _CMDLINE=/opt/oracle-jdk-bin-1.7.0.80/bin/java …
    _SOURCE_REALTIME_TIMESTAMP=1443553625850017

Note that the ThreadContext key-value pair {"MY_KEY": "some value"} is automatically added as field with prefix THREAD_CONTEXT.

You can use the power of systemd journal to filter for interesting messages. Example:

journalctl CODE_FUNC=testMessageWithMDC THREAD_NAME=main will only show messages that are logged from the Java main thread in a method called testMessageWithMDC.

Related Work

• logback-journal
  • Systemd Journal appender for Logback