log4j-systemd-appender

A Log4j 2 appender that sends structured log events directly to the systemd journal via Unix datagram socket, using the native journal protocol. No libsystemd dependency.

Features

• Structured fields: PRIORITY, SYSLOG_IDENTIFIER, THREAD_NAME, LOG4J_LOGGER, LOG4J_APPENDER, CODE_FILE/LINE/FUNC, STACKTRACE, SYSLOG_FACILITY
• Optional ThreadContext (MDC) forwarding with configurable key prefix
• Message truncation when the datagram would exceed maxMessageSize
• On JDK 22+: uses the Foreign Function & Memory API — no native library loaded at runtime
• On JDK 17–21: uses a small JNI shim bundled for glibc and musl (Alpine Linux), x86_64 and aarch64

Requirements

• Java 17 or later
• Linux with systemd (the journal socket at /run/systemd/journal/socket)

Configuration

<?xml version="1.0" encoding="UTF-8"?>
<Configuration packages="org.github.crac.systemd_appender">
    <Appenders>
        <Systemd name="journal"
                 syslogIdentifier="my-app"
                 syslogFacility="3"
                 logSource="false"
                 logStacktrace="true"
                 logThreadName="true"
                 logLoggerName="true"
                 logLoggerAppName="MYAPP"
                 logAppenderName="true"
                 logThreadContext="true"
                 threadContextPrefix="THREAD_CONTEXT_"
                 maxMessageSize="65536"/>
    </Appenders>
    <Loggers>
        <Root level="info">
            <AppenderRef ref="journal"/>
        </Root>
    </Loggers>
</Configuration>

Parameters

Parameter	Type	Default	Description
syslogIdentifier	String	process name	Maps to SYSLOG_IDENTIFIER. Defaults to the executable name derived from ProcessHandle.
syslogFacility	int	(unset)	Maps to SYSLOG_FACILITY. Expects a numeric syslog facility code. Omitted when not set.
logSource	boolean	false	Log source location fields CODE_FILE, CODE_LINE, CODE_FUNC, JAVA_CLASSNAME. Has a performance cost; requires includeLocation on the logger.
logStacktrace	boolean	true	Log the full exception stacktrace to the STACKTRACE field.
logThreadName	boolean	true	Log the thread name to THREAD_NAME.
logLoggerName	boolean	true	Log the logger name. Field is LOG4J_LOGGER by default, or {logLoggerAppName}_LOGGER when logLoggerAppName is set.
logLoggerAppName	String	(unset)	When set, changes the logger-name field from LOG4J_LOGGER to {logLoggerAppName}_LOGGER.
logAppenderName	boolean	true	Log the appender name to LOG4J_APPENDER.
logThreadContext	boolean	true	Forward ThreadContext (MDC) entries as THREAD_CONTEXT_* fields. Keys are uppercased and non-alphanumeric characters replaced with _.
threadContextPrefix	String	THREAD_CONTEXT_	Prefix applied to ThreadContext keys.
maxMessageSize	int	65536	Maximum datagram size in bytes. Messages that would exceed this limit are truncated and suffixed with [TRUNCATED].

System properties

Property	Description
org.github.crac.systemd_appender.jni=true	Force the JNI implementation even on JDK 22+. Useful for verifying that the bundled native library works on a modern JDK.

Building

A standard mvn package compiles the Java sources and the native JNI shim. The native library is compiled for four targets; each compiler is optional — if a compiler is not found the corresponding native variant is silently skipped.

Compiler requirements

Target	Compiler	Debian/Ubuntu	Fedora/RHEL
Linux x86_64 (glibc)	gcc	apt-get install gcc	dnf install gcc
Linux aarch64 (glibc)	aarch64-linux-gnu-gcc	apt-get install gcc-aarch64-linux-gnu	dnf install gcc-aarch64-linux-gnu
Linux x86_64 (musl)	musl-gcc	apt-get install musl-tools	dnf install musl-gcc
Linux aarch64 (musl)	aarch64-linux-musl-gcc	(not in standard repos — see below)	(not in standard repos — see below)

aarch64 musl cross-compiler

There is no standard distribution package for an aarch64-musl cross-compiler. Pre-built toolchains are available from musl.cc:

# Example: download and install to /opt/musl-cross
wget https://musl.cc/aarch64-linux-musl-cross.tgz
tar -xf aarch64-linux-musl-cross.tgz -C /opt
export PATH="/opt/aarch64-linux-musl-cross/bin:$PATH"

Install all compilers (Debian/Ubuntu)

apt-get install gcc gcc-aarch64-linux-gnu musl-tools
# Plus aarch64 musl cross-compiler from musl.cc if needed

Install all compilers (Fedora/RHEL)

dnf install gcc gcc-aarch64-linux-gnu musl-gcc
# Plus aarch64 musl cross-compiler from musl.cc if needed