use java library to grab hostname

[TomonoriSoejima]

I ran into this issue due to lack of hostname command on the container.

2025-04-01 11:21:56,560 [elastic-apm-metadata-0] INFO  co.elastic.apm.agent.impl.metadata.SystemInfo - Failed to execute command "hostname -f" with exit code -1
2025-04-01 11:21:56,560 [elastic-apm-metadata-0] DEBUG co.elastic.apm.agent.impl.metadata.SystemInfo - Command execution error
java.io.IOException: Cannot run program "hostname": error=2, No such file or directory

apm-agent-java/apm-agent-core/src/main/java/co/elastic/apm/agent/impl/metadata/SystemInfo.java

Line 223 in 93f28fe

logger.info("Failed to execute command {} with exit code {}", cmdAsString(cmd), commandOutput.getExitCode());

It would be beneficial to abandon calling this command and do so in java to avoid such error as well as a small performance improvement.

String hostname = InetAddress.getLocalHost().getHostName();

[SylvainJuge]

Hi,

I don't recall exactly but using hostname allows us to capture a value consistent cross-languages, and this Java API tends to not be 100% consistent, for example it might return short host name or fully-qualified named depending on the system, which prevents consistency.

What you could do as a work-around is to set the HOSTNAME environment variable, or hostname configuration option to provide a value explicitly.

[TomonoriSoejima]

Right,

apm-agent-java/apm-agent-core/src/main/java/co/elastic/apm/agent/impl/metadata/SystemInfo.java

Lines 168 to 185 in 93f28fe

	static String fallbackHostnameDiscovery(boolean isWindows) {
	String hostname = discoverHostnameThroughEnv(isWindows);
	if (hostname == null || hostname.isEmpty()) {
	try {
	hostname = InetAddress.getLocalHost().getHostName();
	} catch (Exception e) {
	logger.debug("Network fallback for hostname discovery of localhost failed", e);
	}
	}
	if (hostname == null || hostname.isEmpty() && !isWindows) {
	try (FileReader fileReader = new FileReader(HOST_FILE);
	BufferedReader reader = new BufferedReader(fileReader)) {
	hostname = reader.readLine().trim();
	} catch (IOException e) {
	logger.debug(HOST_FILE + " fallback for hostname discovery of localhost failed", e);
	}
	}
	return hostname;

will be called as fallback in that case.

[SylvainJuge]

Yes, in addition to that I forgot about the /etc/hostname file which can also be used if you are not on Windows.