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Expose service port as a system property, mainly to support use-case where configured port is zero and picked by OS #686

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Expose service port as a system property, mainly to support use-case where configured port is zero and picked by OS #686

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8 changes: 8 additions & 0 deletions jmx_prometheus_javaagent_java6/src/main/java/io/prometheus/jmx/JavaAgent.java
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,14 @@ public static void premain(String agentArgument, Instrumentation instrumentation
new JmxCollector(new File(config.file), JmxCollector.Mode.AGENT).register();
DefaultExports.initialize();
server = new HTTPServer(config.socket, CollectorRegistry.defaultRegistry, true);

// in case port was configured with port 0, then actual port (chosen by operating system) is known only after the socket is open
// in this case, we need a way to expose the actual port number used
// setting it as a system properties allows any other code running on the JVM to discover this
// this information could then be pushed into a discovery service
int actualServerPort = server.getPort();
System.setProperty("jmx_exporter.server.port.configured", String.valueOf(config.port));
System.setProperty("jmx_exporter.server.port.actual", String.valueOf(actualServerPort));
}
catch (IllegalArgumentException e) {
System.err.println("Usage: -javaagent:/path/to/JavaAgent.jar=[host:]<port>:<yaml configuration file> " + e.getMessage());
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