Java -How to get logger to work in shutdown hook?
Again looking at the source, the solution appears to be to define a system property java.util.logging.manager
which is a subclass of LogManager which overrides the reset();
method so the Loggers continue to work on shutdown.
import java.util.logging.LogManager;
import java.util.logging.Logger;
public class Main {
static {
// must be called before any Logger method is used.
System.setProperty("java.util.logging.manager", MyLogManager.class.getName());
}
public static class MyLogManager extends LogManager {
static MyLogManager instance;
public MyLogManager() { instance = this; }
@Override public void reset() { /* don't reset yet. */ }
private void reset0() { super.reset(); }
public static void resetFinally() { instance.reset0(); }
}
public static void main(String... args) {
Logger logger1 = Logger.getLogger("Main1");
logger1.info("Before shutdown");
Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
@Override
public void run() {
try {
Logger logger2 = Logger.getLogger("Main2");
logger2.info("Shutting down 2");
} finally {
MyLogManager.resetFinally();
}
}
}));
}
}
prints
Dec 11, 2012 5:56:55 PM Main main
INFO: Before shutdown
Dec 11, 2012 5:56:55 PM Main$1 run
INFO: Shutting down 2
From this code for LogManager, you can see see there is a shutdown hook which dismantles the handlers and closes them. Logger only works in shutdown if it hasn't been used before so this code is not run.
// This private class is used as a shutdown hook.
// It does a "reset" to close all open handlers.
private class Cleaner extends Thread {
private Cleaner() {
/* Set context class loader to null in order to avoid
* keeping a strong reference to an application classloader.
*/
this.setContextClassLoader(null);
}
public void run() {
// This is to ensure the LogManager.<clinit> is completed
// before synchronized block. Otherwise deadlocks are possible.
LogManager mgr = manager;
// If the global handlers haven't been initialized yet, we
// don't want to initialize them just so we can close them!
synchronized (LogManager.this) {
// Note that death is imminent.
deathImminent = true;
initializedGlobalHandlers = true;
}
// Do a reset to close all active handlers.
reset();
}
}
/**
* Protected constructor. This is protected so that container applications
* (such as J2EE containers) can subclass the object. It is non-public as
* it is intended that there only be one LogManager object, whose value is
* retrieved by calling Logmanager.getLogManager.
*/
protected LogManager() {
// Add a shutdown hook to close the global handlers.
try {
Runtime.getRuntime().addShutdownHook(new Cleaner());
} catch (IllegalStateException e) {
// If the VM is already shutting down,
// We do not need to register shutdownHook.
}
}
From my own testing
Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
@Override
public void run() {
try {
Logger logger2 = Logger.getLogger("Main2");
logger2.info("Shutting down 2");
} catch (Throwable t) {
t.printStackTrace();
}
}
}));
prints
Dec 11, 2012 5:40:15 PM Main$1 run
INFO: Shutting down 2
but if you add
Logger logger1 = Logger.getLogger("Main1");
outside this block you get nothing.