Logs of java.util.logging.Logger are not output when war is deployed to tomcat.

[neut-funahashi]

Spring boot 2.0.4.RELEASE
apache tomcat 8.5.38

SLF4JBridgeHandler.install will not run because the isJulUsingASingleConsoleHandlerAtMost method is false at runtime on org.springframework.boot.logging.Slf4JLoggingSystem class configureJdkLoggingBridgeHandler.

In order to prevent log duplication, how about installing after uninstalling in advance?

It is confirmed that it is output normally with Spring boot 1.3.8.

[wilkinsona]

While not ideal, this is working as designed.

In a multi-application environment, such as standalone Tomcat, bridging JUL into SLF4J results in any logging using JUL (by Tomcat itself or any of the applications) being routed into the application-specific log. To avoid this problem we deliberately do not install the bridge in such environments. Please see #13470 for further details.

We should probably note this behaviour in the documentation. We can use this issue to do that.

[hengyunabc]

@wilkinsona

When there are only two Handlers: java.util.logging.ConsoleHandler and org.slf4j.bridge.SLF4JBridgeHandler , remove these two Handlers?

    private boolean isJulUsingASingleConsoleHandlerAtMost() {
        Logger rootLogger = LogManager.getLogManager().getLogger("");
        Handler[] handlers = rootLogger.getHandlers();
        return handlers.length == 0
                || (handlers.length == 1 && handlers[0] instanceof ConsoleHandler)
                || (handlers.length == 2 && handlers[0] instanceof ConsoleHandler && handlers[1].getClass().getName().equals("org.slf4j.bridge.SLF4JBridgeHandler"));
    }

[wilkinsona]

@hengyunabc Can you explain the benefit of what you have proposed? isJulUsingASingleConsoleHandlerAtMost is used to determine whether or not the SLF4JBridgeHandler should be installed. Why would we install it when it's already present, and in what situation would it already be present?

[hengyunabc]

@wilkinsona We have a problem like this:

In the main method, before the spring boot application starts, we have the initialization code. In this code, slf4j will install a SLF4JBridgeHandler, then the spring boot will skip the install in
Slf4JLoggingSystem.

	public static void main(String[] args) {
		// some init code, will install a SLF4JBridgeHandler
		XXX.init();
		// start spring boot
		SpringApplication.run(DemoApplication.class, args);
	}

Currently, I am removing the SLF4JBridgeHandler before starting the spring boot.

So I think when there are only two Handlers: java.util.logging.ConsoleHandler and org.slf4j.bridge.SLF4JBridgeHandler, can remove these two Handlers?

[wilkinsona]

The problem that you are facing seems to be specific to your application and it calling XXX.init(). Unfortunately, we can't change the behaviour of Slf4jLoggingSystem for everyone to work around the unusual behaviour in your application. Thanks anyway for the suggestion.