logback: Two appenders, multiple loggers, different levels

I want to have two log files in my application (Spring Integration), debug.log and main.log. I want to run main.log at an INFO level and debug.log at a DEBUG level. This is doable with filters on the appenders. I want to log different levels to the appenders based on the source. In other words

<logger name="org.springframework" level="ERROR">
    <appender-ref ref="main" />
</logger>
<logger name="org.springframework" level="DEBUG">
    <appender-ref ref="debug" />
</logger>
<logger name="com.myapp" level="INFO">
    <appender-ref ref="main" />
</logger>
<logger name="com.myapp" level="DEBUG">
    <appender-ref ref="debug" />
</logger>

So to summarise:

1. Spring logger
   • main -> ERROR
   • debug -> DEBUG
2. com.myapp logger
   • main -> INFO
   • debug -> DEBUG

Because of this I have to have the loggers running at DEBUG and a threshold filter on an appender isn't fine grained enough.

Update Added clarity to the question

Create a ThresholdLoggerFilter class which can be put on an appender like:

<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
        <level>INFO</level>
    </filter>
    <filter class="com.myapp.ThresholdLoggerFilter">
        <logger>org.springframework</logger>
        <level>ERROR</level>
    </filter>
    </appender>

The following code works

package com.myapp;

import ch.qos.logback.classic.Level;
import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.filter.Filter;
import ch.qos.logback.core.spi.FilterReply;

public class ThresholdLoggerFilter extends Filter<ILoggingEvent> {
    private Level level;
    private String logger;

    @Override
    public FilterReply decide(ILoggingEvent event) {
        if (!isStarted()) {
            return FilterReply.NEUTRAL;
        }

        if (!event.getLoggerName().startsWith(logger))
            return FilterReply.NEUTRAL;

        if (event.getLevel().isGreaterOrEqual(level)) {
            return FilterReply.NEUTRAL;
        } else {
            return FilterReply.DENY;
        }
    }

    public void setLevel(Level level) {
        this.level = level;
    }

    public void setLogger(String logger) {
        this.logger = logger;
    }

    public void start() {
        if (this.level != null && this.logger != null) {
            super.start();
        }
    }
}

You can also do this somewhat more simply if you are willing to inherit from the root logger, e.g. here we add an extra logger for errors, that logs to stderr. It's only enabled for particular loggers.

<configuration>
    <appender name="CONSOLE-stdout" class="ch.qos.logback.core.ConsoleAppender">
        <target>System.out</target> <!-- the default -->
        <encoder>
            <pattern>%d %-5level [%thread] %logger{0}: %msg%n</pattern>
        </encoder>
    </appender>
    <appender name="CONSOLE-stderr" class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>ERROR</level>
        </filter>

        <target>System.err</target>
        <encoder>
            <pattern>%d %-5level [%thread] %logger{0}: %msg%n</pattern>
        </encoder>
    </appender>
    <root level="DEBUG">
        <appender-ref ref="CONSOLE-stdout" />
    </root>

        <!-- We want error logging from this logger to go to an extra appender 
             It still inherits CONSOLE-stdout from the root logger -->
    <logger name="org.springframework" level="INFO">
        <appender-ref ref="CONSOLE-stderr" />
    </logger>
</configuration>

Adding an additional solution that is simpler than what's already here

None of these solutions worked for me, since I'm not using a framework like Spark or Spring. So I did something a bit simpler that seems to function nicely. While this solution may not work for the OP, perhaps it can be of use to someone wanting something not so bulky.

<property name="pattern" value="%d{yyyy.MMM.dd HH:mm:ss.SSS} [ProgramName] %level - %msg%n" />

<appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>/path/to/your/program.log</file>
    <append>true</append>
    <encoder>
        <pattern>${pattern}</pattern>
    </encoder>
</appender>

<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <target>System.out</target>
    <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
        <level>INFO</level>
    </filter>
    <encoder>
        <pattern>${pattern}</pattern>
    </encoder>
</appender>

<root level="debug">
    <appender-ref ref="FILE" />
    <appender-ref ref="STDOUT" />
</root>

With this configuration, I am able to keep the console rather clean, while outputting DEBUG statements to the log file.