Where does the slf4j log file get saved?

slf4j is only an API. You should have a concrete implementation (for example log4j). This concrete implementation has a config file which tells you where to store the logs.

When slf4j catches a log messages with a logger, it is given to an appender which decides what to do with the message. By default, the ConsoleAppender displays the message in the console.

The default configuration file is :

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">

  <Appenders>
    <!-- By default => console -->
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    </Console>
  </Appenders>

  <Loggers>
    <Root level="error">
      <AppenderRef ref="Console"/>
    </Root>
  </Loggers>
</Configuration>

If you put a configuration file available in the classpath, then your concrete implementation (in your case, log4j) will find and use it. See Log4J documentation.

Example of file appender :

<Appenders>
<File name="File" fileName="${filename}">
  <PatternLayout>
    <pattern>%d %p %C{1.} [%t] %m%n</pattern>
  </PatternLayout>
</File>

...
</Appenders>

Complete example with a file appender :

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">

  <Appenders>
    <File name="File" fileName="${filename}">
      <PatternLayout>
        <pattern>%d %p %C{1.} [%t] %m%n</pattern>
      </PatternLayout>
    </File>
  </Appenders>

  <Loggers>
    <Root level="error">
      <AppenderRef ref="File"/>
    </Root>
  </Loggers>

</Configuration>

As already mentioned its just a facade and it helps to switch between different logger implementation easily. For example if you want to use log4j implementation.

A sample code would looks like below.

If you use maven get the dependencies

    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-api</artifactId>
        <version>1.7.6</version>
    </dependency>
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
        <version>1.7.5</version>
    </dependency>

Have the below in log4j.properties in location src/main/resources/log4j.properties

            log4j.rootLogger=DEBUG, STDOUT, file

            log4j.appender.STDOUT=org.apache.log4j.ConsoleAppender
            log4j.appender.STDOUT.layout=org.apache.log4j.PatternLayout
            log4j.appender.STDOUT.layout.ConversionPattern=%5p [%t] (%F:%L) - %m%n

            log4j.appender.file=org.apache.log4j.RollingFileAppender
            log4j.appender.file.File=mylogs.log
            log4j.appender.file.layout=org.apache.log4j.PatternLayout
            log4j.appender.file.layout.ConversionPattern=%d{dd-MM-yyyy HH:mm:ss} %-5p %c{1}:%L - %m%n

Hello world code below would prints in console and to a log file as per above configuration.

            import org.slf4j.Logger;
            import org.slf4j.LoggerFactory;

            public class HelloWorld {
              public static void main(String[] args) {
                Logger logger = LoggerFactory.getLogger(HelloWorld.class);
                logger.info("Hello World");
              }
            }