public class TestSl4j extends AbstractTest4StdOut {

	
	@Test
	public void demo() throws Exception{
        final Logger logger = LoggerFactory.getLogger(TestSl4j.class);

        // Log in Main Thread
        logger.info("Log in main!");
        assertEquals("[] {} - Log in main!", getLogAndClear());
		
	}
}

public abstract class AbstractTest4StdOut {
    @Rule
    public final SystemOutRule systemOutRule = new SystemOutRule().enableLog();

    protected static final long START_TIME_STAMP = System.currentTimeMillis();

    protected String getLogAndClear() {
        System.out.flush();

        final String out = systemOutRule.getLog();
        systemOutRule.clearLog();

        final int idx = out.indexOf('\n');
        return out.substring(idx + 1, out.length() - 1); // trim first line and last '\n'
    }
}

[ERROR] demo(TestSl4j)  Time elapsed: 0.018 s  <<< ERROR!
java.lang.StringIndexOutOfBoundsException: String index out of range: -1
	at demo(TestSl4j.java:19)