Capturing Log4j output when executing TestNG tests

Use Reporter.log(str) to log the message on the report.

@AfterMethod
public void printLOGonFailure(ITestResult result) {
    if (result.getStatus() == ITestResult.FAILURE) {
        String  str = getLog();
        Reporter.log(str);
    }
}

This site has a explanation on how to do it.. I copied the code part here in case the link goes dead.

Logger.getLogger(this.getClass())
log4j.rootLogger=ERROR,TESTAPPENDER
log4j.appender.TESTAPPENDER=com.my.fantastic.MockedAppender
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;

public class FooTest {
    private Appender appenderMock;
    @Before
    public void setupAppender() {
        appenderMock = mock(Appender.class);
        Logger.getRootLogger().addAppender(appenderMock);
    }
    @After
    public void removeAppender() {
        Logger.getRootLogger().removeAppender(appenderMock);
    }
    @Test
    public void testMethod()  {
        doStuffThatCausesLogging();
        verify(appenderMock).doAppend((LoggingEvent) anyObject());
    }
}

ArgumentCaptor arguments = ArgumentCaptor.forClass(LoggingEvent.class);
verify(appenderMock).doAppend(arguments.capture());

assertThat(arguments.getValue().getLevel(), is(Level.WARN));