Redirect slf4j to string

Question

How can I configure slf4j to redirect all logged information to a Java string?

This is sometimes useful in unit tests, e.g. to test no warnings are printed when loading a servlet, or to make sure a forbidden SQL table is never used.

Answer 1

A bit late, but still...

As logging configurations should be easy to replace when unit testing, you could just configure to log over stdout and then capture that prior to executing the logging subject. Then set the logger to be silent for all but the subject under test.

@Test
public void test()
{
    String log = captureStdOut(() -> {
        // ... invoke method that shouldn't log
    });
    assertThat(log, is(emptyString()));
}



public static String captureStdOut(Runnable r)
{
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    PrintStream out = System.out;
    try {
        System.setOut(new PrintStream(baos, true, StandardCharsets.UTF_8.name()));
        r.run();
        return new String(baos.toByteArray(), StandardCharsets.UTF_8);
    } catch (UnsupportedEncodingException e) {
        throw new RuntimeException("End of the world, Java doesn't recognise UTF-8");
    } finally {
        System.setOut(out);
    }
}

And if using slf4j over log4j in tests, a simple log4j.properties:

log4j.rootLogger=OFF, out
log4j.category.com.acme.YourServlet=INFO, out
log4j.appender.out=org.apache.log4j.ConsoleAppender
log4j.appender.out.layout=org.apache.log4j.PatternLayout
log4j.appender.out.layout.ConversionPattern=%-5p %c{1}:%L - %m%n

Or if you loath configuration as an external dependencies in unit tests, then programmatically configure log4j:

//...

static final String CONSOLE_APPENDER_NAME = "console.appender";

private String pattern = "%d [%p|%c|%C{1}] %m%n";
private Level threshold = Level.ALL;
private Level defaultLevel = Level.OFF;

//...

public void configure()
{
    configureRootLogger();
    configureConsoleAppender();
    configureCustomLevels();
}


private void configureConsoleAppender()
{
    ConsoleAppender console = new ConsoleAppender();
    console.setName(CONSOLE_APPENDER_NAME);
    console.setLayout(new PatternLayout(pattern));
    console.setThreshold(threshold);
    console.activateOptions();
    Logger.getRootLogger().addAppender(console);
}


private void configureRootLogger()
{
    Logger.getRootLogger().getLoggerRepository().resetConfiguration();
    Logger.getRootLogger().setLevel(defaultLevel);
}

Answer 2

As I see it you have two options.

First you could implement a custom Appender (depending on which slf4j implementation you're using) which simply appends each logged statement to a StringBuffer. In this case you probably have to hold a static reference to your StringBuffer so your test classes can access it.

Second you could write your own implementation of ILoggerFactory and Logger. Again your Logger would just append all the messages to internal StringBuffers, although in this case you'd probably have multiple buffers, one for each log level. If you did it this way you'd have an easy way of retrieving the Logger instances since you'd own the factory that was distributing them.