Chapter 13. Logging

13.1. What is slf4j?
13.2. Client-Side Setup
13.3. Server-Side Setup
13.4. Example Usage
13.5. Logger Names

Errai now supports using the slf4j logging api on the server and client. This gives you the flexibility of choosing your own logging back-end for your server-side code, while still allowing a uniform logging interface that can be used in shared packages.

13.1. What is slf4j?

sl4j is logging abstraction. Using the slf4j api, you can add log statements to your code using a fixed api while maintaining the ability to switch the logging implementation at run-time. For example, the slf4j api can be used with java.util.logging (JUL) as the back-end.

13.2. Client-Side Setup

The client-side slf4j code uses the GWT Logging as the back-end. Using slf4j in client-side code has two steps:

Add the errai-common artifact as a maven dependency to your project
Inherit the gwt module org.jboss.errai.common.Logging

The GWT logging back-end works analogously to JUL. See the above GWT Logging link for instructions on how to adjust settings such as the log level.

13.3. Server-Side Setup

On the server you are free to use any logging back-end that has slf4j bindings (or to make your own). Just make sure to add dependencies for the slf4j-api artifact and the slf4j binding you choose. Note: Some application servers provide their own slf4j bindings (such as JBoss AS), in which case you should add your binding dependency as provided scope.

To learn more about how to setup slf4j for your server-side code, see their website .

13.4. Example Usage

Here is sample usage of the slf4j code (which with the above setup can be run on the client or server):

Example 13.1. LogExample.java



import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

import javax.inject.Inject;


public class LogExample {


  public void logStuff() {

    // Get a logger for this class

    @Inject Logger logger;


    // Logging going from most to least detailed

    logger.trace("this is extremely specific!");

    logger.debug("this is still pretty specific");

    logger.info("this is an average log message");

    logger.warn("there might be something fishy here...");

    logger.error("uh oh... abandon ship!", new Exception("I am a logged exception"));

  }

}

13.5. Logger Names

By default, the above example with provide a logger with the fully qualified class name of the enclosing class. To inject a logger with an alternate name, use the NamedLogger annotation:

Example 13.2. NamedLogExample.java



import org.slf4j.Logger;

import javax.inject.Inject;

import org.jboss.errai.common.client.api.NamedLogger;


public class NamedLogExample {


  // Get a logger with the name "Logger!"

  @Inject @NamedLogger("Logger!") logger;

  

  // Get the root logger

  @Inject @NamedLogger rootLogger;


}