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SLF4J logging, different Levels

In SLF4J (Logging) how levels are different in characteristic. i.e. How ERROR message is different than DEBUG message.
import org.apache.log4j.Logger;
public class LogClass {
private static org.apache.log4j.Logger log = Logger.getLogger(LogClass.class);
public static void main(String[] args) {
log.trace("Trace Message!")
log.debug("Debug Message!");
log.info("Info Message!");
log.warn("Warn Message!");
log.error("Error Message!");
log.fatal("Fatal Message!");
}
}
The Output is same regardless of Level, is there any difference in implementation:
Debug Message!
Info Message!
Warn Message!
Error Message!
Fatal Message!
If these levels are producing the same kind of messages then why the implementation didn't have only one method with parameter as level.
Something like:
log("Level","msg");

Starting from the bottom, there's no real benefit to have a log(level, msg) method if you already have all the different methods for all the possible levels. Only if you'd need to log the same message in different levels, but that's a bad practice, since that message should clearly fall into one specific category. And you can always choose how much logging you get out by specifying the level globally or at the package/class.
The message are exactly the same on each level, the only difference is if that message is gonna make to the logging output or not, based on your configuration, and what purpose do you give to each level.

The key purpose to name them levels is to enable you to debug at various levels. Say for example,
INFO level can used to log high level information on the progress of the application during execution.
DEBUG level logged is meant to be even deeper than just high level information. At DEBUG level, you can have more information logged that can include information of what is happening at a module level or component level.
TRACE level is even more granular. You can log message like entering and exiting a method and what information is being returned by each method.
ERROR level is to purely meant to log only errors and exception
You need to be mindful of what kind of message can be logged into their respective level.
To answer your question, these levels can be controlled in log4j.properties or log4j.xml. You can specify at what level the application can debug. If everything goes well in application, I would leave it at INFO level. If something goes wrong and I wanted to dig in deepeur in terms of debugging, I would try to turn on at DEBUG level or TRACE level.
Also, understand that when you run the debugging at DEBUG level, even the INFO level logs will be printed. If you turn on the debugged at TRACE level, even the DEBUG and INFO level logs will be printed. If you turn on debugging at INFO level, only INFO level logs will be printed.
I hope you got some clarify.

Because it is easier to use for you as a user. As the implementation, it might have that very code.

Log4j2, set log level at runtime for Thread specific

We have a webserver and multiple users log in to it. We generally put log level to ERROR or INFO level. But sometimes, for debugging purpose, we need to see logs. There is one way to set it at runtime, but this process is not so good in case of loads of traffic. Important logs will be missed and also we don't know for how much time we need to keep it that way. I have written a wrapper in log4j v1.2, which just ignores the level check if userid belongs to some TestUsersList. So, it opens all logs for a particular user[a thread] only. A snippet is below-
public void trace(Object message) {
Object diagValue = MDC.get(LoggerConstants.IS_ANALYZER_NUMBER);
if (valueToMatch.equals(diagValue)) { // Some condition to check test number
forcedLog(FQCN, Level.TRACE, message, null);
return;
}
if (repository.isDisabled(Level.TRACE_INT))
return;
if (Level.TRACE.isGreaterOrEqual(this.getEffectiveLevel()))
forcedLog(FQCN, Level.TRACE, message, null);
}
But now I have moved to log4j2, I don't want to write this wrapper again. Is there any inbuilt functionality which log4j2 provides for this?

This can be done with filters. Add a logger to the configuration that logs all the messages you want, then add a ThreadContextMapFilter that has a KeyValuePair for each user you want to log.
Then put the user ids in the Thread Context within the code.

What is wrong with this statements?

I have been told by one of my senior that write log statements for two levels , debug and info is wrong , what is wrong with this approach ?
if(LOGGER.isDebugEnabled()){
LOGGER.info("REST SERVICE LOG :QUERY GENERATED FOR ADVANCED SEARCH IS : \n" +queryBuffer );
}
if(LOGGER.isEnabledFor(Level.INFO)){
LOGGER.info("REST SERVICE LOG :QUERY GENERATED FOR ADVANCED SEARCH IS : \n" +queryBuffer );
}
Any suggestions on this please ?

In your example: (Assuming your LOGGER is using one of the standard java logging frameworks)
If LOGGER is configured for TRACE or DEBUG, then the same information will be logged twice.
"REST SERVICE LOG :QUERY GENERATED FOR ADVANCED SEARCH IS :
<query buffer>"
"REST SERVICE LOG :QUERY GENERATED FOR ADVANCED SEARCH IS :
<query buffer>"
That seems wasteful.
If LOGGER is configured for INFO, then the information will be logged once.
"REST SERVICE LOG :QUERY GENERATED FOR ADVANCED SEARCH IS :
<query buffer>"
If LOGGER is configured for a level like ERROR or FATAL, then no information will be logged.
What is your goal for logging here?

How to customize logging for Google App Engine Java?

Google App Engine Java uses java.util.logging to create logging messages. I want to modify the log messages, that are displayed in Developers Console - Monitoring - Logs. The idea is to add some additional output like username without putting it in each log message manually:
log.info("user action");
should result in an logging output like
user "testuser": user action
Therefore I created an own Formatter:
public class TestFormatter extends Formatter {
#Override
public String format(LogRecord record) {
// find out username..
return "user " + username + ": " + record.getMessage();
}
}
Setting this as formatter for the ConsoleHandler in the logging.properties has not effekt:
java.util.logging.ConsoleHandler.formatter = com.example.guestbook.TestFormatter
When deploying it in on the local machine, and trying to add it programmatically like this:
Logger rootLogger = Logger.getLogger("");
Handler[] handlers = rootLogger.getHandlers();
log.info("Handler[] size: " + handlers.length);
for(Handler h : handlers) {
log.info(h.toString());
h.setFormatter(new TestFormatter());
}
I get 2 handler, one ConsoleHandler and one DevLogHandler. But setting the formatter results in the fact that no further logs are displayed. On GAE instead I get 0 handler.
When trying to acces Logger.getGlobal() instead of Logger.getLogger(""), I get 0 Handler on the local instance and a SecurityException: No permission to modify global on GAE. This exception already arises when trying to get the list of Handlers.
Now my question: Is there a way to modify the logs of developer console in such a way? If yes, how?

As I reply I got in the past from a Google ticket I opened for a similar question
I would discourage tampering with the Loggers/Handlers used
internally by GAE.
Besides that, the Global Logger cannot be customized, you can try to it with a Logger with a custom name

Which Appenders should be used in distributed system? How to configure them?

I am trying to add logging component to distributed system. It is written in AspectJ to avoid chaining current source-code. I use socket appender to send logs, but I'd like to try something more effective.
I've heard I should use JMSAppender and AsyncAppender, but I failed to configure it. Should I create Receiver which gathers logs and pass them to database and to GUI (I use ChainSaw)?
I tried to follow turorial1 and tutorial2 , but they aren't clear enough.
Edit:
In a small demo I've prepared I sent 6 logs for a request (simulation of 3 components)
[2012-08-08 15:40:28,957] [request1344433228957] [Component_A] [start]
[2012-08-08 15:40:32,050] [request1344433228957] [Component_B] [start]
[2012-08-08 15:40:32,113] [request1344433228957] [Component_C] [start]
[2012-08-08 15:40:32,113] [request1344433228957] [Component_C] [end - throwing]
[2012-08-08 15:40:32,144] [request1344433228957] [Component_B] [end]
[2012-08-08 15:40:32,175] [request1344433228957] [Component_A] [end]
Using socket Appender. So my log4j.properties is:
log4j.rootLogger=DEBUG, server
log4j.appender.server=org.apache.log4j.net.SocketAppender
log4j.appender.server.Port=4712
log4j.appender.server.RemoteHost=localhost
log4j.appender.server.ReconnectionDelay=1000
so I run
>java -classpath log4j-1.2.17.jar org.apache.log4j.net.SimpleSocketServer 4712 log4j-server.properties
with configuration
log4j.rootLogger=DEBUG, CA, FA
#
log4j.appender.CA=org.apache.log4j.ConsoleAppender
log4j.appender.CA.layout=org.apache.log4j.PatternLayout
log4j.appender.CA.layout.ConversionPattern=[%d] [%t] [%c] [%m]%n
#
log4j.appender.FA=org.apache.log4j.FileAppender
log4j.appender.FA.File=report.log
log4j.appender.FA.layout=org.apache.log4j.PatternLayout
log4j.appender.FA.layout.ConversionPattern=[%d] [%t] [%c] [%m]%n
Then I send my logs from file to Chainsaw:
It is absolutely basic, but I want to learn how to do it better. First of all, I'd like to send logs asynchronously. Then create very simple Receiver, which e.g. can pass logs to a file.
I tried to follow tutorials I listed above, but I failed. So question is: could you provide some example configuration? Example of Receiver.java and log4.properties files?

I would use NFS or CDFS and mount a drive on all the machines. Have each application instance write to a different file. You will be able to find all the logs in one directory (or drive) no matter how many machines you use.
I wouldn't use NFS or CDFS over a global WAN with a high latency e.g. > 50 ms round trip. In this cause I have used JMS (but I didn't use log4j)

My two cents.. Whatever you do, make sure that you use asynchronous mechanism to deliver your logs to the receiver, otherwise it will eventually stall your apps. Another point, to deliver logs reliably you should consider a fail over mechanism built into the appender itself - receivers may go offline for short or long time, if you care for the logs, the fail over is definitely required. We have built similar system you describe (sorry for the add), but if you like you can use our appender (look in downloads), it's free and has the sources. There is also a video tutorial. It has fail over and flexible asynchronous mechanism plus a backup fall back.
How many appenders should you use? One appender per jvm will do all right. Config files should probably be per jvm, not sure how you intend to implement the receiver, in any case the appenders need to find your receiver which is usually host port pair at least. Regarding the database, my experience is very sour with RDBMS (we are moving to nosql) but if you don't go above couple of hundred million records, most commercial databases will do with some effort. Not a simple task I must say, took us couple of years to build commercial quality system you just drawn with few skinny rectangles :)

Finally I've found how to configure it. I put 2 files into src folder.
jndi.properties
topic.logTopic=logTopic
and log4j-jms.properties
log4j.rootLogger=INFO, stdout, jms
## Be sure that ActiveMQ messages are not logged to 'jms' appender
log4j.logger.org.apache.activemq=INFO, stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=
## Configure 'jms' appender. You'll also need jndi.properties file in order to make it work
log4j.appender.jms=org.apache.log4j.net.JMSAppender
log4j.appender.jms.InitialContextFactoryName=org.apache.activemq.jndi.ActiveMQInitialContextFactory
log4j.appender.jms.ProviderURL=tcp://localhost:61616
log4j.appender.jms.TopicBindingName=logTopic
log4j.appender.jms.TopicConnectionFactoryBindingName=ConnectionFactory
Then I run my program with VM argument
-Dlog4j.configuration=log4j-jms.properties
and receive logs in class Receiver.java
public class Receiver implements MessageListener {
PrintWriter pw = new PrintWriter("result.log");
Connection conn;
Session sess;
MessageConsumer consumer;
public Receiver() throws Exception {
ActiveMQConnectionFactory factory = new ActiveMQConnectionFactory("tcp://localhost:61616");
Connection conn = factory.createConnection();
Session sess = conn.createSession(false, Session.AUTO_ACKNOWLEDGE);
conn.start();
MessageConsumer consumer = sess.createConsumer(sess.createTopic("logTopic"));
consumer.setMessageListener(this);
}
public static void main(String[] args) throws Exception {
new Receiver();
}
public void onMessage(Message message) {
try {
LoggingEvent event = (LoggingEvent) ((ActiveMQObjectMessage) message).getObject();
DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss,SSS");
String nowAsString = df.format(new Date(event.getTimeStamp()));
pw.println("["+ nowAsString + "]" +
" [" + event.getThreadName()+"]" +
" ["+ event.getLoggerName() + "]" +
" ["+ event.getMessage()+"]");
pw.flush();
} catch (Exception e) {
e.printStackTrace();
}
}
}

I'd recommend syslog and the built in syslog appender. Use TCP for reliable logging (+Asyc appender maybe) or UDP for fire-and-forget logging.
I have a rsyslog config if you need.