Java logger that automatically determines caller's class name

Question

public static Logger getLogger() {
    final Throwable t = new Throwable();
    final StackTraceElement methodCaller = t.getStackTrace()[1];
    final Logger logger = Logger.getLogger(methodCaller.getClassName());
    logger.setLevel(ResourceManager.LOGLEVEL);
    return logger;
}

This method would return a logger that knows the class it's logging for. Any ideas against it?

Many years later: https://github.com/yanchenko/droidparts/blob/master/droidparts/src/org/droidparts/util/L.java

Answer 1

I guess it adds a lot of overhead for every class. Every class has to be 'looked up'. You create new Throwable objects to do that... These throwables don't come for free.

Answer 2

Creating a stack trace is a relatively slow operation. Your caller already knows what class and method it is in, so the effort is wasted. This aspect of your solution is inefficient.

Even if you use static class information, you should not fetch the Logger again for each message. From the author of Log4j,Ceki Gülcü:

The most common error in wrapper classes is the invocation of the Logger.getLogger method on each log request. This is guaranteed to wreak havoc on your application's performance. Really!!!

This is the conventional, efficient idiom for getting a Logger is during class initialization:

private static final Logger log = Logger.getLogger(MyClass.class);

Note that this gives you a separate Logger for each type in a hierarchy. If you come up with a method that invokes getClass() on an instance, you will see messages logged by a base type showing up under the subtype's logger. Maybe this is desirable in some cases, but I find it confusing (and I tend to favor composition over inheritance anyway).

Obviously, using the dynamic type via getClass() will require you to obtain the logger at least once per instance, rather than once per class like the recommended idiom using static type information.

Answer 3

The MethodHandles class (as of Java 7) includes a Lookup class that, from a static context, can find and return the name of the current class. Consider the following example:

import java.lang.invoke.MethodHandles;

public class Main {
  private static final Class clazz = MethodHandles.lookup().lookupClass();
  private static final String CLASSNAME = clazz.getSimpleName();

  public static void main( String args[] ) {
    System.out.println( CLASSNAME );
  }
}

When run this produces:

Main

For a logger, you could use:

private static Logger LOGGER = 
  Logger.getLogger(MethodHandles.lookup().lookupClass().getSimpleName());

Answer 4

We actually have something quite similar in a LogUtils class. Yes, it's kind of icky, but the advantages are worth it as far as I'm concerned. We wanted to make sure we didn't have any overhead from it being repeatedly called though, so ours (somewhat hackily) ensures that it can ONLY be called from a static initializer context, a la:

private static final Logger LOG = LogUtils.loggerForThisClass();

It will fail if it's invoked from a normal method, or from an instance initializer (i.e. if the 'static' was left off above) to reduce the risk of performance overhead. The method is:

public static Logger loggerForThisClass() {
    // We use the third stack element; second is this method, first is .getStackTrace()
    StackTraceElement myCaller = Thread.currentThread().getStackTrace()[2];
    Assert.equal("<clinit>", myCaller.getMethodName());
    return Logger.getLogger(myCaller.getClassName());
}

Anyone who asks what advantage does this have over

= Logger.getLogger(MyClass.class);

has probably never had to deal with someone who copies and pastes that line from somewhere else and forgets to change the class name, leaving you dealing with a class which sends all its stuff to another logger.

Answer 5

Assuming you are keeping static refs to the loggers, here's a standalone static singleton:

public class LoggerUtils extends SecurityManager
{
    public static Logger getLogger()
    {
        String className = new LoggerUtils().getClassName();
        Logger logger = Logger.getLogger(className);
        return logger;
    }

    private String getClassName()
    {
        return getClassContext()[2].getName();
    }
}

Usage is nice and clean:

Logger logger = LoggerUtils.getLogger();

Answer 6

For every class that you use this with, you're going to have to look up the Logger anyway, so you might as well just use a static Logger in those classes.

private static final Logger logger = Logger.getLogger(MyClass.class.getName());

Then you just reference that logger when you need to do your log messages. Your method does the same thing that the static Log4J Logger does already so why reinvent the wheel?

Answer 7

Then the best thing is mix of two .

public class LoggerUtil {

    public static Level level=Level.ALL;

    public static java.util.logging.Logger getLogger() {
        final Throwable t = new Throwable();
        final StackTraceElement methodCaller = t.getStackTrace()[1];
        final java.util.logging.Logger logger = java.util.logging.Logger.getLogger(methodCaller.getClassName());
        logger.setLevel(level);

        return logger;
    }
}

And then in every class:

private static final Logger LOG = LoggerUtil.getLogger();

in code :

LOG.fine("debug that !...");

You get static logger that you can just copy&paste in every class and with no overhead ...

Alaa

Answer 8

From reading through all the other feedback on this site, I created the following for use with Log4j:

package com.edsdev.testapp.util;

import java.util.concurrent.ConcurrentHashMap;

import org.apache.log4j.Level;
import org.apache.log4j.Priority;

public class Logger extends SecurityManager {

private static ConcurrentHashMap<String, org.apache.log4j.Logger> loggerMap = new ConcurrentHashMap<String, org.apache.log4j.Logger>();

public static org.apache.log4j.Logger getLog() {
    String className = new Logger().getClassName();
    if (!loggerMap.containsKey(className)) {
        loggerMap.put(className, org.apache.log4j.Logger.getLogger(className));
    }
    return loggerMap.get(className);
}
public String getClassName() {
    return getClassContext()[3].getName();
}
public static void trace(Object message) {
    getLog().trace(message);
}
public static void trace(Object message, Throwable t) {
    getLog().trace(message, t);
}
public static boolean isTraceEnabled() {
    return getLog().isTraceEnabled();
}
public static void debug(Object message) {
    getLog().debug(message);
}
public static void debug(Object message, Throwable t) {
    getLog().debug(message, t);
}
public static void error(Object message) {
    getLog().error(message);
}
public static void error(Object message, Throwable t) {
    getLog().error(message, t);
}
public static void fatal(Object message) {
    getLog().fatal(message);
}
public static void fatal(Object message, Throwable t) {
    getLog().fatal(message, t);
}
public static void info(Object message) {
    getLog().info(message);
}
public static void info(Object message, Throwable t) {
    getLog().info(message, t);
}
public static boolean isDebugEnabled() {
    return getLog().isDebugEnabled();
}
public static boolean isEnabledFor(Priority level) {
    return getLog().isEnabledFor(level);
}
public static boolean isInfoEnabled() {
    return getLog().isInfoEnabled();
}
public static void setLevel(Level level) {
    getLog().setLevel(level);
}
public static void warn(Object message) {
    getLog().warn(message);
}
public static void warn(Object message, Throwable t) {
    getLog().warn(message, t);
}

}

Now in your code all you need is

Logger.debug("This is a test");

or

Logger.error("Look what happened Ma!", e);

If you need more exposure to log4j methods, just delegate them from the Logger class listed above.

Answer 9

You could of course just use Log4J with the appropriate pattern layout:

For example, for the class name "org.apache.xyz.SomeClass", the pattern %C{1} will output "SomeClass".

http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/PatternLayout.html

Answer 10

I prefer creating a (static) Logger for each class (with it's explicit class name). I than use the logger as is.

Answer 11

You don't need to create a new Throwable object. You can just call Thread.currentThread().getStackTrace()[1]

Answer 12

I just have the following line at the beginning of most of my classes.

  private static final Logger log = 
     LoggerFactory.getLogger(new Throwable().getStackTrace()[0].getClassName());

yes there is some overhead the very first time an object of that class is created, but I work mostly in webapps, so adding microseconds onto a 20 second startup isn't really a problem.

Answer 13

Google Flogger logging API supports this e.g.

private static final FluentLogger logger = FluentLogger.forEnclosingClass();

See https://github.com/google/flogger for more details.

Answer 14

Why not?

public static Logger getLogger(Object o) {
  final Logger logger = Logger.getLogger(o.getClass());
  logger.setLevel(ResourceManager.LOGLEVEL);
  return logger;
}

And then when you need a logger for a class:

getLogger(this).debug("Some log message")

Answer 15

This mechanism puts in a lot of extra effort at runtime.

If you use Eclipse as your IDE, consider using Log4e. This handy plugin will generate logger declarations for you using your favourite logging framework. A fraction more effort at coding time, but much less work at runtime.

Answer 16

Unless you really need your Logger to be static, you could use

final Logger logger = LoggerFactory.getLogger(getClass());

Answer 17

Please see my static getLogger() implementation (use same "sun.*" magic on JDK 7 as default java Logger doit)

• note static logging methods (with static import) without ugly log property...

  import static my.pakg.Logger.*;

And their speed is equivalent to native Java implementation (checked with 1 million of log traces)

package my.pkg;

import java.text.MessageFormat;
import java.util.Arrays;
import java.util.IllegalFormatException;
import java.util.logging.Level;
import java.util.logging.LogRecord;

import sun.misc.JavaLangAccess;
import sun.misc.SharedSecrets;


public class Logger {
static final int CLASS_NAME = 0;
static final int METHOD_NAME = 1;

// Private method to infer the caller's class and method names
protected static String[] getClassName() {
    JavaLangAccess access = SharedSecrets.getJavaLangAccess();
    Throwable throwable = new Throwable();
    int depth = access.getStackTraceDepth(throwable);

    boolean lookingForLogger = true;
    for (int i = 0; i < depth; i++) {
        // Calling getStackTraceElement directly prevents the VM
        // from paying the cost of building the entire stack frame.
        StackTraceElement frame = access.getStackTraceElement(throwable, i);
        String cname = frame.getClassName();
        boolean isLoggerImpl = isLoggerImplFrame(cname);
        if (lookingForLogger) {
            // Skip all frames until we have found the first logger frame.
            if (isLoggerImpl) {
                lookingForLogger = false;
            }
        } else {
            if (!isLoggerImpl) {
                // skip reflection call
                if (!cname.startsWith("java.lang.reflect.") && !cname.startsWith("sun.reflect.")) {
                    // We've found the relevant frame.
                    return new String[] {cname, frame.getMethodName()};
                }
            }
        }
    }
    return new String[] {};
    // We haven't found a suitable frame, so just punt.  This is
    // OK as we are only committed to making a "best effort" here.
}

protected static String[] getClassNameJDK5() {
    // Get the stack trace.
    StackTraceElement stack[] = (new Throwable()).getStackTrace();
    // First, search back to a method in the Logger class.
    int ix = 0;
    while (ix < stack.length) {
        StackTraceElement frame = stack[ix];
        String cname = frame.getClassName();
        if (isLoggerImplFrame(cname)) {
            break;
        }
        ix++;
    }
    // Now search for the first frame before the "Logger" class.
    while (ix < stack.length) {
        StackTraceElement frame = stack[ix];
        String cname = frame.getClassName();
        if (isLoggerImplFrame(cname)) {
            // We've found the relevant frame.
            return new String[] {cname, frame.getMethodName()};
        }
        ix++;
    }
    return new String[] {};
    // We haven't found a suitable frame, so just punt.  This is
    // OK as we are only committed to making a "best effort" here.
}


private static boolean isLoggerImplFrame(String cname) {
    // the log record could be created for a platform logger
    return (
            cname.equals("my.package.Logger") ||
            cname.equals("java.util.logging.Logger") ||
            cname.startsWith("java.util.logging.LoggingProxyImpl") ||
            cname.startsWith("sun.util.logging."));
}

protected static java.util.logging.Logger getLogger(String name) {
    return java.util.logging.Logger.getLogger(name);
}

protected static boolean log(Level level, String msg, Object... args) {
    return log(level, null, msg, args);
}

protected static boolean log(Level level, Throwable thrown, String msg, Object... args) {
    String[] values = getClassName();
    java.util.logging.Logger log = getLogger(values[CLASS_NAME]);
    if (level != null && log.isLoggable(level)) {
        if (msg != null) {
            log.log(getRecord(level, thrown, values[CLASS_NAME], values[METHOD_NAME], msg, args));
        }
        return true;
    }
    return false;
}

protected static LogRecord getRecord(Level level, Throwable thrown, String className, String methodName, String msg, Object... args) {
    LogRecord record = new LogRecord(level, format(msg, args));
    record.setSourceClassName(className);
    record.setSourceMethodName(methodName);
    if (thrown != null) {
        record.setThrown(thrown);
    }
    return record;
}

private static String format(String msg, Object... args) {
    if (msg == null || args == null || args.length == 0) {
        return msg;
    } else if (msg.indexOf('%') >= 0) {
        try {
            return String.format(msg, args);
        } catch (IllegalFormatException esc) {
            // none
        }
    } else if (msg.indexOf('{') >= 0) {
        try {
            return MessageFormat.format(msg, args);
        } catch (IllegalArgumentException exc) {
            // none
        }
    }
    if (args.length == 1) {
        Object param = args[0];
        if (param != null && param.getClass().isArray()) {
            return msg + Arrays.toString((Object[]) param);
        } else if (param instanceof Throwable){
            return msg;
        } else {
            return msg + param;
        }
    } else {
        return msg + Arrays.toString(args);
    }
}

public static void severe(String msg, Object... args) {
    log(Level.SEVERE, msg, args);
}

public static void warning(String msg, Object... args) {
    log(Level.WARNING, msg, args);
}

public static void info(Throwable thrown, String format, Object... args) {
    log(Level.INFO, thrown, format, args);
}

public static void warning(Throwable thrown, String format, Object... args) {
    log(Level.WARNING, thrown, format, args);
}

public static void warning(Throwable thrown) {
    log(Level.WARNING, thrown, thrown.getMessage());
}

public static void severe(Throwable thrown, String format, Object... args) {
    log(Level.SEVERE, thrown, format, args);
}

public static void severe(Throwable thrown) {
    log(Level.SEVERE, thrown, thrown.getMessage());
}

public static void info(String msg, Object... args) {
    log(Level.INFO, msg, args);
}

public static void fine(String msg, Object... args) {
    log(Level.FINE, msg, args);
}

public static void finer(String msg, Object... args) {
    log(Level.FINER, msg, args);
}

public static void finest(String msg, Object... args) {
    log(Level.FINEST, msg, args);
}

public static boolean isLoggableFinest() {
    return isLoggable(Level.FINEST);
}

public static boolean isLoggableFiner() {
    return isLoggable(Level.FINER);
}

public static boolean isLoggableFine() {
    return isLoggable(Level.FINE);
}

public static boolean isLoggableInfo() {
    return isLoggable(Level.INFO);
}

public static boolean isLoggableWarning() {
    return isLoggable(Level.WARNING);
}
public static boolean isLoggableSevere() {
    return isLoggable(Level.SEVERE);
}

private static boolean isLoggable(Level level) {
    return log(level, null);
}

}

Answer 18

Take a look at Logger class from jcabi-log. It does exactly what you're looking for, providing a collection of static methods. You don't need to embed loggers into classes any more:

import com.jcabi.log.Logger;
class Foo {
  public void bar() {
    Logger.info(this, "doing something...");
  }
}

Logger sends all logs to SLF4J, which you can redirect to any other logging facility, in runtime.

Answer 19

A good alternative is to use (one of) the lombok logs annotations : https://projectlombok.org/features/Log.html

It generate the corresponding log statement with the current class.

Answer 20

A nice way to do this from Java 7 onwards:

private static final Logger logger = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());

The logger can be static and that fine. Here its using the SLF4J API

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

But in principal can be used with any logging framework. If the logger needs a string argument add toString()