Logging in Java

This article has been written by Orange Software Expert members (see authors), and is targeting Java (and JVM-based languages) developers, and more specifically server-side developers (JEE or else).

It first gives a clear answer to "which logging library should I use", then gives several technical tips and tools to take the most benefit from your application logs.

Our only goal is to share and spread best practices about logs.

Which logging library(ies)?

In Java, there are lots of logging libraries:

Library Comment Our advise
utils.logging (aka JUL) Functionally and technically, it lags behind competitors do not use it
Log4J 1 Too old, not maintained anymore do not use it
Apache commons-logging Too old, not maintained anymore do not use it
Log4J 2 Seems okay, but clearly not very popular no concrete feedback for now
SLF4J + logback See reasons below this is our choice !

Two libraries?!

SLF4J is a logging facade API (abstract interface): this is the logging API you will use in your code. This component ensures the interoperability with all your dependencies (that might rely on other logging libraries).

Logback is the backend implementation: this is the underlying library that implements the facade and processes your logs (pushes them to stdout, flat file, with rolling policies, forward them to a centralized system, work asynchronously, ...). It shall never be used directly from your application code. By the way SLF4J is also able to work with Log4J 2 as the backend implementation (instead of Logback).

All logback documentation here.

Why this choice?

  1. Interoperability: SLF4J has bridges for most other libraries (JUL, commons-logging, Log4J, ...)
  2. Reliability: both projects are still actively developed, tested, documented and supported, with a large user community
  3. Performance: SLF4J + Logback support a feature called « parameterized logging », that significantly boosts logging performance for disabled logging statement, by not concatenating logging messages (which IS costly)
  4. DevOps: logback supports externalized configuration, and automatic reload (changing logging levels without restarting your application)
  5. ... and other reasons

By the way, this choice is now the most popular choice in Java development, and is even the default setup when using full-stack web development frameworks such as Dropwizard or Spring Boot.

Also notice that this choice is perfectly compatible with 12 factor apps recommendations about logs; effective logs routing will be implemented with logback configuration (apart from application logic).

For more info about the great history of logging libraries in Java, please read:

Note: your application is using Log4J 1 ?

Top recommendations

Never implement your own logging abstraction layer

Directly use the provided logging facade API: SLF4J.

Be aware of the parameters evaluation constraint

Logging as follows:

logger.debug("Entry number: " + i + " is " + String.valueOf(entry[i]));

incurs the cost of constructing the message parameter, that is converting both integer i and entry[i] to a String, and concatenating intermediate strings. This is regardless of whether the message will be logged or not.

SLF4J introduces a better pattern for addressing this issue:

Object entry = new SomeObject();
logger.debug("The entry is {}.", entry);

Only after evaluating whether to log or not, and only if the decision is positive, will the logger implementation format the message and replace the {} pair with the string value of entry.

In other words, this form does not incur the cost of parameter construction when the log statement is disabled.

Caution: in some cases, even this pattern may cause parameters evaluation.


logger.debug("My JSON structure is {}.", data.toJson());

will always cause the toJson() method to be evaluated...

In such a case, the recommended pattern would be to protect the logging instruction with level checking:

if(logger.isDebugEnabled()) {
  logger.debug("My JSON structure is {}.", data.toJson());

For more info, please read the parameterized logging documentation.

Take care of the performance cost of log caller context

Read carefully the Conversion Word from layouts documentation, and avoid directives with the warning "Generating the xxx information is not particularly fast. Thus, its use should be avoided unless execution speed is not an issue." (i.e. class, file, line, method).

The same recommendation applies to any other kind of logback outputs, for example avoid the use of "callerData" LoggingEvents provider in the logstash-logback-encoder module.

Enrich your logs!


Quoting splunk "logging best practices":

Unique identifiers such as transaction IDs, user IDs, session IDs, request IDs are tremendously helpful when debugging, and even more helpful when you are gathering analytics. Unique IDs can point you to the exact transaction, [isolate logs from a single request or logs related to a given user]. Without them, you might only have a time range to use. When possible, carry these IDs through multiple touch points and avoid changing the format of these IDs between modules. That way, you can track transactions through the system and follow them across machines, networks, and services.

With SLF4J, the standard way of enriching your logging context with IDs is using Mapped Diagnostic Context.

Tag logs with (unique) request IDs

For instance, wouldn't it be cool to mark every log produced during the processing of a request with a (unique) request ID ? Then, with a tool such as Kibana, it would become so easy to isolate logs from a single request (ElasticSearch query "requestId: 123456789")...

Technically there are several options to implement this:

Tag logs with user IDs

Similarly, it's also very helpful to mark every log produced during the processing of an authenticated request with the user ID.

With Kibana you will then be able to filter out in one click all logs related to a single user !

Warning: for security/privacy reasons, it is highly recommended that the user ID is a technical ID and not a personal information (such as login or email address).

You'll find a simple implementation in our orange-mathoms-logging library.

Make a signature hash of your errors

When your system will be in production, you'll have issues. And issues are (generally) ERROR logs with stack traces. But...

... the idea we propose is to generate a short, unique ID that identifies your stack trace.

Thus, the same error occurring twice will have the same ID, allowing you to count, compare, track history of this issue.

You could even join this signature to your client error messages for traceability.

Imagine the situation...

The user (quite upset): I just had this « Internal error [#B23F6545] occurred while calling catalog » error... your app sucks ! You: no prob’, I simply type-in this « #B23F6545 » thing in my Kibana and... bing ! I get the exact stack trace, call flow, occurrences count, ... you’ll have a fix in instants !
Audience: (applause)

... well, guess what ? You'll also find a simple implementation in our orange-mathoms-logging library.

How to ship logs to Logstash?

You’re using ELK in your project ? Lucky you !

With the logstash-logback-encoder library, you’ll be able to:

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