Java Logs: 4 Types of Logs You Need to Know

Logging is an important topic in software development, especially if you need to analyze bugs and other unexpected events in your production environment. Implementing your logging often seems easy. But as you probably experienced yourself, logging is far more complex than it might seem. That's why you can find lots of articles about it here on the blog.

As an experienced developer, you should know the different logging frameworks available, common pitfalls and best practices, and, of course, the types of log files used in common deployment scenarios. In this article, I will focus on the latter. I will tell you about four different types of logs that you can find in almost all development or production environments.

Application Logs

Let's start with the most common type of log: the application log.

Most developers think of this log when they talk about logging. The reason for that is simple. Their application produces this log. It contains all kinds of error messages, warnings, or other events written by the application. These messages can provide logical, high-level information that is connected to specific use cases. Typical examples are:

• The stacktrace of an exception that occurred in a use case.
• Warning messages about the slow response times of an external system.
• Information that a use case was triggered or completed.

The application log is probably the most important type of log mentioned in this article because you have full control over it. That means that you are responsible for writing the messages.

Each log message needs to provide useful information that helps you understand the behavior of your application. If you want to learn more about application logs, you should take a look at Eric's article in which he describes a great set of best practices for application logging.

Web and Application Server Logs

The next interesting and important type is the log of your web or application server.

Most enterprise applications get deployed to a web server, e.g. Apache Tomcat, or an application server, e.g. Wildfly or WebSphere Liberty. Their log files provide you with information about technical issues and the current status of the server.

The configuration and capabilities of each server differ, and I will not explain them in details in this article. But please make sure to check the documentation of your server and configure it accordingly. Getting the right information from your server can make the difference between identifying potential problems in time and a server outage.

Garbage Collector Logs

The garbage collector provides automatic memory management. It keeps track of all objects instantiated within a JVM and removes the ones that are no longer used. That is a highly complex task, and there are several different garbage collector implementations available.

In general, the garbage collection process in Java is very efficient. It's so efficient that there are lots of applications that use the default implementation with its standard configuration. But if your application has to handle the high load or uses lots of complex object structures, the performance of your application might decrease over time. That often happens when the garbage collector needs to spend more and more time to manage the available memory. The garbage collector log provides you with the necessary information to analyze all garbage collector activities.

The garbage collector log is deactivated by default. You need to activate it with a set of command line properties.

If you're using JDK 8 or earlier, you can do that using the following properties:

-XX:+PrintGCDetails -Xloggc:

Starting with JDK 9, you need to use the following properties instead:

class="prettyprint"-Xlog:gc*:file=<gc-log-file-path>

After you started your application with these command line properties, your garbage collector will write detailed information about all operations. Unfortunately, the content and format of the generated garbage collector log depend on the vendor and version of your JVM and the garbage collection algorithm.

System Logs

All of the previously discussed logs are written by the JVM or by an application that's running within the JVM. In addition to these logs, you should also be familiar with the logs written by your operating system.

As a Java developer, you will most likely never write to these logs yourself. But, your operating system writes certain events to these logs. Other applications that are part of your deployment might do that as well. So, the system logs are a good place to get an overview of external events.

If you're running your application on Windows, this is the event log. On Linux, you should know the syslog service and its successors.

Windows Event Log

Windows' event log contains important hardware and software events in the categories application, security, setup, system, and forwarded events.

The events of the application and the system category can be especially helpful to understand events that occurred outside of your application. The application category contains events that were reported by other applications running on your local Windows system. Hardware and driver related events belong to the system category.

You can learn more about Windows' event log in this article.

Linux System Logging

Linux's syslog service and its successors provide a highly configurable logging system. It writes messages triggered by applications or the operating system to a file or sends them to a remote system.

The used logging service and its configuration depend on your Linux distribution. Please check the documentation to learn more about your specific operating system.

Ian Shields wrote a great introduction to Linux system logging. I recommend you read his Learn Linux, 101: System logging guide, if you're looking for a general introduction.

Conclusion

Logging is an important topic with lots of different facets. In this article, I summarized four important logs that you should know as a Java developer:

• The application log is the most obvious one. It's written by your application and you are responsible for its content.
• If you deploy your application into a web or an application server, you also need to know about the logs provided by that server.
• Garbage collector logs can provide important information if you need to analyze performance issues.
• You can check the system logs to get an overview of all external events that might have affected your application and caused the service incident.