If you’re new to the concept of logging levels, we recommend you have a look at our tutorial on log levels, where we explain in great detail how they work and the best ways to use them. The log levels can help in reducing the information noise and reduce alert fatigue. You can think of the log levels as a way to filter the critical information about your system state and the one that is purely informative. It will tell you if you can continue sleeping during the on-call night or you need to jump out of bed right away and hit another personal best in running between your bedroom and laptop in the living room. If the log levels are used properly in your application all you need is to look at the severity first. It is a simple, yet very powerful way of distinguishing log events from each other. No matter which method of initialization we will chose, once we have the Logger instance we can use it to generate our log events: ("This is an INFO level log message") Log4j 2 Log LevelsĪ log level or log severity is a piece of information telling how important a given log event is. Private static final Logger LOGGER = LoggerFactory.getLogger(SomeFanc圜lassName.class) The above part of the code uses the Log4j 2 classes to create the Logger object, but you can also base the initialization on SLF4J, so you can always change the logging framework of your choice: In the majority of the cases it will be a private, static object inside a class, for example: private static final Logger LOGGER = LogManager.getLogger(SomeFanc圜lassName.class) The Logger is the main entity that our application uses to create LogRecord, so basically to log what we want to output as the log message. It is less verbose and works, but is also less flexible if you want to use slightly more complicated functionalities. Public static void main(String args) %-5level - %msg%n rootLogger.level = info LOGGER = LogManager.getLogger(Log4JDefaultConfig.class) Let’s start with the code that we will be using for the test: package Keep in mind that the Log4j 1.x is deprecated and no longer maintained, so when choosing the logging library of your choice you should go for Log4j 2.x. For the purpose of this tutorial I created a simple Java project that uses Log4j 2, you can find it in our Github account. R ead our Logback tutorial if you’re curious to learn more about how this library works. Log4j 2 promises to improve the first version of the Log4j library and fix some of the issues identified in the Logback framework. Log4j 2 and its predecessor the Log4j are the most common and widely known logging framework for Java. What Is Log4j 2 and Why Should You Use It In our recent blog post about the Log4jShell vulnerability, we explained how you would know if you are affected by the problem, what steps to follow to fix it, and what we, at Sematext, have done to protect our system and users. The vulnerability was registered on the National Vulnerability Database as CVE-2021-44228 with a severity code of 10 and allows a system running Apache Log4j 2 version 2.14.1 or below to be compromised, giving an attacker full control to execute arbitrary code on the vulnerable server. On December 9, 2021, a critical vulnerability was disclosed and nicknamed Log4Shell. However, before diving into that, let’s address an issue that impacted the community using this framework. Today, we will look into Log4j 2, the latest version of the widely known Log4j library developed under the Apache Software Foundation. Logging in your Java applications can be achieved in multiple ways – for example, you can just write data to a file, but there are far better ways on how to do that, as we explained in our Java logging tutorial. Java logs combined with JVM metrics and traces give you full visibility into your Java applications. When it comes to troubleshooting application performance, the more information you have the better.
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