log4j 1.x to log4j 2.x migration

December 3, 2022

What's inside this article ⌄

How to migrate from log4j 1.x to log4j 2.x
How to make log4j bridge migration
How to use log4j 2.x if log4j 1.x classes still required
How to use log4j bridge with slf4j
Log4j-1.2-api and slf4j

Even though you may want to merely copy and paste the code below, I suggest you firstly explore carefully the article describing how Java logging works, what is slf4j, what is log4j, what is bridge and how it works: How Java logging should work, and then get back to here.

Suppose you have these two at-a-glance controversial requirements:

You need to use log4j 2.x because of security department
You have dependencies with log4j 1.x support only

To satisfy these needs, we’re doing the following things:

locking slf4j version (slf4j-api)
importing log4j 2.x jars (log4j-api; log4j-core)
connecting log4j 2.x to slf4j (log4j-slf4j-impl)
providing log4j 1.x api support without using log4j 1.x (log4j-1.2-api)

So you need to ensure that you don’t have any transitive log4j dependencies from other libraries, then add this bunch of dependencies to your pom.xml:

<log4j2.version>2.17.2</log4j2.version>
<slf4j.version>1.7.30</slf4j.version>

<!-- Lock slf4j facade version -->
<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>${slf4j.version}</version>
</dependency>
<!-- log4j 2.x-->
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-api</artifactId>
    <version>${log4j2.version}</version>
</dependency>
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>${log4j2.version}</version>
</dependency>
<!-- log4j 2.x to slf4j binding -->
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-slf4j-impl</artifactId>
    <version>${log4j2.version}</version>
</dependency>
<!-- Compatibility of log4j 1.x related code -->
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-1.2-api</artifactId>
    <version>${log4j2.version}</version>
</dependency>