SLF4J is a log interface, and Logback and Log4j2 are implementation frameworks; 1. SLF4J is a unified interface, providing decoupling capabilities, which facilitates the replacement of the underlying log system in the later stage; 2. Logback is developed by the SLF4J author, fast startup, good performance, natural integration, flexible configuration, suitable for Spring Boot projects; 3. Log4j2 has powerful functions, outstanding asynchronous logging performance, suitable for high-concurrency scenarios, but complex configuration and attention to security vulnerabilities; Selection suggestions: Spring Boot uses Logback by default, and Log4j2 is selected for high-performance requirements. SLF4J is required for code decoupling. Old projects can be considered for upgrading or migration.

There are many log frameworks in Java, and the common ones are Log4j2, SLF4J and Logback. They each have their own characteristics and are suitable for different usage scenarios. This article will talk about the differences and applicable suggestions of these mainstream logging frameworks.

SLF4J: Unified log interface

SLF4J (Simple Logging Facade for Java) itself is not a log implementation, but a facade , used to unify the calling methods of different log implementations. Its core value lies in decoupling , so that you do not need to bind to specific log implementations during development.

• You can first use the SLF4J API in your code
• Then select the specific log implementation to use during deployment, such as Logback or Log4j2
• This facilitates the replacement of the underlying log system later without modifying the business code

A typical way of using it is:

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

public class MyClass {
    private static final Logger logger = LoggerFactory.getLogger(MyClass.class);
    public void doSomething() {
        logger.info("Doing something...");
    }
}

If you use multiple log dependencies in your project, SLF4J will prompt you to "multiple bindings". At this time, you need to eliminate unnecessary bindings to avoid conflicts.

Logback: a high-performance implementation natively supported by SLF4J

Logback was developed by Ceki Gülcü (the author of SLF4J) and is one of the most natural and best-performing log implementations with SLF4J . It is divided into three modules: logback-core, logback-classic and logback-access.

Key advantages include:

• Fast startup speed and good performance
• Support automatic reloading of configuration files
• Provide rich Appenders (such as asynchronous writes, database records, etc.)
• Simple configuration, either XML or programming

To give a simple example, you can start using it by simply introducing the following dependencies (Maven):

 <dependency>
    <groupId>ch.qos.logback</groupId>
    <artifactId>logback-classic</artifactId>
    <version>1.2.11</version>
</dependency>

Then put a logback-spring.xml file in src/main/resources to complete the basic configuration.

Log4j2: Powerful but slightly complex

Log4j2 is Apache's old logging framework. Compared with Logback, its advantages lies in its stronger functions and better multi-threading performance , especially in high concurrency scenarios.

Its main features include:

• Supports asynchronous logs (AsyncLogger), and their performance is significantly improved compared to synchronous logs.
• Plug-in architecture, strong scalability
• Support automatic reload configuration
• More flexible configuration methods (XML, JSON, YAML)

However, the configuration of Log4j2 is relatively complicated, and Logback is used by default in Spring Boot projects. If you want to change to Log4j2, you also need to manually exclude the default dependencies and add an adapter.

In addition, Log4j2 has been widely discussed for the famous Log4j vulnerability (CVE-2021-44228), so you need to pay attention to version security issues when using it.

How to choose?

Which framework to choose actually depends on your project requirements and technology stack:

• If you are using Spring Boot, Logback is recommended by default, because it has high integration and can be used out of the box.
• If you need extreme performance, especially asynchronous log processing capabilities, you can consider Log4j2
• If you want the code to not rely on specific log implementations, then you must use SLF4J as the interface layer
• If you are maintaining an old project and may already be using Log4j1.x, it is recommended to upgrade to Log4j2 or migrate to Logback

Finally, I would like to remind you: No matter which one you choose, you should pay attention to details such as log level control, output format, and log cleaning strategy, otherwise it will easily cause excessive disk usage or difficulty in troubleshooting.

Basically that's it.

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