Apache Log4j to SLF4J Adapter allows applications coded to the Log4j 2 API to be routed to SLF4J. Use of this adapter may cause some loss of performance as the Log4j 2 Messages must be formatted before they can be passed to SLF4J.

Bytecode (Java 8) for Apache Log4j to SLF4J Adapter is provided in a separate JAR file like log4j-to-slf4j-2.14.1.jar.

Source Code files for Apache Log4j IOStreams are provided in both binary packge like apache-log4j-2.14.1-bin.zip and source package like apache-log4j-2.14.1-src.zip. You can download them at Apache Log4j Website.

You can also browse Source Code files for Apache Log4j to SLF4J Adapter 2.14.1 below.

⏎ org/apache/logging/slf4j/SLF4JLoggerContext.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache license, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the license for the specific language governing permissions and
 * limitations under the license.
 */
package org.apache.logging.slf4j;

import org.apache.logging.log4j.message.MessageFactory;
import org.apache.logging.log4j.spi.ExtendedLogger;
import org.apache.logging.log4j.spi.LoggerContext;
import org.apache.logging.log4j.spi.LoggerRegistry;
import org.slf4j.LoggerFactory;

/**
 *
 */
public class SLF4JLoggerContext implements LoggerContext {
    private final LoggerRegistry<ExtendedLogger> loggerRegistry = new LoggerRegistry<>();

    @Override
    public Object getExternalContext() {
        return null;
    }

    @Override
    public ExtendedLogger getLogger(final String name) {
        if (!loggerRegistry.hasLogger(name)) {
            loggerRegistry.putIfAbsent(name, null, new SLF4JLogger(name, LoggerFactory.getLogger(name)));
        }
        return loggerRegistry.getLogger(name);
    }

    @Override
    public ExtendedLogger getLogger(final String name, final MessageFactory messageFactory) {
        // FIXME according to LOG4J2-1180, the below line should be:
        // FIXME if (!loggerRegistry.hasLogger(name, messageFactory)) {
        if (!loggerRegistry.hasLogger(name)) {
            // FIXME: should be loggerRegistry.putIfAbsent(name, messageFactory,
            loggerRegistry.putIfAbsent(name, null,
                    new SLF4JLogger(name, messageFactory, LoggerFactory.getLogger(name)));
        }
        // FIXME should be return loggerRegistry.getLogger(name, messageFactory);
        return loggerRegistry.getLogger(name);

        // TODO applying the above fixes causes (log4j-to-slf4j) LoggerTest to fail
    }

    @Override
    public boolean hasLogger(final String name) {
        return loggerRegistry.hasLogger(name);
    }

    @Override
    public boolean hasLogger(final String name, final MessageFactory messageFactory) {
        return loggerRegistry.hasLogger(name, messageFactory);
    }

    @Override
    public boolean hasLogger(final String name, final Class<? extends MessageFactory> messageFactoryClass) {
        return loggerRegistry.hasLogger(name, messageFactoryClass);
    }
}

⏎ org/apache/logging/slf4j/SLF4JLoggerContext.java

Or download all of them as a single archive file:

File name: log4j-to-slf4j-2.14.1-sources.jar
File size: 15755 bytes
Release date: 2021-03-06
Download 

⇒ Source Code for Apache Log4j IOStreams

⇐ Source Code for Apache Log4j SLF4J Binding

⇑ Downloading and Reviewing Apache Log4j Packages

⇑⇑ FAQ for Apache Log4j