Apache Log4j Core Implementation provides the functional components of the logging system. Users are free to create their own plugins and include them in the logging configuration. Apache Log4j Core is a required module to use Apache Log4j.

Bytecode (Java 8) for Apache Log4j Core Implementation is provided in a separate JAR file like log4j-core-2.14.1.jar.

Source Code files for Apache Log4j API 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 Core Implementation 2.14.1 below.

⏎ org/apache/logging/log4j/core/config/ConfigurationFactory.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.log4j.core.config;

import org.apache.logging.log4j.Level;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.core.LoggerContext;
import org.apache.logging.log4j.core.config.builder.api.ConfigurationBuilderFactory;
import org.apache.logging.log4j.core.config.composite.CompositeConfiguration;
import org.apache.logging.log4j.core.config.plugins.util.PluginManager;
import org.apache.logging.log4j.core.config.plugins.util.PluginType;
import org.apache.logging.log4j.core.lookup.Interpolator;
import org.apache.logging.log4j.core.lookup.StrSubstitutor;
import org.apache.logging.log4j.core.net.UrlConnectionFactory;
import org.apache.logging.log4j.core.util.AuthorizationProvider;
import org.apache.logging.log4j.core.util.BasicAuthorizationProvider;
import org.apache.logging.log4j.core.util.FileUtils;
import org.apache.logging.log4j.core.util.Loader;
import org.apache.logging.log4j.core.util.NetUtils;
import org.apache.logging.log4j.core.util.ReflectionUtil;
import org.apache.logging.log4j.status.StatusLogger;
import org.apache.logging.log4j.util.LoaderUtil;
import org.apache.logging.log4j.util.PropertiesUtil;
import org.apache.logging.log4j.util.Strings;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.UnsupportedEncodingException;
import java.net.URI;
import java.net.URISyntaxException;
import java.net.URL;
import java.net.URLConnection;
import java.net.URLDecoder;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Factory class for parsed {@link Configuration} objects from a configuration file.
 * ConfigurationFactory allows the configuration implementation to be
 * dynamically chosen in 1 of 3 ways:
 * <ol>
 * <li>A system property named "log4j.configurationFactory" can be set with the
 * name of the ConfigurationFactory to be used.</li>
 * <li>
 * {@linkplain #setConfigurationFactory(ConfigurationFactory)} can be called
 * with the instance of the ConfigurationFactory to be used. This must be called
 * before any other calls to Log4j.</li>
 * <li>
 * A ConfigurationFactory implementation can be added to the classpath and configured as a plugin in the
 * {@link #CATEGORY ConfigurationFactory} category. The {@link Order} annotation should be used to configure the
 * factory to be the first one inspected. See
 * {@linkplain org.apache.logging.log4j.core.config.xml.XmlConfigurationFactory} for an example.</li>
 * </ol>
 *
 * If the ConfigurationFactory that was added returns null on a call to
 * getConfiguration then any other ConfigurationFactories found as plugins will
 * be called in their respective order. DefaultConfiguration is always called
 * last if no configuration has been returned.
 */
public abstract class ConfigurationFactory extends ConfigurationBuilderFactory {

    public ConfigurationFactory() {
        // TEMP For breakpoints
    }

    /**
     * Allows the ConfigurationFactory class to be specified as a system property.
     */
    public static final String CONFIGURATION_FACTORY_PROPERTY = "log4j.configurationFactory";

    /**
     * Allows the location of the configuration file to be specified as a system property.
     */
    public static final String CONFIGURATION_FILE_PROPERTY = "log4j.configurationFile";

    public static final String LOG4J1_CONFIGURATION_FILE_PROPERTY = "log4j.configuration";

    public static final String LOG4J1_EXPERIMENTAL = "log4j1.compatibility";

    public static final String AUTHORIZATION_PROVIDER = "log4j2.authorizationProvider";

    /**
     * Plugin category used to inject a ConfigurationFactory {@link org.apache.logging.log4j.core.config.plugins.Plugin}
     * class.
     *
     * @since 2.1
     */
    public static final String CATEGORY = "ConfigurationFactory";

    /**
     * Allows subclasses access to the status logger without creating another instance.
     */
    protected static final Logger LOGGER = StatusLogger.getLogger();

    /**
     * File name prefix for test configurations.
     */
    protected static final String TEST_PREFIX = "log4j2-test";

    /**
     * File name prefix for standard configurations.
     */
    protected static final String DEFAULT_PREFIX = "log4j2";

    protected static final String LOG4J1_VERSION = "1";
    protected static final String LOG4J2_VERSION = "2";

    /**
     * The name of the classloader URI scheme.
     */
    private static final String CLASS_LOADER_SCHEME = "classloader";

    /**
     * The name of the classpath URI scheme, synonymous with the classloader URI scheme.
     */
    private static final String CLASS_PATH_SCHEME = "classpath";

    private static final String OVERRIDE_PARAM = "override";

    private static volatile List<ConfigurationFactory> factories;

    private static ConfigurationFactory configFactory = new Factory();

    protected final StrSubstitutor substitutor = new StrSubstitutor(new Interpolator());

    private static final Lock LOCK = new ReentrantLock();

    private static final String HTTPS = "https";
    private static final String HTTP = "http";

    private static volatile AuthorizationProvider authorizationProvider;

    /**
     * Returns the ConfigurationFactory.
     * @return the ConfigurationFactory.
     */
    public static ConfigurationFactory getInstance() {
        // volatile works in Java 1.6+, so double-checked locking also works properly
        //noinspection DoubleCheckedLocking
        if (factories == null) {
            LOCK.lock();
            try {
                if (factories == null) {
                    final List<ConfigurationFactory> list = new ArrayList<>();
                    PropertiesUtil props = PropertiesUtil.getProperties();
                    final String factoryClass = props.getStringProperty(CONFIGURATION_FACTORY_PROPERTY);
                    if (factoryClass != null) {
                        addFactory(list, factoryClass);
                    }
                    final PluginManager manager = new PluginManager(CATEGORY);
                    manager.collectPlugins();
                    final Map<String, PluginType<?>> plugins = manager.getPlugins();
                    final List<Class<? extends ConfigurationFactory>> ordered = new ArrayList<>(plugins.size());
                    for (final PluginType<?> type : plugins.values()) {
                        try {
                            ordered.add(type.getPluginClass().asSubclass(ConfigurationFactory.class));
                        } catch (final Exception ex) {
                            LOGGER.warn("Unable to add class {}", type.getPluginClass(), ex);
                        }
                    }
                    Collections.sort(ordered, OrderComparator.getInstance());
                    for (final Class<? extends ConfigurationFactory> clazz : ordered) {
                        addFactory(list, clazz);
                    }
                    // see above comments about double-checked locking
                    //noinspection NonThreadSafeLazyInitialization
                    factories = Collections.unmodifiableList(list);
                    authorizationProvider = authorizationProvider(props);
                }
            } finally {
                LOCK.unlock();
            }
        }

        LOGGER.debug("Using configurationFactory {}", configFactory);
        return configFactory;
    }

    public static AuthorizationProvider authorizationProvider(PropertiesUtil props) {
        final String authClass = props.getStringProperty(AUTHORIZATION_PROVIDER);
        AuthorizationProvider provider = null;
        if (authClass != null) {
            try {
                Object obj = LoaderUtil.newInstanceOf(authClass);
                if (obj instanceof AuthorizationProvider) {
                    provider = (AuthorizationProvider) obj;
                } else {
                    LOGGER.warn("{} is not an AuthorizationProvider, using default", obj.getClass().getName());
                }
            } catch (Exception ex) {
                LOGGER.warn("Unable to create {}, using default: {}", authClass, ex.getMessage());
            }
        }
        if (provider == null) {
            provider = new BasicAuthorizationProvider(props);
        }
        return provider;
    }

    public static AuthorizationProvider getAuthorizationProvider() {
        return authorizationProvider;
    }

    private static void addFactory(final Collection<ConfigurationFactory> list, final String factoryClass) {
        try {
            addFactory(list, Loader.loadClass(factoryClass).asSubclass(ConfigurationFactory.class));
        } catch (final Exception ex) {
            LOGGER.error("Unable to load class {}", factoryClass, ex);
        }
    }

    private static void addFactory(final Collection<ConfigurationFactory> list,
                                   final Class<? extends ConfigurationFactory> factoryClass) {
        try {
            list.add(ReflectionUtil.instantiate(factoryClass));
        } catch (final Exception ex) {
            LOGGER.error("Unable to create instance of {}", factoryClass.getName(), ex);
        }
    }

    /**
     * Sets the configuration factory. This method is not intended for general use and may not be thread safe.
     * @param factory the ConfigurationFactory.
     */
    public static void setConfigurationFactory(final ConfigurationFactory factory) {
        configFactory = factory;
    }

    /**
     * Resets the ConfigurationFactory to the default. This method is not intended for general use and may
     * not be thread safe.
     */
    public static void resetConfigurationFactory() {
        configFactory = new Factory();
    }

    /**
     * Removes the ConfigurationFactory. This method is not intended for general use and may not be thread safe.
     * @param factory The factory to remove.
     */
    public static void removeConfigurationFactory(final ConfigurationFactory factory) {
        if (configFactory == factory) {
            configFactory = new Factory();
        }
    }

    protected abstract String[] getSupportedTypes();

    protected String getTestPrefix() {
        return TEST_PREFIX;
    }

    protected String getDefaultPrefix() {
        return DEFAULT_PREFIX;
    }

    protected String getVersion() {
        return LOG4J2_VERSION;
    }

    protected boolean isActive() {
        return true;
    }

    public abstract Configuration getConfiguration(final LoggerContext loggerContext, ConfigurationSource source);

    /**
     * Returns the Configuration.
     * @param loggerContext The logger context
     * @param name The configuration name.
     * @param configLocation The configuration location.
     * @return The Configuration.
     */
    public Configuration getConfiguration(final LoggerContext loggerContext, final String name, final URI configLocation) {
        if (!isActive()) {
            return null;
        }
        if (configLocation != null) {
            final ConfigurationSource source = ConfigurationSource.fromUri(configLocation);
            if (source != null) {
                return getConfiguration(loggerContext, source);
            }
        }
        return null;
    }

    /**
     * Returns the Configuration obtained using a given ClassLoader.
     * @param loggerContext The logger context
     * @param name The configuration name.
     * @param configLocation A URI representing the location of the configuration.
     * @param loader The default ClassLoader to use. If this is {@code null}, then the
     *               {@linkplain LoaderUtil#getThreadContextClassLoader() default ClassLoader} will be used.
     *
     * @return The Configuration.
     */
    public Configuration getConfiguration(final LoggerContext loggerContext, final String name, final URI configLocation, final ClassLoader loader) {
        if (!isActive()) {
            return null;
        }
        if (loader == null) {
            return getConfiguration(loggerContext, name, configLocation);
        }
        if (isClassLoaderUri(configLocation)) {
            final String path = extractClassLoaderUriPath(configLocation);
            final ConfigurationSource source = ConfigurationSource.fromResource(path, loader);
            if (source != null) {
                final Configuration configuration = getConfiguration(loggerContext, source);
                if (configuration != null) {
                    return configuration;
                }
            }
        }
        return getConfiguration(loggerContext, name, configLocation);
    }

    static boolean isClassLoaderUri(final URI uri) {
        if (uri == null) {
            return false;
        }
        final String scheme = uri.getScheme();
        return scheme == null || scheme.equals(CLASS_LOADER_SCHEME) || scheme.equals(CLASS_PATH_SCHEME);
    }

    static String extractClassLoaderUriPath(final URI uri) {
        return uri.getScheme() == null ? uri.getPath() : uri.getSchemeSpecificPart();
    }

    /**
     * Loads the configuration from the location represented by the String.
     * @param config The configuration location.
     * @param loader The default ClassLoader to use.
     * @return The InputSource to use to read the configuration.
     */
    protected ConfigurationSource getInputFromString(final String config, final ClassLoader loader) {
        try {
            final URL url = new URL(config);
            URLConnection urlConnection = UrlConnectionFactory.createConnection(url);
            File file = FileUtils.fileFromUri(url.toURI());
            if (file != null) {
                return new ConfigurationSource(urlConnection.getInputStream(), FileUtils.fileFromUri(url.toURI()));
            } else {
                return new ConfigurationSource(urlConnection.getInputStream(), url, urlConnection.getLastModified());
            }
        } catch (final Exception ex) {
            final ConfigurationSource source = ConfigurationSource.fromResource(config, loader);
            if (source == null) {
                try {
                    final File file = new File(config);
                    return new ConfigurationSource(new FileInputStream(file), file);
                } catch (final FileNotFoundException fnfe) {
                    // Ignore the exception
                    LOGGER.catching(Level.DEBUG, fnfe);
                }
            }
            return source;
        }
    }

    /**
     * Default Factory.
     */
    private static class Factory extends ConfigurationFactory {

        private static final String ALL_TYPES = "*";

        /**
         * Default Factory Constructor.
         * @param name The configuration name.
         * @param configLocation The configuration location.
         * @return The Configuration.
         */
        @Override
        public Configuration getConfiguration(final LoggerContext loggerContext, final String name, final URI configLocation) {

            if (configLocation == null) {
                final String configLocationStr = this.substitutor.replace(PropertiesUtil.getProperties()
                        .getStringProperty(CONFIGURATION_FILE_PROPERTY));
                if (configLocationStr != null) {
                    String[] sources = parseConfigLocations(configLocationStr);
                    if (sources.length > 1) {
                        final List<AbstractConfiguration> configs = new ArrayList<>();
                        for (final String sourceLocation : sources) {
                            final Configuration config = getConfiguration(loggerContext, sourceLocation.trim());
                            if (config != null) {
                                if (config instanceof AbstractConfiguration) {
                                    configs.add((AbstractConfiguration) config);
                                } else {
                                    LOGGER.error("Failed to created configuration at {}", sourceLocation);
                                    return null;
                                }
                            } else {
                                LOGGER.warn("Unable to create configuration for {}, ignoring", sourceLocation);
                            }
                        }
                        if (configs.size() > 1) {
                            return new CompositeConfiguration(configs);
                        } else if (configs.size() == 1) {
                            return configs.get(0);
                        }
                    }
                    return getConfiguration(loggerContext, configLocationStr);
                } else {
                    final String log4j1ConfigStr = this.substitutor.replace(PropertiesUtil.getProperties()
                            .getStringProperty(LOG4J1_CONFIGURATION_FILE_PROPERTY));
                    if (log4j1ConfigStr != null) {
                        System.setProperty(LOG4J1_EXPERIMENTAL, "true");
                        return getConfiguration(LOG4J1_VERSION, loggerContext, log4j1ConfigStr);
                    }
                }
                for (final ConfigurationFactory factory : getFactories()) {
                    final String[] types = factory.getSupportedTypes();
                    if (types != null) {
                        for (final String type : types) {
                            if (type.equals(ALL_TYPES)) {
                                final Configuration config = factory.getConfiguration(loggerContext, name, configLocation);
                                if (config != null) {
                                    return config;
                                }
                            }
                        }
                    }
                }
            } else {
                String[] sources = parseConfigLocations(configLocation);
                if (sources.length > 1) {
                    final List<AbstractConfiguration> configs = new ArrayList<>();
                    for (final String sourceLocation : sources) {
                        final Configuration config = getConfiguration(loggerContext, sourceLocation.trim());
                        if (config instanceof AbstractConfiguration) {
                            configs.add((AbstractConfiguration) config);
                        } else {
                            LOGGER.error("Failed to created configuration at {}", sourceLocation);
                            return null;
                        }
                    }
                    return new CompositeConfiguration(configs);
                }
                // configLocation != null
                final String configLocationStr = configLocation.toString();
                for (final ConfigurationFactory factory : getFactories()) {
                    final String[] types = factory.getSupportedTypes();
                    if (types != null) {
                        for (final String type : types) {
                            if (type.equals(ALL_TYPES) || configLocationStr.endsWith(type)) {
                                final Configuration config = factory.getConfiguration(loggerContext, name, configLocation);
                                if (config != null) {
                                    return config;
                                }
                            }
                        }
                    }
                }
            }

            Configuration config = getConfiguration(loggerContext, true, name);
            if (config == null) {
                config = getConfiguration(loggerContext, true, null);
                if (config == null) {
                    config = getConfiguration(loggerContext, false, name);
                    if (config == null) {
                        config = getConfiguration(loggerContext, false, null);
                    }
                }
            }
            if (config != null) {
                return config;
            }
            LOGGER.warn("No Log4j 2 configuration file found. " +
                    "Using default configuration (logging only errors to the console), " +
                    "or user programmatically provided configurations. " +
                    "Set system property 'log4j2.debug' " +
                    "to show Log4j 2 internal initialization logging. " +
                    "See https://logging.apache.org/log4j/2.x/manual/configuration.html for instructions on how to configure Log4j 2");
            return new DefaultConfiguration();
        }

        private Configuration getConfiguration(final LoggerContext loggerContext, final String configLocationStr) {
            return getConfiguration(null, loggerContext, configLocationStr);
        }

        private Configuration getConfiguration(String requiredVersion, final LoggerContext loggerContext,
                final String configLocationStr) {
            ConfigurationSource source = null;
            try {
                source = ConfigurationSource.fromUri(NetUtils.toURI(configLocationStr));
            } catch (final Exception ex) {
                // Ignore the error and try as a String.
                LOGGER.catching(Level.DEBUG, ex);
            }
            if (source == null) {
                final ClassLoader loader = LoaderUtil.getThreadContextClassLoader();
                source = getInputFromString(configLocationStr, loader);
            }
            if (source != null) {
                for (final ConfigurationFactory factory : getFactories()) {
                    if (requiredVersion != null && !factory.getVersion().equals(requiredVersion)) {
                        continue;
                    }
                    final String[] types = factory.getSupportedTypes();
                    if (types != null) {
                        for (final String type : types) {
                            if (type.equals(ALL_TYPES) || configLocationStr.endsWith(type)) {
                                final Configuration config = factory.getConfiguration(loggerContext, source);
                                if (config != null) {
                                    return config;
                                }
                            }
                        }
                    }
                }
            }
            return null;
        }

        private Configuration getConfiguration(final LoggerContext loggerContext, final boolean isTest, final String name) {
            final boolean named = Strings.isNotEmpty(name);
            final ClassLoader loader = LoaderUtil.getThreadContextClassLoader();
            for (final ConfigurationFactory factory : getFactories()) {
                String configName;
                final String prefix = isTest ? factory.getTestPrefix() : factory.getDefaultPrefix();
                final String [] types = factory.getSupportedTypes();
                if (types == null) {
                    continue;
                }

                for (final String suffix : types) {
                    if (suffix.equals(ALL_TYPES)) {
                        continue;
                    }
                    configName = named ? prefix + name + suffix : prefix + suffix;

                    final ConfigurationSource source = ConfigurationSource.fromResource(configName, loader);
                    if (source != null) {
                        if (!factory.isActive()) {
                            LOGGER.warn("Found configuration file {} for inactive ConfigurationFactory {}", configName, factory.getClass().getName());
                        }
                        return factory.getConfiguration(loggerContext, source);
                    }
                }
            }
            return null;
        }

        @Override
        public String[] getSupportedTypes() {
            return null;
        }

        @Override
        public Configuration getConfiguration(final LoggerContext loggerContext, final ConfigurationSource source) {
            if (source != null) {
                final String config = source.getLocation();
                for (final ConfigurationFactory factory : getFactories()) {
                    final String[] types = factory.getSupportedTypes();
                    if (types != null) {
                        for (final String type : types) {
                            if (type.equals(ALL_TYPES) || config != null && config.endsWith(type)) {
                                final Configuration c = factory.getConfiguration(loggerContext, source);
                                if (c != null) {
                                    LOGGER.debug("Loaded configuration from {}", source);
                                    return c;
                                }
                                LOGGER.error("Cannot determine the ConfigurationFactory to use for {}", config);
                                return null;
                            }
                        }
                    }
                }
            }
            LOGGER.error("Cannot process configuration, input source is null");
            return null;
        }

        private String[] parseConfigLocations(URI configLocations) {
            final String[] uris = configLocations.toString().split("\\?");
            final List<String> locations = new ArrayList<>();
            if (uris.length > 1) {
                locations.add(uris[0]);
                final String[] pairs = configLocations.getQuery().split("&");
                for (String pair : pairs) {
                    final int idx = pair.indexOf("=");
                    try {
                        final String key = idx > 0 ? URLDecoder.decode(pair.substring(0, idx), "UTF-8") : pair;
                        if (key.equalsIgnoreCase(OVERRIDE_PARAM)) {
                            locations.add(URLDecoder.decode(pair.substring(idx + 1), "UTF-8"));
                        }
                    } catch (UnsupportedEncodingException ex) {
                        LOGGER.warn("Invalid query parameter in {}", configLocations);
                    }
                }
                return locations.toArray(new String[0]);
            }
            return new String[] {uris[0]};
        }

        private String[] parseConfigLocations(String configLocations) {
            final String[] uris = configLocations.split(",");
            if (uris.length > 1) {
                return uris;
            }
            try {
                return parseConfigLocations(new URI(configLocations));
            } catch (URISyntaxException ex) {
                LOGGER.warn("Error parsing URI {}", configLocations);
            }
            return new String[] {configLocations};
        }
    }

    static List<ConfigurationFactory> getFactories() {
        return factories;
    }
}

⏎ org/apache/logging/log4j/core/config/ConfigurationFactory.java

Or download all of them as a single archive file:

File name: log4j-core-2.14.1-sources.jar
File size: 1281358 bytes
Release date: 2021-03-06
Download 

⇒ Source Code for Apache Log4j JDK Logging Adapter

⇐ Source Code for Apache Log4j API

⇑ Downloading and Reviewing Apache Log4j Packages

⇑⇑ FAQ for Apache Log4j