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/plugins/util/PluginRegistry.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.plugins.util;

import java.io.IOException;
import java.net.URI;
import java.net.URL;
import java.text.DecimalFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicReference;

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.core.config.plugins.Plugin;
import org.apache.logging.log4j.core.config.plugins.PluginAliases;
import org.apache.logging.log4j.core.config.plugins.processor.PluginCache;
import org.apache.logging.log4j.core.config.plugins.processor.PluginEntry;
import org.apache.logging.log4j.core.config.plugins.processor.PluginProcessor;
import org.apache.logging.log4j.core.util.Loader;
import org.apache.logging.log4j.status.StatusLogger;
import org.apache.logging.log4j.util.Strings;

/**
 * Registry singleton for PluginType maps partitioned by source type and then by category names.
 */
public class PluginRegistry {

    private static final Logger LOGGER = StatusLogger.getLogger();

    private static volatile PluginRegistry INSTANCE;
    private static final Object INSTANCE_LOCK = new Object();

    /**
     * Contains plugins found in Log4j2Plugins.dat cache files in the main CLASSPATH.
     */
    private final AtomicReference<Map<String, List<PluginType<?>>>> pluginsByCategoryRef =
        new AtomicReference<>();

    /**
     * Contains plugins found in Log4j2Plugins.dat cache files in OSGi Bundles.
     */
    private final ConcurrentMap<Long, Map<String, List<PluginType<?>>>> pluginsByCategoryByBundleId =
        new ConcurrentHashMap<>();

    /**
     * Contains plugins found by searching for annotated classes at runtime.
     */
    private final ConcurrentMap<String, Map<String, List<PluginType<?>>>> pluginsByCategoryByPackage =
        new ConcurrentHashMap<>();

    private PluginRegistry() {
    }

    /**
     * Returns the global PluginRegistry instance.
     *
     * @return the global PluginRegistry instance.
     * @since 2.1
     */
    public static PluginRegistry getInstance() {
        PluginRegistry result = INSTANCE;
        if (result == null) {
            synchronized (INSTANCE_LOCK) {
                result = INSTANCE;
                if (result == null) {
                    INSTANCE = result = new PluginRegistry();
                }
            }
        }
        return result;
    }

    /**
     * Resets the registry to an empty state.
     */
    public void clear() {
        pluginsByCategoryRef.set(null);
        pluginsByCategoryByPackage.clear();
        pluginsByCategoryByBundleId.clear();
    }

    /**
     * @since 2.1
     */
    public Map<Long, Map<String, List<PluginType<?>>>> getPluginsByCategoryByBundleId() {
        return pluginsByCategoryByBundleId;
    }

    /**
     * @since 2.1
     */
    public Map<String, List<PluginType<?>>> loadFromMainClassLoader() {
        final Map<String, List<PluginType<?>>> existing = pluginsByCategoryRef.get();
        if (existing != null) {
            // already loaded
            return existing;
        }
        final Map<String, List<PluginType<?>>> newPluginsByCategory = decodeCacheFiles(Loader.getClassLoader());

        // Note multiple threads could be calling this method concurrently. Both will do the work,
        // but only one will be allowed to store the result in the AtomicReference.
        // Return the map produced by whichever thread won the race, so all callers will get the same result.
        if (pluginsByCategoryRef.compareAndSet(null, newPluginsByCategory)) {
            return newPluginsByCategory;
        }
        return pluginsByCategoryRef.get();
    }

    /**
     * @since 2.1
     */
    public void clearBundlePlugins(final long bundleId) {
        pluginsByCategoryByBundleId.remove(bundleId);
    }

    /**
     * @since 2.1
     */
    public Map<String, List<PluginType<?>>> loadFromBundle(final long bundleId, final ClassLoader loader) {
        Map<String, List<PluginType<?>>> existing = pluginsByCategoryByBundleId.get(bundleId);
        if (existing != null) {
            // already loaded from this classloader
            return existing;
        }
        final Map<String, List<PluginType<?>>> newPluginsByCategory = decodeCacheFiles(loader);

        // Note multiple threads could be calling this method concurrently. Both will do the work,
        // but only one will be allowed to store the result in the outer map.
        // Return the inner map produced by whichever thread won the race, so all callers will get the same result.
        existing = pluginsByCategoryByBundleId.putIfAbsent(bundleId, newPluginsByCategory);
        if (existing != null) {
            return existing;
        }
        return newPluginsByCategory;
    }

    private Map<String, List<PluginType<?>>> decodeCacheFiles(final ClassLoader loader) {
        final long startTime = System.nanoTime();
        final PluginCache cache = new PluginCache();
        try {
            final Enumeration<URL> resources = loader.getResources(PluginProcessor.PLUGIN_CACHE_FILE);
            if (resources == null) {
                LOGGER.info("Plugin preloads not available from class loader {}", loader);
            } else {
                cache.loadCacheFiles(resources);
            }
        } catch (final IOException ioe) {
            LOGGER.warn("Unable to preload plugins", ioe);
        }
        final Map<String, List<PluginType<?>>> newPluginsByCategory = new HashMap<>();
        int pluginCount = 0;
        for (final Map.Entry<String, Map<String, PluginEntry>> outer : cache.getAllCategories().entrySet()) {
            final String categoryLowerCase = outer.getKey();
            final List<PluginType<?>> types = new ArrayList<>(outer.getValue().size());
            newPluginsByCategory.put(categoryLowerCase, types);
            for (final Map.Entry<String, PluginEntry> inner : outer.getValue().entrySet()) {
                final PluginEntry entry = inner.getValue();
                final String className = entry.getClassName();
                try {
                    final Class<?> clazz = loader.loadClass(className);
                    final PluginType<?> type = new PluginType<>(entry, clazz, entry.getName());
                    types.add(type);
                    ++pluginCount;
                } catch (final ClassNotFoundException e) {
                    LOGGER.info("Plugin [{}] could not be loaded due to missing classes.", className, e);
                } catch (final LinkageError e) {
                    LOGGER.info("Plugin [{}] could not be loaded due to linkage error.", className, e);
                }
            }
        }
        final int numPlugins = pluginCount;
        LOGGER.debug(() -> {
            final long endTime = System.nanoTime();
            StringBuilder sb = new StringBuilder("Took ");
            final DecimalFormat numFormat = new DecimalFormat("#0.000000");
            sb.append(numFormat.format((endTime - startTime) * 1e-9));
            sb.append(" seconds to load ").append(numPlugins);
            sb.append(" plugins from ").append(loader);
            return sb.toString();
        });
        return newPluginsByCategory;
    }

    /**
     * @since 2.1
     */
    public Map<String, List<PluginType<?>>> loadFromPackage(final String pkg) {
        if (Strings.isBlank(pkg)) {
            // happens when splitting an empty string
            return Collections.emptyMap();
        }
        Map<String, List<PluginType<?>>> existing = pluginsByCategoryByPackage.get(pkg);
        if (existing != null) {
            // already loaded this package
            return existing;
        }

        final long startTime = System.nanoTime();
        final ResolverUtil resolver = new ResolverUtil();
        final ClassLoader classLoader = Loader.getClassLoader();
        if (classLoader != null) {
            resolver.setClassLoader(classLoader);
        }
        resolver.findInPackage(new PluginTest(), pkg);

        final Map<String, List<PluginType<?>>> newPluginsByCategory = new HashMap<>();
        for (final Class<?> clazz : resolver.getClasses()) {
            final Plugin plugin = clazz.getAnnotation(Plugin.class);
            final String categoryLowerCase = plugin.category().toLowerCase();
            List<PluginType<?>> list = newPluginsByCategory.get(categoryLowerCase);
            if (list == null) {
                newPluginsByCategory.put(categoryLowerCase, list = new ArrayList<>());
            }
            final PluginEntry mainEntry = new PluginEntry();
            final String mainElementName = plugin.elementType().equals(
                Plugin.EMPTY) ? plugin.name() : plugin.elementType();
            mainEntry.setKey(plugin.name().toLowerCase());
            mainEntry.setName(plugin.name());
            mainEntry.setCategory(plugin.category());
            mainEntry.setClassName(clazz.getName());
            mainEntry.setPrintable(plugin.printObject());
            mainEntry.setDefer(plugin.deferChildren());
            final PluginType<?> mainType = new PluginType<>(mainEntry, clazz, mainElementName);
            list.add(mainType);
            final PluginAliases pluginAliases = clazz.getAnnotation(PluginAliases.class);
            if (pluginAliases != null) {
                for (final String alias : pluginAliases.value()) {
                    final PluginEntry aliasEntry = new PluginEntry();
                    final String aliasElementName = plugin.elementType().equals(
                        Plugin.EMPTY) ? alias.trim() : plugin.elementType();
                    aliasEntry.setKey(alias.trim().toLowerCase());
                    aliasEntry.setName(plugin.name());
                    aliasEntry.setCategory(plugin.category());
                    aliasEntry.setClassName(clazz.getName());
                    aliasEntry.setPrintable(plugin.printObject());
                    aliasEntry.setDefer(plugin.deferChildren());
                    final PluginType<?> aliasType = new PluginType<>(aliasEntry, clazz, aliasElementName);
                    list.add(aliasType);
                }
            }
        }
        LOGGER.debug(() -> {
            final long endTime = System.nanoTime();
            StringBuilder sb = new StringBuilder("Took ");
            final DecimalFormat numFormat = new DecimalFormat("#0.000000");
            sb.append(numFormat.format((endTime - startTime) * 1e-9));
            sb.append(" seconds to load ").append(resolver.getClasses().size());
            sb.append(" plugins from package ").append(pkg);
            return sb.toString();
        });

        // Note multiple threads could be calling this method concurrently. Both will do the work,
        // but only one will be allowed to store the result in the outer map.
        // Return the inner map produced by whichever thread won the race, so all callers will get the same result.
        existing = pluginsByCategoryByPackage.putIfAbsent(pkg, newPluginsByCategory);
        if (existing != null) {
            return existing;
        }
        return newPluginsByCategory;
    }

    /**
     * A Test that checks to see if each class is annotated with the 'Plugin' annotation. If it
     * is, then the test returns true, otherwise false.
     *
     * @since 2.1
     */
    public static class PluginTest implements ResolverUtil.Test {
        @Override
        public boolean matches(final Class<?> type) {
            return type != null && type.isAnnotationPresent(Plugin.class);
        }

        @Override
        public String toString() {
            return "annotated with @" + Plugin.class.getSimpleName();
        }

        @Override
        public boolean matches(final URI resource) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean doesMatchClass() {
            return true;
        }

        @Override
        public boolean doesMatchResource() {
            return false;
        }
    }
}

⏎ org/apache/logging/log4j/core/config/plugins/util/PluginRegistry.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