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/util/DefaultShutdownCallbackRegistry.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.util;

import java.lang.ref.Reference;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.Collection;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.core.AbstractLifeCycle;
import org.apache.logging.log4j.core.LifeCycle2;
import org.apache.logging.log4j.status.StatusLogger;

/**
 * ShutdownRegistrationStrategy that simply uses {@link Runtime#addShutdownHook(Thread)}. If no strategy is specified,
 * this one is used for shutdown hook registration.
 *
 * @since 2.1
 */
public class DefaultShutdownCallbackRegistry implements ShutdownCallbackRegistry, LifeCycle2, Runnable {
    /** Status logger. */
    protected static final Logger LOGGER = StatusLogger.getLogger();

    private final AtomicReference<State> state = new AtomicReference<>(State.INITIALIZED);
    private final ThreadFactory threadFactory;

    // use references to prevent memory leaks
    private final Collection<Reference<Cancellable>> hooks = new CopyOnWriteArrayList<>();
    private Reference<Thread> shutdownHookRef;

    /**
     * Constructs a DefaultShutdownRegistrationStrategy.
     */
    public DefaultShutdownCallbackRegistry() {
        this(Executors.defaultThreadFactory());
    }

    /**
     * Constructs a DefaultShutdownRegistrationStrategy using the given {@link ThreadFactory}.
     *
     * @param threadFactory the ThreadFactory to use to create a {@link Runtime} shutdown hook thread
     */
    protected DefaultShutdownCallbackRegistry(final ThreadFactory threadFactory) {
        this.threadFactory = threadFactory;
    }

    /**
     * Executes the registered shutdown callbacks.
     */
    @Override
    public void run() {
        if (state.compareAndSet(State.STARTED, State.STOPPING)) {
            for (final Reference<Cancellable> hookRef : hooks) {
                Cancellable hook = hookRef.get();
                if (hook != null) {
                    try {
                        hook.run();
                    } catch (final Throwable t1) {
                        try {
                            LOGGER.error(SHUTDOWN_HOOK_MARKER, "Caught exception executing shutdown hook {}", hook, t1);
                        } catch (final Throwable t2) {
                            System.err.println("Caught exception " + t2.getClass() + " logging exception " + t1.getClass());
                            t1.printStackTrace();
                        }
                    }
                }
            }
            state.set(State.STOPPED);
        }
    }

    private static class RegisteredCancellable implements Cancellable {
        private Runnable callback;
        private Collection<Reference<Cancellable>> registered;

        RegisteredCancellable(final Runnable callback, final Collection<Reference<Cancellable>> registered) {
            this.callback = callback;
            this.registered = registered;
        }

        @Override
        public void cancel() {
            callback = null;
            Collection<Reference<Cancellable>> references = registered;
            if (references != null) {
                registered = null;
                references.removeIf(ref -> {
                    Cancellable value = ref.get();
                    return value == null || value == RegisteredCancellable.this;
                });
            }
        }

        @Override
        public void run() {
            final Runnable runnableHook = callback;
            if (runnableHook != null) {
                runnableHook.run();
                callback = null;
            }
        }

        @Override
        public String toString() {
            return String.valueOf(callback);
        }
    }

    @Override
    public Cancellable addShutdownCallback(final Runnable callback) {
        if (isStarted()) {
            final Cancellable receipt = new RegisteredCancellable(callback, hooks);
            hooks.add(new SoftReference<>(receipt));
            return receipt;
        }
        throw new IllegalStateException("Cannot add new shutdown hook as this is not started. Current state: " +
            state.get().name());
    }

    @Override
    public void initialize() {
    }

    /**
     * Registers the shutdown thread only if this is initialized.
     */
    @Override
    public void start() {
        if (state.compareAndSet(State.INITIALIZED, State.STARTING)) {
            try {
                addShutdownHook(threadFactory.newThread(this));
                state.set(State.STARTED);
            } catch (final IllegalStateException ex) {
                state.set(State.STOPPED);
                throw ex;
            } catch (final Exception e) {
                LOGGER.catching(e);
                state.set(State.STOPPED);
            }
        }
    }

    private void addShutdownHook(final Thread thread) {
        shutdownHookRef = new WeakReference<>(thread);
        Runtime.getRuntime().addShutdownHook(thread);
    }

    @Override
    public void stop() {
        stop(AbstractLifeCycle.DEFAULT_STOP_TIMEOUT, AbstractLifeCycle.DEFAULT_STOP_TIMEUNIT);
    }

    /**
     * Cancels the shutdown thread only if this is started.
     */
    @Override
    public boolean stop(final long timeout, final TimeUnit timeUnit) {
        if (state.compareAndSet(State.STARTED, State.STOPPING)) {
            try {
                removeShutdownHook();
            } finally {
                state.set(State.STOPPED);
            }
        }
        return true;
    }

    private void removeShutdownHook() {
        final Thread shutdownThread = shutdownHookRef.get();
        if (shutdownThread != null) {
            Runtime.getRuntime().removeShutdownHook(shutdownThread);
            shutdownHookRef.enqueue();
        }
    }

    @Override
    public State getState() {
        return state.get();
    }

    /**
     * Indicates if this can accept shutdown hooks.
     *
     * @return true if this can accept shutdown hooks
     */
    @Override
    public boolean isStarted() {
        return state.get() == State.STARTED;
    }

    @Override
    public boolean isStopped() {
        return state.get() == State.STOPPED;
    }

}

⏎ org/apache/logging/log4j/core/util/DefaultShutdownCallbackRegistry.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