Source Code for Apache Log4j API

Apache Log4j API provides the interface that applications should code to and provides the adapter components required for implementers to create a logging implementation. Apache Log4j API is a required module to use Apache Log4j.

Bytecode (Java 8) for Apache Log4j API is provided in a separate JAR file like log4j-api-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 API 2.14.1 below.

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org/apache/logging/log4j/util/LoaderUtil.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.util;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.net.URL;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Collection;
import java.util.Enumeration;
import java.util.LinkedHashSet;
import java.util.Objects;

/**
 * <em>Consider this class private.</em> Utility class for ClassLoaders.
 *
 * @see ClassLoader
 * @see RuntimePermission
 * @see Thread#getContextClassLoader()
 * @see ClassLoader#getSystemClassLoader()
 */
public final class LoaderUtil {

    /**
     * System property to set to ignore the thread context ClassLoader.
     *
     * @since 2.1
     */
    public static final String IGNORE_TCCL_PROPERTY = "log4j.ignoreTCL";

    private static final SecurityManager SECURITY_MANAGER = System.getSecurityManager();

    // this variable must be lazily loaded; otherwise, we get a nice circular class loading problem where LoaderUtil
    // wants to use PropertiesUtil, but then PropertiesUtil wants to use LoaderUtil.
    private static Boolean ignoreTCCL;

    private static final boolean GET_CLASS_LOADER_DISABLED;

    private static final PrivilegedAction<ClassLoader> TCCL_GETTER = new ThreadContextClassLoaderGetter();

    static {
        if (SECURITY_MANAGER != null) {
            boolean getClassLoaderDisabled;
            try {
                SECURITY_MANAGER.checkPermission(new RuntimePermission("getClassLoader"));
                getClassLoaderDisabled = false;
            } catch (final SecurityException ignored) {
                getClassLoaderDisabled = true;
            }
            GET_CLASS_LOADER_DISABLED = getClassLoaderDisabled;
        } else {
            GET_CLASS_LOADER_DISABLED = false;
        }
    }

    private LoaderUtil() {
    }

    /**
     * Gets the current Thread ClassLoader. Returns the system ClassLoader if the TCCL is {@code null}. If the system
     * ClassLoader is {@code null} as well, then the ClassLoader for this class is returned. If running with a
     * {@link SecurityManager} that does not allow access to the Thread ClassLoader or system ClassLoader, then the
     * ClassLoader for this class is returned.
     *
     * @return the current ThreadContextClassLoader.
     */
    public static ClassLoader getThreadContextClassLoader() {
        if (GET_CLASS_LOADER_DISABLED) {
            // we can at least get this class's ClassLoader regardless of security context
            // however, if this is null, there's really no option left at this point
            return LoaderUtil.class.getClassLoader();
        }
        return SECURITY_MANAGER == null ? TCCL_GETTER.run() : AccessController.doPrivileged(TCCL_GETTER);
    }

    /**
     *
     */
    private static class ThreadContextClassLoaderGetter implements PrivilegedAction<ClassLoader> {
        @Override
        public ClassLoader run() {
            final ClassLoader cl = Thread.currentThread().getContextClassLoader();
            if (cl != null) {
                return cl;
            }
            final ClassLoader ccl = LoaderUtil.class.getClassLoader();
            return ccl == null && !GET_CLASS_LOADER_DISABLED ? ClassLoader.getSystemClassLoader() : ccl;
        }
    }

    public static ClassLoader[] getClassLoaders() {
        final Collection<ClassLoader> classLoaders = new LinkedHashSet<>();
        final ClassLoader tcl = getThreadContextClassLoader();
        if (tcl != null) {
            classLoaders.add(tcl);
        }
        accumulateClassLoaders(LoaderUtil.class.getClassLoader(), classLoaders);
        accumulateClassLoaders(tcl == null ? null : tcl.getParent(), classLoaders);
        final ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
		if (systemClassLoader != null) {
            classLoaders.add(systemClassLoader);
        }
        return classLoaders.toArray(new ClassLoader[classLoaders.size()]);
    }

    /**
     * Adds the provided loader to the loaders collection, and traverses up the tree until either a null
     * value or a classloader which has already been added is encountered.
     */
    private static void accumulateClassLoaders(ClassLoader loader, Collection<ClassLoader> loaders) {
        // Some implementations may use null to represent the bootstrap class loader.
        if (loader != null && loaders.add(loader)) {
            accumulateClassLoaders(loader.getParent(), loaders);
        }
    }

    /**
     * Determines if a named Class can be loaded or not.
     *
     * @param className The class name.
     * @return {@code true} if the class could be found or {@code false} otherwise.
     * @since 2.7
     */
    public static boolean isClassAvailable(final String className) {
        try {
            final Class<?> clazz = loadClass(className);
            return clazz != null;
        } catch (final ClassNotFoundException | LinkageError e) {
            return false;
        } catch (final Throwable e) {
            LowLevelLogUtil.logException("Unknown error checking for existence of class: " + className, e);
            return false;
        }
    }

    /**
     * Loads a class by name. This method respects the {@link #IGNORE_TCCL_PROPERTY} Log4j property. If this property is
     * specified and set to anything besides {@code false}, then the default ClassLoader will be used.
     *
     * @param className The class name.
     * @return the Class for the given name.
     * @throws ClassNotFoundException if the specified class name could not be found
     * @since 2.1
     */
    public static Class<?> loadClass(final String className) throws ClassNotFoundException {
        if (isIgnoreTccl()) {
            return Class.forName(className);
        }
        try {
            ClassLoader tccl = getThreadContextClassLoader();
            if (tccl != null) {
                return tccl.loadClass(className);
            }
        } catch (final Throwable ignored) {
        }
        return Class.forName(className);
    }

    /**
     * Loads and instantiates a Class using the default constructor.
     *
     * @param clazz The class.
     * @return new instance of the class.
     * @throws IllegalAccessException if the class can't be instantiated through a public constructor
     * @throws InstantiationException if there was an exception whilst instantiating the class
     * @throws InvocationTargetException if there was an exception whilst constructing the class
     * @since 2.7
     */
    public static <T> T newInstanceOf(final Class<T> clazz)
            throws InstantiationException, IllegalAccessException, InvocationTargetException {
        try {
            return clazz.getConstructor().newInstance();
        } catch (final NoSuchMethodException ignored) {
            // FIXME: looking at the code for Class.newInstance(), this seems to do the same thing as above
            return clazz.newInstance();
        }
    }

    /**
     * Loads and instantiates a Class using the default constructor.
     *
     * @param className The class name.
     * @return new instance of the class.
     * @throws ClassNotFoundException if the class isn't available to the usual ClassLoaders
     * @throws IllegalAccessException if the class can't be instantiated through a public constructor
     * @throws InstantiationException if there was an exception whilst instantiating the class
     * @throws NoSuchMethodException if there isn't a no-args constructor on the class
     * @throws InvocationTargetException if there was an exception whilst constructing the class
     * @since 2.1
     */
    @SuppressWarnings("unchecked")
    public static <T> T newInstanceOf(final String className) throws ClassNotFoundException, IllegalAccessException,
            InstantiationException, NoSuchMethodException, InvocationTargetException {
        return newInstanceOf((Class<T>) loadClass(className));
    }

    /**
     * Loads and instantiates a derived class using its default constructor.
     *
     * @param className The class name.
     * @param clazz The class to cast it to.
     * @param <T> The type of the class to check.
     * @return new instance of the class cast to {@code T}
     * @throws ClassNotFoundException if the class isn't available to the usual ClassLoaders
     * @throws IllegalAccessException if the class can't be instantiated through a public constructor
     * @throws InstantiationException if there was an exception whilst instantiating the class
     * @throws NoSuchMethodException if there isn't a no-args constructor on the class
     * @throws InvocationTargetException if there was an exception whilst constructing the class
     * @throws ClassCastException if the constructed object isn't type compatible with {@code T}
     * @since 2.1
     */
    public static <T> T newCheckedInstanceOf(final String className, final Class<T> clazz)
            throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, InstantiationException,
            IllegalAccessException {
        return clazz.cast(newInstanceOf(className));
    }

    /**
     * Loads and instantiates a class given by a property name.
     *
     * @param propertyName The property name to look up a class name for.
     * @param clazz        The class to cast it to.
     * @param <T>          The type to cast it to.
     * @return new instance of the class given in the property or {@code null} if the property was unset.
     * @throws ClassNotFoundException    if the class isn't available to the usual ClassLoaders
     * @throws IllegalAccessException    if the class can't be instantiated through a public constructor
     * @throws InstantiationException    if there was an exception whilst instantiating the class
     * @throws NoSuchMethodException     if there isn't a no-args constructor on the class
     * @throws InvocationTargetException if there was an exception whilst constructing the class
     * @throws ClassCastException        if the constructed object isn't type compatible with {@code T}
     * @since 2.5
     */
    public static <T> T newCheckedInstanceOfProperty(final String propertyName, final Class<T> clazz)
        throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, InstantiationException,
        IllegalAccessException {
        final String className = PropertiesUtil.getProperties().getStringProperty(propertyName);
        if (className == null) {
            return null;
        }
        return newCheckedInstanceOf(className, clazz);
    }

    private static boolean isIgnoreTccl() {
        // we need to lazily initialize this, but concurrent access is not an issue
        if (ignoreTCCL == null) {
            final String ignoreTccl = PropertiesUtil.getProperties().getStringProperty(IGNORE_TCCL_PROPERTY, null);
            ignoreTCCL = ignoreTccl != null && !"false".equalsIgnoreCase(ignoreTccl.trim());
        }
        return ignoreTCCL;
    }

    /**
     * Finds classpath {@linkplain URL resources}.
     *
     * @param resource the name of the resource to find.
     * @return a Collection of URLs matching the resource name. If no resources could be found, then this will be empty.
     * @since 2.1
     */
    public static Collection<URL> findResources(final String resource) {
        final Collection<UrlResource> urlResources = findUrlResources(resource);
        final Collection<URL> resources = new LinkedHashSet<>(urlResources.size());
        for (final UrlResource urlResource : urlResources) {
            resources.add(urlResource.getUrl());
        }
        return resources;
    }

    static Collection<UrlResource> findUrlResources(final String resource) {
        // @formatter:off
        final ClassLoader[] candidates = {
                getThreadContextClassLoader(), 
                LoaderUtil.class.getClassLoader(),
                GET_CLASS_LOADER_DISABLED ? null : ClassLoader.getSystemClassLoader()};
        // @formatter:on
        final Collection<UrlResource> resources = new LinkedHashSet<>();
        for (final ClassLoader cl : candidates) {
            if (cl != null) {
                try {
                    final Enumeration<URL> resourceEnum = cl.getResources(resource);
                    while (resourceEnum.hasMoreElements()) {
                        resources.add(new UrlResource(cl, resourceEnum.nextElement()));
                    }
                } catch (final IOException e) {
                    LowLevelLogUtil.logException(e);
                }
            }
        }
        return resources;
    }

    /**
     * {@link URL} and {@link ClassLoader} pair.
     */
    static class UrlResource {
        private final ClassLoader classLoader;
        private final URL url;

        UrlResource(final ClassLoader classLoader, final URL url) {
            this.classLoader = classLoader;
            this.url = url;
        }

        public ClassLoader getClassLoader() {
            return classLoader;
        }

        public URL getUrl() {
            return url;
        }

        @Override
        public boolean equals(final Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }

            final UrlResource that = (UrlResource) o;

            if (classLoader != null ? !classLoader.equals(that.classLoader) : that.classLoader != null) {
                return false;
            }
            if (url != null ? !url.equals(that.url) : that.url != null) {
                return false;
            }

            return true;
        }

        @Override
        public int hashCode() {
            return Objects.hashCode(classLoader) + Objects.hashCode(url);
        }
    }
}

org/apache/logging/log4j/util/LoaderUtil.java

 

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Release date: 2021-03-06
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