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The Pax Logging API Library is to allow for the Pax Logging Service to be reloaded without stopping the many dependent bundles. It also contains the OSGi Log Service API and the Knopflerfish Log API.
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/*
 * 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.util.Deque;
import java.util.NoSuchElementException;
import java.util.function.Predicate;

import org.apache.logging.log4j.status.StatusLogger;
import org.ops4j.pax.logging.spi.support.OsgiUtil;
import org.osgi.framework.Version;

/**
 * Consider this class private. Provides various methods to determine the caller class. Background
 */
public final class StackLocatorUtil {
    private static IStackLocator stackLocator = null;
    private static volatile boolean errorLogged;

    static {
        String version = OsgiUtil.systemOrContextProperty(null, "java.specification.version");
        Version v = Version.parseVersion(version);
        if (v == null) {
            // fallback to JDK8
            stackLocator = StackLocator.getInstance();
        } else {
            if (v.getMajor() >= 9) {
                // JDK9+, avoid sun.reflect.Reflection.getCallerClass
                stackLocator = StackLocator9.getInstance();
            } else {
                // JDK8
                stackLocator = StackLocator.getInstance();
            }
        }
    }

    private StackLocatorUtil() {
    }

    // TODO: return Object.class instead of null (though it will have a null ClassLoader)
    // (MS) I believe this would work without any modifications elsewhere, but I could be wrong

    // migrated from ReflectiveCallerClassUtility
    @PerformanceSensitive
    public static Class getCallerClass(final int depth) {
        return stackLocator.getCallerClass(depth + 1);
    }

    public static StackTraceElement getStackTraceElement(final int depth) {
        return stackLocator.getStackTraceElement(depth + 1);
    }

    /**
     * Equivalent to {@link #getCallerClass(String, String)} with an empty {@code pkg}.
     */
    // migrated from ClassLoaderContextSelector
    @PerformanceSensitive
    public static Class getCallerClass(final String fqcn) {
        return getCallerClass(fqcn, Strings.EMPTY);
    }

    /**
     * Search for a calling class.
     *
     * @param fqcn Root class name whose caller to search for.
     * @param pkg Package name prefix that must be matched after the {@code fqcn} has been found.
     * @return The caller class that was matched, or null if one could not be located.
     */
    @PerformanceSensitive
    public static Class getCallerClass(final String fqcn, final String pkg) {
        return stackLocator.getCallerClass(fqcn, pkg);
    }

    /**
     * Gets the ClassLoader of the class that called this method at the location up the call stack by the given
     * stack frame depth.
     * 
     * This method returns {@code null} if:
     * 
     * 
     * {@code sun.reflect.Reflection.getCallerClass(int)} is not present.
     * An exception is caught calling {@code sun.reflect.Reflection.getCallerClass(int)}.
     * Some Class implementations may use null to represent the bootstrap class loader.
     * 
     *
     * @param depth The stack frame count to walk.
     * @return A class or null.
     * @throws IndexOutOfBoundsException if depth is negative.
     */
    @PerformanceSensitive
    public static ClassLoader getCallerClassLoader(final int depth) {
        final Class callerClass = stackLocator.getCallerClass(depth + 1);
        return callerClass != null ? callerClass.getClassLoader() : null;
    }

    /**
     * Search for a calling class.
     *
     * @param sentinelClass Sentinel class at which to begin searching
     * @param callerPredicate Predicate checked after the sentinelClass is found
     * @return the first matching class after sentinelClass is found.
     */
    @PerformanceSensitive
    public static Class getCallerClass(final Class sentinelClass, final Predicate> callerPredicate) {
        return stackLocator.getCallerClass(sentinelClass, callerPredicate);
    }

    // added for use in LoggerAdapter implementations mainly
    @PerformanceSensitive
    public static Class getCallerClass(final Class anchor) {
        return stackLocator.getCallerClass(anchor);
    }

    // migrated from ThrowableProxy
    @PerformanceSensitive
    public static Deque> getCurrentStackTrace() {
        return stackLocator.getCurrentStackTrace();
    }

    public static StackTraceElement calcLocation(final String fqcnOfLogger) {
        try {
            return stackLocator.calcLocation(fqcnOfLogger);
        } catch (NoSuchElementException ex) {
            if (!errorLogged) {
                errorLogged = true;
                StatusLogger.getLogger().warn("Unable to locate stack trace element for {}", fqcnOfLogger, ex);
            }
            return null;
        }
    }
}