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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
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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.cassandra.utils;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.Iterator;
import java.util.Map;
import java.util.function.Predicate;

public class ReflectionUtils
{
    private ReflectionUtils()
    {

    }

    public static Field getModifiersField() throws NoSuchFieldException
    {
        return getField(Field.class, "modifiers");
    }

    public static Field getField(Class clazz, String fieldName) throws NoSuchFieldException
    {
        // below code works before Java 12
        try
        {
            return clazz.getDeclaredField(fieldName);
        }
        catch (NoSuchFieldException e)
        {
            // this is mitigation for JDK 17 (https://bugs.openjdk.org/browse/JDK-8210522)
            try
            {
                Method getDeclaredFields0 = Class.class.getDeclaredMethod("getDeclaredFields0", boolean.class);
                getDeclaredFields0.setAccessible(true);
                Field[] fields = (Field[]) getDeclaredFields0.invoke(clazz, false);
                for (Field field : fields)
                {
                    if (fieldName.equals(field.getName()))
                    {
                        return field;
                    }
                }
            }
            catch (ReflectiveOperationException ex)
            {
                e.addSuppressed(ex);
            }
            throw e;
        }
    }

    /**
     * Used by the in-jvm dtest framework to remove entries from private map fields that otherwise would prevent
     * collection of classloaders (which causes metaspace OOMs) or otherwise interfere with instance restart.
     * @param clazz The class which has the map field to clear
     * @param instance an instance of the class to clear (pass null for a static member)
     * @param mapName the name of the map field to clear
     * @param shouldRemove a predicate which determines if the entry in question should be removed
     * @param  The type of the map key
     * @param  The type of the map value
     */
    public static  void clearMapField(Class clazz, Object instance, String mapName, Predicate> shouldRemove) {
        try
        {
            Field mapField = getField(clazz, mapName);
            mapField.setAccessible(true);
            // noinspection unchecked
            Map map = (Map) mapField.get(instance);
            // Because multiple instances can be shutting down at once,
            // synchronize on the map to avoid ConcurrentModificationException
            synchronized (map)
            {
                // This could be done with a simple `map.entrySet.removeIf()` call
                // but for debugging purposes it is much easier to keep it like this.
                Iterator> it = map.entrySet().iterator();
                while (it.hasNext()) {
                    Map.Entry entry = it.next();
                    if (shouldRemove.test(entry))
                    {
                        it.remove();
                    }
                }
            }
        }
        catch (NoSuchFieldException | IllegalAccessException ex)
        {
            throw new RuntimeException(String.format("Could not clear map field %s in class %s", mapName, clazz), ex);
        }
    }
}