/*
 * Copyright 2014 - 2020 Rafael Winterhalter
 *
 * Licensed 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 net.bytebuddy;

import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
import net.bytebuddy.build.CachedReturnPlugin;
import net.bytebuddy.utility.CompoundList;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.*;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;


A cache for storing types without strongly referencing any class loader or type.


Note: In order to clean obsolete class loader references from the map, expungeStaleEntries() must be called regularly. This can happen in a different thread, in custom intervals or on every use of the cache by creating an instance of WithInlineExpunction. This cache is fully thread-safe. 

Important: The behavior of a type cache might not be as expected. A class is only eligible for garbage collection once its class
loader is eligible for garbage collection. At the same time, a garbage collector is only eligible for garbage collection once all of
its classes are eligible for garbage collection. If this cache referenced the cached type strongly, this would never be the case which
is why this cache maintains either strong or weak references. In the latter case, a type is typically retained until the last instance of
the type is not eligible for garbage collection. With soft references, the type is typically retained until the next full garbage collection
where all instances of the type are eligible for garbage collection.

Type parameters:
<T> – The type of the key that is used for identifying stored classes per class loader. Such keys must not strongly reference any
           types or class loaders without potentially corrupting the garbage eligibility of stored classes. As the storage is segmented
           by class loader, it is normally sufficient to store types by their name.
See Also:
WithInlineExpunction
SimpleKey/**
 * <p>
 * A cache for storing types without strongly referencing any class loader or type.
 * </p>
 * <p>
 * <b>Note</b>: In order to clean obsolete class loader references from the map, {@link TypeCache#expungeStaleEntries()} must be called
 * regularly. This can happen in a different thread, in custom intervals or on every use of the cache by creating an instance of
 * {@link WithInlineExpunction}. This cache is fully thread-safe.
 * </p>
 * <p>
 * <b>Important</b>: The behavior of a type cache might not be as expected. A class is only eligible for garbage collection once its class
 * loader is eligible for garbage collection. At the same time, a garbage collector is only eligible for garbage collection once all of
 * its classes are eligible for garbage collection. If this cache referenced the cached type strongly, this would never be the case which
 * is why this cache maintains either strong or weak references. In the latter case, a type is typically retained until the last instance of
 * the type is not eligible for garbage collection. With soft references, the type is typically retained until the next full garbage collection
 * where all instances of the type are eligible for garbage collection.
 * </p>
 *
 * @param <T> The type of the key that is used for identifying stored classes per class loader. Such keys must not strongly reference any
 *            types or class loaders without potentially corrupting the garbage eligibility of stored classes. As the storage is segmented
 *            by class loader, it is normally sufficient to store types by their name.
 * @see WithInlineExpunction
 * @see SimpleKey
 */
public class TypeCache<T> extends ReferenceQueue<ClassLoader> {

    
Indicates that a type was not found.
/**
     * Indicates that a type was not found.
     */
    private static final Class<?> NOT_FOUND = null;

    
The reference type to use for stored types.
/**
     * The reference type to use for stored types.
     */
    protected final Sort sort;

    
The underlying map containing cached objects.
/**
     * The underlying map containing cached objects.
     */
    protected final ConcurrentMap<StorageKey, ConcurrentMap<T, Reference<Class<?>>>> cache;

    
Creates a new type cache.
Params:
sort – The reference type to use for stored types./**
     * Creates a new type cache.
     *
     * @param sort The reference type to use for stored types.
     */
    public TypeCache(Sort sort) {
        this.sort = sort;
        cache = new ConcurrentHashMap<StorageKey, ConcurrentMap<T, Reference<Class<?>>>>();
    }

    
Finds a stored type or returns null if no type was stored. 
Params:
classLoader – The class loader for which this type is stored.
key –         The key for the type in question.
Returns:
The stored type or null if no type was stored./**
     * Finds a stored type or returns {@code null} if no type was stored.
     *
     * @param classLoader The class loader for which this type is stored.
     * @param key         The key for the type in question.
     * @return The stored type or {@code null} if no type was stored.
     */
    @SuppressFBWarnings(value = "GC_UNRELATED_TYPES", justification = "Cross-comparison is intended")
    public Class<?> find(ClassLoader classLoader, T key) {
        ConcurrentMap<T, Reference<Class<?>>> storage = cache.get(new LookupKey(classLoader));
        if (storage == null) {
            return NOT_FOUND;
        } else {
            Reference<Class<?>> reference = storage.get(key);
            if (reference == null) {
                return NOT_FOUND;
            } else {
                return reference.get();
            }
        }
    }

    
Inserts a new type into the cache. If a type with the same class loader and key was inserted previously, the cache is not updated.
Params:
classLoader – The class loader for which this type is stored.
key –         The key for the type in question.
type –        The type to insert of no previous type was stored in the cache.
Returns:
The supplied type or a previously submitted type for the same class loader and key combination./**
     * Inserts a new type into the cache. If a type with the same class loader and key was inserted previously, the cache is not updated.
     *
     * @param classLoader The class loader for which this type is stored.
     * @param key         The key for the type in question.
     * @param type        The type to insert of no previous type was stored in the cache.
     * @return The supplied type or a previously submitted type for the same class loader and key combination.
     */
    @SuppressFBWarnings(value = "GC_UNRELATED_TYPES", justification = "Cross-comparison is intended")
    public Class<?> insert(ClassLoader classLoader, T key, Class<?> type) {
        ConcurrentMap<T, Reference<Class<?>>> storage = cache.get(new LookupKey(classLoader));
        if (storage == null) {
            storage = new ConcurrentHashMap<T, Reference<Class<?>>>();
            ConcurrentMap<T, Reference<Class<?>>> previous = cache.putIfAbsent(new StorageKey(classLoader, this), storage);
            if (previous != null) {
                storage = previous;
            }
        }
        Reference<Class<?>> reference = sort.wrap(type), previous = storage.putIfAbsent(key, reference);
        while (previous != null) {
            Class<?> previousType = previous.get();
            if (previousType != null) {
                return previousType;
            } else if (storage.remove(key, previous)) {
                previous = storage.putIfAbsent(key, reference);
            } else {
                previous = storage.get(key);
                if (previous == null) {
                    previous = storage.putIfAbsent(key, reference);
                }
            }
        }
        return type;
    }

    
Finds an existing type or inserts a new one if the previous type was not found.
Params:
classLoader – The class loader for which this type is stored.
key –         The key for the type in question.
lazy –        A lazy creator for the type to insert of no previous type was stored in the cache.
Returns:
The lazily created type or a previously submitted type for the same class loader and key combination./**
     * Finds an existing type or inserts a new one if the previous type was not found.
     *
     * @param classLoader The class loader for which this type is stored.
     * @param key         The key for the type in question.
     * @param lazy        A lazy creator for the type to insert of no previous type was stored in the cache.
     * @return The lazily created type or a previously submitted type for the same class loader and key combination.
     */
    public Class<?> findOrInsert(ClassLoader classLoader, T key, Callable<Class<?>> lazy) {
        Class<?> type = find(classLoader, key);
        if (type != null) {
            return type;
        } else {
            try {
                return insert(classLoader, key, lazy.call());
            } catch (Throwable throwable) {
                throw new IllegalArgumentException("Could not create type", throwable);
            }
        }
    }

    
Finds an existing type or inserts a new one if the previous type was not found.
Params:
classLoader – The class loader for which this type is stored.
key –         The key for the type in question.
lazy –        A lazy creator for the type to insert of no previous type was stored in the cache.
monitor –     A monitor to lock before creating the lazy type.
Returns:
The lazily created type or a previously submitted type for the same class loader and key combination./**
     * Finds an existing type or inserts a new one if the previous type was not found.
     *
     * @param classLoader The class loader for which this type is stored.
     * @param key         The key for the type in question.
     * @param lazy        A lazy creator for the type to insert of no previous type was stored in the cache.
     * @param monitor     A monitor to lock before creating the lazy type.
     * @return The lazily created type or a previously submitted type for the same class loader and key combination.
     */
    public Class<?> findOrInsert(ClassLoader classLoader, T key, Callable<Class<?>> lazy, Object monitor) {
        Class<?> type = find(classLoader, key);
        if (type != null) {
            return type;
        } else {
            synchronized (monitor) {
                return findOrInsert(classLoader, key, lazy);
            }
        }
    }

    
Removes any stale class loader entries from the cache.
/**
     * Removes any stale class loader entries from the cache.
     */
    public void expungeStaleEntries() {
        Reference<?> reference;
        while ((reference = poll()) != null) {
            cache.remove(reference);
        }
    }

    
Clears the entire cache.
/**
     * Clears the entire cache.
     */
    public void clear() {
        cache.clear();
    }

    
Determines the storage format for a cached type.
/**
     * Determines the storage format for a cached type.
     */
    public enum Sort {

        
Creates a cache where cached types are wrapped by WeakReferences. /**
         * Creates a cache where cached types are wrapped by {@link WeakReference}s.
         */
        WEAK {
            @Override
            protected Reference<Class<?>> wrap(Class<?> type) {
                return new WeakReference<Class<?>>(type);
            }
        },

        
Creates a cache where cached types are wrapped by SoftReferences. /**
         * Creates a cache where cached types are wrapped by {@link SoftReference}s.
         */
        SOFT {
            @Override
            protected Reference<Class<?>> wrap(Class<?> type) {
                return new SoftReference<Class<?>>(type);
            }
        };

        
Wraps a type as a Reference. 
Params:
type – The type to wrap.
Returns:
The reference that represents the type./**
         * Wraps a type as a {@link Reference}.
         *
         * @param type The type to wrap.
         * @return The reference that represents the type.
         */
        protected abstract Reference<Class<?>> wrap(Class<?> type);
    }

    
A key used for looking up a previously inserted class loader cache.
/**
     * A key used for looking up a previously inserted class loader cache.
     */
    protected static class LookupKey {

        
The referenced class loader.
/**
         * The referenced class loader.
         */
        private final ClassLoader classLoader;

        
The class loader's identity hash code.
/**
         * The class loader's identity hash code.
         */
        private final int hashCode;

        
Creates a new lookup key.
Params:
classLoader – The represented class loader./**
         * Creates a new lookup key.
         *
         * @param classLoader The represented class loader.
         */
        protected LookupKey(ClassLoader classLoader) {
            this.classLoader = classLoader;
            hashCode = System.identityHashCode(classLoader);
        }

        @Override
        public int hashCode() {
            return hashCode;
        }

        @Override
        @SuppressFBWarnings(value = "EQ_CHECK_FOR_OPERAND_NOT_COMPATIBLE_WITH_THIS", justification = "Cross-comparison is intended")
        public boolean equals(Object other) {
            if (this == other) {
                return true;
            } else if (other instanceof LookupKey) {
                return classLoader == ((LookupKey) other).classLoader;
            } else if (other instanceof StorageKey) {
                StorageKey storageKey = (StorageKey) other;
                return hashCode == storageKey.hashCode && classLoader == storageKey.get();
            } else {
                return false;
            }
        }
    }

    
A key used for storing a class loader cache reference.
/**
     * A key used for storing a class loader cache reference.
     */
    protected static class StorageKey extends WeakReference<ClassLoader> {

        
The class loader's identity hash code.
/**
         * The class loader's identity hash code.
         */
        private final int hashCode;

        
Creates a new storage key.
Params:
classLoader –    The represented class loader.
referenceQueue – The reference queue to notify upon a garbage collection./**
         * Creates a new storage key.
         *
         * @param classLoader    The represented class loader.
         * @param referenceQueue The reference queue to notify upon a garbage collection.
         */
        protected StorageKey(ClassLoader classLoader, ReferenceQueue<? super ClassLoader> referenceQueue) {
            super(classLoader, referenceQueue);
            hashCode = System.identityHashCode(classLoader);
        }

        @Override
        public int hashCode() {
            return hashCode;
        }

        @Override
        @SuppressFBWarnings(value = "EQ_CHECK_FOR_OPERAND_NOT_COMPATIBLE_WITH_THIS", justification = "Cross-comparison is intended")
        public boolean equals(Object other) {
            if (this == other) {
                return true;
            } else if (other instanceof LookupKey) {
                LookupKey lookupKey = (LookupKey) other;
                return hashCode == lookupKey.hashCode && get() == lookupKey.classLoader;
            } else if (other instanceof StorageKey) {
                StorageKey storageKey = (StorageKey) other;
                return hashCode == storageKey.hashCode && get() == storageKey.get();
            } else {
                return false;
            }
        }
    }

    
An implementation of a TypeCache where obsolete references are cleared upon any call. 
Type parameters:
<S> – The type of the key that is used for identifying stored classes per class loader. Such keys must not strongly reference any
           types or class loaders without potentially corrupting the garbage eligibility of stored classes. As the storage is segmented
           by class loader, it is normally sufficient to store types by their name.
See Also:
TypeCache/**
     * An implementation of a {@link TypeCache} where obsolete references are cleared upon any call.
     *
     * @param <S> The type of the key that is used for identifying stored classes per class loader. Such keys must not strongly reference any
     *            types or class loaders without potentially corrupting the garbage eligibility of stored classes. As the storage is segmented
     *            by class loader, it is normally sufficient to store types by their name.
     * @see TypeCache
     */
    public static class WithInlineExpunction<S> extends TypeCache<S> {

        
Creates a new type cache with inlined expunction.
Params:
sort – The reference type to use for stored types./**
         * Creates a new type cache with inlined expunction.
         *
         * @param sort The reference type to use for stored types.
         */
        public WithInlineExpunction(Sort sort) {
            super(sort);
        }

        
{@inheritDoc}
/**
         * {@inheritDoc}
         */
        public Class<?> find(ClassLoader classLoader, S key) {
            try {
                return super.find(classLoader, key);
            } finally {
                expungeStaleEntries();
            }
        }

        
{@inheritDoc}
/**
         * {@inheritDoc}
         */
        public Class<?> insert(ClassLoader classLoader, S key, Class<?> type) {
            try {
                return super.insert(classLoader, key, type);
            } finally {
                expungeStaleEntries();
            }
        }

        
{@inheritDoc}
/**
         * {@inheritDoc}
         */
        public Class<?> findOrInsert(ClassLoader classLoader, S key, Callable<Class<?>> builder) {
            try {
                return super.findOrInsert(classLoader, key, builder);
            } finally {
                expungeStaleEntries();
            }
        }

        
{@inheritDoc}
/**
         * {@inheritDoc}
         */
        public Class<?> findOrInsert(ClassLoader classLoader, S key, Callable<Class<?>> builder, Object monitor) {
            try {
                return super.findOrInsert(classLoader, key, builder, monitor);
            } finally {
                expungeStaleEntries();
            }
        }
    }

    
A simple key based on a collection of types where no type is strongly referenced.
/**
     * A simple key based on a collection of types where no type is strongly referenced.
     */
    public static class SimpleKey {

        
The referenced types.
/**
         * The referenced types.
         */
        private final Set<String> types;

        
Creates a simple cache key..
Params:
type –           The first type to be represented by this key.
additionalType – Any additional types to be represented by this key./**
         * Creates a simple cache key..
         *
         * @param type           The first type to be represented by this key.
         * @param additionalType Any additional types to be represented by this key.
         */
        public SimpleKey(Class<?> type, Class<?>... additionalType) {
            this(type, Arrays.asList(additionalType));
        }

        
Creates a simple cache key..
Params:
type –            The first type to be represented by this key.
additionalTypes – Any additional types to be represented by this key./**
         * Creates a simple cache key..
         *
         * @param type            The first type to be represented by this key.
         * @param additionalTypes Any additional types to be represented by this key.
         */
        public SimpleKey(Class<?> type, Collection<? extends Class<?>> additionalTypes) {
            this(CompoundList.of(type, new ArrayList<Class<?>>(additionalTypes)));
        }

        
Creates a simple cache key..
Params:
types – Any types to be represented by this key./**
         * Creates a simple cache key..
         *
         * @param types Any types to be represented by this key.
         */
        public SimpleKey(Collection<? extends Class<?>> types) {
            this.types = new HashSet<String>();
            for (Class<?> type : types) {
                this.types.add(type.getName());
            }
        }

        @Override
        @CachedReturnPlugin.Enhance("hashCode")
        public int hashCode() {
            return types.hashCode();
        }

        @Override
        public boolean equals(Object other) {
            if (this == other) {
                return true;
            } else if (other == null || getClass() != other.getClass()) {
                return false;
            }
            SimpleKey simpleKey = (SimpleKey) other;
            return types.equals(simpleKey.types);
        }
    }
}