001// Copyright 2010 The Apache Software Foundation
002//
003// Licensed under the Apache License, Version 2.0 (the "License");
004// you may not use this file except in compliance with the License.
005// You may obtain a copy of the License at
006//
007// http://www.apache.org/licenses/LICENSE-2.0
008//
009// Unless required by applicable law or agreed to in writing, software
010// distributed under the License is distributed on an "AS IS" BASIS,
011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012// See the License for the specific language governing permissions and
013// limitations under the License.
014
015package org.apache.tapestry5.ioc.services;
016
017/**
018 * Provides access to per-thread (and, by extension, per-request) data, managed by the {@link PerthreadManager}.
019 * A PerThreadValue stores a particular type of information.
020 *
021 * @see org.apache.tapestry5.ioc.services.PerthreadManager#createValue()
022 * @since 5.2.0
023 */
024public interface PerThreadValue<T>
025{
026    /**
027     * Is a value stored (even null)?
028     */
029    boolean exists();
030
031    /**
032     * Reads the current per-thread value, or returns null if no value has been stored.
033     */
034    T get();
035
036    /**
037     * Gets the current per-thread value if it exists (even if null), or the defaultValue
038     * if no value has been stored.
039     */
040    T get(T defaultValue);
041
042    /**
043     * Sets the current per-thread value, then returns that value.
044     */
045    T set(T newValue);
046}