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    
015    package 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     */
024    public 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    }