001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements.  See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache License, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License.  You may obtain a copy of the License at
008     *
009     *     http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the License for the specific language governing permissions and
015     * limitations under the License.
016     */
017    package org.apache.commons.chain;
018    
019    
020    /**
021     * <p>A {@link Filter} is a specialized {@link Command} that also expects
022     * the {@link Chain} that is executing it to call the
023     * <code>postprocess()</code> method if it called the <code>execute()</code>
024     * method.  This promise must be fulfilled in spite of any possible
025     * exceptions thrown by the <code>execute()</code> method of this
026     * {@link Command}, or any subsequent {@link Command} whose
027     * <code>execute()</code> method was called.  The owning {@link Chain}
028     * must call the <code>postprocess()</code> method of each {@link Filter}
029     * in a {@link Chain} in reverse order of the invocation of their
030     * <code>execute()</code> methods.</p>
031     *
032     * <p>The most common use case for a {@link Filter}, as opposed to a
033     * {@link Command}, is where potentially expensive resources must be acquired
034     * and held until the processing of a particular request has been completed,
035     * even if execution is delegated to a subsequent {@link Command} via the
036     * <code>execute()</code> returning <code>false</code>.  A {@link Filter}
037     * can reliably release such resources in the <code>postprocess()</code>
038     * method, which is guaranteed to be called by the owning {@link Chain}.</p>
039     *
040     * @author Craig R. McClanahan
041     * @version $Revision: 480477 $ $Date: 2006-11-29 08:34:52 +0000 (Wed, 29 Nov 2006) $
042     */
043    
044    public interface Filter extends Command {
045    
046    
047        /**
048         * <p>Execute any cleanup activities, such as releasing resources that
049         * were acquired during the <code>execute()</code> method of this
050         * {@link Filter} instance.</p>
051         *
052         * @param context The {@link Context} to be processed by this
053         *  {@link Filter}
054         * @param exception The <code>Exception</code> (if any) that was thrown
055         *  by the last {@link Command} that was executed; otherwise
056         *  <code>null</code>
057         *
058         * @exception IllegalArgumentException if <code>context</code>
059         *  is <code>null</code>
060         *
061         * @return If a non-null <code>exception</code> was "handled" by this
062         *  method (and therefore need not be rethrown), return <code>true</code>;
063         *  otherwise return <code>false</code>
064         */
065       boolean postprocess(Context context, Exception exception);
066    
067    
068    }