[6785] | 1 | /**************************************************************** |
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| 2 | * Licensed to the Apache Software Foundation (ASF) under one * |
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| 3 | * or more contributor license agreements. See the NOTICE file * |
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| 4 | * distributed with this work for additional information * |
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| 5 | * regarding copyright ownership. The ASF licenses this file * |
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| 6 | * to you under the Apache License, Version 2.0 (the * |
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| 7 | * "License"); you may not use this file except in compliance * |
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| 8 | * with the License. You may obtain a copy of the License at * |
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| 9 | * * |
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| 10 | * http://www.apache.org/licenses/LICENSE-2.0 * |
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| 11 | * * |
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| 12 | * Unless required by applicable law or agreed to in writing, * |
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| 13 | * software distributed under the License is distributed on an * |
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| 14 | * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY * |
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| 15 | * KIND, either express or implied. See the License for the * |
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| 16 | * specific language governing permissions and limitations * |
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| 17 | * under the License. * |
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| 18 | ****************************************************************/ |
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| 19 | |
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| 20 | package org.apache.james.mime4j.storage; |
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| 21 | |
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| 22 | import java.io.IOException; |
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| 23 | import java.io.InputStream; |
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| 24 | |
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| 25 | /** |
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| 26 | * <p> |
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| 27 | * A wrapper around another {@link Storage} that also maintains a reference |
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| 28 | * counter. The inner storage gets deleted only if the reference counter reaches |
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| 29 | * zero. |
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| 30 | * </p> |
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| 31 | * <p> |
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| 32 | * Reference counting is used to delete the storage when it is no longer needed. |
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| 33 | * So, any users of this class should note: |
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| 34 | * </p> |
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| 35 | * <ul> |
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| 36 | * <li>The reference count is set up one on construction. In all other cases, |
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| 37 | * {@link #addReference()} should be called when the storage is shared.</li> |
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| 38 | * <li>The caller of {@link #addReference()} should ensure that |
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| 39 | * {@link #delete()} is called once and only once.</li> |
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| 40 | * <li>Sharing the {@link Storage} instance passed into |
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| 41 | * {@link #MultiReferenceStorage(Storage)} may lead to miscounting and premature |
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| 42 | * deletion</li> |
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| 43 | * </ul> |
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| 44 | */ |
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| 45 | public class MultiReferenceStorage implements Storage { |
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| 46 | |
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| 47 | private final Storage storage; |
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| 48 | private int referenceCounter; |
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| 49 | |
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| 50 | /** |
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| 51 | * Creates a new <code>MultiReferenceStorage</code> instance for the given |
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| 52 | * back-end. The reference counter is initially set to one so the caller |
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| 53 | * does not have to call {@link #addReference()} after this constructor. |
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| 54 | * |
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| 55 | * @param storage |
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| 56 | * storage back-end that should be reference counted. |
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| 57 | * @throws IllegalArgumentException |
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| 58 | * when storage is null |
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| 59 | */ |
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| 60 | public MultiReferenceStorage(Storage storage) { |
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| 61 | if (storage == null) |
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| 62 | throw new IllegalArgumentException(); |
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| 63 | |
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| 64 | this.storage = storage; |
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| 65 | this.referenceCounter = 1; // caller holds first reference |
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| 66 | } |
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| 67 | |
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| 68 | /** |
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| 69 | * Increments the reference counter. |
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| 70 | * |
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| 71 | * @throws IllegalStateException |
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| 72 | * if the reference counter is zero which implies that the |
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| 73 | * backing storage has already been deleted. |
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| 74 | */ |
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| 75 | public void addReference() { |
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| 76 | incrementCounter(); |
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| 77 | } |
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| 78 | |
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| 79 | /** |
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| 80 | * Decrements the reference counter and deletes the inner |
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| 81 | * <code>Storage</code> object if the reference counter reaches zero. |
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| 82 | * <p> |
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| 83 | * A client that holds a reference to this object must make sure not to |
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| 84 | * invoke this method a second time. |
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| 85 | * |
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| 86 | * @throws IllegalStateException |
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| 87 | * if the reference counter is zero which implies that the |
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| 88 | * backing storage has already been deleted. |
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| 89 | */ |
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| 90 | public void delete() { |
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| 91 | if (decrementCounter()) { |
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| 92 | storage.delete(); |
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| 93 | } |
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| 94 | } |
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| 95 | |
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| 96 | /** |
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| 97 | * Returns the input stream of the inner <code>Storage</code> object. |
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| 98 | * |
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| 99 | * @return an input stream. |
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| 100 | */ |
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| 101 | public InputStream getInputStream() throws IOException { |
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| 102 | return storage.getInputStream(); |
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| 103 | } |
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| 104 | |
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| 105 | /** |
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| 106 | * Synchronized increment of reference count. |
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| 107 | * |
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| 108 | * @throws IllegalStateException |
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| 109 | * when counter is already zero |
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| 110 | */ |
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| 111 | private synchronized void incrementCounter() { |
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| 112 | if (referenceCounter == 0) |
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| 113 | throw new IllegalStateException("storage has been deleted"); |
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| 114 | |
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| 115 | referenceCounter++; |
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| 116 | } |
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| 117 | |
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| 118 | /** |
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| 119 | * Synchronized decrement of reference count. |
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| 120 | * |
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| 121 | * @return true when counter has reached zero, false otherwise |
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| 122 | * @throws IllegalStateException |
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| 123 | * when counter is already zero |
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| 124 | */ |
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| 125 | private synchronized boolean decrementCounter() { |
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| 126 | if (referenceCounter == 0) |
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| 127 | throw new IllegalStateException("storage has been deleted"); |
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| 128 | |
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| 129 | return --referenceCounter == 0; |
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| 130 | } |
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| 131 | } |
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