| 1 | /* |
| 2 | * Copyright 2002-2007 the original author or authors. |
| 3 | * |
| 4 | * Licensed under the Apache License, Version 2.0 (the "License"); |
| 5 | * you may not use this file except in compliance with the License. |
| 6 | * You may obtain a copy of the License at |
| 7 | * |
| 8 | * http://www.apache.org/licenses/LICENSE-2.0 |
| 9 | * |
| 10 | * Unless required by applicable law or agreed to in writing, software |
| 11 | * distributed under the License is distributed on an "AS IS" BASIS, |
| 12 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 13 | * See the License for the specific language governing permissions and |
| 14 | * limitations under the License. |
| 15 | */ |
| 16 | |
| 17 | package org.springframework.batch.repeat.support; |
| 18 | |
| 19 | import java.util.Comparator; |
| 20 | import java.util.NoSuchElementException; |
| 21 | import java.util.concurrent.BlockingQueue; |
| 22 | import java.util.concurrent.PriorityBlockingQueue; |
| 23 | import java.util.concurrent.Semaphore; |
| 24 | |
| 25 | import org.springframework.batch.repeat.RepeatStatus; |
| 26 | |
| 27 | /** |
| 28 | * An implementation of the {@link ResultQueue} that throttles the number of |
| 29 | * expected results, limiting it to a maximum at any given time. |
| 30 | * |
| 31 | * @author Dave Syer |
| 32 | */ |
| 33 | public class ResultHolderResultQueue implements ResultQueue<ResultHolder> { |
| 34 | |
| 35 | // Accumulation of result objects as they finish. |
| 36 | private final BlockingQueue<ResultHolder> results; |
| 37 | |
| 38 | // Accumulation of dummy objects flagging expected results in the future. |
| 39 | private final Semaphore waits; |
| 40 | |
| 41 | private final Object lock = new Object(); |
| 42 | |
| 43 | private volatile int count = 0; |
| 44 | |
| 45 | /** |
| 46 | * @param throttleLimit the maximum number of results that can be expected |
| 47 | * at any given time. |
| 48 | */ |
| 49 | public ResultHolderResultQueue(int throttleLimit) { |
| 50 | results = new PriorityBlockingQueue<ResultHolder>(throttleLimit, new ResultHolderComparator()); |
| 51 | waits = new Semaphore(throttleLimit); |
| 52 | } |
| 53 | |
| 54 | public boolean isEmpty() { |
| 55 | return results.isEmpty(); |
| 56 | } |
| 57 | |
| 58 | /* |
| 59 | * (non-Javadoc) |
| 60 | * |
| 61 | * @see org.springframework.batch.repeat.support.ResultQueue#isExpecting() |
| 62 | */ |
| 63 | public boolean isExpecting() { |
| 64 | // Base the decision about whether we expect more results on a |
| 65 | // counter of the number of expected results actually collected. |
| 66 | // Do not synchronize! Otherwise put and expect can deadlock. |
| 67 | return count > 0; |
| 68 | } |
| 69 | |
| 70 | /** |
| 71 | * Tell the queue to expect one more result. Blocks until a new result is |
| 72 | * available if already expecting too many (as determined by the throttle |
| 73 | * limit). |
| 74 | * |
| 75 | * @see ResultQueue#expect() |
| 76 | */ |
| 77 | public void expect() throws InterruptedException { |
| 78 | waits.acquire(); |
| 79 | // Don't acquire the lock in a synchronized block - might deadlock |
| 80 | synchronized (lock) { |
| 81 | count++; |
| 82 | } |
| 83 | } |
| 84 | |
| 85 | public void put(ResultHolder holder) throws IllegalArgumentException { |
| 86 | if (!isExpecting()) { |
| 87 | throw new IllegalArgumentException("Not expecting a result. Call expect() before put()."); |
| 88 | } |
| 89 | results.add(holder); |
| 90 | // Take from the waits queue now to allow another result to |
| 91 | // accumulate. But don't decrement the counter. |
| 92 | waits.release(); |
| 93 | synchronized (lock) { |
| 94 | lock.notifyAll(); |
| 95 | } |
| 96 | } |
| 97 | |
| 98 | /** |
| 99 | * Get the next result as soon as it becomes available. <br/> |
| 100 | * <br/> |
| 101 | * Release result immediately if: |
| 102 | * <ul> |
| 103 | * <li>There is a result that is continuable.</li> |
| 104 | * </ul> |
| 105 | * Otherwise block if either: |
| 106 | * <ul> |
| 107 | * <li>There is no result (as per contract of {@link ResultQueue}).</li> |
| 108 | * <li>The number of results is less than the number expected.</li> |
| 109 | * </ul> |
| 110 | * Error if either: |
| 111 | * <ul> |
| 112 | * <li>Not expecting.</li> |
| 113 | * <li>Interrupted.</li> |
| 114 | * </ul> |
| 115 | * |
| 116 | * @see ResultQueue#take() |
| 117 | */ |
| 118 | public ResultHolder take() throws NoSuchElementException, InterruptedException { |
| 119 | if (!isExpecting()) { |
| 120 | throw new NoSuchElementException("Not expecting a result. Call expect() before take()."); |
| 121 | } |
| 122 | ResultHolder value; |
| 123 | synchronized (lock) { |
| 124 | value = results.take(); |
| 125 | if (isContinuable(value)) { |
| 126 | // Decrement the counter only when the result is collected. |
| 127 | count--; |
| 128 | return value; |
| 129 | } |
| 130 | } |
| 131 | results.put(value); |
| 132 | synchronized (lock) { |
| 133 | while (count > results.size()) { |
| 134 | lock.wait(); |
| 135 | } |
| 136 | value = results.take(); |
| 137 | count--; |
| 138 | } |
| 139 | return value; |
| 140 | } |
| 141 | |
| 142 | private boolean isContinuable(ResultHolder value) { |
| 143 | return value.getResult() != null && value.getResult().isContinuable(); |
| 144 | } |
| 145 | |
| 146 | /** |
| 147 | * Compares ResultHolders so that one that is continuable ranks lowest. |
| 148 | * |
| 149 | * @author Dave Syer |
| 150 | * |
| 151 | */ |
| 152 | private static class ResultHolderComparator implements Comparator<ResultHolder> { |
| 153 | public int compare(ResultHolder h1, ResultHolder h2) { |
| 154 | RepeatStatus result1 = h1.getResult(); |
| 155 | RepeatStatus result2 = h2.getResult(); |
| 156 | if (result1 == null && result2 == null) { |
| 157 | return 0; |
| 158 | } |
| 159 | if (result1 == null) { |
| 160 | return -1; |
| 161 | } |
| 162 | else if (result2 == null) { |
| 163 | return 1; |
| 164 | } |
| 165 | if ((result1.isContinuable() && result2.isContinuable()) |
| 166 | || (!result1.isContinuable() && !result2.isContinuable())) { |
| 167 | return 0; |
| 168 | } |
| 169 | if (result1.isContinuable()) { |
| 170 | return -1; |
| 171 | } |
| 172 | return 1; |
| 173 | } |
| 174 | } |
| 175 | |
| 176 | } |