| 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.NoSuchElementException; |
| 20 | import java.util.concurrent.BlockingQueue; |
| 21 | import java.util.concurrent.LinkedBlockingQueue; |
| 22 | import java.util.concurrent.Semaphore; |
| 23 | |
| 24 | /** |
| 25 | * An implementation of the {@link ResultQueue} that throttles the number of |
| 26 | * expected results, limiting it to a maximum at any given time. |
| 27 | * |
| 28 | * @author Dave Syer |
| 29 | */ |
| 30 | public class ThrottleLimitResultQueue<T> implements ResultQueue<T> { |
| 31 | |
| 32 | // Accumulation of result objects as they finish. |
| 33 | private final BlockingQueue<T> results; |
| 34 | |
| 35 | // Accumulation of dummy objects flagging expected results in the future. |
| 36 | private final Semaphore waits; |
| 37 | |
| 38 | private final Object lock = new Object(); |
| 39 | |
| 40 | private volatile int count = 0; |
| 41 | |
| 42 | /** |
| 43 | * @param throttleLimit the maximum number of results that can be expected |
| 44 | * at any given time. |
| 45 | */ |
| 46 | public ThrottleLimitResultQueue(int throttleLimit) { |
| 47 | results = new LinkedBlockingQueue<T>(); |
| 48 | waits = new Semaphore(throttleLimit); |
| 49 | } |
| 50 | |
| 51 | public boolean isEmpty() { |
| 52 | return results.isEmpty(); |
| 53 | } |
| 54 | |
| 55 | /* |
| 56 | * (non-Javadoc) |
| 57 | * |
| 58 | * @see org.springframework.batch.repeat.support.ResultQueue#isExpecting() |
| 59 | */ |
| 60 | public boolean isExpecting() { |
| 61 | // Base the decision about whether we expect more results on a |
| 62 | // counter of the number of expected results actually collected. |
| 63 | // Do not synchronize! Otherwise put and expect can deadlock. |
| 64 | return count > 0; |
| 65 | } |
| 66 | |
| 67 | /** |
| 68 | * Tell the queue to expect one more result. Blocks until a new result is |
| 69 | * available if already expecting too many (as determined by the throttle |
| 70 | * limit). |
| 71 | * |
| 72 | * @see ResultQueue#expect() |
| 73 | */ |
| 74 | public void expect() throws InterruptedException { |
| 75 | synchronized (lock) { |
| 76 | waits.acquire(); |
| 77 | count++; |
| 78 | } |
| 79 | } |
| 80 | |
| 81 | public void put(T holder) throws IllegalArgumentException { |
| 82 | if (!isExpecting()) { |
| 83 | throw new IllegalArgumentException("Not expecting a result. Call expect() before put()."); |
| 84 | } |
| 85 | // There should be no need to block here, or to use offer() |
| 86 | results.add(holder); |
| 87 | // Take from the waits queue now to allow another result to |
| 88 | // accumulate. But don't decrement the counter. |
| 89 | waits.release(); |
| 90 | } |
| 91 | |
| 92 | public T take() throws NoSuchElementException, InterruptedException { |
| 93 | if (!isExpecting()) { |
| 94 | throw new NoSuchElementException("Not expecting a result. Call expect() before take()."); |
| 95 | } |
| 96 | T value; |
| 97 | synchronized (lock) { |
| 98 | value = results.take(); |
| 99 | // Decrement the counter only when the result is collected. |
| 100 | count--; |
| 101 | } |
| 102 | return value; |
| 103 | } |
| 104 | |
| 105 | } |