1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one 3 * or more contributor license agreements. See the NOTICE file 4 * distributed with this work for additional information 5 * regarding copyright ownership. The ASF licenses this file 6 * to you under the Apache License, Version 2.0 (the 7 * "License"); you may not use this file except in compliance 8 * with the License. You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 */ 18 19 package org.apache.giraph.utils; 20 21 import java.util.concurrent.Semaphore; 22 23 import static com.google.common.base.Preconditions.checkState; 24 25 /** 26 * Implementation of a semaphore where number of available permits can change 27 */ 28 public final class AdjustableSemaphore extends Semaphore { 29 /** Maximum number of available permits */ 30 private int maxPermits; 31 32 /** 33 * Constructor 34 * @param permits initial number of available permits 35 */ 36 public AdjustableSemaphore(int permits) { 37 super(permits); 38 maxPermits = permits; 39 } 40 41 @edu.umd.cs.findbugs.annotations.SuppressWarnings( 42 "UG_SYNC_SET_UNSYNC_GET") 43 public int getMaxPermits() { 44 return maxPermits; 45 } 46 47 /** 48 * Adjusts the maximum number of available permits. 49 * 50 * @param newMax max number of permits 51 */ 52 public synchronized void setMaxPermits(int newMax) { 53 checkState(newMax >= 0, "setMaxPermits: number of permits cannot be " + 54 "less than 0"); 55 int delta = newMax - this.maxPermits; 56 if (delta > 0) { 57 // Releasing semaphore to make room for 'delta' more permits 58 release(delta); 59 } else if (delta < 0) { 60 // Reducing number of permits in the semaphore 61 reducePermits(-delta); 62 } 63 this.maxPermits = newMax; 64 } 65 }