1 /* 2 * Copyright (c) 2003, 2014, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 package sun.management.snmp.util; 26 27 import com.sun.jmx.snmp.SnmpOid; 28 29 import java.io.Serializable; 30 31 import java.util.Comparator; 32 import java.util.Arrays; 33 import java.util.TreeMap; 34 import java.util.List; 35 import java.util.Iterator; 36 37 import java.lang.ref.WeakReference; 38 39 /** 40 * This abstract class implements a weak cache that holds table data. 41 * <p>The table data is stored in an instance of 42 * {@link SnmpCachedData}, which is kept in a {@link WeakReference}. 43 * If the WeakReference is null or empty, the cached data is recomputed.</p> 44 * 45 * <p><b>NOTE: This class is not synchronized, subclasses must implement 46 * the appropriate synchronization when needed.</b></p> 47 **/ 48 @SuppressWarnings("serial") // JDK implementation class 49 public abstract class SnmpTableCache implements Serializable { 50 51 /** 52 * Interval of time in ms during which the cached table data 53 * is considered valid. 54 **/ 55 protected long validity; 56 57 /** 58 * A weak refernce holding cached table data. 59 **/ 60 protected transient WeakReference<SnmpCachedData> datas; 61 62 /** 63 * true if the given cached table data is obsolete. 64 **/ 65 protected boolean isObsolete(SnmpCachedData cached) { 66 if (cached == null) return true; 67 if (validity < 0) return false; 68 return ((System.currentTimeMillis() - cached.lastUpdated) > validity); 69 } 70 71 /** 72 * Returns the cached table data. 73 * Returns null if the cached data is obsolete, or if there is no 74 * cached data, or if the cached data was garbage collected. 75 * @return a still valid cached data or null. 76 **/ 77 protected SnmpCachedData getCachedDatas() { 78 if (datas == null) return null; 79 final SnmpCachedData cached = datas.get(); 80 if ((cached == null) || isObsolete(cached)) return null; 81 return cached; 82 } 83 84 /** 85 * Returns the cached table data, if it is still valid, 86 * or recompute it if it is obsolete. 87 * <p> 88 * When cache data is recomputed, store it in the weak reference, 89 * unless {@link #validity} is 0: then the data will not be stored 90 * at all.<br> 91 * This method calls {@link #isObsolete(SnmpCachedData)} to determine 92 * whether the cached data is obsolete, and { 93 * {@link #updateCachedDatas(Object)} to recompute it. 94 * </p> 95 * @param context A context object. 96 * @return the valid cached data, or the recomputed table data. 97 **/ 98 protected synchronized SnmpCachedData getTableDatas(Object context) { 99 final SnmpCachedData cached = getCachedDatas(); 100 if (cached != null) return cached; 101 final SnmpCachedData computedDatas = updateCachedDatas(context); 102 if (validity != 0) datas = new WeakReference<>(computedDatas); 103 return computedDatas; 104 } 105 106 /** 107 * Recompute cached data. 108 * @param context A context object, as passed to 109 * {@link #getTableDatas(Object)} 110 **/ 111 protected abstract SnmpCachedData updateCachedDatas(Object context); 112 113 /** 114 * Return a table handler that holds the table data. 115 * This method should return the cached table data if it is still 116 * valid, recompute it and cache the new value if it's not. 117 **/ 118 public abstract SnmpTableHandler getTableHandler(); 119 120 }