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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 public abstract class SnmpTableCache implements Serializable {
49 
50     /**
51      * Interval of time in ms during which the cached table data
52      * is considered valid.
53      **/
54     protected long validity;
55 
56     /**
57      * A weak refernce holding cached table data.
58      **/
59     protected transient WeakReference<SnmpCachedData> datas;
60 
61     /**
62      * true if the given cached table data is obsolete.
63      **/
isObsolete(SnmpCachedData cached)64     protected boolean isObsolete(SnmpCachedData cached) {
65         if (cached   == null) return true;
66         if (validity < 0)     return false;
67         return ((System.currentTimeMillis() - cached.lastUpdated) > validity);
68     }
69 
70     /**
71      * Returns the cached table data.
72      * Returns null if the cached data is obsolete, or if there is no
73      * cached data, or if the cached data was garbage collected.
74      * @return a still valid cached data or null.
75      **/
getCachedDatas()76     protected SnmpCachedData getCachedDatas() {
77         if (datas == null) return null;
78         final SnmpCachedData cached = datas.get();
79         if ((cached == null) || isObsolete(cached)) return null;
80         return cached;
81     }
82 
83     /**
84      * Returns the cached table data, if it is still valid,
85      * or recompute it if it is obsolete.
86      * <p>
87      * When cache data is recomputed, store it in the weak reference,
88      * unless {@link #validity} is 0: then the data will not be stored
89      * at all.<br>
90      * This method calls {@link #isObsolete(SnmpCachedData)} to determine
91      * whether the cached data is obsolete, and {
92      * {@link #updateCachedDatas(Object)} to recompute it.
93      * </p>
94      * @param context A context object.
95      * @return the valid cached data, or the recomputed table data.
96      **/
getTableDatas(Object context)97     protected synchronized SnmpCachedData getTableDatas(Object context) {
98         final SnmpCachedData cached   = getCachedDatas();
99         if (cached != null) return cached;
100         final SnmpCachedData computedDatas = updateCachedDatas(context);
101         if (validity != 0) datas = new WeakReference<>(computedDatas);
102         return computedDatas;
103     }
104 
105     /**
106      * Recompute cached data.
107      * @param context A context object, as passed to
108      *        {@link #getTableDatas(Object)}
109      **/
updateCachedDatas(Object context)110     protected abstract SnmpCachedData updateCachedDatas(Object context);
111 
112     /**
113      * Return a table handler that holds the table data.
114      * This method should return the cached table data if it is still
115      * valid, recompute it and cache the new value if it's not.
116      **/
getTableHandler()117     public abstract SnmpTableHandler getTableHandler();
118 
119 }
120