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25 
26 package sun.rmi.log;
27 
28 import java.io.*;
29 import sun.rmi.server.MarshalOutputStream;
30 import sun.rmi.server.MarshalInputStream;
31 
32 /**
33  * A LogHandler represents snapshots and update records as serializable
34  * objects.
35  *
36  * This implementation does not know how to create an initial snaphot or
37  * apply an update to a snapshot.  The client must specifiy these methods
38  * via a subclass.
39  *
40  * @see ReliableLog
41  *
42  * @author Ann Wollrath
43  */
44 public abstract
45 class LogHandler {
46 
47     /**
48      * Creates a LogHandler for a ReliableLog.
49      */
LogHandler()50     public LogHandler() {}
51 
52     /**
53      * Creates and returns the initial state of data structure that needs
54      * to be stably stored. This method is called when a ReliableLog is
55      * created.
56      * @return the initial state
57      * @exception Exception can raise any exception
58      */
59     public abstract
initialSnapshot()60     Object initialSnapshot() throws Exception;
61 
62     /**
63      * Writes the snapshot object to a stream.  This callback is
64      * invoked when the client calls the snaphot method of ReliableLog.
65      * @param out the output stream
66      * @param value the snapshot
67      * @exception Exception can raise any exception
68      */
69     public
snapshot(OutputStream out, Object value)70     void snapshot(OutputStream out, Object value) throws Exception {
71         MarshalOutputStream s = new MarshalOutputStream(out);
72         s.writeObject(value);
73         s.flush();
74     }
75 
76     /**
77      * Read the snapshot object from a stream and returns the snapshot.
78      * This callback is invoked when the client calls the recover method
79      * of ReliableLog.
80      * @param in the input stream
81      * @return the state (snapshot)
82      * @exception Exception can raise any exception
83      */
84 
85     public
recover(InputStream in)86     Object recover(InputStream in) throws Exception {
87         MarshalInputStream s = new MarshalInputStream(in);
88         return s.readObject();
89     }
90 
91     /**
92      * Writes the representation (a serializable object) of an update
93      * to a stream.  This callback is invoked when the client calls the
94      * update method of ReliableLog.
95      * @param out the output stream
96      * @param value the snapshot
97      * @exception Exception can raise any exception
98      */
99     public
writeUpdate(LogOutputStream out, Object value)100     void writeUpdate(LogOutputStream out, Object value) throws Exception {
101 
102         MarshalOutputStream s = new MarshalOutputStream(out);
103         s.writeObject(value);
104         s.flush();
105     }
106 
107     /**
108      * Reads a stably logged update (a serializable object) from a
109      * stream.  This callback is invoked during recovery, once for
110      * every record in the log.  After reading the update, this method
111      * invokes the applyUpdate (abstract) method in order to obtain
112      * the new snapshot value.  It then returns the new snapshot.
113      *
114      * @param in the input stream
115      * @param state the current state
116      * @return the new state
117      * @exception Exception can raise any exception
118      */
119     public
readUpdate(LogInputStream in, Object state)120     Object readUpdate(LogInputStream in, Object state) throws Exception {
121         MarshalInputStream  s = new MarshalInputStream(in);
122         return applyUpdate(s.readObject(), state);
123     }
124 
125     /**
126      * Reads a stably logged update (a serializable object) from a stream.
127      * This callback is invoked during recovery, once for every record in the
128      * log.  After reading the update, this method is invoked in order to
129      * obtain the new snapshot value.  The method should apply the update
130      * object to the current state <code>state</code> and return the new
131      * state (the new snapshot value).
132      * @param update the update object
133      * @param state the current state
134      * @return the new state
135      * @exception Exception can raise any exception
136      */
137     public abstract
applyUpdate(Object update, Object state)138     Object applyUpdate(Object update, Object state) throws Exception;
139 
140 }
141