1 /*
2  Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
3 
4  This program is free software; you can redistribute it and/or modify
5  it under the terms of the GNU General Public License, version 2.0,
6  as published by the Free Software Foundation.
7 
8  This program is also distributed with certain software (including
9  but not limited to OpenSSL) that is licensed under separate terms,
10  as designated in a particular file or component or in included license
11  documentation.  The authors of MySQL hereby grant you an additional
12  permission to link the program and your derivative works with the
13  separately licensed software that they have included with MySQL.
14 
15  This program is distributed in the hope that it will be useful,
16  but WITHOUT ANY WARRANTY; without even the implied warranty of
17  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
18  GNU General Public License, version 2.0, for more details.
19 
20  You should have received a copy of the GNU General Public License
21  along with this program; if not, write to the Free Software
22  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA
23 */
24 /*
25  * Wrapper.java
26  */
27 
28 package com.mysql.jtie;
29 
30 /**
31  * A base class for Java peer classes representing a C/C++ object type.
32  *
33  * This class declares only non-public members, which enable the JTie
34  * runtime to map a Java </code>Wrapper</code> instance to a C/C++ object.
35  *
36  * A Java peer class extending </code>Wrapper</code>
37  * <ol>
38  * <li> must provide a no-arg constructor (may be of any access modifier,
39  *      but preferred private),
40  * <li> will declare a native method for each C/C++ (member and non-member)
41  *      function to be mapped to Java, and
42  * <li> ought not to have any non-static fields (i.e., state by its own).
43  * </ol>
44  *
45  * Please, note that JTie provides no guarantees on the association between
46  * </code>Wrapper</code> instances and their underlying C/C++ objects.
47  * In particular,
48  * <ol>
49  * <li> multiple </code>Wrapper</code> instances may exist for the same
50  *      C/C++ object at any time and
51  * <li> a C/C++ object may have been deleted by the application while
52  *      corresponding Java </code>Wrapper</code> instances still exist
53  *      (i.e., referential integrity cannot be assumed).
54  * </ol>
55  */
56 public class Wrapper {
57 
58     /*
59      * A native function called during static initialization of this class
60      * for preloaded caching of field/method Ids.
61      *
62      * Caution, this leads easily to circular dependencies: If the underlying
63      * native library has not been loaded when this function is called, the
64      * result is in a non-descriptive UnsatisfiedLinkError.
65      *
66      * For this, and since preloading cannot be expected to provide a large
67      * performance gain, this feature is not implemented at this time.
68      *
69      * static private native void initIds();
70      * static {
71      *     initIds();
72      * }
73      */
74 
75     /**
76      * The address of the native delegate object.
77      */
78     // comments: this field
79     // - is allowed to be private, for access from JNI is always possible;
80     // - is private for better security and consistency assurances;
81     // - must not be final if it is to be nullified when deleting the
82     //   native delegate object through this instance from JNI;
83     // - is not required to be final under the hashcode() and equals()
84     //   consistency requirement;
85     // - XXX consider (and benchmark) declaring this field volatile
86     //   - a write then happens-before every subsequent read of that field
87     //   - writes and reads of volatile longs and doubles are always atomic
88     private volatile long cdelegate;
89 
90     /**
91      * Creates an unattached wrapper instance.
92      */
93     // comments:
94     // - field access from JNI is fast, hence no initialization of this
95     //   cdelegate field in a c'tor;
Wrapper()96     protected Wrapper() {
97         //System.out.println("<-> jtie.Wrapper()");
98     };
99 
100     /**
101      * Copies a wrapper instance.
102      */
Wrapper(Wrapper o)103     protected Wrapper(Wrapper o) {
104         //System.out.println("<-> jtie.Wrapper(Wrapper)");
105         cdelegate = o.cdelegate;
106     };
107 
108     /**
109      * Indicates whether some other object is a wrapper that refers to the
110      * same native delegate instance.
111      * @see Object#equals(Object)
112      */
113     // all equals() requirements are met due to pure delegation semantics
equals(Object obj)114     public final boolean equals(Object obj) {
115         if (this == obj)
116             return true;
117         if (!(obj instanceof Wrapper))
118             return false;
119         final Wrapper wo = (Wrapper)obj;
120         return (cdelegate == wo.cdelegate);
121     }
122 
123     /**
124      * Returns a hash code value for the object.
125      * @see Object#hashCode()
126      */
127     // all hashCode() requirements are met due to pure delegation semantics
hashCode()128     public final int hashCode() {
129         return (int)cdelegate; // precision loss ok, for only a hash
130     }
131 
132     /**
133      * Returns a string representation of the object.
134      * @see Object#toString()
135      */
136     // overrides inherited toString() for full precision of cdelegate
toString()137     public String toString() {
138         return (getClass().getName() + '@' + Long.toHexString(cdelegate));
139     }
140 }
141