NIOLockFile


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30  
31  
32  package org.hsqldb.persist;
33  
34  import java.nio.channels.FileChannel  ;
35  import java.nio.channels.FileLock  ;
36  
37  /**
38   * A LockFile variant that capitalizes upon the
39   * availability of {@link java.nio.channels.FileLock FileLock}.
40   *
41   * @author boucherb@users
42   * @version 1.7.2
43   * @since 1.7.2
44   *
45   */
46  final class NIOLockFile extends LockFile {
47  
48      // From the java.nio.channels.FileLock API docs:
49      //
50      // Some network filesystems do not implement file locks on regions
51      // that extend past a certain position, often 2**30 or 2**31.
52      // In general, great care should be taken when locking files that
53      // reside on network filesystems.
54      static final long MAX_NFS_LOCK_REGION = (1L << 30);
55      static final long MIN_LOCK_REGION     = MAGIC.length + 8;
56  
57      /**
58       * A <code>FileChannel</code> object obtained from the super
59       * <code>raf</code> attribute. <p>
60       *
61       * The <code>fc</code> attribute is used to obtain this object's
62       * {@link #fl FileLock} attribute.
63       */
64      private FileChannel   fc;
65  
66      /**
67       * The <code>FileLock</code> object used to lock this object's
68       * lock file.
69       */
70      private FileLock   fl;
71  
72      /**
73       * Tries to obtain a valid NIO lock upon this object's lock file using
74       * this object's {@link #fl FileLock} attribute.
75       *
76       * @return true if a valid lock is obtained, else false.
77       * @throws Exception if an error occurs while attempting to obtain the lock
78       *
79       */
80      protected boolean lockImpl() throws Exception   {
81  
82          boolean isValid;
83  
84          if (fl != null && fl.isValid()) {
85              return true;
86          }
87  
88          trace("lockImpl(): fc = raf.getChannel()");
89  
90          fc = raf.getChannel();
91  
92          trace("lockImpl(): fl = fc.tryLock()");
93  
94          fl = null;
95  
96          try {
97              fl = fc.tryLock(0, MIN_LOCK_REGION, false);
98  
99              trace("lockImpl(): fl = " + fl);
100         } catch (Exception   e) {
101 
102             // This will not work with a localized JVM
103             /*
104             if (-1 == e.toString().indexOf("No locks available")) {
105                 throw e;
106             } else {
107                 trace(e.toString());
108             }
109             */
110             trace(e.toString());
111         }
112 
113 // In an ideal world, maybe?:
114 //        try {
115 //            fl = fc.tryLock();
116 //
117 //            trace("lockImpl(): fl = " + fl);
118 //        } catch (Exception e) {
119 //            trace(e.toString());
120 //
121 //            try {
122 //               fl = fc.tryLock(0, MAX_NFS_LOCK_REGION, false);
123 //
124 //               trace("lockImpl(): fl = " + fl);
125 //               trace("Warning: possibly attempting to lock on NFS");
126 //            } catch (Exception e2) {
127 //                trace(e2.toString());
128 //
129 //                try {
130 //                    fl = fc.tryLock(0, MIN_LOCK_REGION, false);
131 //
132 //                    trace("lockImpl(): fl = " + fl);
133 //                    trace("Warning: backed off to min lock region");
134 //                    trace("Warning: lock file may be unusable on reuse");
135 //                } catch (Exception e3) {
136 //                      trace(e3.toString());
137 //                }
138 //            }
139 //        }
140         trace("lockImpl(): f.deleteOnExit()");
141         f.deleteOnExit();
142 
143         isValid = fl != null && fl.isValid();
144 
145         trace("lockImpl():isValid(): " + isValid);
146 
147         return isValid;
148     }
149 
150     /**
151      * Tries to release any valid lock held upon this object's lock file using
152      * this object's {@link #fl FileLock} attribute.
153      *
154      * @return true if a valid lock is released, else false
155      * @throws Exception if na error occurs while attempting to release the lock
156      */
157     protected boolean releaseImpl() throws Exception   {
158 
159         // PRE: we know that this method is only called
160         // if isLocked() is true.
161         trace("releaseImpl(): fl = " + fl);
162 
163         if (fl != null) {
164             trace("releaseImpl(): fl.release()");
165             fl.release();
166             trace("tryRelease(): fl = " + fl);
167 
168             fl = null;
169         }
170 
171         trace("releaseImpl(): fc = " + fc);
172 
173         if (fc != null) {
174             trace("releaseImpl(): fc.close()");
175             fc.close();
176 
177             fc = null;
178         }
179 
180         // CHECKME:
181         // possibly overcomes some regarding full and
182         // true release of FileLock and maybe related
183         // NIO resources?
184         // System.gc();
185         return true;
186     }
187 
188     /**
189      * Retrieves whether this object's {@link #fl FileLock} attribute represents
190      * a valid lock upon this object's lock file.
191      *
192      * @return true if this object's {@link #fl FileLock} attribute is valid,
193      *      else false
194      */
195     public boolean isValid() {
196         return super.isValid() && (fl != null && fl.isValid());
197     }
198 
199     /**
200      * Retrieves the String value: "fl =" + fl
201      * @return the String value: "fl =" + fl
202      */
203     protected String   toStringImpl() {
204         return "fl =" + fl;
205     }
206 }
207
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