View Javadoc
1   /*
2    * Copyright (c) 2000, 2013, 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  
26  package java.nio.charset;
27  
28  import java.nio.ByteBuffer;
29  import java.nio.CharBuffer;
30  import java.nio.charset.spi.CharsetProvider;
31  import java.security.AccessController;
32  import java.security.PrivilegedAction;
33  import java.util.Collections;
34  import java.util.HashSet;
35  import java.util.Iterator;
36  import java.util.Locale;
37  import java.util.Map;
38  import java.util.NoSuchElementException;
39  import java.util.Set;
40  import java.util.ServiceLoader;
41  import java.util.ServiceConfigurationError;
42  import java.util.SortedMap;
43  import java.util.TreeMap;
44  import sun.misc.ASCIICaseInsensitiveComparator;
45  import sun.nio.cs.StandardCharsets;
46  import sun.nio.cs.ThreadLocalCoders;
47  import sun.security.action.GetPropertyAction;
48  
49  
50  /**
51   * A named mapping between sequences of sixteen-bit Unicode <a
52   * href="../../lang/Character.html#unicode">code units</a> and sequences of
53   * bytes.  This class defines methods for creating decoders and encoders and
54   * for retrieving the various names associated with a charset.  Instances of
55   * this class are immutable.
56   *
57   * <p> This class also defines static methods for testing whether a particular
58   * charset is supported, for locating charset instances by name, and for
59   * constructing a map that contains every charset for which support is
60   * available in the current Java virtual machine.  Support for new charsets can
61   * be added via the service-provider interface defined in the {@link
62   * java.nio.charset.spi.CharsetProvider} class.
63   *
64   * <p> All of the methods defined in this class are safe for use by multiple
65   * concurrent threads.
66   *
67   *
68   * <a name="names"></a><a name="charenc"></a>
69   * <h2>Charset names</h2>
70   *
71   * <p> Charsets are named by strings composed of the following characters:
72   *
73   * <ul>
74   *
75   *   <li> The uppercase letters <tt>'A'</tt> through <tt>'Z'</tt>
76   *        (<tt>'&#92;u0041'</tt>&nbsp;through&nbsp;<tt>'&#92;u005a'</tt>),
77   *
78   *   <li> The lowercase letters <tt>'a'</tt> through <tt>'z'</tt>
79   *        (<tt>'&#92;u0061'</tt>&nbsp;through&nbsp;<tt>'&#92;u007a'</tt>),
80   *
81   *   <li> The digits <tt>'0'</tt> through <tt>'9'</tt>
82   *        (<tt>'&#92;u0030'</tt>&nbsp;through&nbsp;<tt>'&#92;u0039'</tt>),
83   *
84   *   <li> The dash character <tt>'-'</tt>
85   *        (<tt>'&#92;u002d'</tt>,&nbsp;<small>HYPHEN-MINUS</small>),
86   *
87   *   <li> The plus character <tt>'+'</tt>
88   *        (<tt>'&#92;u002b'</tt>,&nbsp;<small>PLUS SIGN</small>),
89   *
90   *   <li> The period character <tt>'.'</tt>
91   *        (<tt>'&#92;u002e'</tt>,&nbsp;<small>FULL STOP</small>),
92   *
93   *   <li> The colon character <tt>':'</tt>
94   *        (<tt>'&#92;u003a'</tt>,&nbsp;<small>COLON</small>), and
95   *
96   *   <li> The underscore character <tt>'_'</tt>
97   *        (<tt>'&#92;u005f'</tt>,&nbsp;<small>LOW&nbsp;LINE</small>).
98   *
99   * </ul>
100  *
101  * A charset name must begin with either a letter or a digit.  The empty string
102  * is not a legal charset name.  Charset names are not case-sensitive; that is,
103  * case is always ignored when comparing charset names.  Charset names
104  * generally follow the conventions documented in <a
105  * href="http://www.ietf.org/rfc/rfc2278.txt"><i>RFC&nbsp;2278:&nbsp;IANA Charset
106  * Registration Procedures</i></a>.
107  *
108  * <p> Every charset has a <i>canonical name</i> and may also have one or more
109  * <i>aliases</i>.  The canonical name is returned by the {@link #name() name} method
110  * of this class.  Canonical names are, by convention, usually in upper case.
111  * The aliases of a charset are returned by the {@link #aliases() aliases}
112  * method.
113  *
114  * <p><a name="hn">Some charsets have an <i>historical name</i> that is defined for
115  * compatibility with previous versions of the Java platform.</a>  A charset's
116  * historical name is either its canonical name or one of its aliases.  The
117  * historical name is returned by the <tt>getEncoding()</tt> methods of the
118  * {@link java.io.InputStreamReader#getEncoding InputStreamReader} and {@link
119  * java.io.OutputStreamWriter#getEncoding OutputStreamWriter} classes.
120  *
121  * <p><a name="iana"> </a>If a charset listed in the <a
122  * href="http://www.iana.org/assignments/character-sets"><i>IANA Charset
123  * Registry</i></a> is supported by an implementation of the Java platform then
124  * its canonical name must be the name listed in the registry. Many charsets
125  * are given more than one name in the registry, in which case the registry
126  * identifies one of the names as <i>MIME-preferred</i>.  If a charset has more
127  * than one registry name then its canonical name must be the MIME-preferred
128  * name and the other names in the registry must be valid aliases.  If a
129  * supported charset is not listed in the IANA registry then its canonical name
130  * must begin with one of the strings <tt>"X-"</tt> or <tt>"x-"</tt>.
131  *
132  * <p> The IANA charset registry does change over time, and so the canonical
133  * name and the aliases of a particular charset may also change over time.  To
134  * ensure compatibility it is recommended that no alias ever be removed from a
135  * charset, and that if the canonical name of a charset is changed then its
136  * previous canonical name be made into an alias.
137  *
138  *
139  * <h2>Standard charsets</h2>
140  *
141  *
142  *
143  * <p><a name="standard">Every implementation of the Java platform is required to support the
144  * following standard charsets.</a>  Consult the release documentation for your
145  * implementation to see if any other charsets are supported.  The behavior
146  * of such optional charsets may differ between implementations.
147  *
148  * <blockquote><table width="80%" summary="Description of standard charsets">
149  * <tr><th align="left">Charset</th><th align="left">Description</th></tr>
150  * <tr><td valign=top><tt>US-ASCII</tt></td>
151  *     <td>Seven-bit ASCII, a.k.a. <tt>ISO646-US</tt>,
152  *         a.k.a. the Basic Latin block of the Unicode character set</td></tr>
153  * <tr><td valign=top><tt>ISO-8859-1&nbsp;&nbsp;</tt></td>
154  *     <td>ISO Latin Alphabet No. 1, a.k.a. <tt>ISO-LATIN-1</tt></td></tr>
155  * <tr><td valign=top><tt>UTF-8</tt></td>
156  *     <td>Eight-bit UCS Transformation Format</td></tr>
157  * <tr><td valign=top><tt>UTF-16BE</tt></td>
158  *     <td>Sixteen-bit UCS Transformation Format,
159  *         big-endian byte&nbsp;order</td></tr>
160  * <tr><td valign=top><tt>UTF-16LE</tt></td>
161  *     <td>Sixteen-bit UCS Transformation Format,
162  *         little-endian byte&nbsp;order</td></tr>
163  * <tr><td valign=top><tt>UTF-16</tt></td>
164  *     <td>Sixteen-bit UCS Transformation Format,
165  *         byte&nbsp;order identified by an optional byte-order mark</td></tr>
166  * </table></blockquote>
167  *
168  * <p> The <tt>UTF-8</tt> charset is specified by <a
169  * href="http://www.ietf.org/rfc/rfc2279.txt"><i>RFC&nbsp;2279</i></a>; the
170  * transformation format upon which it is based is specified in
171  * Amendment&nbsp;2 of ISO&nbsp;10646-1 and is also described in the <a
172  * href="http://www.unicode.org/unicode/standard/standard.html"><i>Unicode
173  * Standard</i></a>.
174  *
175  * <p> The <tt>UTF-16</tt> charsets are specified by <a
176  * href="http://www.ietf.org/rfc/rfc2781.txt"><i>RFC&nbsp;2781</i></a>; the
177  * transformation formats upon which they are based are specified in
178  * Amendment&nbsp;1 of ISO&nbsp;10646-1 and are also described in the <a
179  * href="http://www.unicode.org/unicode/standard/standard.html"><i>Unicode
180  * Standard</i></a>.
181  *
182  * <p> The <tt>UTF-16</tt> charsets use sixteen-bit quantities and are
183  * therefore sensitive to byte order.  In these encodings the byte order of a
184  * stream may be indicated by an initial <i>byte-order mark</i> represented by
185  * the Unicode character <tt>'&#92;uFEFF'</tt>.  Byte-order marks are handled
186  * as follows:
187  *
188  * <ul>
189  *
190  *   <li><p> When decoding, the <tt>UTF-16BE</tt> and <tt>UTF-16LE</tt>
191  *   charsets interpret the initial byte-order marks as a <small>ZERO-WIDTH
192  *   NON-BREAKING SPACE</small>; when encoding, they do not write
193  *   byte-order marks. </p></li>
194 
195  *
196  *   <li><p> When decoding, the <tt>UTF-16</tt> charset interprets the
197  *   byte-order mark at the beginning of the input stream to indicate the
198  *   byte-order of the stream but defaults to big-endian if there is no
199  *   byte-order mark; when encoding, it uses big-endian byte order and writes
200  *   a big-endian byte-order mark. </p></li>
201  *
202  * </ul>
203  *
204  * In any case, byte order marks occurring after the first element of an
205  * input sequence are not omitted since the same code is used to represent
206  * <small>ZERO-WIDTH NON-BREAKING SPACE</small>.
207  *
208  * <p> Every instance of the Java virtual machine has a default charset, which
209  * may or may not be one of the standard charsets.  The default charset is
210  * determined during virtual-machine startup and typically depends upon the
211  * locale and charset being used by the underlying operating system. </p>
212  *
213  * <p>The {@link StandardCharsets} class defines constants for each of the
214  * standard charsets.
215  *
216  * <h2>Terminology</h2>
217  *
218  * <p> The name of this class is taken from the terms used in
219  * <a href="http://www.ietf.org/rfc/rfc2278.txt"><i>RFC&nbsp;2278</i></a>.
220  * In that document a <i>charset</i> is defined as the combination of
221  * one or more coded character sets and a character-encoding scheme.
222  * (This definition is confusing; some other software systems define
223  * <i>charset</i> as a synonym for <i>coded character set</i>.)
224  *
225  * <p> A <i>coded character set</i> is a mapping between a set of abstract
226  * characters and a set of integers.  US-ASCII, ISO&nbsp;8859-1,
227  * JIS&nbsp;X&nbsp;0201, and Unicode are examples of coded character sets.
228  *
229  * <p> Some standards have defined a <i>character set</i> to be simply a
230  * set of abstract characters without an associated assigned numbering.
231  * An alphabet is an example of such a character set.  However, the subtle
232  * distinction between <i>character set</i> and <i>coded character set</i>
233  * is rarely used in practice; the former has become a short form for the
234  * latter, including in the Java API specification.
235  *
236  * <p> A <i>character-encoding scheme</i> is a mapping between one or more
237  * coded character sets and a set of octet (eight-bit byte) sequences.
238  * UTF-8, UTF-16, ISO&nbsp;2022, and EUC are examples of
239  * character-encoding schemes.  Encoding schemes are often associated with
240  * a particular coded character set; UTF-8, for example, is used only to
241  * encode Unicode.  Some schemes, however, are associated with multiple
242  * coded character sets; EUC, for example, can be used to encode
243  * characters in a variety of Asian coded character sets.
244  *
245  * <p> When a coded character set is used exclusively with a single
246  * character-encoding scheme then the corresponding charset is usually
247  * named for the coded character set; otherwise a charset is usually named
248  * for the encoding scheme and, possibly, the locale of the coded
249  * character sets that it supports.  Hence <tt>US-ASCII</tt> is both the
250  * name of a coded character set and of the charset that encodes it, while
251  * <tt>EUC-JP</tt> is the name of the charset that encodes the
252  * JIS&nbsp;X&nbsp;0201, JIS&nbsp;X&nbsp;0208, and JIS&nbsp;X&nbsp;0212
253  * coded character sets for the Japanese language.
254  *
255  * <p> The native character encoding of the Java programming language is
256  * UTF-16.  A charset in the Java platform therefore defines a mapping
257  * between sequences of sixteen-bit UTF-16 code units (that is, sequences
258  * of chars) and sequences of bytes. </p>
259  *
260  *
261  * @author Mark Reinhold
262  * @author JSR-51 Expert Group
263  * @since 1.4
264  *
265  * @see CharsetDecoder
266  * @see CharsetEncoder
267  * @see java.nio.charset.spi.CharsetProvider
268  * @see java.lang.Character
269  */
270 
271 public abstract class Charset
272     implements Comparable<Charset>
273 {
274 
275     /* -- Static methods -- */
276 
277     private static volatile String bugLevel = null;
278 
279     static boolean atBugLevel(String bl) {              // package-private
280         String level = bugLevel;
281         if (level == null) {
282             if (!sun.misc.VM.isBooted())
283                 return false;
284             bugLevel = level = AccessController.doPrivileged(
285                 new GetPropertyAction("sun.nio.cs.bugLevel", ""));
286         }
287         return level.equals(bl);
288     }
289 
290     /**
291      * Checks that the given string is a legal charset name. </p>
292      *
293      * @param  s
294      *         A purported charset name
295      *
296      * @throws  IllegalCharsetNameException
297      *          If the given name is not a legal charset name
298      */
299     private static void checkName(String s) {
300         int n = s.length();
301         if (!atBugLevel("1.4")) {
302             if (n == 0)
303                 throw new IllegalCharsetNameException(s);
304         }
305         for (int i = 0; i < n; i++) {
306             char c = s.charAt(i);
307             if (c >= 'A' && c <= 'Z') continue;
308             if (c >= 'a' && c <= 'z') continue;
309             if (c >= '0' && c <= '9') continue;
310             if (c == '-' && i != 0) continue;
311             if (c == '+' && i != 0) continue;
312             if (c == ':' && i != 0) continue;
313             if (c == '_' && i != 0) continue;
314             if (c == '.' && i != 0) continue;
315             throw new IllegalCharsetNameException(s);
316         }
317     }
318 
319     /* The standard set of charsets */
320     private static CharsetProvider standardProvider = new StandardCharsets();
321 
322     // Cache of the most-recently-returned charsets,
323     // along with the names that were used to find them
324     //
325     private static volatile Object[] cache1 = null; // "Level 1" cache
326     private static volatile Object[] cache2 = null; // "Level 2" cache
327 
328     private static void cache(String charsetName, Charset cs) {
329         cache2 = cache1;
330         cache1 = new Object[] { charsetName, cs };
331     }
332 
333     // Creates an iterator that walks over the available providers, ignoring
334     // those whose lookup or instantiation causes a security exception to be
335     // thrown.  Should be invoked with full privileges.
336     //
337     private static Iterator<CharsetProvider> providers() {
338         return new Iterator<CharsetProvider>() {
339 
340                 ClassLoader cl = ClassLoader.getSystemClassLoader();
341                 ServiceLoader<CharsetProvider> sl =
342                     ServiceLoader.load(CharsetProvider.class, cl);
343                 Iterator<CharsetProvider> i = sl.iterator();
344 
345                 CharsetProvider next = null;
346 
347                 private boolean getNext() {
348                     while (next == null) {
349                         try {
350                             if (!i.hasNext())
351                                 return false;
352                             next = i.next();
353                         } catch (ServiceConfigurationError sce) {
354                             if (sce.getCause() instanceof SecurityException) {
355                                 // Ignore security exceptions
356                                 continue;
357                             }
358                             throw sce;
359                         }
360                     }
361                     return true;
362                 }
363 
364                 public boolean hasNext() {
365                     return getNext();
366                 }
367 
368                 public CharsetProvider next() {
369                     if (!getNext())
370                         throw new NoSuchElementException();
371                     CharsetProvider n = next;
372                     next = null;
373                     return n;
374                 }
375 
376                 public void remove() {
377                     throw new UnsupportedOperationException();
378                 }
379 
380             };
381     }
382 
383     // Thread-local gate to prevent recursive provider lookups
384     private static ThreadLocal<ThreadLocal<?>> gate =
385             new ThreadLocal<ThreadLocal<?>>();
386 
387     private static Charset lookupViaProviders(final String charsetName) {
388 
389         // The runtime startup sequence looks up standard charsets as a
390         // consequence of the VM's invocation of System.initializeSystemClass
391         // in order to, e.g., set system properties and encode filenames.  At
392         // that point the application class loader has not been initialized,
393         // however, so we can't look for providers because doing so will cause
394         // that loader to be prematurely initialized with incomplete
395         // information.
396         //
397         if (!sun.misc.VM.isBooted())
398             return null;
399 
400         if (gate.get() != null)
401             // Avoid recursive provider lookups
402             return null;
403         try {
404             gate.set(gate);
405 
406             return AccessController.doPrivileged(
407                 new PrivilegedAction<Charset>() {
408                     public Charset run() {
409                         for (Iterator<CharsetProvider> i = providers();
410                              i.hasNext();) {
411                             CharsetProvider cp = i.next();
412                             Charset cs = cp.charsetForName(charsetName);
413                             if (cs != null)
414                                 return cs;
415                         }
416                         return null;
417                     }
418                 });
419 
420         } finally {
421             gate.set(null);
422         }
423     }
424 
425     /* The extended set of charsets */
426     private static class ExtendedProviderHolder {
427         static final CharsetProvider extendedProvider = extendedProvider();
428         // returns ExtendedProvider, if installed
429         private static CharsetProvider extendedProvider() {
430             return AccessController.doPrivileged(
431                        new PrivilegedAction<CharsetProvider>() {
432                            public CharsetProvider run() {
433                                 try {
434                                     Class<?> epc
435                                         = Class.forName("sun.nio.cs.ext.ExtendedCharsets");
436                                     return (CharsetProvider)epc.newInstance();
437                                 } catch (ClassNotFoundException x) {
438                                     // Extended charsets not available
439                                     // (charsets.jar not present)
440                                 } catch (InstantiationException |
441                                          IllegalAccessException x) {
442                                   throw new Error(x);
443                                 }
444                                 return null;
445                             }
446                         });
447         }
448     }
449 
450     private static Charset lookupExtendedCharset(String charsetName) {
451         CharsetProvider ecp = ExtendedProviderHolder.extendedProvider;
452         return (ecp != null) ? ecp.charsetForName(charsetName) : null;
453     }
454 
455     private static Charset lookup(String charsetName) {
456         if (charsetName == null)
457             throw new IllegalArgumentException("Null charset name");
458         Object[] a;
459         if ((a = cache1) != null && charsetName.equals(a[0]))
460             return (Charset)a[1];
461         // We expect most programs to use one Charset repeatedly.
462         // We convey a hint to this effect to the VM by putting the
463         // level 1 cache miss code in a separate method.
464         return lookup2(charsetName);
465     }
466 
467     private static Charset lookup2(String charsetName) {
468         Object[] a;
469         if ((a = cache2) != null && charsetName.equals(a[0])) {
470             cache2 = cache1;
471             cache1 = a;
472             return (Charset)a[1];
473         }
474         Charset cs;
475         if ((cs = standardProvider.charsetForName(charsetName)) != null ||
476             (cs = lookupExtendedCharset(charsetName))           != null ||
477             (cs = lookupViaProviders(charsetName))              != null)
478         {
479             cache(charsetName, cs);
480             return cs;
481         }
482 
483         /* Only need to check the name if we didn't find a charset for it */
484         checkName(charsetName);
485         return null;
486     }
487 
488     /**
489      * Tells whether the named charset is supported.
490      *
491      * @param  charsetName
492      *         The name of the requested charset; may be either
493      *         a canonical name or an alias
494      *
495      * @return  <tt>true</tt> if, and only if, support for the named charset
496      *          is available in the current Java virtual machine
497      *
498      * @throws IllegalCharsetNameException
499      *         If the given charset name is illegal
500      *
501      * @throws  IllegalArgumentException
502      *          If the given <tt>charsetName</tt> is null
503      */
504     public static boolean isSupported(String charsetName) {
505         return (lookup(charsetName) != null);
506     }
507 
508     /**
509      * Returns a charset object for the named charset.
510      *
511      * @param  charsetName
512      *         The name of the requested charset; may be either
513      *         a canonical name or an alias
514      *
515      * @return  A charset object for the named charset
516      *
517      * @throws  IllegalCharsetNameException
518      *          If the given charset name is illegal
519      *
520      * @throws  IllegalArgumentException
521      *          If the given <tt>charsetName</tt> is null
522      *
523      * @throws  UnsupportedCharsetException
524      *          If no support for the named charset is available
525      *          in this instance of the Java virtual machine
526      */
527     public static Charset forName(String charsetName) {
528         Charset cs = lookup(charsetName);
529         if (cs != null)
530             return cs;
531         throw new UnsupportedCharsetException(charsetName);
532     }
533 
534     // Fold charsets from the given iterator into the given map, ignoring
535     // charsets whose names already have entries in the map.
536     //
537     private static void put(Iterator<Charset> i, Map<String,Charset> m) {
538         while (i.hasNext()) {
539             Charset cs = i.next();
540             if (!m.containsKey(cs.name()))
541                 m.put(cs.name(), cs);
542         }
543     }
544 
545     /**
546      * Constructs a sorted map from canonical charset names to charset objects.
547      *
548      * <p> The map returned by this method will have one entry for each charset
549      * for which support is available in the current Java virtual machine.  If
550      * two or more supported charsets have the same canonical name then the
551      * resulting map will contain just one of them; which one it will contain
552      * is not specified. </p>
553      *
554      * <p> The invocation of this method, and the subsequent use of the
555      * resulting map, may cause time-consuming disk or network I/O operations
556      * to occur.  This method is provided for applications that need to
557      * enumerate all of the available charsets, for example to allow user
558      * charset selection.  This method is not used by the {@link #forName
559      * forName} method, which instead employs an efficient incremental lookup
560      * algorithm.
561      *
562      * <p> This method may return different results at different times if new
563      * charset providers are dynamically made available to the current Java
564      * virtual machine.  In the absence of such changes, the charsets returned
565      * by this method are exactly those that can be retrieved via the {@link
566      * #forName forName} method.  </p>
567      *
568      * @return An immutable, case-insensitive map from canonical charset names
569      *         to charset objects
570      */
571     public static SortedMap<String,Charset> availableCharsets() {
572         return AccessController.doPrivileged(
573             new PrivilegedAction<SortedMap<String,Charset>>() {
574                 public SortedMap<String,Charset> run() {
575                     TreeMap<String,Charset> m =
576                         new TreeMap<String,Charset>(
577                             ASCIICaseInsensitiveComparator.CASE_INSENSITIVE_ORDER);
578                     put(standardProvider.charsets(), m);
579                     CharsetProvider ecp = ExtendedProviderHolder.extendedProvider;
580                     if (ecp != null)
581                         put(ecp.charsets(), m);
582                     for (Iterator<CharsetProvider> i = providers(); i.hasNext();) {
583                         CharsetProvider cp = i.next();
584                         put(cp.charsets(), m);
585                     }
586                     return Collections.unmodifiableSortedMap(m);
587                 }
588             });
589     }
590 
591     private static volatile Charset defaultCharset;
592 
593     /**
594      * Returns the default charset of this Java virtual machine.
595      *
596      * <p> The default charset is determined during virtual-machine startup and
597      * typically depends upon the locale and charset of the underlying
598      * operating system.
599      *
600      * @return  A charset object for the default charset
601      *
602      * @since 1.5
603      */
604     public static Charset defaultCharset() {
605         if (defaultCharset == null) {
606             synchronized (Charset.class) {
607                 String csn = AccessController.doPrivileged(
608                     new GetPropertyAction("file.encoding"));
609                 Charset cs = lookup(csn);
610                 if (cs != null)
611                     defaultCharset = cs;
612                 else
613                     defaultCharset = forName("UTF-8");
614             }
615         }
616         return defaultCharset;
617     }
618 
619 
620     /* -- Instance fields and methods -- */
621 
622     private final String name;          // tickles a bug in oldjavac
623     private final String[] aliases;     // tickles a bug in oldjavac
624     private Set<String> aliasSet = null;
625 
626     /**
627      * Initializes a new charset with the given canonical name and alias
628      * set.
629      *
630      * @param  canonicalName
631      *         The canonical name of this charset
632      *
633      * @param  aliases
634      *         An array of this charset's aliases, or null if it has no aliases
635      *
636      * @throws IllegalCharsetNameException
637      *         If the canonical name or any of the aliases are illegal
638      */
639     protected Charset(String canonicalName, String[] aliases) {
640         checkName(canonicalName);
641         String[] as = (aliases == null) ? new String[0] : aliases;
642         for (int i = 0; i < as.length; i++)
643             checkName(as[i]);
644         this.name = canonicalName;
645         this.aliases = as;
646     }
647 
648     /**
649      * Returns this charset's canonical name.
650      *
651      * @return  The canonical name of this charset
652      */
653     public final String name() {
654         return name;
655     }
656 
657     /**
658      * Returns a set containing this charset's aliases.
659      *
660      * @return  An immutable set of this charset's aliases
661      */
662     public final Set<String> aliases() {
663         if (aliasSet != null)
664             return aliasSet;
665         int n = aliases.length;
666         HashSet<String> hs = new HashSet<String>(n);
667         for (int i = 0; i < n; i++)
668             hs.add(aliases[i]);
669         aliasSet = Collections.unmodifiableSet(hs);
670         return aliasSet;
671     }
672 
673     /**
674      * Returns this charset's human-readable name for the default locale.
675      *
676      * <p> The default implementation of this method simply returns this
677      * charset's canonical name.  Concrete subclasses of this class may
678      * override this method in order to provide a localized display name. </p>
679      *
680      * @return  The display name of this charset in the default locale
681      */
682     public String displayName() {
683         return name;
684     }
685 
686     /**
687      * Tells whether or not this charset is registered in the <a
688      * href="http://www.iana.org/assignments/character-sets">IANA Charset
689      * Registry</a>.
690      *
691      * @return  <tt>true</tt> if, and only if, this charset is known by its
692      *          implementor to be registered with the IANA
693      */
694     public final boolean isRegistered() {
695         return !name.startsWith("X-") && !name.startsWith("x-");
696     }
697 
698     /**
699      * Returns this charset's human-readable name for the given locale.
700      *
701      * <p> The default implementation of this method simply returns this
702      * charset's canonical name.  Concrete subclasses of this class may
703      * override this method in order to provide a localized display name. </p>
704      *
705      * @param  locale
706      *         The locale for which the display name is to be retrieved
707      *
708      * @return  The display name of this charset in the given locale
709      */
710     public String displayName(Locale locale) {
711         return name;
712     }
713 
714     /**
715      * Tells whether or not this charset contains the given charset.
716      *
717      * <p> A charset <i>C</i> is said to <i>contain</i> a charset <i>D</i> if,
718      * and only if, every character representable in <i>D</i> is also
719      * representable in <i>C</i>.  If this relationship holds then it is
720      * guaranteed that every string that can be encoded in <i>D</i> can also be
721      * encoded in <i>C</i> without performing any replacements.
722      *
723      * <p> That <i>C</i> contains <i>D</i> does not imply that each character
724      * representable in <i>C</i> by a particular byte sequence is represented
725      * in <i>D</i> by the same byte sequence, although sometimes this is the
726      * case.
727      *
728      * <p> Every charset contains itself.
729      *
730      * <p> This method computes an approximation of the containment relation:
731      * If it returns <tt>true</tt> then the given charset is known to be
732      * contained by this charset; if it returns <tt>false</tt>, however, then
733      * it is not necessarily the case that the given charset is not contained
734      * in this charset.
735      *
736      * @param   cs
737      *          The given charset
738      *
739      * @return  <tt>true</tt> if the given charset is contained in this charset
740      */
741     public abstract boolean contains(Charset cs);
742 
743     /**
744      * Constructs a new decoder for this charset.
745      *
746      * @return  A new decoder for this charset
747      */
748     public abstract CharsetDecoder newDecoder();
749 
750     /**
751      * Constructs a new encoder for this charset.
752      *
753      * @return  A new encoder for this charset
754      *
755      * @throws  UnsupportedOperationException
756      *          If this charset does not support encoding
757      */
758     public abstract CharsetEncoder newEncoder();
759 
760     /**
761      * Tells whether or not this charset supports encoding.
762      *
763      * <p> Nearly all charsets support encoding.  The primary exceptions are
764      * special-purpose <i>auto-detect</i> charsets whose decoders can determine
765      * which of several possible encoding schemes is in use by examining the
766      * input byte sequence.  Such charsets do not support encoding because
767      * there is no way to determine which encoding should be used on output.
768      * Implementations of such charsets should override this method to return
769      * <tt>false</tt>. </p>
770      *
771      * @return  <tt>true</tt> if, and only if, this charset supports encoding
772      */
773     public boolean canEncode() {
774         return true;
775     }
776 
777     /**
778      * Convenience method that decodes bytes in this charset into Unicode
779      * characters.
780      *
781      * <p> An invocation of this method upon a charset <tt>cs</tt> returns the
782      * same result as the expression
783      *
784      * <pre>
785      *     cs.newDecoder()
786      *       .onMalformedInput(CodingErrorAction.REPLACE)
787      *       .onUnmappableCharacter(CodingErrorAction.REPLACE)
788      *       .decode(bb); </pre>
789      *
790      * except that it is potentially more efficient because it can cache
791      * decoders between successive invocations.
792      *
793      * <p> This method always replaces malformed-input and unmappable-character
794      * sequences with this charset's default replacement byte array.  In order
795      * to detect such sequences, use the {@link
796      * CharsetDecoder#decode(java.nio.ByteBuffer)} method directly.  </p>
797      *
798      * @param  bb  The byte buffer to be decoded
799      *
800      * @return  A char buffer containing the decoded characters
801      */
802     public final CharBuffer decode(ByteBuffer bb) {
803         try {
804             return ThreadLocalCoders.decoderFor(this)
805                 .onMalformedInput(CodingErrorAction.REPLACE)
806                 .onUnmappableCharacter(CodingErrorAction.REPLACE)
807                 .decode(bb);
808         } catch (CharacterCodingException x) {
809             throw new Error(x);         // Can't happen
810         }
811     }
812 
813     /**
814      * Convenience method that encodes Unicode characters into bytes in this
815      * charset.
816      *
817      * <p> An invocation of this method upon a charset <tt>cs</tt> returns the
818      * same result as the expression
819      *
820      * <pre>
821      *     cs.newEncoder()
822      *       .onMalformedInput(CodingErrorAction.REPLACE)
823      *       .onUnmappableCharacter(CodingErrorAction.REPLACE)
824      *       .encode(bb); </pre>
825      *
826      * except that it is potentially more efficient because it can cache
827      * encoders between successive invocations.
828      *
829      * <p> This method always replaces malformed-input and unmappable-character
830      * sequences with this charset's default replacement string.  In order to
831      * detect such sequences, use the {@link
832      * CharsetEncoder#encode(java.nio.CharBuffer)} method directly.  </p>
833      *
834      * @param  cb  The char buffer to be encoded
835      *
836      * @return  A byte buffer containing the encoded characters
837      */
838     public final ByteBuffer encode(CharBuffer cb) {
839         try {
840             return ThreadLocalCoders.encoderFor(this)
841                 .onMalformedInput(CodingErrorAction.REPLACE)
842                 .onUnmappableCharacter(CodingErrorAction.REPLACE)
843                 .encode(cb);
844         } catch (CharacterCodingException x) {
845             throw new Error(x);         // Can't happen
846         }
847     }
848 
849     /**
850      * Convenience method that encodes a string into bytes in this charset.
851      *
852      * <p> An invocation of this method upon a charset <tt>cs</tt> returns the
853      * same result as the expression
854      *
855      * <pre>
856      *     cs.encode(CharBuffer.wrap(s)); </pre>
857      *
858      * @param  str  The string to be encoded
859      *
860      * @return  A byte buffer containing the encoded characters
861      */
862     public final ByteBuffer encode(String str) {
863         return encode(CharBuffer.wrap(str));
864     }
865 
866     /**
867      * Compares this charset to another.
868      *
869      * <p> Charsets are ordered by their canonical names, without regard to
870      * case. </p>
871      *
872      * @param  that
873      *         The charset to which this charset is to be compared
874      *
875      * @return A negative integer, zero, or a positive integer as this charset
876      *         is less than, equal to, or greater than the specified charset
877      */
878     public final int compareTo(Charset that) {
879         return (name().compareToIgnoreCase(that.name()));
880     }
881 
882     /**
883      * Computes a hashcode for this charset.
884      *
885      * @return  An integer hashcode
886      */
887     public final int hashCode() {
888         return name().hashCode();
889     }
890 
891     /**
892      * Tells whether or not this object is equal to another.
893      *
894      * <p> Two charsets are equal if, and only if, they have the same canonical
895      * names.  A charset is never equal to any other type of object.  </p>
896      *
897      * @return  <tt>true</tt> if, and only if, this charset is equal to the
898      *          given object
899      */
900     public final boolean equals(Object ob) {
901         if (!(ob instanceof Charset))
902             return false;
903         if (this == ob)
904             return true;
905         return name.equals(((Charset)ob).name());
906     }
907 
908     /**
909      * Returns a string describing this charset.
910      *
911      * @return  A string describing this charset
912      */
913     public final String toString() {
914         return name();
915     }
916 
917 }