1 /*
   2  * Copyright (c) 2000, 2010, 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.AccessControlException;
  33 import java.security.PrivilegedAction;
  34 import java.util.Collections;
  35 import java.util.HashSet;
  36 import java.util.Iterator;
  37 import java.util.Locale;
  38 import java.util.Map;
  39 import java.util.NoSuchElementException;
  40 import java.util.Set;
  41 import java.util.ServiceLoader;
  42 import java.util.ServiceConfigurationError;
  43 import java.util.SortedMap;
  44 import java.util.TreeMap;
  45 import sun.misc.ASCIICaseInsensitiveComparator;
  46 import sun.nio.cs.StandardCharsets;
  47 import sun.nio.cs.ThreadLocalCoders;
  48 import sun.security.action.GetPropertyAction;
  49 
  50 
  51 /**
  52  * A named mapping between sequences of sixteen-bit Unicode <a
  53  * href="../../lang/Character.html#unicode">code units</a> and sequences of
  54  * bytes.  This class defines methods for creating decoders and encoders and
  55  * for retrieving the various names associated with a charset.  Instances of
  56  * this class are immutable.
  57  *
  58  * <p> This class also defines static methods for testing whether a particular
  59  * charset is supported, for locating charset instances by name, and for
  60  * constructing a map that contains every charset for which support is
  61  * available in the current Java virtual machine.  Support for new charsets can
  62  * be added via the service-provider interface defined in the {@link
  63  * java.nio.charset.spi.CharsetProvider} class.
  64  *
  65  * <p> All of the methods defined in this class are safe for use by multiple
  66  * concurrent threads.
  67  *
  68  *
  69  * <a name="names"><a name="charenc">
  70  * <h4>Charset names</h4>
  71  *
  72  * <p> Charsets are named by strings composed of the following characters:
  73  *
  74  * <ul>
  75  *
  76  *   <li> The uppercase letters <tt>'A'</tt> through <tt>'Z'</tt>
  77  *        (<tt>'&#92;u0041'</tt>&nbsp;through&nbsp;<tt>'&#92;u005a'</tt>),
  78  *
  79  *   <li> The lowercase letters <tt>'a'</tt> through <tt>'z'</tt>
  80  *        (<tt>'&#92;u0061'</tt>&nbsp;through&nbsp;<tt>'&#92;u007a'</tt>),
  81  *
  82  *   <li> The digits <tt>'0'</tt> through <tt>'9'</tt>
  83  *        (<tt>'&#92;u0030'</tt>&nbsp;through&nbsp;<tt>'&#92;u0039'</tt>),
  84  *
  85  *   <li> The dash character <tt>'-'</tt>
  86  *        (<tt>'&#92;u002d'</tt>,&nbsp;<small>HYPHEN-MINUS</small>),
  87  *
  88  *   <li> The plus character <tt>'+'</tt>
  89  *        (<tt>'&#92;u002b'</tt>,&nbsp;<small>PLUS SIGN</small>),
  90  *
  91  *   <li> The period character <tt>'.'</tt>
  92  *        (<tt>'&#92;u002e'</tt>,&nbsp;<small>FULL STOP</small>),
  93  *
  94  *   <li> The colon character <tt>':'</tt>
  95  *        (<tt>'&#92;u003a'</tt>,&nbsp;<small>COLON</small>), and
  96  *
  97  *   <li> The underscore character <tt>'_'</tt>
  98  *        (<tt>'&#92;u005f'</tt>,&nbsp;<small>LOW&nbsp;LINE</small>).
  99  *
 100  * </ul>
 101  *
 102  * A charset name must begin with either a letter or a digit.  The empty string
 103  * is not a legal charset name.  Charset names are not case-sensitive; that is,
 104  * case is always ignored when comparing charset names.  Charset names
 105  * generally follow the conventions documented in <a
 106  * href="http://www.ietf.org/rfc/rfc2278.txt"><i>RFC&nbsp;2278:&nbsp;IANA Charset
 107  * Registration Procedures</i></a>.
 108  *
 109  * <p> Every charset has a <i>canonical name</i> and may also have one or more
 110  * <i>aliases</i>.  The canonical name is returned by the {@link #name() name} method
 111  * of this class.  Canonical names are, by convention, usually in upper case.
 112  * The aliases of a charset are returned by the {@link #aliases() aliases}
 113  * method.
 114  *
 115  * <a name="hn">
 116  *
 117  * <p> Some charsets have an <i>historical name</i> that is defined for
 118  * compatibility with previous versions of the Java platform.  A charset's
 119  * historical name is either its canonical name or one of its aliases.  The
 120  * historical name is returned by the <tt>getEncoding()</tt> methods of the
 121  * {@link java.io.InputStreamReader#getEncoding InputStreamReader} and {@link
 122  * java.io.OutputStreamWriter#getEncoding OutputStreamWriter} classes.
 123  *
 124  * <a name="iana">
 125  *
 126  * <p> If a charset listed in the <a
 127  * href="http://www.iana.org/assignments/character-sets"><i>IANA Charset
 128  * Registry</i></a> is supported by an implementation of the Java platform then
 129  * its canonical name must be the name listed in the registry.  Many charsets
 130  * are given more than one name in the registry, in which case the registry
 131  * identifies one of the names as <i>MIME-preferred</i>.  If a charset has more
 132  * than one registry name then its canonical name must be the MIME-preferred
 133  * name and the other names in the registry must be valid aliases.  If a
 134  * supported charset is not listed in the IANA registry then its canonical name
 135  * must begin with one of the strings <tt>"X-"</tt> or <tt>"x-"</tt>.
 136  *
 137  * <p> The IANA charset registry does change over time, and so the canonical
 138  * name and the aliases of a particular charset may also change over time.  To
 139  * ensure compatibility it is recommended that no alias ever be removed from a
 140  * charset, and that if the canonical name of a charset is changed then its
 141  * previous canonical name be made into an alias.
 142  *
 143  *
 144  * <h4>Standard charsets</h4>
 145  *
 146  * <a name="standard">
 147  *
 148  * <p> Every implementation of the Java platform is required to support the
 149  * following standard charsets.  Consult the release documentation for your
 150  * implementation to see if any other charsets are supported.  The behavior
 151  * of such optional charsets may differ between implementations.
 152  *
 153  * <blockquote><table width="80%" summary="Description of standard charsets">
 154  * <tr><th><p align="left">Charset</p></th><th><p align="left">Description</p></th></tr>
 155  * <tr><td valign=top><tt>US-ASCII</tt></td>
 156  *     <td>Seven-bit ASCII, a.k.a. <tt>ISO646-US</tt>,
 157  *         a.k.a. the Basic Latin block of the Unicode character set</td></tr>
 158  * <tr><td valign=top><tt>ISO-8859-1&nbsp;&nbsp;</tt></td>
 159  *     <td>ISO Latin Alphabet No. 1, a.k.a. <tt>ISO-LATIN-1</tt></td></tr>
 160  * <tr><td valign=top><tt>UTF-8</tt></td>
 161  *     <td>Eight-bit UCS Transformation Format</td></tr>
 162  * <tr><td valign=top><tt>UTF-16BE</tt></td>
 163  *     <td>Sixteen-bit UCS Transformation Format,
 164  *         big-endian byte&nbsp;order</td></tr>
 165  * <tr><td valign=top><tt>UTF-16LE</tt></td>
 166  *     <td>Sixteen-bit UCS Transformation Format,
 167  *         little-endian byte&nbsp;order</td></tr>
 168  * <tr><td valign=top><tt>UTF-16</tt></td>
 169  *     <td>Sixteen-bit UCS Transformation Format,
 170  *         byte&nbsp;order identified by an optional byte-order mark</td></tr>
 171  * </table></blockquote>
 172  *
 173  * <p> The <tt>UTF-8</tt> charset is specified by <a
 174  * href="http://www.ietf.org/rfc/rfc2279.txt"><i>RFC&nbsp;2279</i></a>; the
 175  * transformation format upon which it is based is specified in
 176  * Amendment&nbsp;2 of ISO&nbsp;10646-1 and is also described in the <a
 177  * href="http://www.unicode.org/unicode/standard/standard.html"><i>Unicode
 178  * Standard</i></a>.
 179  *
 180  * <p> The <tt>UTF-16</tt> charsets are specified by <a
 181  * href="http://www.ietf.org/rfc/rfc2781.txt"><i>RFC&nbsp;2781</i></a>; the
 182  * transformation formats upon which they are based are specified in
 183  * Amendment&nbsp;1 of ISO&nbsp;10646-1 and are also described in the <a
 184  * href="http://www.unicode.org/unicode/standard/standard.html"><i>Unicode
 185  * Standard</i></a>.
 186  *
 187  * <p> The <tt>UTF-16</tt> charsets use sixteen-bit quantities and are
 188  * therefore sensitive to byte order.  In these encodings the byte order of a
 189  * stream may be indicated by an initial <i>byte-order mark</i> represented by
 190  * the Unicode character <tt>'&#92;uFEFF'</tt>.  Byte-order marks are handled
 191  * as follows:
 192  *
 193  * <ul>
 194  *
 195  *   <li><p> When decoding, the <tt>UTF-16BE</tt> and <tt>UTF-16LE</tt>
 196  *   charsets interpret the initial byte-order marks as a <small>ZERO-WIDTH
 197  *   NON-BREAKING SPACE</small>; when encoding, they do not write
 198  *   byte-order marks. </p></li>
 199 
 200  *
 201  *   <li><p> When decoding, the <tt>UTF-16</tt> charset interprets the
 202  *   byte-order mark at the beginning of the input stream to indicate the
 203  *   byte-order of the stream but defaults to big-endian if there is no
 204  *   byte-order mark; when encoding, it uses big-endian byte order and writes
 205  *   a big-endian byte-order mark. </p></li>
 206  *
 207  * </ul>
 208  *
 209  * In any case, byte order marks occuring after the first element of an
 210  * input sequence are not omitted since the same code is used to represent
 211  * <small>ZERO-WIDTH NON-BREAKING SPACE</small>.
 212  *
 213  * <p> Every instance of the Java virtual machine has a default charset, which
 214  * may or may not be one of the standard charsets.  The default charset is
 215  * determined during virtual-machine startup and typically depends upon the
 216  * locale and charset being used by the underlying operating system. </p>
 217  *
 218  * <p>The {@link StandardCharsets} class defines constants for each of the
 219  * standard charsets.
 220  *
 221  * <h4>Terminology</h4>
 222  *
 223  * <p> The name of this class is taken from the terms used in
 224  * <a href="http://www.ietf.org/rfc/rfc2278.txt"><i>RFC&nbsp;2278</i></a>.
 225  * In that document a <i>charset</i> is defined as the combination of
 226  * one or more coded character sets and a character-encoding scheme.
 227  * (This definition is confusing; some other software systems define
 228  * <i>charset</i> as a synonym for <i>coded character set</i>.)
 229  *
 230  * <p> A <i>coded character set</i> is a mapping between a set of abstract
 231  * characters and a set of integers.  US-ASCII, ISO&nbsp;8859-1,
 232  * JIS&nbsp;X&nbsp;0201, and Unicode are examples of coded character sets.
 233  *
 234  * <p> Some standards have defined a <i>character set</i> to be simply a
 235  * set of abstract characters without an associated assigned numbering.
 236  * An alphabet is an example of such a character set.  However, the subtle
 237  * distinction between <i>character set</i> and <i>coded character set</i>
 238  * is rarely used in practice; the former has become a short form for the
 239  * latter, including in the Java API specification.
 240  *
 241  * <p> A <i>character-encoding scheme</i> is a mapping between one or more
 242  * coded character sets and a set of octet (eight-bit byte) sequences.
 243  * UTF-8, UTF-16, ISO&nbsp;2022, and EUC are examples of
 244  * character-encoding schemes.  Encoding schemes are often associated with
 245  * a particular coded character set; UTF-8, for example, is used only to
 246  * encode Unicode.  Some schemes, however, are associated with multiple
 247  * coded character sets; EUC, for example, can be used to encode
 248  * characters in a variety of Asian coded character sets.
 249  *
 250  * <p> When a coded character set is used exclusively with a single
 251  * character-encoding scheme then the corresponding charset is usually
 252  * named for the coded character set; otherwise a charset is usually named
 253  * for the encoding scheme and, possibly, the locale of the coded
 254  * character sets that it supports.  Hence <tt>US-ASCII</tt> is both the
 255  * name of a coded character set and of the charset that encodes it, while
 256  * <tt>EUC-JP</tt> is the name of the charset that encodes the
 257  * JIS&nbsp;X&nbsp;0201, JIS&nbsp;X&nbsp;0208, and JIS&nbsp;X&nbsp;0212
 258  * coded character sets for the Japanese language.
 259  *
 260  * <p> The native character encoding of the Java programming language is
 261  * UTF-16.  A charset in the Java platform therefore defines a mapping
 262  * between sequences of sixteen-bit UTF-16 code units (that is, sequences
 263  * of chars) and sequences of bytes. </p>
 264  *
 265  *
 266  * @author Mark Reinhold
 267  * @author JSR-51 Expert Group
 268  * @since 1.4
 269  *
 270  * @see CharsetDecoder
 271  * @see CharsetEncoder
 272  * @see java.nio.charset.spi.CharsetProvider
 273  * @see java.lang.Character
 274  */
 275 
 276 public abstract class Charset
 277     implements Comparable<Charset>
 278 {
 279 
 280     /* -- Static methods -- */
 281 
 282     private static volatile String bugLevel = null;
 283 
 284     static boolean atBugLevel(String bl) {              // package-private
 285         String level = bugLevel;
 286         if (level == null) {
 287             if (!sun.misc.VM.isBooted())
 288                 return false;
 289             bugLevel = level = AccessController.doPrivileged(
 290                 new GetPropertyAction("sun.nio.cs.bugLevel", ""));
 291         }
 292         return level.equals(bl);
 293     }
 294 
 295     /**
 296      * Checks that the given string is a legal charset name. </p>
 297      *
 298      * @param  s
 299      *         A purported charset name
 300      *
 301      * @throws  IllegalCharsetNameException
 302      *          If the given name is not a legal charset name
 303      */
 304     private static void checkName(String s) {
 305         int n = s.length();
 306         if (!atBugLevel("1.4")) {
 307             if (n == 0)
 308                 throw new IllegalCharsetNameException(s);
 309         }
 310         for (int i = 0; i < n; i++) {
 311             char c = s.charAt(i);
 312             if (c >= 'A' && c <= 'Z') continue;
 313             if (c >= 'a' && c <= 'z') continue;
 314             if (c >= '0' && c <= '9') continue;
 315             if (c == '-' && i != 0) continue;
 316             if (c == '+' && i != 0) continue;
 317             if (c == ':' && i != 0) continue;
 318             if (c == '_' && i != 0) continue;
 319             if (c == '.' && i != 0) continue;
 320             throw new IllegalCharsetNameException(s);
 321         }
 322     }
 323 
 324     /* The standard set of charsets */
 325     private static CharsetProvider standardProvider = new StandardCharsets();
 326 
 327     // Cache of the most-recently-returned charsets,
 328     // along with the names that were used to find them
 329     //
 330     private static volatile Object[] cache1 = null; // "Level 1" cache
 331     private static volatile Object[] cache2 = null; // "Level 2" cache
 332 
 333     private static void cache(String charsetName, Charset cs) {
 334         cache2 = cache1;
 335         cache1 = new Object[] { charsetName, cs };
 336     }
 337 
 338     // Creates an iterator that walks over the available providers, ignoring
 339     // those whose lookup or instantiation causes a security exception to be
 340     // thrown.  Should be invoked with full privileges.
 341     //
 342     private static Iterator providers() {
 343         return new Iterator() {
 344 
 345                 ClassLoader cl = ClassLoader.getSystemClassLoader();
 346                 ServiceLoader<CharsetProvider> sl =
 347                     ServiceLoader.load(CharsetProvider.class, cl);
 348                 Iterator<CharsetProvider> i = sl.iterator();
 349 
 350                 Object next = null;
 351 
 352                 private boolean getNext() {
 353                     while (next == null) {
 354                         try {
 355                             if (!i.hasNext())
 356                                 return false;
 357                             next = i.next();
 358                         } catch (ServiceConfigurationError sce) {
 359                             if (sce.getCause() instanceof SecurityException) {
 360                                 // Ignore security exceptions
 361                                 continue;
 362                             }
 363                             throw sce;
 364                         }
 365                     }
 366                     return true;
 367                 }
 368 
 369                 public boolean hasNext() {
 370                     return getNext();
 371                 }
 372 
 373                 public Object next() {
 374                     if (!getNext())
 375                         throw new NoSuchElementException();
 376                     Object n = next;
 377                     next = null;
 378                     return n;
 379                 }
 380 
 381                 public void remove() {
 382                     throw new UnsupportedOperationException();
 383                 }
 384 
 385             };
 386     }
 387 
 388     // Thread-local gate to prevent recursive provider lookups
 389     private static ThreadLocal<ThreadLocal> gate = new ThreadLocal<ThreadLocal>();
 390 
 391     private static Charset lookupViaProviders(final String charsetName) {
 392 
 393         // The runtime startup sequence looks up standard charsets as a
 394         // consequence of the VM's invocation of System.initializeSystemClass
 395         // in order to, e.g., set system properties and encode filenames.  At
 396         // that point the application class loader has not been initialized,
 397         // however, so we can't look for providers because doing so will cause
 398         // that loader to be prematurely initialized with incomplete
 399         // information.
 400         //
 401         if (!sun.misc.VM.isBooted())
 402             return null;
 403 
 404         if (gate.get() != null)
 405             // Avoid recursive provider lookups
 406             return null;
 407         try {
 408             gate.set(gate);
 409 
 410             return AccessController.doPrivileged(
 411                 new PrivilegedAction<Charset>() {
 412                     public Charset run() {
 413                         for (Iterator i = providers(); i.hasNext();) {
 414                             CharsetProvider cp = (CharsetProvider)i.next();
 415                             Charset cs = cp.charsetForName(charsetName);
 416                             if (cs != null)
 417                                 return cs;
 418                         }
 419                         return null;
 420                     }
 421                 });
 422 
 423         } finally {
 424             gate.set(null);
 425         }
 426     }
 427 
 428     /* The extended set of charsets */
 429     private static Object extendedProviderLock = new Object();
 430     private static boolean extendedProviderProbed = false;
 431     private static CharsetProvider extendedProvider = null;
 432 
 433     private static void probeExtendedProvider() {
 434         AccessController.doPrivileged(new PrivilegedAction<Object>() {
 435                 public Object run() {
 436                     try {
 437                         Class epc
 438                             = Class.forName("sun.nio.cs.ext.ExtendedCharsets");
 439                         extendedProvider = (CharsetProvider)epc.newInstance();
 440                     } catch (ClassNotFoundException x) {
 441                         // Extended charsets not available
 442                         // (charsets.jar not present)
 443                     } catch (InstantiationException x) {
 444                         throw new Error(x);
 445                     } catch (IllegalAccessException x) {
 446                         throw new Error(x);
 447                     }
 448                     return null;
 449                 }
 450             });
 451     }
 452 
 453     private static Charset lookupExtendedCharset(String charsetName) {
 454         CharsetProvider ecp = null;
 455         synchronized (extendedProviderLock) {
 456             if (!extendedProviderProbed) {
 457                 probeExtendedProvider();
 458                 extendedProviderProbed = true;
 459             }
 460             ecp = extendedProvider;
 461         }
 462         return (ecp != null) ? ecp.charsetForName(charsetName) : null;
 463     }
 464 
 465     private static Charset lookup(String charsetName) {
 466         if (charsetName == null)
 467             throw new IllegalArgumentException("Null charset name");
 468 
 469         Object[] a;
 470         if ((a = cache1) != null && charsetName.equals(a[0]))
 471             return (Charset)a[1];
 472         // We expect most programs to use one Charset repeatedly.
 473         // We convey a hint to this effect to the VM by putting the
 474         // level 1 cache miss code in a separate method.
 475         return lookup2(charsetName);
 476     }
 477 
 478     private static Charset lookup2(String charsetName) {
 479         Object[] a;
 480         if ((a = cache2) != null && charsetName.equals(a[0])) {
 481             cache2 = cache1;
 482             cache1 = a;
 483             return (Charset)a[1];
 484         }
 485 
 486         Charset cs;
 487         if ((cs = standardProvider.charsetForName(charsetName)) != null ||
 488             (cs = lookupExtendedCharset(charsetName))           != null ||
 489             (cs = lookupViaProviders(charsetName))              != null)
 490         {
 491             cache(charsetName, cs);
 492             return cs;
 493         }
 494 
 495         /* Only need to check the name if we didn't find a charset for it */
 496         checkName(charsetName);
 497         return null;
 498     }
 499 
 500     /**
 501      * Tells whether the named charset is supported. </p>
 502      *
 503      * @param  charsetName
 504      *         The name of the requested charset; may be either
 505      *         a canonical name or an alias
 506      *
 507      * @return  <tt>true</tt> if, and only if, support for the named charset
 508      *          is available in the current Java virtual machine
 509      *
 510      * @throws IllegalCharsetNameException
 511      *         If the given charset name is illegal
 512      *
 513      * @throws  IllegalArgumentException
 514      *          If the given <tt>charsetName</tt> is null
 515      */
 516     public static boolean isSupported(String charsetName) {
 517         return (lookup(charsetName) != null);
 518     }
 519 
 520     /**
 521      * Returns a charset object for the named charset. </p>
 522      *
 523      * @param  charsetName
 524      *         The name of the requested charset; may be either
 525      *         a canonical name or an alias
 526      *
 527      * @return  A charset object for the named charset
 528      *
 529      * @throws  IllegalCharsetNameException
 530      *          If the given charset name is illegal
 531      *
 532      * @throws  IllegalArgumentException
 533      *          If the given <tt>charsetName</tt> is null
 534      *
 535      * @throws  UnsupportedCharsetException
 536      *          If no support for the named charset is available
 537      *          in this instance of the Java virtual machine
 538      */
 539     public static Charset forName(String charsetName) {
 540         Charset cs = lookup(charsetName);
 541         if (cs != null)
 542             return cs;
 543         throw new UnsupportedCharsetException(charsetName);
 544     }
 545 
 546     // Fold charsets from the given iterator into the given map, ignoring
 547     // charsets whose names already have entries in the map.
 548     //
 549     private static void put(Iterator<Charset> i, Map<String,Charset> m) {
 550         while (i.hasNext()) {
 551             Charset cs = i.next();
 552             if (!m.containsKey(cs.name()))
 553                 m.put(cs.name(), cs);
 554         }
 555     }
 556 
 557     /**
 558      * Constructs a sorted map from canonical charset names to charset objects.
 559      *
 560      * <p> The map returned by this method will have one entry for each charset
 561      * for which support is available in the current Java virtual machine.  If
 562      * two or more supported charsets have the same canonical name then the
 563      * resulting map will contain just one of them; which one it will contain
 564      * is not specified. </p>
 565      *
 566      * <p> The invocation of this method, and the subsequent use of the
 567      * resulting map, may cause time-consuming disk or network I/O operations
 568      * to occur.  This method is provided for applications that need to
 569      * enumerate all of the available charsets, for example to allow user
 570      * charset selection.  This method is not used by the {@link #forName
 571      * forName} method, which instead employs an efficient incremental lookup
 572      * algorithm.
 573      *
 574      * <p> This method may return different results at different times if new
 575      * charset providers are dynamically made available to the current Java
 576      * virtual machine.  In the absence of such changes, the charsets returned
 577      * by this method are exactly those that can be retrieved via the {@link
 578      * #forName forName} method.  </p>
 579      *
 580      * @return An immutable, case-insensitive map from canonical charset names
 581      *         to charset objects
 582      */
 583     public static SortedMap<String,Charset> availableCharsets() {
 584         return AccessController.doPrivileged(
 585             new PrivilegedAction<SortedMap<String,Charset>>() {
 586                 public SortedMap<String,Charset> run() {
 587                     TreeMap<String,Charset> m =
 588                         new TreeMap<String,Charset>(
 589                             ASCIICaseInsensitiveComparator.CASE_INSENSITIVE_ORDER);
 590                     put(standardProvider.charsets(), m);
 591                     for (Iterator i = providers(); i.hasNext();) {
 592                         CharsetProvider cp = (CharsetProvider)i.next();
 593                         put(cp.charsets(), m);
 594                     }
 595                     return Collections.unmodifiableSortedMap(m);
 596                 }
 597             });
 598     }
 599 
 600     private static volatile Charset defaultCharset;
 601 
 602     /**
 603      * Returns the default charset of this Java virtual machine.
 604      *
 605      * <p> The default charset is determined during virtual-machine startup and
 606      * typically depends upon the locale and charset of the underlying
 607      * operating system.
 608      *
 609      * @return  A charset object for the default charset
 610      *
 611      * @since 1.5
 612      */
 613     public static Charset defaultCharset() {
 614         if (defaultCharset == null) {
 615             synchronized (Charset.class) {
 616                 String csn = AccessController.doPrivileged(
 617                     new GetPropertyAction("file.encoding"));
 618                 Charset cs = lookup(csn);
 619                 if (cs != null)
 620                     defaultCharset = cs;
 621                 else
 622                     defaultCharset = forName("UTF-8");
 623             }
 624         }
 625         return defaultCharset;
 626     }
 627 
 628 
 629     /* -- Instance fields and methods -- */
 630 
 631     private final String name;          // tickles a bug in oldjavac
 632     private final String[] aliases;     // tickles a bug in oldjavac
 633     private Set<String> aliasSet = null;
 634 
 635     /**
 636      * Initializes a new charset with the given canonical name and alias
 637      * set. </p>
 638      *
 639      * @param  canonicalName
 640      *         The canonical name of this charset
 641      *
 642      * @param  aliases
 643      *         An array of this charset's aliases, or null if it has no aliases
 644      *
 645      * @throws IllegalCharsetNameException
 646      *         If the canonical name or any of the aliases are illegal
 647      */
 648     protected Charset(String canonicalName, String[] aliases) {
 649         checkName(canonicalName);
 650         String[] as = (aliases == null) ? new String[0] : aliases;
 651         for (int i = 0; i < as.length; i++)
 652             checkName(as[i]);
 653         this.name = canonicalName;
 654         this.aliases = as;
 655     }
 656 
 657     /**
 658      * Returns this charset's canonical name. </p>
 659      *
 660      * @return  The canonical name of this charset
 661      */
 662     public final String name() {
 663         return name;
 664     }
 665 
 666     /**
 667      * Returns a set containing this charset's aliases. </p>
 668      *
 669      * @return  An immutable set of this charset's aliases
 670      */
 671     public final Set<String> aliases() {
 672         if (aliasSet != null)
 673             return aliasSet;
 674         int n = aliases.length;
 675         HashSet<String> hs = new HashSet<String>(n);
 676         for (int i = 0; i < n; i++)
 677             hs.add(aliases[i]);
 678         aliasSet = Collections.unmodifiableSet(hs);
 679         return aliasSet;
 680     }
 681 
 682     /**
 683      * Returns this charset's human-readable name for the default locale.
 684      *
 685      * <p> The default implementation of this method simply returns this
 686      * charset's canonical name.  Concrete subclasses of this class may
 687      * override this method in order to provide a localized display name. </p>
 688      *
 689      * @return  The display name of this charset in the default locale
 690      */
 691     public String displayName() {
 692         return name;
 693     }
 694 
 695     /**
 696      * Tells whether or not this charset is registered in the <a
 697      * href="http://www.iana.org/assignments/character-sets">IANA Charset
 698      * Registry</a>.  </p>
 699      *
 700      * @return  <tt>true</tt> if, and only if, this charset is known by its
 701      *          implementor to be registered with the IANA
 702      */
 703     public final boolean isRegistered() {
 704         return !name.startsWith("X-") && !name.startsWith("x-");
 705     }
 706 
 707     /**
 708      * Returns this charset's human-readable name for the given locale.
 709      *
 710      * <p> The default implementation of this method simply returns this
 711      * charset's canonical name.  Concrete subclasses of this class may
 712      * override this method in order to provide a localized display name. </p>
 713      *
 714      * @param  locale
 715      *         The locale for which the display name is to be retrieved
 716      *
 717      * @return  The display name of this charset in the given locale
 718      */
 719     public String displayName(Locale locale) {
 720         return name;
 721     }
 722 
 723     /**
 724      * Tells whether or not this charset contains the given charset.
 725      *
 726      * <p> A charset <i>C</i> is said to <i>contain</i> a charset <i>D</i> if,
 727      * and only if, every character representable in <i>D</i> is also
 728      * representable in <i>C</i>.  If this relationship holds then it is
 729      * guaranteed that every string that can be encoded in <i>D</i> can also be
 730      * encoded in <i>C</i> without performing any replacements.
 731      *
 732      * <p> That <i>C</i> contains <i>D</i> does not imply that each character
 733      * representable in <i>C</i> by a particular byte sequence is represented
 734      * in <i>D</i> by the same byte sequence, although sometimes this is the
 735      * case.
 736      *
 737      * <p> Every charset contains itself.
 738      *
 739      * <p> This method computes an approximation of the containment relation:
 740      * If it returns <tt>true</tt> then the given charset is known to be
 741      * contained by this charset; if it returns <tt>false</tt>, however, then
 742      * it is not necessarily the case that the given charset is not contained
 743      * in this charset.
 744      *
 745      * @return  <tt>true</tt> if the given charset is contained in this charset
 746      */
 747     public abstract boolean contains(Charset cs);
 748 
 749     /**
 750      * Constructs a new decoder for this charset. </p>
 751      *
 752      * @return  A new decoder for this charset
 753      */
 754     public abstract CharsetDecoder newDecoder();
 755 
 756     /**
 757      * Constructs a new encoder for this charset. </p>
 758      *
 759      * @return  A new encoder for this charset
 760      *
 761      * @throws  UnsupportedOperationException
 762      *          If this charset does not support encoding
 763      */
 764     public abstract CharsetEncoder newEncoder();
 765 
 766     /**
 767      * Tells whether or not this charset supports encoding.
 768      *
 769      * <p> Nearly all charsets support encoding.  The primary exceptions are
 770      * special-purpose <i>auto-detect</i> charsets whose decoders can determine
 771      * which of several possible encoding schemes is in use by examining the
 772      * input byte sequence.  Such charsets do not support encoding because
 773      * there is no way to determine which encoding should be used on output.
 774      * Implementations of such charsets should override this method to return
 775      * <tt>false</tt>. </p>
 776      *
 777      * @return  <tt>true</tt> if, and only if, this charset supports encoding
 778      */
 779     public boolean canEncode() {
 780         return true;
 781     }
 782 
 783     /**
 784      * Convenience method that decodes bytes in this charset into Unicode
 785      * characters.
 786      *
 787      * <p> An invocation of this method upon a charset <tt>cs</tt> returns the
 788      * same result as the expression
 789      *
 790      * <pre>
 791      *     cs.newDecoder()
 792      *       .onMalformedInput(CodingErrorAction.REPLACE)
 793      *       .onUnmappableCharacter(CodingErrorAction.REPLACE)
 794      *       .decode(bb); </pre>
 795      *
 796      * except that it is potentially more efficient because it can cache
 797      * decoders between successive invocations.
 798      *
 799      * <p> This method always replaces malformed-input and unmappable-character
 800      * sequences with this charset's default replacement byte array.  In order
 801      * to detect such sequences, use the {@link
 802      * CharsetDecoder#decode(java.nio.ByteBuffer)} method directly.  </p>
 803      *
 804      * @param  bb  The byte buffer to be decoded
 805      *
 806      * @return  A char buffer containing the decoded characters
 807      */
 808     public final CharBuffer decode(ByteBuffer bb) {
 809         try {
 810             return ThreadLocalCoders.decoderFor(this)
 811                 .onMalformedInput(CodingErrorAction.REPLACE)
 812                 .onUnmappableCharacter(CodingErrorAction.REPLACE)
 813                 .decode(bb);
 814         } catch (CharacterCodingException x) {
 815             throw new Error(x);         // Can't happen
 816         }
 817     }
 818 
 819     /**
 820      * Convenience method that encodes Unicode characters into bytes in this
 821      * charset.
 822      *
 823      * <p> An invocation of this method upon a charset <tt>cs</tt> returns the
 824      * same result as the expression
 825      *
 826      * <pre>
 827      *     cs.newEncoder()
 828      *       .onMalformedInput(CodingErrorAction.REPLACE)
 829      *       .onUnmappableCharacter(CodingErrorAction.REPLACE)
 830      *       .encode(bb); </pre>
 831      *
 832      * except that it is potentially more efficient because it can cache
 833      * encoders between successive invocations.
 834      *
 835      * <p> This method always replaces malformed-input and unmappable-character
 836      * sequences with this charset's default replacement string.  In order to
 837      * detect such sequences, use the {@link
 838      * CharsetEncoder#encode(java.nio.CharBuffer)} method directly.  </p>
 839      *
 840      * @param  cb  The char buffer to be encoded
 841      *
 842      * @return  A byte buffer containing the encoded characters
 843      */
 844     public final ByteBuffer encode(CharBuffer cb) {
 845         try {
 846             return ThreadLocalCoders.encoderFor(this)
 847                 .onMalformedInput(CodingErrorAction.REPLACE)
 848                 .onUnmappableCharacter(CodingErrorAction.REPLACE)
 849                 .encode(cb);
 850         } catch (CharacterCodingException x) {
 851             throw new Error(x);         // Can't happen
 852         }
 853     }
 854 
 855     /**
 856      * Convenience method that encodes a string into bytes in this charset.
 857      *
 858      * <p> An invocation of this method upon a charset <tt>cs</tt> returns the
 859      * same result as the expression
 860      *
 861      * <pre>
 862      *     cs.encode(CharBuffer.wrap(s)); </pre>
 863      *
 864      * @param  str  The string to be encoded
 865      *
 866      * @return  A byte buffer containing the encoded characters
 867      */
 868     public final ByteBuffer encode(String str) {
 869         return encode(CharBuffer.wrap(str));
 870     }
 871 
 872     /**
 873      * Compares this charset to another.
 874      *
 875      * <p> Charsets are ordered by their canonical names, without regard to
 876      * case. </p>
 877      *
 878      * @param  that
 879      *         The charset to which this charset is to be compared
 880      *
 881      * @return A negative integer, zero, or a positive integer as this charset
 882      *         is less than, equal to, or greater than the specified charset
 883      */
 884     public final int compareTo(Charset that) {
 885         return (name().compareToIgnoreCase(that.name()));
 886     }
 887 
 888     /**
 889      * Computes a hashcode for this charset. </p>
 890      *
 891      * @return  An integer hashcode
 892      */
 893     public final int hashCode() {
 894         return name().hashCode();
 895     }
 896 
 897     /**
 898      * Tells whether or not this object is equal to another.
 899      *
 900      * <p> Two charsets are equal if, and only if, they have the same canonical
 901      * names.  A charset is never equal to any other type of object.  </p>
 902      *
 903      * @return  <tt>true</tt> if, and only if, this charset is equal to the
 904      *          given object
 905      */
 906     public final boolean equals(Object ob) {
 907         if (!(ob instanceof Charset))
 908             return false;
 909         if (this == ob)
 910             return true;
 911         return name.equals(((Charset)ob).name());
 912     }
 913 
 914     /**
 915      * Returns a string describing this charset. </p>
 916      *
 917      * @return  A string describing this charset
 918      */
 919     public final String toString() {
 920         return name();
 921     }
 922 
 923 }