1 /*
   2  * Copyright (c) 2000, 2017, 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.Arrays;
  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.Objects;
  41 import java.util.Set;
  42 import java.util.ServiceLoader;
  43 import java.util.ServiceConfigurationError;
  44 import java.util.SortedMap;
  45 import java.util.TreeMap;
  46 import jdk.internal.misc.VM;
  47 import sun.nio.cs.StandardCharsets;
  48 import sun.nio.cs.ThreadLocalCoders;
  49 import sun.security.action.GetPropertyAction;
  50 
  51 
  52 /**
  53  * A named mapping between sequences of sixteen-bit Unicode <a
  54  * href="../../lang/Character.html#unicode">code units</a> and sequences of
  55  * bytes.  This class defines methods for creating decoders and encoders and
  56  * for retrieving the various names associated with a charset.  Instances of
  57  * this class are immutable.
  58  *
  59  * <p> This class also defines static methods for testing whether a particular
  60  * charset is supported, for locating charset instances by name, and for
  61  * constructing a map that contains every charset for which support is
  62  * available in the current Java virtual machine.  Support for new charsets can
  63  * be added via the service-provider interface defined in the {@link
  64  * java.nio.charset.spi.CharsetProvider} class.
  65  *
  66  * <p> All of the methods defined in this class are safe for use by multiple
  67  * concurrent threads.
  68  *
  69  *
  70  * <a id="names"></a><a id="charenc"></a>
  71  * <h2>Charset names</h2>
  72  *
  73  * <p> Charsets are named by strings composed of the following characters:
  74  *
  75  * <ul>
  76  *
  77  *   <li> The uppercase letters {@code 'A'} through {@code 'Z'}
  78  *        (<code>'\u0041'</code>&nbsp;through&nbsp;<code>'\u005a'</code>),
  79  *
  80  *   <li> The lowercase letters {@code 'a'} through {@code 'z'}
  81  *        (<code>'\u0061'</code>&nbsp;through&nbsp;<code>'\u007a'</code>),
  82  *
  83  *   <li> The digits {@code '0'} through {@code '9'}
  84  *        (<code>'\u0030'</code>&nbsp;through&nbsp;<code>'\u0039'</code>),
  85  *
  86  *   <li> The dash character {@code '-'}
  87  *        (<code>'\u002d'</code>,&nbsp;<small>HYPHEN-MINUS</small>),
  88  *
  89  *   <li> The plus character {@code '+'}
  90  *        (<code>'\u002b'</code>,&nbsp;<small>PLUS SIGN</small>),
  91  *
  92  *   <li> The period character {@code '.'}
  93  *        (<code>'\u002e'</code>,&nbsp;<small>FULL STOP</small>),
  94  *
  95  *   <li> The colon character {@code ':'}
  96  *        (<code>'\u003a'</code>,&nbsp;<small>COLON</small>), and
  97  *
  98  *   <li> The underscore character {@code '_'}
  99  *        (<code>'\u005f'</code>,&nbsp;<small>LOW&nbsp;LINE</small>).
 100  *
 101  * </ul>
 102  *
 103  * A charset name must begin with either a letter or a digit.  The empty string
 104  * is not a legal charset name.  Charset names are not case-sensitive; that is,
 105  * case is always ignored when comparing charset names.  Charset names
 106  * generally follow the conventions documented in <a
 107  * href="http://www.ietf.org/rfc/rfc2278.txt"><i>RFC&nbsp;2278:&nbsp;IANA Charset
 108  * Registration Procedures</i></a>.
 109  *
 110  * <p> Every charset has a <i>canonical name</i> and may also have one or more
 111  * <i>aliases</i>.  The canonical name is returned by the {@link #name() name} method
 112  * of this class.  Canonical names are, by convention, usually in upper case.
 113  * The aliases of a charset are returned by the {@link #aliases() aliases}
 114  * method.
 115  *
 116  * <p><a id="hn">Some charsets have an <i>historical name</i> that is defined for
 117  * compatibility with previous versions of the Java platform.</a>  A charset's
 118  * historical name is either its canonical name or one of its aliases.  The
 119  * historical name is returned by the {@code getEncoding()} methods of the
 120  * {@link java.io.InputStreamReader#getEncoding InputStreamReader} and {@link
 121  * java.io.OutputStreamWriter#getEncoding OutputStreamWriter} classes.
 122  *
 123  * <p><a id="iana"> </a>If a charset listed in the <a
 124  * href="http://www.iana.org/assignments/character-sets"><i>IANA Charset
 125  * Registry</i></a> is supported by an implementation of the Java platform then
 126  * its canonical name must be the name listed in the registry. Many charsets
 127  * are given more than one name in the registry, in which case the registry
 128  * identifies one of the names as <i>MIME-preferred</i>.  If a charset has more
 129  * than one registry name then its canonical name must be the MIME-preferred
 130  * name and the other names in the registry must be valid aliases.  If a
 131  * supported charset is not listed in the IANA registry then its canonical name
 132  * must begin with one of the strings {@code "X-"} or {@code "x-"}.
 133  *
 134  * <p> The IANA charset registry does change over time, and so the canonical
 135  * name and the aliases of a particular charset may also change over time.  To
 136  * ensure compatibility it is recommended that no alias ever be removed from a
 137  * charset, and that if the canonical name of a charset is changed then its
 138  * previous canonical name be made into an alias.
 139  *
 140  *
 141  * <h2>Standard charsets</h2>
 142  *
 143  *
 144  *
 145  * <p><a id="standard">Every implementation of the Java platform is required to support the
 146  * following standard charsets.</a>  Consult the release documentation for your
 147  * implementation to see if any other charsets are supported.  The behavior
 148  * of such optional charsets may differ between implementations.
 149  *
 150  * <blockquote><table class="striped" style="width:80%">
 151  * <caption style="display:none">Description of standard charsets</caption>
 152  * <thead>
 153  * <tr><th style="text-align:left">Charset</th><th style="text-align:left">Description</th></tr>
 154  * </thead>
 155  * <tbody>
 156  * <tr><td style="vertical-align:top">{@code US-ASCII}</td>
 157  *     <td>Seven-bit ASCII, a.k.a. {@code ISO646-US},
 158  *         a.k.a. the Basic Latin block of the Unicode character set</td></tr>
 159  * <tr><td style="vertical-align:top"><code>ISO-8859-1&nbsp;&nbsp;</code></td>
 160  *     <td>ISO Latin Alphabet No. 1, a.k.a. {@code ISO-LATIN-1}</td></tr>
 161  * <tr><td style="vertical-align:top">{@code UTF-8}</td>
 162  *     <td>Eight-bit UCS Transformation Format</td></tr>
 163  * <tr><td style="vertical-align:top">{@code UTF-16BE}</td>
 164  *     <td>Sixteen-bit UCS Transformation Format,
 165  *         big-endian byte&nbsp;order</td></tr>
 166  * <tr><td style="vertical-align:top">{@code UTF-16LE}</td>
 167  *     <td>Sixteen-bit UCS Transformation Format,
 168  *         little-endian byte&nbsp;order</td></tr>
 169  * <tr><td style="vertical-align:top">{@code UTF-16}</td>
 170  *     <td>Sixteen-bit UCS Transformation Format,
 171  *         byte&nbsp;order identified by an optional byte-order mark</td></tr>
 172  * </tbody>
 173  * </table></blockquote>
 174  *
 175  * <p> The {@code UTF-8} charset is specified by <a
 176  * href="http://www.ietf.org/rfc/rfc2279.txt"><i>RFC&nbsp;2279</i></a>; the
 177  * transformation format upon which it is based is specified in
 178  * Amendment&nbsp;2 of ISO&nbsp;10646-1 and is 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 {@code UTF-16} charsets are specified by <a
 183  * href="http://www.ietf.org/rfc/rfc2781.txt"><i>RFC&nbsp;2781</i></a>; the
 184  * transformation formats upon which they are based are specified in
 185  * Amendment&nbsp;1 of ISO&nbsp;10646-1 and are also described in the <a
 186  * href="http://www.unicode.org/unicode/standard/standard.html"><i>Unicode
 187  * Standard</i></a>.
 188  *
 189  * <p> The {@code UTF-16} charsets use sixteen-bit quantities and are
 190  * therefore sensitive to byte order.  In these encodings the byte order of a
 191  * stream may be indicated by an initial <i>byte-order mark</i> represented by
 192  * the Unicode character <code>'\uFEFF'</code>.  Byte-order marks are handled
 193  * as follows:
 194  *
 195  * <ul>
 196  *
 197  *   <li><p> When decoding, the {@code UTF-16BE} and {@code UTF-16LE}
 198  *   charsets interpret the initial byte-order marks as a <small>ZERO-WIDTH
 199  *   NON-BREAKING SPACE</small>; when encoding, they do not write
 200  *   byte-order marks. </p></li>
 201 
 202  *
 203  *   <li><p> When decoding, the {@code UTF-16} charset interprets the
 204  *   byte-order mark at the beginning of the input stream to indicate the
 205  *   byte-order of the stream but defaults to big-endian if there is no
 206  *   byte-order mark; when encoding, it uses big-endian byte order and writes
 207  *   a big-endian byte-order mark. </p></li>
 208  *
 209  * </ul>
 210  *
 211  * In any case, byte order marks occurring after the first element of an
 212  * input sequence are not omitted since the same code is used to represent
 213  * <small>ZERO-WIDTH NON-BREAKING SPACE</small>.
 214  *
 215  * <p> Every instance of the Java virtual machine has a default charset, which
 216  * may or may not be one of the standard charsets.  The default charset is
 217  * determined during virtual-machine startup and typically depends upon the
 218  * locale and charset being used by the underlying operating system. </p>
 219  *
 220  * <p>The {@link StandardCharsets} class defines constants for each of the
 221  * standard charsets.
 222  *
 223  * <h2>Terminology</h2>
 224  *
 225  * <p> The name of this class is taken from the terms used in
 226  * <a href="http://www.ietf.org/rfc/rfc2278.txt"><i>RFC&nbsp;2278</i></a>.
 227  * In that document a <i>charset</i> is defined as the combination of
 228  * one or more coded character sets and a character-encoding scheme.
 229  * (This definition is confusing; some other software systems define
 230  * <i>charset</i> as a synonym for <i>coded character set</i>.)
 231  *
 232  * <p> A <i>coded character set</i> is a mapping between a set of abstract
 233  * characters and a set of integers.  US-ASCII, ISO&nbsp;8859-1,
 234  * JIS&nbsp;X&nbsp;0201, and Unicode are examples of coded character sets.
 235  *
 236  * <p> Some standards have defined a <i>character set</i> to be simply a
 237  * set of abstract characters without an associated assigned numbering.
 238  * An alphabet is an example of such a character set.  However, the subtle
 239  * distinction between <i>character set</i> and <i>coded character set</i>
 240  * is rarely used in practice; the former has become a short form for the
 241  * latter, including in the Java API specification.
 242  *
 243  * <p> A <i>character-encoding scheme</i> is a mapping between one or more
 244  * coded character sets and a set of octet (eight-bit byte) sequences.
 245  * UTF-8, UTF-16, ISO&nbsp;2022, and EUC are examples of
 246  * character-encoding schemes.  Encoding schemes are often associated with
 247  * a particular coded character set; UTF-8, for example, is used only to
 248  * encode Unicode.  Some schemes, however, are associated with multiple
 249  * coded character sets; EUC, for example, can be used to encode
 250  * characters in a variety of Asian coded character sets.
 251  *
 252  * <p> When a coded character set is used exclusively with a single
 253  * character-encoding scheme then the corresponding charset is usually
 254  * named for the coded character set; otherwise a charset is usually named
 255  * for the encoding scheme and, possibly, the locale of the coded
 256  * character sets that it supports.  Hence {@code US-ASCII} is both the
 257  * name of a coded character set and of the charset that encodes it, while
 258  * {@code EUC-JP} is the name of the charset that encodes the
 259  * JIS&nbsp;X&nbsp;0201, JIS&nbsp;X&nbsp;0208, and JIS&nbsp;X&nbsp;0212
 260  * coded character sets for the Japanese language.
 261  *
 262  * <p> The native character encoding of the Java programming language is
 263  * UTF-16.  A charset in the Java platform therefore defines a mapping
 264  * between sequences of sixteen-bit UTF-16 code units (that is, sequences
 265  * of chars) and sequences of bytes. </p>
 266  *
 267  *
 268  * @author Mark Reinhold
 269  * @author JSR-51 Expert Group
 270  * @since 1.4
 271  *
 272  * @see CharsetDecoder
 273  * @see CharsetEncoder
 274  * @see java.nio.charset.spi.CharsetProvider
 275  * @see java.lang.Character
 276  */
 277 
 278 public abstract class Charset
 279     implements Comparable<Charset>
 280 {
 281 
 282     /* -- Static methods -- */
 283 
 284     private static String bugLevel;
 285 
 286     static boolean atBugLevel(String bl) {              // package-private
 287         String level = bugLevel;
 288         if (level == null) {
 289             if (!VM.isBooted())
 290                 return false;
 291             bugLevel = level = GetPropertyAction
 292                     .privilegedGetProperty("sun.nio.cs.bugLevel", "");
 293         }
 294         return level.equals(bl);
 295     }
 296 
 297     /**
 298      * Checks that the given string is a legal charset name. </p>
 299      *
 300      * @param  s
 301      *         A purported charset name
 302      *
 303      * @throws  IllegalCharsetNameException
 304      *          If the given name is not a legal charset name
 305      */
 306     private static void checkName(String s) {
 307         int n = s.length();
 308         if (n == 0 && !atBugLevel("1.4")) {
 309             throw new IllegalCharsetNameException(s);
 310         }
 311         for (int i = 0; i < n; i++) {
 312             char c = s.charAt(i);
 313             if (c >= 'A' && c <= 'Z') continue;
 314             if (c >= 'a' && c <= 'z') continue;
 315             if (c >= '0' && c <= '9') 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             if (c == '.' && i != 0) continue;
 321             throw new IllegalCharsetNameException(s);
 322         }
 323     }
 324 
 325     /* The standard set of charsets */
 326     private static final CharsetProvider standardProvider = new StandardCharsets();
 327 
 328     private static final String[] zeroAliases = new String[0];
 329 
 330     // Cache of the most-recently-returned charsets,
 331     // along with the names that were used to find them
 332     //
 333     private static volatile Object[] cache1; // "Level 1" cache
 334     private static volatile Object[] cache2; // "Level 2" cache
 335 
 336     private static void cache(String charsetName, Charset cs) {
 337         cache2 = cache1;
 338         cache1 = new Object[] { charsetName, cs };
 339     }
 340 
 341     // Creates an iterator that walks over the available providers, ignoring
 342     // those whose lookup or instantiation causes a security exception to be
 343     // thrown.  Should be invoked with full privileges.
 344     //
 345     private static Iterator<CharsetProvider> providers() {
 346         return new Iterator<>() {
 347                 ClassLoader cl = ClassLoader.getSystemClassLoader();
 348                 ServiceLoader<CharsetProvider> sl =
 349                     ServiceLoader.load(CharsetProvider.class, cl);
 350                 Iterator<CharsetProvider> i = sl.iterator();
 351                 CharsetProvider next = null;
 352 
 353                 private boolean getNext() {
 354                     while (next == null) {
 355                         try {
 356                             if (!i.hasNext())
 357                                 return false;
 358                             next = i.next();
 359                         } catch (ServiceConfigurationError sce) {
 360                             if (sce.getCause() instanceof SecurityException) {
 361                                 // Ignore security exceptions
 362                                 continue;
 363                             }
 364                             throw sce;
 365                         }
 366                     }
 367                     return true;
 368                 }
 369 
 370                 public boolean hasNext() {
 371                     return getNext();
 372                 }
 373 
 374                 public CharsetProvider next() {
 375                     if (!getNext())
 376                         throw new NoSuchElementException();
 377                     CharsetProvider n = next;
 378                     next = null;
 379                     return n;
 380                 }
 381 
 382                 public void remove() {
 383                     throw new UnsupportedOperationException();
 384                 }
 385 
 386             };
 387     }
 388 
 389     // Thread-local gate to prevent recursive provider lookups
 390     private static ThreadLocal<ThreadLocal<?>> gate =
 391             new ThreadLocal<ThreadLocal<?>>();
 392 
 393     private static Charset lookupViaProviders(final String charsetName) {
 394 
 395         // The runtime startup sequence looks up standard charsets as a
 396         // consequence of the VM's invocation of System.initializeSystemClass
 397         // in order to, e.g., set system properties and encode filenames.  At
 398         // that point the application class loader has not been initialized,
 399         // however, so we can't look for providers because doing so will cause
 400         // that loader to be prematurely initialized with incomplete
 401         // information.
 402         //
 403         if (!VM.isBooted())
 404             return null;
 405 
 406         if (gate.get() != null)
 407             // Avoid recursive provider lookups
 408             return null;
 409         try {
 410             gate.set(gate);
 411 
 412             return AccessController.doPrivileged(
 413                 new PrivilegedAction<>() {
 414                     public Charset run() {
 415                         for (Iterator<CharsetProvider> i = providers();
 416                              i.hasNext();) {
 417                             CharsetProvider cp = i.next();
 418                             Charset cs = cp.charsetForName(charsetName);
 419                             if (cs != null)
 420                                 return cs;
 421                         }
 422                         return null;
 423                     }
 424                 });
 425 
 426         } finally {
 427             gate.set(null);
 428         }
 429     }
 430 
 431     /* The extended set of charsets */
 432     private static class ExtendedProviderHolder {
 433         static final CharsetProvider[] extendedProviders = extendedProviders();
 434         // returns ExtendedProvider, if installed
 435         private static CharsetProvider[] extendedProviders() {
 436             return AccessController.doPrivileged(new PrivilegedAction<>() {
 437                     public CharsetProvider[] run() {
 438                         CharsetProvider[] cps = new CharsetProvider[1];
 439                         int n = 0;
 440                         ServiceLoader<CharsetProvider> sl =
 441                             ServiceLoader.loadInstalled(CharsetProvider.class);
 442                         for (CharsetProvider cp : sl) {
 443                             if (n + 1 > cps.length) {
 444                                 cps = Arrays.copyOf(cps, cps.length << 1);
 445                             }
 446                             cps[n++] = cp;
 447                         }
 448                         return n == cps.length ? cps : Arrays.copyOf(cps, n);
 449                     }});
 450         }
 451     }
 452 
 453     private static Charset lookupExtendedCharset(String charsetName) {
 454         if (!VM.isBooted())  // see lookupViaProviders()
 455             return null;
 456         CharsetProvider[] ecps = ExtendedProviderHolder.extendedProviders;
 457         for (CharsetProvider cp : ecps) {
 458             Charset cs = cp.charsetForName(charsetName);
 459             if (cs != null)
 460                 return cs;
 461         }
 462         return null;
 463     }
 464 
 465     private static Charset lookup(String charsetName) {
 466         if (charsetName == null)
 467             throw new IllegalArgumentException("Null charset name");
 468         Object[] a;
 469         if ((a = cache1) != null && charsetName.equals(a[0]))
 470             return (Charset)a[1];
 471         // We expect most programs to use one Charset repeatedly.
 472         // We convey a hint to this effect to the VM by putting the
 473         // level 1 cache miss code in a separate method.
 474         return lookup2(charsetName);
 475     }
 476 
 477     private static Charset lookup2(String charsetName) {
 478         Object[] a;
 479         if ((a = cache2) != null && charsetName.equals(a[0])) {
 480             cache2 = cache1;
 481             cache1 = a;
 482             return (Charset)a[1];
 483         }
 484         Charset cs;
 485         if ((cs = standardProvider.charsetForName(charsetName)) != null ||
 486             (cs = lookupExtendedCharset(charsetName))           != null ||
 487             (cs = lookupViaProviders(charsetName))              != null)
 488         {
 489             cache(charsetName, cs);
 490             return cs;
 491         }
 492 
 493         /* Only need to check the name if we didn't find a charset for it */
 494         checkName(charsetName);
 495         return null;
 496     }
 497 
 498     /**
 499      * Tells whether the named charset is supported.
 500      *
 501      * @param  charsetName
 502      *         The name of the requested charset; may be either
 503      *         a canonical name or an alias
 504      *
 505      * @return  {@code true} if, and only if, support for the named charset
 506      *          is available in the current Java virtual machine
 507      *
 508      * @throws IllegalCharsetNameException
 509      *         If the given charset name is illegal
 510      *
 511      * @throws  IllegalArgumentException
 512      *          If the given {@code charsetName} is null
 513      */
 514     public static boolean isSupported(String charsetName) {
 515         return (lookup(charsetName) != null);
 516     }
 517 
 518     /**
 519      * Returns a charset object for the named charset.
 520      *
 521      * @param  charsetName
 522      *         The name of the requested charset; may be either
 523      *         a canonical name or an alias
 524      *
 525      * @return  A charset object for the named charset
 526      *
 527      * @throws  IllegalCharsetNameException
 528      *          If the given charset name is illegal
 529      *
 530      * @throws  IllegalArgumentException
 531      *          If the given {@code charsetName} is null
 532      *
 533      * @throws  UnsupportedCharsetException
 534      *          If no support for the named charset is available
 535      *          in this instance of the Java virtual machine
 536      */
 537     public static Charset forName(String charsetName) {
 538         Charset cs = lookup(charsetName);
 539         if (cs != null)
 540             return cs;
 541         throw new UnsupportedCharsetException(charsetName);
 542     }
 543 
 544     // Fold charsets from the given iterator into the given map, ignoring
 545     // charsets whose names already have entries in the map.
 546     //
 547     private static void put(Iterator<Charset> i, Map<String,Charset> m) {
 548         while (i.hasNext()) {
 549             Charset cs = i.next();
 550             if (!m.containsKey(cs.name()))
 551                 m.put(cs.name(), cs);
 552         }
 553     }
 554 
 555     /**
 556      * Constructs a sorted map from canonical charset names to charset objects.
 557      *
 558      * <p> The map returned by this method will have one entry for each charset
 559      * for which support is available in the current Java virtual machine.  If
 560      * two or more supported charsets have the same canonical name then the
 561      * resulting map will contain just one of them; which one it will contain
 562      * is not specified. </p>
 563      *
 564      * <p> The invocation of this method, and the subsequent use of the
 565      * resulting map, may cause time-consuming disk or network I/O operations
 566      * to occur.  This method is provided for applications that need to
 567      * enumerate all of the available charsets, for example to allow user
 568      * charset selection.  This method is not used by the {@link #forName
 569      * forName} method, which instead employs an efficient incremental lookup
 570      * algorithm.
 571      *
 572      * <p> This method may return different results at different times if new
 573      * charset providers are dynamically made available to the current Java
 574      * virtual machine.  In the absence of such changes, the charsets returned
 575      * by this method are exactly those that can be retrieved via the {@link
 576      * #forName forName} method.  </p>
 577      *
 578      * @return An immutable, case-insensitive map from canonical charset names
 579      *         to charset objects
 580      */
 581     public static SortedMap<String,Charset> availableCharsets() {
 582         return AccessController.doPrivileged(
 583             new PrivilegedAction<>() {
 584                 public SortedMap<String,Charset> run() {
 585                     TreeMap<String,Charset> m =
 586                         new TreeMap<>(
 587                             String.CASE_INSENSITIVE_ORDER);
 588                     put(standardProvider.charsets(), m);
 589                     CharsetProvider[] ecps = ExtendedProviderHolder.extendedProviders;
 590                     for (CharsetProvider ecp :ecps) {
 591                         put(ecp.charsets(), m);
 592                     }
 593                     for (Iterator<CharsetProvider> i = providers(); i.hasNext();) {
 594                         CharsetProvider cp = i.next();
 595                         put(cp.charsets(), m);
 596                     }
 597                     return Collections.unmodifiableSortedMap(m);
 598                 }
 599             });
 600     }
 601 
 602     private static volatile Charset defaultCharset;
 603 
 604     /**
 605      * Returns the default charset of this Java virtual machine.
 606      *
 607      * <p> The default charset is determined during virtual-machine startup and
 608      * typically depends upon the locale and charset of the underlying
 609      * operating system.
 610      *
 611      * @return  A charset object for the default charset
 612      *
 613      * @since 1.5
 614      */
 615     public static Charset defaultCharset() {
 616         if (defaultCharset == null) {
 617             synchronized (Charset.class) {
 618                 String csn = GetPropertyAction
 619                         .privilegedGetProperty("file.encoding");
 620                 Charset cs = lookup(csn);
 621                 if (cs != null)
 622                     defaultCharset = cs;
 623                 else
 624                     defaultCharset = forName("UTF-8");
 625             }
 626         }
 627         return defaultCharset;
 628     }
 629 
 630 
 631     /* -- Instance fields and methods -- */
 632 
 633     private final String name;          // tickles a bug in oldjavac
 634     private final String[] aliases;     // tickles a bug in oldjavac
 635     private Set<String> aliasSet = null;
 636 
 637     /**
 638      * Initializes a new charset with the given canonical name and alias
 639      * set.
 640      *
 641      * @param  canonicalName
 642      *         The canonical name of this charset
 643      *
 644      * @param  aliases
 645      *         An array of this charset's aliases, or null if it has no aliases
 646      *
 647      * @throws IllegalCharsetNameException
 648      *         If the canonical name or any of the aliases are illegal
 649      */
 650     protected Charset(String canonicalName, String[] aliases) {
 651         checkName(canonicalName);
 652         String[] as = Objects.requireNonNullElse(aliases, zeroAliases);
 653         for (int i = 0; i < as.length; i++)
 654             checkName(as[i]);
 655         this.name = canonicalName;
 656         this.aliases = as;
 657     }
 658 
 659     /**
 660      * Returns this charset's canonical name.
 661      *
 662      * @return  The canonical name of this charset
 663      */
 664     public final String name() {
 665         return name;
 666     }
 667 
 668     /**
 669      * Returns a set containing this charset's aliases.
 670      *
 671      * @return  An immutable set of this charset's aliases
 672      */
 673     public final Set<String> aliases() {
 674         if (aliasSet != null)
 675             return aliasSet;
 676         int n = aliases.length;
 677         HashSet<String> hs = new HashSet<>(n);
 678         for (int i = 0; i < n; i++)
 679             hs.add(aliases[i]);
 680         aliasSet = Collections.unmodifiableSet(hs);
 681         return aliasSet;
 682     }
 683 
 684     /**
 685      * Returns this charset's human-readable name for the default locale.
 686      *
 687      * <p> The default implementation of this method simply returns this
 688      * charset's canonical name.  Concrete subclasses of this class may
 689      * override this method in order to provide a localized display name. </p>
 690      *
 691      * @return  The display name of this charset in the default locale
 692      */
 693     public String displayName() {
 694         return name;
 695     }
 696 
 697     /**
 698      * Tells whether or not this charset is registered in the <a
 699      * href="http://www.iana.org/assignments/character-sets">IANA Charset
 700      * Registry</a>.
 701      *
 702      * @return  {@code true} if, and only if, this charset is known by its
 703      *          implementor to be registered with the IANA
 704      */
 705     public final boolean isRegistered() {
 706         return !name.startsWith("X-") && !name.startsWith("x-");
 707     }
 708 
 709     /**
 710      * Returns this charset's human-readable name for the given locale.
 711      *
 712      * <p> The default implementation of this method simply returns this
 713      * charset's canonical name.  Concrete subclasses of this class may
 714      * override this method in order to provide a localized display name. </p>
 715      *
 716      * @param  locale
 717      *         The locale for which the display name is to be retrieved
 718      *
 719      * @return  The display name of this charset in the given locale
 720      */
 721     public String displayName(Locale locale) {
 722         return name;
 723     }
 724 
 725     /**
 726      * Tells whether or not this charset contains the given charset.
 727      *
 728      * <p> A charset <i>C</i> is said to <i>contain</i> a charset <i>D</i> if,
 729      * and only if, every character representable in <i>D</i> is also
 730      * representable in <i>C</i>.  If this relationship holds then it is
 731      * guaranteed that every string that can be encoded in <i>D</i> can also be
 732      * encoded in <i>C</i> without performing any replacements.
 733      *
 734      * <p> That <i>C</i> contains <i>D</i> does not imply that each character
 735      * representable in <i>C</i> by a particular byte sequence is represented
 736      * in <i>D</i> by the same byte sequence, although sometimes this is the
 737      * case.
 738      *
 739      * <p> Every charset contains itself.
 740      *
 741      * <p> This method computes an approximation of the containment relation:
 742      * If it returns {@code true} then the given charset is known to be
 743      * contained by this charset; if it returns {@code false}, however, then
 744      * it is not necessarily the case that the given charset is not contained
 745      * in this charset.
 746      *
 747      * @param   cs
 748      *          The given charset
 749      *
 750      * @return  {@code true} if the given charset is contained in this charset
 751      */
 752     public abstract boolean contains(Charset cs);
 753 
 754     /**
 755      * Constructs a new decoder for this charset.
 756      *
 757      * @return  A new decoder for this charset
 758      */
 759     public abstract CharsetDecoder newDecoder();
 760 
 761     /**
 762      * Constructs a new encoder for this charset.
 763      *
 764      * @return  A new encoder for this charset
 765      *
 766      * @throws  UnsupportedOperationException
 767      *          If this charset does not support encoding
 768      */
 769     public abstract CharsetEncoder newEncoder();
 770 
 771     /**
 772      * Tells whether or not this charset supports encoding.
 773      *
 774      * <p> Nearly all charsets support encoding.  The primary exceptions are
 775      * special-purpose <i>auto-detect</i> charsets whose decoders can determine
 776      * which of several possible encoding schemes is in use by examining the
 777      * input byte sequence.  Such charsets do not support encoding because
 778      * there is no way to determine which encoding should be used on output.
 779      * Implementations of such charsets should override this method to return
 780      * {@code false}. </p>
 781      *
 782      * @return  {@code true} if, and only if, this charset supports encoding
 783      */
 784     public boolean canEncode() {
 785         return true;
 786     }
 787 
 788     /**
 789      * Convenience method that decodes bytes in this charset into Unicode
 790      * characters.
 791      *
 792      * <p> An invocation of this method upon a charset {@code cs} returns the
 793      * same result as the expression
 794      *
 795      * <pre>
 796      *     cs.newDecoder()
 797      *       .onMalformedInput(CodingErrorAction.REPLACE)
 798      *       .onUnmappableCharacter(CodingErrorAction.REPLACE)
 799      *       .decode(bb); </pre>
 800      *
 801      * except that it is potentially more efficient because it can cache
 802      * decoders between successive invocations.
 803      *
 804      * <p> This method always replaces malformed-input and unmappable-character
 805      * sequences with this charset's default replacement byte array.  In order
 806      * to detect such sequences, use the {@link
 807      * CharsetDecoder#decode(java.nio.ByteBuffer)} method directly.  </p>
 808      *
 809      * @param  bb  The byte buffer to be decoded
 810      *
 811      * @return  A char buffer containing the decoded characters
 812      */
 813     public final CharBuffer decode(ByteBuffer bb) {
 814         try {
 815             return ThreadLocalCoders.decoderFor(this)
 816                 .onMalformedInput(CodingErrorAction.REPLACE)
 817                 .onUnmappableCharacter(CodingErrorAction.REPLACE)
 818                 .decode(bb);
 819         } catch (CharacterCodingException x) {
 820             throw new Error(x);         // Can't happen
 821         }
 822     }
 823 
 824     /**
 825      * Convenience method that encodes Unicode characters into bytes in this
 826      * charset.
 827      *
 828      * <p> An invocation of this method upon a charset {@code cs} returns the
 829      * same result as the expression
 830      *
 831      * <pre>
 832      *     cs.newEncoder()
 833      *       .onMalformedInput(CodingErrorAction.REPLACE)
 834      *       .onUnmappableCharacter(CodingErrorAction.REPLACE)
 835      *       .encode(bb); </pre>
 836      *
 837      * except that it is potentially more efficient because it can cache
 838      * encoders between successive invocations.
 839      *
 840      * <p> This method always replaces malformed-input and unmappable-character
 841      * sequences with this charset's default replacement string.  In order to
 842      * detect such sequences, use the {@link
 843      * CharsetEncoder#encode(java.nio.CharBuffer)} method directly.  </p>
 844      *
 845      * @param  cb  The char buffer to be encoded
 846      *
 847      * @return  A byte buffer containing the encoded characters
 848      */
 849     public final ByteBuffer encode(CharBuffer cb) {
 850         try {
 851             return ThreadLocalCoders.encoderFor(this)
 852                 .onMalformedInput(CodingErrorAction.REPLACE)
 853                 .onUnmappableCharacter(CodingErrorAction.REPLACE)
 854                 .encode(cb);
 855         } catch (CharacterCodingException x) {
 856             throw new Error(x);         // Can't happen
 857         }
 858     }
 859 
 860     /**
 861      * Convenience method that encodes a string into bytes in this charset.
 862      *
 863      * <p> An invocation of this method upon a charset {@code cs} returns the
 864      * same result as the expression
 865      *
 866      * <pre>
 867      *     cs.encode(CharBuffer.wrap(s)); </pre>
 868      *
 869      * @param  str  The string to be encoded
 870      *
 871      * @return  A byte buffer containing the encoded characters
 872      */
 873     public final ByteBuffer encode(String str) {
 874         return encode(CharBuffer.wrap(str));
 875     }
 876 
 877     /**
 878      * Compares this charset to another.
 879      *
 880      * <p> Charsets are ordered by their canonical names, without regard to
 881      * case. </p>
 882      *
 883      * @param  that
 884      *         The charset to which this charset is to be compared
 885      *
 886      * @return A negative integer, zero, or a positive integer as this charset
 887      *         is less than, equal to, or greater than the specified charset
 888      */
 889     public final int compareTo(Charset that) {
 890         return (name().compareToIgnoreCase(that.name()));
 891     }
 892 
 893     /**
 894      * Computes a hashcode for this charset.
 895      *
 896      * @return  An integer hashcode
 897      */
 898     public final int hashCode() {
 899         return name().hashCode();
 900     }
 901 
 902     /**
 903      * Tells whether or not this object is equal to another.
 904      *
 905      * <p> Two charsets are equal if, and only if, they have the same canonical
 906      * names.  A charset is never equal to any other type of object.  </p>
 907      *
 908      * @return  {@code true} if, and only if, this charset is equal to the
 909      *          given object
 910      */
 911     public final boolean equals(Object ob) {
 912         if (!(ob instanceof Charset))
 913             return false;
 914         if (this == ob)
 915             return true;
 916         return name.equals(((Charset)ob).name());
 917     }
 918 
 919     /**
 920      * Returns a string describing this charset.
 921      *
 922      * @return  A string describing this charset
 923      */
 924     public final String toString() {
 925         return name();
 926     }
 927 
 928 }
--- EOF ---