001/* System.java -- useful methods to interface with the system
002   Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007
003   Free Software Foundation, Inc.
004
005This file is part of GNU Classpath.
006
007GNU Classpath is free software; you can redistribute it and/or modify
008it under the terms of the GNU General Public License as published by
009the Free Software Foundation; either version 2, or (at your option)
010any later version.
011
012GNU Classpath is distributed in the hope that it will be useful, but
013WITHOUT ANY WARRANTY; without even the implied warranty of
014MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
015General Public License for more details.
016
017You should have received a copy of the GNU General Public License
018along with GNU Classpath; see the file COPYING.  If not, write to the
019Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02002110-1301 USA.
021
022Linking this library statically or dynamically with other modules is
023making a combined work based on this library.  Thus, the terms and
024conditions of the GNU General Public License cover the whole
025combination.
026
027As a special exception, the copyright holders of this library give you
028permission to link this library with independent modules to produce an
029executable, regardless of the license terms of these independent
030modules, and to copy and distribute the resulting executable under
031terms of your choice, provided that you also meet, for each linked
032independent module, the terms and conditions of the license of that
033module.  An independent module is a module which is not derived from
034or based on this library.  If you modify this library, you may extend
035this exception to your version of the library, but you are not
036obligated to do so.  If you do not wish to do so, delete this
037exception statement from your version. */
038
039
040package java.lang;
041
042import gnu.classpath.SystemProperties;
043
044import java.io.BufferedInputStream;
045import java.io.BufferedOutputStream;
046import java.io.FileDescriptor;
047import java.io.FileInputStream;
048import java.io.FileOutputStream;
049import java.io.IOException;
050import java.io.InputStream;
051import java.io.PrintStream;
052import java.nio.channels.Channel;
053import java.nio.channels.spi.SelectorProvider;
054import java.util.AbstractCollection;
055import java.util.ArrayList;
056import java.util.Collection;
057import java.util.Collections;
058import java.util.HashMap;
059import java.util.Iterator;
060import java.util.List;
061import java.util.Map;
062import java.util.Set;
063import java.util.Properties;
064import java.util.PropertyPermission;
065
066/**
067 * System represents system-wide resources; things that represent the
068 * general environment.  As such, all methods are static.
069 *
070 * @author John Keiser
071 * @author Eric Blake (ebb9@email.byu.edu)
072 * @since 1.0
073 * @status still missing 1.4 functionality
074 */
075public final class System
076{
077  // WARNING: System is a CORE class in the bootstrap cycle. See the comments
078  // in vm/reference/java/lang/Runtime for implications of this fact.
079
080  /**
081   * The standard InputStream. This is assigned at startup and starts its
082   * life perfectly valid. Although it is marked final, you can change it
083   * using {@link #setIn(InputStream)} through some hefty VM magic.
084   *
085   * <p>This corresponds to the C stdin and C++ cin variables, which
086   * typically input from the keyboard, but may be used to pipe input from
087   * other processes or files.  That should all be transparent to you,
088   * however.
089   */
090  public static final InputStream in
091    = new BufferedInputStream(new FileInputStream(FileDescriptor.in));
092  /**
093   * The standard output PrintStream.  This is assigned at startup and
094   * starts its life perfectly valid. Although it is marked final, you can
095   * change it using {@link #setOut(PrintStream)} through some hefty VM magic.
096   *
097   * <p>This corresponds to the C stdout and C++ cout variables, which
098   * typically output normal messages to the screen, but may be used to pipe
099   * output to other processes or files.  That should all be transparent to
100   * you, however.
101   */
102  public static final PrintStream out
103    = new PrintStream(new BufferedOutputStream(new FileOutputStream(FileDescriptor.out)), true);
104  /**
105   * The standard output PrintStream.  This is assigned at startup and
106   * starts its life perfectly valid. Although it is marked final, you can
107   * change it using {@link #setErr(PrintStream)} through some hefty VM magic.
108   *
109   * <p>This corresponds to the C stderr and C++ cerr variables, which
110   * typically output error messages to the screen, but may be used to pipe
111   * output to other processes or files.  That should all be transparent to
112   * you, however.
113   */
114  public static final PrintStream err
115    = new PrintStream(new BufferedOutputStream(new FileOutputStream(FileDescriptor.err)), true);
116
117  /**
118   * A cached copy of the environment variable map.
119   */
120  private static Map<String,String> environmentMap;
121
122  /**
123   * This class is uninstantiable.
124   */
125  private System()
126  {
127  }
128
129  /**
130   * Set {@link #in} to a new InputStream. This uses some VM magic to change
131   * a "final" variable, so naturally there is a security check,
132   * <code>RuntimePermission("setIO")</code>.
133   *
134   * @param in the new InputStream
135   * @throws SecurityException if permission is denied
136   * @since 1.1
137   */
138  public static void setIn(InputStream in)
139  {
140    SecurityManager sm = SecurityManager.current; // Be thread-safe.
141    if (sm != null)
142      sm.checkPermission(new RuntimePermission("setIO"));
143    setIn0(in);
144  }
145
146  /**
147   * Set {@link #out} to a new PrintStream. This uses some VM magic to change
148   * a "final" variable, so naturally there is a security check,
149   * <code>RuntimePermission("setIO")</code>.
150   *
151   * @param out the new PrintStream
152   * @throws SecurityException if permission is denied
153   * @since 1.1
154   */
155  public static void setOut(PrintStream out)
156  {
157    SecurityManager sm = SecurityManager.current; // Be thread-safe.
158    if (sm != null)
159      sm.checkPermission(new RuntimePermission("setIO"));
160    
161    setOut0(out);
162  }
163
164  /**
165   * Set {@link #err} to a new PrintStream. This uses some VM magic to change
166   * a "final" variable, so naturally there is a security check,
167   * <code>RuntimePermission("setIO")</code>.
168   *
169   * @param err the new PrintStream
170   * @throws SecurityException if permission is denied
171   * @since 1.1
172   */
173  public static void setErr(PrintStream err)
174  {
175    SecurityManager sm = SecurityManager.current; // Be thread-safe.
176    if (sm != null)
177      sm.checkPermission(new RuntimePermission("setIO"));
178    setErr0(err);
179  }
180
181  /**
182   * Set the current SecurityManager. If a security manager already exists,
183   * then <code>RuntimePermission("setSecurityManager")</code> is checked
184   * first. Since this permission is denied by the default security manager,
185   * setting the security manager is often an irreversible action.
186   *
187   * @param sm the new SecurityManager
188   * @throws SecurityException if permission is denied
189   */
190  public static synchronized void setSecurityManager(SecurityManager sm)
191  {
192    // Implementation note: the field lives in SecurityManager because of
193    // bootstrap initialization issues. This method is synchronized so that
194    // no other thread changes it to null before this thread makes the change.
195    if (SecurityManager.current != null)
196      SecurityManager.current.checkPermission
197        (new RuntimePermission("setSecurityManager"));
198    SecurityManager.current = sm;
199  }
200
201  /**
202   * Get the current SecurityManager. If the SecurityManager has not been
203   * set yet, then this method returns null.
204   *
205   * @return the current SecurityManager, or null
206   */
207  public static SecurityManager getSecurityManager()
208  {
209    return SecurityManager.current;
210  }
211
212  /**
213   * Get the current time, measured in the number of milliseconds from the
214   * beginning of Jan. 1, 1970. This is gathered from the system clock, with
215   * any attendant incorrectness (it may be timezone dependent).
216   *
217   * @return the current time
218   * @see java.util.Date
219   */
220  public static native long currentTimeMillis();
221
222  /**
223   * Get the current time, measured in nanoseconds.  The result is as
224   * precise as possible, and is measured against a fixed epoch.
225   * However, unlike currentTimeMillis(), the epoch chosen is
226   * arbitrary and may vary by platform, etc.
227   * @since 1.5
228   */
229  public static native long nanoTime();
230
231  /**
232   * Copy one array onto another from <code>src[srcStart]</code> ...
233   * <code>src[srcStart+len-1]</code> to <code>dest[destStart]</code> ...
234   * <code>dest[destStart+len-1]</code>. First, the arguments are validated:
235   * neither array may be null, they must be of compatible types, and the
236   * start and length must fit within both arrays. Then the copying starts,
237   * and proceeds through increasing slots.  If src and dest are the same
238   * array, this will appear to copy the data to a temporary location first.
239   * An ArrayStoreException in the middle of copying will leave earlier
240   * elements copied, but later elements unchanged.
241   *
242   * @param src the array to copy elements from
243   * @param srcStart the starting position in src
244   * @param dest the array to copy elements to
245   * @param destStart the starting position in dest
246   * @param len the number of elements to copy
247   * @throws NullPointerException if src or dest is null
248   * @throws ArrayStoreException if src or dest is not an array, if they are
249   *         not compatible array types, or if an incompatible runtime type
250   *         is stored in dest
251   * @throws IndexOutOfBoundsException if len is negative, or if the start or
252   *         end copy position in either array is out of bounds
253   */
254  public static native void arraycopy(Object src, int srcStart,
255                                      Object dest, int destStart, int len);
256
257  /**
258   * Get a hash code computed by the VM for the Object. This hash code will
259   * be the same as Object's hashCode() method.  It is usually some
260   * convolution of the pointer to the Object internal to the VM.  It
261   * follows standard hash code rules, in that it will remain the same for a
262   * given Object for the lifetime of that Object.
263   *
264   * @param o the Object to get the hash code for
265   * @return the VM-dependent hash code for this Object
266   * @since 1.1
267   */
268  public static native int identityHashCode(Object o);
269
270  /**
271   * Get all the system properties at once. A security check may be performed,
272   * <code>checkPropertiesAccess</code>. Note that a security manager may
273   * allow getting a single property, but not the entire group.
274   *
275   * <p>The required properties include:
276   * <dl>
277   * <dt>java.version</dt>         <dd>Java version number</dd>
278   * <dt>java.vendor</dt>          <dd>Java vendor specific string</dd>
279   * <dt>java.vendor.url</dt>      <dd>Java vendor URL</dd>
280   * <dt>java.home</dt>            <dd>Java installation directory</dd>
281   * <dt>java.vm.specification.version</dt> <dd>VM Spec version</dd>
282   * <dt>java.vm.specification.vendor</dt>  <dd>VM Spec vendor</dd>
283   * <dt>java.vm.specification.name</dt>    <dd>VM Spec name</dd>
284   * <dt>java.vm.version</dt>      <dd>VM implementation version</dd>
285   * <dt>java.vm.vendor</dt>       <dd>VM implementation vendor</dd>
286   * <dt>java.vm.name</dt>         <dd>VM implementation name</dd>
287   * <dt>java.specification.version</dt>    <dd>Java Runtime Environment version</dd>
288   * <dt>java.specification.vendor</dt>     <dd>Java Runtime Environment vendor</dd>
289   * <dt>java.specification.name</dt>       <dd>Java Runtime Environment name</dd>
290   * <dt>java.class.version</dt>   <dd>Java class version number</dd>
291   * <dt>java.class.path</dt>      <dd>Java classpath</dd>
292   * <dt>java.library.path</dt>    <dd>Path for finding Java libraries</dd>
293   * <dt>java.io.tmpdir</dt>       <dd>Default temp file path</dd>
294   * <dt>java.compiler</dt>        <dd>Name of JIT to use</dd>
295   * <dt>java.ext.dirs</dt>        <dd>Java extension path</dd>
296   * <dt>os.name</dt>              <dd>Operating System Name</dd>
297   * <dt>os.arch</dt>              <dd>Operating System Architecture</dd>
298   * <dt>os.version</dt>           <dd>Operating System Version</dd>
299   * <dt>file.separator</dt>       <dd>File separator ("/" on Unix)</dd>
300   * <dt>path.separator</dt>       <dd>Path separator (":" on Unix)</dd>
301   * <dt>line.separator</dt>       <dd>Line separator ("\n" on Unix)</dd>
302   * <dt>user.name</dt>            <dd>User account name</dd>
303   * <dt>user.home</dt>            <dd>User home directory</dd>
304   * <dt>user.dir</dt>             <dd>User's current working directory</dd>
305   * </dl>
306   *
307   * In addition, gnu defines several other properties, where ? stands for
308   * each character in '0' through '9':
309   * <dl>
310   * <dt>gnu.classpath.home</dt>         <dd>Path to the classpath libraries.</dd>
311   * <dt>gnu.classpath.version</dt>      <dd>Version of the classpath libraries.</dd>
312   * <dt>gnu.classpath.vm.shortname</dt> <dd>Succinct version of the VM name;
313   *     used for finding property files in file system</dd>
314   * <dt>gnu.classpath.home.url</dt>     <dd> Base URL; used for finding
315   *     property files in file system</dd>
316   * <dt>gnu.cpu.endian</dt>             <dd>big or little</dd>
317   * <dt>gnu.java.io.encoding_scheme_alias.ISO-8859-?</dt>   <dd>8859_?</dd>
318   * <dt>gnu.java.io.encoding_scheme_alias.iso-8859-?</dt>   <dd>8859_?</dd>
319   * <dt>gnu.java.io.encoding_scheme_alias.iso8859_?</dt>    <dd>8859_?</dd>
320   * <dt>gnu.java.io.encoding_scheme_alias.iso-latin-_?</dt> <dd>8859_?</dd>
321   * <dt>gnu.java.io.encoding_scheme_alias.latin?</dt>       <dd>8859_?</dd>
322   * <dt>gnu.java.io.encoding_scheme_alias.UTF-8</dt>        <dd>UTF8</dd>
323   * <dt>gnu.java.io.encoding_scheme_alias.utf-8</dt>        <dd>UTF8</dd>
324   * <dt>gnu.java.util.zoneinfo.dir</dt>    <dd>Root of zoneinfo tree</dd>
325   * </dl>
326   *
327   * @return the system properties, will never be null
328   * @throws SecurityException if permission is denied
329   */
330  public static Properties getProperties()
331  {
332    SecurityManager sm = SecurityManager.current; // Be thread-safe.
333    if (sm != null)
334      sm.checkPropertiesAccess();
335    return SystemProperties.getProperties();
336  }
337
338  /**
339   * Set all the system properties at once. A security check may be performed,
340   * <code>checkPropertiesAccess</code>. Note that a security manager may
341   * allow setting a single property, but not the entire group. An argument
342   * of null resets the properties to the startup default.
343   *
344   * @param properties the new set of system properties
345   * @throws SecurityException if permission is denied
346   */
347  public static void setProperties(Properties properties)
348  {
349    SecurityManager sm = SecurityManager.current; // Be thread-safe.
350    if (sm != null)
351      sm.checkPropertiesAccess();
352    SystemProperties.setProperties(properties);
353  }
354
355  /**
356   * Get a single system property by name. A security check may be performed,
357   * <code>checkPropertyAccess(key)</code>.
358   *
359   * @param key the name of the system property to get
360   * @return the property, or null if not found
361   * @throws SecurityException if permission is denied
362   * @throws NullPointerException if key is null
363   * @throws IllegalArgumentException if key is ""
364   */
365  public static String getProperty(String key)
366  {
367    SecurityManager sm = SecurityManager.current; // Be thread-safe.
368    if (sm != null)
369      sm.checkPropertyAccess(key);
370    else if (key.length() == 0)
371      throw new IllegalArgumentException("key can't be empty");
372    return SystemProperties.getProperty(key);
373  }
374
375  /**
376   * Get a single system property by name. A security check may be performed,
377   * <code>checkPropertyAccess(key)</code>.
378   *
379   * @param key the name of the system property to get
380   * @param def the default
381   * @return the property, or def if not found
382   * @throws SecurityException if permission is denied
383   * @throws NullPointerException if key is null
384   * @throws IllegalArgumentException if key is ""
385   */
386  public static String getProperty(String key, String def)
387  {
388    SecurityManager sm = SecurityManager.current; // Be thread-safe.
389    if (sm != null)
390      sm.checkPropertyAccess(key);
391    return SystemProperties.getProperty(key, def);
392  }
393
394  /**
395   * Set a single system property by name. A security check may be performed,
396   * <code>checkPropertyAccess(key, "write")</code>.
397   *
398   * @param key the name of the system property to set
399   * @param value the new value
400   * @return the previous value, or null
401   * @throws SecurityException if permission is denied
402   * @throws NullPointerException if key is null
403   * @throws IllegalArgumentException if key is ""
404   * @since 1.2
405   */
406  public static String setProperty(String key, String value)
407  {
408    SecurityManager sm = SecurityManager.current; // Be thread-safe.
409    if (sm != null)
410      sm.checkPermission(new PropertyPermission(key, "write"));
411    return SystemProperties.setProperty(key, value);
412  }
413
414  /**
415   * Remove a single system property by name. A security check may be
416   * performed, <code>checkPropertyAccess(key, "write")</code>.
417   *
418   * @param key the name of the system property to remove
419   * @return the previous value, or null
420   * @throws SecurityException if permission is denied
421   * @throws NullPointerException if key is null
422   * @throws IllegalArgumentException if key is ""
423   * @since 1.5
424   */
425  public static String clearProperty(String key)
426  {
427    SecurityManager sm = SecurityManager.current; // Be thread-safe.
428    if (sm != null)
429      sm.checkPermission(new PropertyPermission(key, "write"));
430    // This handles both the null pointer exception and the illegal
431    // argument exception.
432    if (key.length() == 0)
433      throw new IllegalArgumentException("key can't be empty");
434    return SystemProperties.remove(key);
435  }
436
437  /**
438   * Gets the value of an environment variable.
439   *
440   * @param name the name of the environment variable
441   * @return the string value of the variable or null when the
442   *         environment variable is not defined.
443   * @throws NullPointerException
444   * @throws SecurityException if permission is denied
445   * @since 1.5
446   * @specnote This method was deprecated in some JDK releases, but
447   *           was restored in 1.5.
448   */
449  public static String getenv(String name)
450  {
451    if (name == null)
452      throw new NullPointerException();
453    SecurityManager sm = SecurityManager.current; // Be thread-safe.
454    if (sm != null)
455      sm.checkPermission(new RuntimePermission("getenv." + name));
456    return getenv0(name);
457  }
458
459  /**
460   * <p>
461   * Returns an unmodifiable view of the system environment variables.
462   * If the underlying system does not support environment variables,
463   * an empty map is returned.
464   * </p>
465   * <p>
466   * The returned map is read-only and does not accept queries using
467   * null keys or values, or those of a type other than <code>String</code>.
468   * Attempts to modify the map will throw an
469   * <code>UnsupportedOperationException</code>, while attempts
470   * to pass in a null value will throw a
471   * <code>NullPointerException</code>.  Types other than <code>String</code>
472   * throw a <code>ClassCastException</code>.
473   * </p>
474   * <p>
475   * As the returned map is generated using data from the underlying
476   * platform, it may not comply with the <code>equals()</code>
477   * and <code>hashCode()</code> contracts.  It is also likely that
478   * the keys of this map will be case-sensitive.
479   * </p>
480   * <p>
481   * Use of this method may require a security check for the
482   * RuntimePermission "getenv.*".
483   * </p>
484   *
485   * @return a map of the system environment variables.
486   * @throws SecurityException if the checkPermission method of
487   *         an installed security manager prevents access to
488   *         the system environment variables.
489   * @since 1.5
490   */
491  public static Map<String, String> getenv()
492  {
493    SecurityManager sm = SecurityManager.current; // Be thread-safe.
494    if (sm != null)
495      sm.checkPermission(new RuntimePermission("getenv.*"));
496    if (environmentMap == null)
497      {
498        // List<String> environ = (List<String>)VMSystem.environ();
499        // FIXME
500        List<String> environ = new ArrayList<String>();
501        Map<String,String> variables = new EnvironmentMap();
502        for (String pair : environ)
503          {
504            String[] parts = pair.split("=");
505            variables.put(parts[0], parts[1]);
506          }
507        environmentMap = Collections.unmodifiableMap(variables);
508      }
509    return environmentMap;
510  }
511
512  /**
513   * Terminate the Virtual Machine. This just calls
514   * <code>Runtime.getRuntime().exit(status)</code>, and never returns.
515   * Obviously, a security check is in order, <code>checkExit</code>.
516   *
517   * @param status the exit status; by convention non-zero is abnormal
518   * @throws SecurityException if permission is denied
519   * @see Runtime#exit(int)
520   */
521  public static void exit(int status)
522  {
523    Runtime.getRuntime().exit(status);
524  }
525
526  /**
527   * Calls the garbage collector. This is only a hint, and it is up to the
528   * implementation what this hint suggests, but it usually causes a
529   * best-effort attempt to reclaim unused memory from discarded objects.
530   * This calls <code>Runtime.getRuntime().gc()</code>.
531   *
532   * @see Runtime#gc()
533   */
534  public static void gc()
535  {
536    Runtime.getRuntime().gc();
537  }
538
539  /**
540   * Runs object finalization on pending objects. This is only a hint, and
541   * it is up to the implementation what this hint suggests, but it usually
542   * causes a best-effort attempt to run finalizers on all objects ready
543   * to be reclaimed. This calls
544   * <code>Runtime.getRuntime().runFinalization()</code>.
545   *
546   * @see Runtime#runFinalization()
547   */
548  public static void runFinalization()
549  {
550    Runtime.getRuntime().runFinalization();
551  }
552
553  /**
554   * Tell the Runtime whether to run finalization before exiting the
555   * JVM.  This is inherently unsafe in multi-threaded applications,
556   * since it can force initialization on objects which are still in use
557   * by live threads, leading to deadlock; therefore this is disabled by
558   * default. There may be a security check, <code>checkExit(0)</code>. This
559   * calls <code>Runtime.getRuntime().runFinalizersOnExit()</code>.
560   *
561   * @param finalizeOnExit whether to run finalizers on exit
562   * @throws SecurityException if permission is denied
563   * @see Runtime#runFinalizersOnExit()
564   * @since 1.1
565   * @deprecated never rely on finalizers to do a clean, thread-safe,
566   *             mop-up from your code
567   */
568  public static void runFinalizersOnExit(boolean finalizeOnExit)
569  {
570    Runtime.getRuntime().runFinalizersOnExit(finalizeOnExit);
571  }
572
573  /**
574   * Load a code file using its explicit system-dependent filename. A security
575   * check may be performed, <code>checkLink</code>. This just calls
576   * <code>Runtime.getRuntime().load(filename)</code>.
577   *
578   * <p>
579   * The library is loaded using the class loader associated with the
580   * class associated with the invoking method.
581   *
582   * @param filename the code file to load
583   * @throws SecurityException if permission is denied
584   * @throws UnsatisfiedLinkError if the file cannot be loaded
585   * @see Runtime#load(String)
586   */
587  public static void load(String filename)
588  {
589    Runtime.getRuntime().load(filename);
590  }
591
592  /**
593   * Load a library using its explicit system-dependent filename. A security
594   * check may be performed, <code>checkLink</code>. This just calls
595   * <code>Runtime.getRuntime().load(filename)</code>.
596   *
597   * <p>
598   * The library is loaded using the class loader associated with the
599   * class associated with the invoking method.
600   *
601   * @param libname the library file to load
602   * @throws SecurityException if permission is denied
603   * @throws UnsatisfiedLinkError if the file cannot be loaded
604   * @see Runtime#load(String)
605   */
606  public static void loadLibrary(String libname)
607  {
608    Runtime.getRuntime().loadLibrary(libname);
609  }
610
611  /**
612   * Convert a library name to its platform-specific variant.
613   *
614   * @param libname the library name, as used in <code>loadLibrary</code>
615   * @return the platform-specific mangling of the name
616   * @since 1.2
617   */
618  public static String mapLibraryName(String libname)
619  {
620    // XXX Fix this!!!!
621    return Runtime.nativeGetLibname("", libname);
622  }
623
624  /**
625   * Set {@link #in} to a new InputStream.
626   *
627   * @param in the new InputStream
628   * @see #setIn(InputStream)
629   */
630  private static native void setIn0(InputStream in);
631
632  /**
633   * Set {@link #out} to a new PrintStream.
634   *
635   * @param out the new PrintStream
636   * @see #setOut(PrintStream)
637   */
638  private static native void setOut0(PrintStream out);
639
640  /**
641   * Set {@link #err} to a new PrintStream.
642   *
643   * @param err the new PrintStream
644   * @see #setErr(PrintStream)
645   */
646  private static native void setErr0(PrintStream err);
647
648  /**
649   * Gets the value of an environment variable.
650   *
651   * @see #getenv(String)
652   */
653  static native String getenv0(String name);
654
655  /**
656   * Returns the inherited channel of the VM.
657   *
658   * This wraps the inheritedChannel() call of the system's default
659   * {@link SelectorProvider}.
660   *
661   * @return the inherited channel of the VM
662   *
663   * @throws IOException If an I/O error occurs
664   * @throws SecurityException If an installed security manager denies access
665   *         to RuntimePermission("inheritedChannel")
666   *
667   * @since 1.5
668   */
669  public static Channel inheritedChannel()
670    throws IOException
671  {
672    return SelectorProvider.provider().inheritedChannel();
673  }
674
675  /**
676   * This is a specialised <code>Collection</code>, providing
677   * the necessary provisions for the collections used by the
678   * environment variable map.  Namely, it prevents
679   * querying anything but <code>String</code>s.
680   *
681   * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
682   */
683  private static class EnvironmentCollection
684    extends AbstractCollection<String>
685  {
686
687    /**
688     * The wrapped collection.
689     */
690    protected Collection<String> c;
691
692    /**
693     * Constructs a new environment collection, which
694     * wraps the elements of the supplied collection.
695     *
696     * @param coll the collection to use as a base for
697     *             this collection.
698     */
699    public EnvironmentCollection(Collection<String> coll)
700    {
701      c = coll;
702    }
703        
704    /**
705     * Blocks queries containing a null object or an object which
706     * isn't of type <code>String</code>.  All other queries
707     * are forwarded to the underlying collection.
708     *
709     * @param obj the object to look for.
710     * @return true if the object exists in the collection.
711     * @throws NullPointerException if the specified object is null.
712     * @throws ClassCastException if the specified object is not a String.
713     */
714    public boolean contains(Object obj)
715    {
716      if (obj == null)
717          throw new
718            NullPointerException("This collection does not support " +
719                                 "null values.");
720      if (!(obj instanceof String))
721          throw new
722            ClassCastException("This collection only supports Strings.");
723      return c.contains(obj);
724    }
725    
726    /**
727     * Blocks queries where the collection contains a null object or
728     * an object which isn't of type <code>String</code>.  All other
729     * queries are forwarded to the underlying collection.
730     *
731     * @param coll the collection of objects to look for.
732     * @return true if the collection contains all elements in the collection.
733     * @throws NullPointerException if the collection is null.
734     * @throws NullPointerException if any collection entry is null.
735     * @throws ClassCastException if any collection entry is not a String.
736     */
737    public boolean containsAll(Collection<?> coll)
738    {
739      for (Object o: coll)
740        {
741          if (o == null)
742              throw new
743                NullPointerException("This collection does not support " +
744                                     "null values.");
745          if (!(o instanceof String))
746              throw new
747                ClassCastException("This collection only supports Strings.");
748        }
749      return c.containsAll(coll);
750    }
751
752    /**
753     * This returns an iterator over the map elements, with the
754     * same provisions as for the collection and underlying map.
755     *
756     * @return an iterator over the map elements.
757     */
758    public Iterator<String> iterator()
759    {
760      return c.iterator();
761    }
762    
763    /**
764     * Blocks the removal of elements from the collection.
765     *
766     * @return true if the removal was sucessful.
767     * @throws NullPointerException if the collection is null.
768     * @throws NullPointerException if any collection entry is null.
769     * @throws ClassCastException if any collection entry is not a String.
770     */
771    public boolean remove(Object key)
772    {
773      if (key == null)
774          throw new
775            NullPointerException("This collection does not support " +
776                                 "null values.");
777      if (!(key instanceof String))
778          throw new
779            ClassCastException("This collection only supports Strings.");
780      return c.contains(key);
781    }   
782        
783    /**
784     * Blocks the removal of all elements in the specified
785     * collection from the collection.
786     *
787     * @param coll the collection of elements to remove.
788     * @return true if the elements were removed.
789     * @throws NullPointerException if the collection is null.
790     * @throws NullPointerException if any collection entry is null.
791     * @throws ClassCastException if any collection entry is not a String.
792     */
793    public boolean removeAll(Collection<?> coll)
794    {
795      for (Object o: coll)
796        {
797          if (o == null)
798              throw new
799                NullPointerException("This collection does not support " +
800                                     "null values.");
801          if (!(o instanceof String))
802            throw new
803              ClassCastException("This collection only supports Strings.");
804        }
805      return c.removeAll(coll);
806    }
807    
808    /**
809     * Blocks the retention of all elements in the specified
810     * collection from the collection.
811     *
812     * @param c the collection of elements to retain.
813     * @return true if the other elements were removed.
814     * @throws NullPointerException if the collection is null.
815     * @throws NullPointerException if any collection entry is null.
816     * @throws ClassCastException if any collection entry is not a String.
817     */
818    public boolean retainAll(Collection<?> coll)
819    {
820      for (Object o: coll)
821        {
822          if (o == null)
823              throw new
824                NullPointerException("This collection does not support " +
825                                     "null values.");
826          if (!(o instanceof String))
827            throw new
828              ClassCastException("This collection only supports Strings.");
829        }
830      return c.containsAll(coll);
831    }
832
833    /**
834     * This simply calls the same method on the wrapped
835     * collection.
836     *
837     * @return the size of the underlying collection.
838     */
839    public int size()
840    {
841      return c.size();
842    }
843
844  } // class EnvironmentCollection<String>
845
846  /**
847   * This is a specialised <code>HashMap</code>, which
848   * prevents the addition or querying of anything other than
849   * <code>String</code> objects. 
850   *
851   * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
852   */
853  static class EnvironmentMap
854    extends HashMap<String,String>
855  {
856    
857    /**
858     * Cache the entry set.
859     */
860    private transient Set<Map.Entry<String,String>> entries;
861
862    /**
863     * Cache the key set.
864     */
865    private transient Set<String> keys;
866
867    /**
868     * Cache the value collection.
869     */
870    private transient Collection<String> values;
871
872    /**
873     * Constructs a new empty <code>EnvironmentMap</code>.
874     */
875    EnvironmentMap()
876    {
877      super();
878    }
879
880    /**
881     * Constructs a new <code>EnvironmentMap</code> containing
882     * the contents of the specified map.
883     *
884     * @param m the map to be added to this.
885     * @throws NullPointerException if a key or value is null.
886     * @throws ClassCastException if a key or value is not a String.
887     */    
888    EnvironmentMap(Map<String,String> m)
889    {
890      super(m);
891    }
892
893    /**
894     * Blocks queries containing a null key or one which is not
895     * of type <code>String</code>.  All other queries
896     * are forwarded to the superclass.
897     *
898     * @param key the key to look for in the map.
899     * @return true if the key exists in the map.
900     * @throws NullPointerException if the specified key is null.
901     */
902    public boolean containsKey(Object key)
903    {
904      if (key == null)
905        throw new
906          NullPointerException("This map does not support null keys.");
907      if (!(key instanceof String))
908        throw new
909          ClassCastException("This map only allows queries using Strings.");
910      return super.containsKey(key);
911    }
912    
913    /**
914     * Blocks queries using a null or non-<code>String</code> value.
915     * All other queries are forwarded to the superclass.
916     *
917     * @param value the value to look for in the map.
918     * @return true if the value exists in the map.
919     * @throws NullPointerException if the specified value is null.
920     */
921    public boolean containsValue(Object value)
922    {
923      if (value == null)
924          throw new
925            NullPointerException("This map does not support null values.");
926      if (!(value instanceof String))
927        throw new
928          ClassCastException("This map only allows queries using Strings.");
929      return super.containsValue(value);
930    }
931
932    /**
933     * Returns a set view of the map entries, with the same
934     * provisions as for the underlying map.
935     *
936     * @return a set containing the map entries.
937     */
938    public Set<Map.Entry<String,String>> entrySet()
939    {
940      if (entries == null)
941        entries = super.entrySet();
942      return entries;
943    }
944
945    /**
946     * Blocks queries containing a null or non-<code>String</code> key.
947     * All other queries are passed on to the superclass.
948     *
949     * @param key the key to retrieve the value for.
950     * @return the value associated with the given key.
951     * @throws NullPointerException if the specified key is null.
952     * @throws ClassCastException if the specified key is not a String.
953     */
954    public String get(Object key)
955    {
956      if (key == null)
957        throw new
958          NullPointerException("This map does not support null keys.");
959      if (!(key instanceof String))
960        throw new
961          ClassCastException("This map only allows queries using Strings.");
962      return super.get(key);
963    }
964    
965    /**
966     * Returns a set view of the keys, with the same
967     * provisions as for the underlying map.
968     *
969     * @return a set containing the keys.
970     */
971    public Set<String> keySet()
972    {
973      if (keys == null)
974        keys = new EnvironmentSet(super.keySet());
975      return keys;
976    }
977
978    /**
979     * Associates the given key to the given value. If the
980     * map already contains the key, its value is replaced.
981     * The map does not accept null keys or values, or keys
982     * and values not of type {@link String}.
983     *
984     * @param key the key to map.
985     * @param value the value to be mapped.
986     * @return the previous value of the key, or null if there was no mapping
987     * @throws NullPointerException if a key or value is null.
988     * @throws ClassCastException if a key or value is not a String.
989     */
990    public String put(String key, String value)
991    {
992      if (key == null)
993        throw new NullPointerException("A new key is null.");
994      if (value == null)
995        throw new NullPointerException("A new value is null.");
996      if (!(key instanceof String))
997        throw new ClassCastException("A new key is not a String.");
998      if (!(value instanceof String))
999        throw new ClassCastException("A new value is not a String.");
1000      return super.put(key, value);
1001    }
1002
1003    /**
1004     * Removes a key-value pair from the map.  The queried key may not
1005     * be null or of a type other than a <code>String</code>.
1006     *
1007     * @param key the key of the entry to remove.
1008     * @return the removed value.
1009     * @throws NullPointerException if the specified key is null.
1010     * @throws ClassCastException if the specified key is not a String.
1011     */
1012    public String remove(Object key)
1013    {
1014      if (key == null)
1015        throw new
1016          NullPointerException("This map does not support null keys.");
1017      if (!(key instanceof String))
1018        throw new
1019          ClassCastException("This map only allows queries using Strings.");
1020      return super.remove(key);
1021    }
1022    
1023    /**
1024     * Returns a collection view of the values, with the same
1025     * provisions as for the underlying map.
1026     *
1027     * @return a collection containing the values.
1028     */
1029    public Collection<String> values()
1030    {
1031      if (values == null)
1032        values = new EnvironmentCollection(super.values());
1033      return values;
1034    }
1035    
1036  }
1037
1038  /**
1039   * This is a specialised <code>Set</code>, providing
1040   * the necessary provisions for the collections used by the
1041   * environment variable map.  Namely, it prevents
1042   * modifications and the use of queries with null
1043   * or non-<code>String</code> values.
1044   *
1045   * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
1046   */
1047  private static class EnvironmentSet
1048    extends EnvironmentCollection
1049    implements Set<String>
1050  {
1051
1052    /**
1053     * Constructs a new environment set, which
1054     * wraps the elements of the supplied set.
1055     *
1056     * @param set the set to use as a base for
1057     *             this set.
1058     */
1059    public EnvironmentSet(Set<String> set)
1060    {
1061      super(set);
1062    }
1063
1064    /**
1065     * This simply calls the same method on the wrapped
1066     * collection.
1067     *
1068     * @param obj the object to compare with.
1069     * @return true if the two objects are equal.
1070     */
1071    public boolean equals(Object obj)
1072    {
1073      return c.equals(obj);
1074    }
1075
1076    /**
1077     * This simply calls the same method on the wrapped
1078     * collection.
1079     *
1080     * @return the hashcode of the collection.
1081     */
1082    public int hashCode()
1083    {
1084      return c.hashCode();
1085    }
1086
1087  } // class EnvironmentSet<String>
1088
1089} // class System