runtime.java

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/* Runtime.java -- access to the VM process
   Copyright (C) 1998, 2002 Free Software Foundation

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.

GNU Classpath is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

You should have received a copy of the GNU General Public License
along with GNU Classpath; see the file COPYING.  If not, write to the
Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA.

Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
conditions of the GNU General Public License cover the whole
combination.

As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent
modules, and to copy and distribute the resulting executable under
terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
module.  An independent module is a module which is not derived from
or based on this library.  If you modify this library, you may extend
this exception to your version of the library, but you are not
obligated to do so.  If you do not wish to do so, delete this
exception statement from your version. */

package java.lang;

import java.io.File;
import java.io.InputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Properties;
import java.util.Set;
import java.util.StringTokenizer;

/**
 * Runtime represents the Virtual Machine.
 *
 * @author John Keiser
 * @author Eric Blake <[e-mail]>
 * @status still missing 1.4 functionality
 */
// No idea why this class isn't final, since you can't build a subclass!
public class Runtime
{
  /**
   * The library path, to search when loading libraries. We can also safely use
   * this as a lock for synchronization.
   */
  private final String[] libpath;

  /**
   * The current security manager. This is located here instead of in
   * Runtime, to avoid security problems, as well as bootstrap issues.
   * Make sure to access it in a thread-safe manner; it is package visible
   * to avoid overhead in java.lang.
   */
  static SecurityManager securityManager;

  /**
   * The default properties defined by the system. This is likewise located
   * here instead of in Runtime, to avoid bootstrap issues; it is package
   * visible to avoid overhead in java.lang. Note that System will add a
   * few more properties to this collection, but that after that, it is
   * treated as read-only.
   *
   * No matter what class you start initialization with, it defers to the
   * superclass, therefore Object.<clinit> will be the first Java code
   * executed. From there, the bootstrap sequence, up to the point that
   * native libraries are loaded (as of March 24, when I traced this
   * manually) is as follows:
   *
   * Object.<clinit> uses a String literal, possibly triggering initialization
   *  String.<clinit> calls WeakHashMap.<init>, triggering initialization
   *   AbstractMap, WeakHashMap, WeakHashMap$1 have no dependencies
   *  String.<clinit> calls CaseInsensitiveComparator.<init>, triggering
   *      initialization
   *   CaseInsensitiveComparator has no dependencies
   * Object.<clinit> calls System.loadLibrary, triggering initialization
   *  System.<clinit> calls System.loadLibrary
   *  System.loadLibrary calls Runtime.getRuntime, triggering initialization
   *   Runtime.<clinit> calls Properties.<init>, triggering initialization
   *    Dictionary, Hashtable, and Properties have no dependencies
   *   Runtime.<clinit> calls insertSystemProperties; the VM must make sure
   *      that there are not any harmful dependencies
   *   Runtime.<clinit> calls Runtime.<init>
   *    Runtime.<init> calls StringTokenizer.<init>, triggering initialization
   *     StringTokenizer has no dependencies
   *  System.loadLibrary calls Runtime.loadLibrary
   *   Runtime.loadLibrary should be able to load the library, although it
   *       will probably set off another string of initializations from
   *       ClassLoader first
   */
  static Properties defaultProperties = new Properties();
  static
  {
    insertSystemProperties(defaultProperties);
  }

  /**
   * The thread that started the exit sequence. Access to this field must
   * be thread-safe; lock on libpath to avoid deadlock with user code.
   * <code>runFinalization()</code> may want to look at this to see if ALL
   * finalizers should be run, because the virtual machine is about to halt.
   */
  private Thread exitSequence;

  /**
   * All shutdown hooks. This is initialized lazily, and set to null once all
   * shutdown hooks have run. Access to this field must be thread-safe; lock
   * on libpath to avoid deadlock with user code.
   */
  private Set shutdownHooks;

  /** True if we should finalize on exit.  */
  private boolean finalizeOnExit;

  /**
   * The one and only runtime instance. This must appear after the default
   * properties have been initialized by the VM.
   */
  private static final Runtime current = new Runtime();

  /**
   * Not instantiable by a user, this should only create one instance.
   */
  private Runtime()
  {
    if (current != null)
      throw new InternalError("Attempt to recreate Runtime");

    // We don't use libpath in the libgcj implementation.  We still
    // set it to something to allow the various synchronizations to
    // work.
    libpath = new String[0];

    init ();
  }

  /**
   * Get the current Runtime object for this JVM. This is necessary to access
   * the many instance methods of this class.
   *
   * @return the current Runtime object
   */
  public static Runtime getRuntime()
  {
    return current;
  }

  /**
   * Exit the Java runtime. This method will either throw a SecurityException
   * or it will never return. The status code is returned to the system; often
   * a non-zero status code indicates an abnormal exit. Of course, there is a
   * security check, <code>checkExit(status)</code>.
   *
   * <p>First, all shutdown hooks are run, in unspecified order, and
   * concurrently. Next, if finalization on exit has been enabled, all pending
   * finalizers are run. Finally, the system calls <code>halt</code>.
   *
   * <p>If this is run a second time after shutdown has already started, there
   * are two actions. If shutdown hooks are still executing, it blocks
   * indefinitely. Otherwise, if the status is nonzero it halts immediately;
   * if it is zero, it blocks indefinitely. This is typically called by
   * <code>System.exit</code>.
   *
   * @param status the status to exit with
   * @throws SecurityException if permission is denied
   * @see #addShutdownHook(Thread)
   * @see #runFinalizersOnExit(boolean)
   * @see #runFinalization()
   * @see #halt(int)
   */
  public void exit(int status)
  {
    SecurityManager sm = securityManager; // Be thread-safe!
    if (sm != null)
      sm.checkExit(status);
    boolean first = false;
    synchronized (libpath) // Synch on libpath, not this, to avoid deadlock.
      {
        if (exitSequence == null)
          {
            first = true;
            exitSequence = Thread.currentThread();
            if (shutdownHooks != null)
              {
                Iterator i = shutdownHooks.iterator();
                while (i.hasNext()) // Start all shutdown hooks.
                  try
                    {
                      ((Thread) i.next()).start();
                    }
                  catch (IllegalThreadStateException e)
                    {
                      i.remove();
                    }
              }
          }
      }
    if (first)
      {
        if (shutdownHooks != null)
          {
            // Check progress of all shutdown hooks. As a hook completes,
            // remove it from the set. If a hook calls exit, it removes
            // itself from the set, then waits indefinitely on the
            // exitSequence thread. Once the set is empty, set it to null to
            // signal all finalizer threads that halt may be called.
            while (! shutdownHooks.isEmpty())
              {
                Thread[] hooks;
                synchronized (libpath)
                  {
                    hooks = new Thread[shutdownHooks.size()];
                    shutdownHooks.toArray(hooks);
                  }
                for (int i = hooks.length; --i >= 0; )
                  if (! hooks[i].isAlive())
                    synchronized (libpath)
                      {
                        shutdownHooks.remove(hooks[i]);
                      }
                try
                  {
                    exitSequence.sleep(1); // Give other threads a chance.
                  }
                catch (InterruptedException e)
                  {
                    // Ignore, the next loop just starts sooner.
                  }
              }
            synchronized (libpath)
              {
                shutdownHooks = null;
              }
          }
        // XXX Right now, it is the VM that knows whether runFinalizersOnExit
        // is true; so the VM must look at exitSequence to decide whether
        // this should be run on every object.
        runFinalization();
      }
    else
      synchronized (libpath)
        {
          if (shutdownHooks != null)
            {
              shutdownHooks.remove(Thread.currentThread());
              status = 0; // Change status to enter indefinite wait.
            }
        }
    
    if (first || status > 0)
      halt(status);
    while (true)
      try
        {
          exitSequence.join();
        }
      catch (InterruptedException e)
        {
          // Ignore, we've suspended indefinitely to let all shutdown
          // hooks complete, and to let any non-zero exits through, because
          // this is a duplicate call to exit(0).
        }
  }

  /**
   * Register a new shutdown hook. This is invoked when the program exits
   * normally (because all non-daemon threads ended, or because
   * <code>System.exit</code> was invoked), or when the user terminates
   * the virtual machine (such as by typing ^C, or logging off). There is
   * a security check to add hooks,
   * <code>RuntimePermission("shutdownHooks")<code>.
   *
   * <p>The hook must be an initialized, but unstarted Thread. The threads
   * are run concurrently, and started in an arbitrary order; and user
   * threads or daemons may still be running. Once shutdown hooks have
   * started, they must all complete, or else you must use <code>halt</code>,
   * to actually finish the shutdown sequence. Attempts to modify hooks
   * after shutdown has started result in IllegalStateExceptions.
   *
   * <p>It is imperative that you code shutdown hooks defensively, as you
   * do not want to deadlock, and have no idea what other hooks will be
   * running concurrently. It is also a good idea to finish quickly, as the
   * virtual machine really wants to shut down!
   *
   * <p>There are no guarantees that such hooks will run, as there are ways
   * to forcibly kill a process. But in such a drastic case, shutdown hooks
   * would do little for you in the first place.
   *
   * @param hook an initialized, unstarted Thread
   * @throws IllegalArgumentException if the hook is already registered or run
   * @throws IllegalStateException if the virtual machine is already in
   *         the shutdown sequence
   * @throws SecurityException if permission is denied
   * @since 1.3
   * @see #removeShutdownHook(Thread)
   * @see #exit(int)
   * @see #halt(int)
   */
  public void addShutdownHook(Thread hook)
  {
    SecurityManager sm = securityManager; // Be thread-safe!
    if (sm != null)
      sm.checkPermission(new RuntimePermission("shutdownHooks"));
    if (hook.isAlive() || hook.getThreadGroup() == null)
      throw new IllegalArgumentException();
    synchronized (libpath)
      {
        if (exitSequence != null)
          throw new IllegalStateException();
        if (shutdownHooks == null)
          shutdownHooks = new HashSet(); // Lazy initialization.
        if (! shutdownHooks.add(hook))
          throw new IllegalArgumentException();
      }
  }

  /**
   * De-register a shutdown hook. As when you registered it, there is a
   * security check to remove hooks,
   * <code>RuntimePermission("shutdownHooks")<code>.
   *
   * @param hook the hook to remove
   * @return true if the hook was successfully removed, false if it was not
   *         registered in the first place
   * @throws IllegalStateException if the virtual machine is already in
   *         the shutdown sequence
   * @throws SecurityException if permission is denied
   * @since 1.3
   * @see #addShutdownHook(Thread)
   * @see #exit(int)
   * @see #halt(int)
   */
  public boolean removeShutdownHook(Thread hook)
  {
    SecurityManager sm = securityManager; // Be thread-safe!
    if (sm != null)
      sm.checkPermission(new RuntimePermission("shutdownHooks"));
    synchronized (libpath)
      {
        if (exitSequence != null)
          throw new IllegalStateException();
        if (shutdownHooks != null)
          return shutdownHooks.remove(hook);
      }
    return false;
  }

  /**
   * Forcibly terminate the virtual machine. This call never returns. It is
   * much more severe than <code>exit</code>, as it bypasses all shutdown
   * hooks and initializers. Use caution in calling this! Of course, there is
   * a security check, <code>checkExit(status)</code>.
   *
   * @param status the status to exit with
   * @throws SecurityException if permission is denied
   * @since 1.3
   * @see #exit(int)
   * @see #addShutdownHook(Thread)
   */
  public void halt(int status)
  {
    SecurityManager sm = securityManager; // Be thread-safe!
    if (sm != null)
      sm.checkExit(status);
    exitInternal(status);
  }

  /**
   * Tell the VM to run the finalize() method on every single Object before
   * it exits.  Note that the JVM may still exit abnormally and not perform
   * this, so you still don't have a guarantee. And besides that, this is
   * inherently unsafe in multi-threaded code, as it may result in deadlock
   * as multiple threads compete to manipulate objects. This value defaults to