java/lang/VMSystem.java

/* VMSystem.java -- helper for java.lang.system
   Copyright (C) 1998, 2002, 2004, 2010  Free Software Foundation, Inc.

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., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301 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
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exception statement from your version. */

package java.lang;

import java.util.List;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.PrintStream;

/**
 * VMSystem is a package-private helper class for System that the
 * VM must implement.
 *
 * @author John Keiser
 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
 */
final class VMSystem
{

  private VMSystem() {} // Prohibits instantiation.

  /**
   * Copy one array onto another from <code>src[srcStart]</code> ...
   * <code>src[srcStart+len-1]</code> to <code>dest[destStart]</code> ...
   * <code>dest[destStart+len-1]</code>. First, the arguments are validated:
   * neither array may be null, they must be of compatible types, and the
   * start and length must fit within both arrays. Then the copying starts,
   * and proceeds through increasing slots.  If src and dest are the same
   * array, this will appear to copy the data to a temporary location first.
   * An ArrayStoreException in the middle of copying will leave earlier
   * elements copied, but later elements unchanged.
   *
   * @param src the array to copy elements from
   * @param srcStart the starting position in src
   * @param dest the array to copy elements to
   * @param destStart the starting position in dest
   * @param len the number of elements to copy
   * @throws NullPointerException if src or dest is null
   * @throws ArrayStoreException if src or dest is not an array, if they are
   *         not compatible array types, or if an incompatible runtime type
   *         is stored in dest
   * @throws IndexOutOfBoundsException if len is negative, or if the start or
   *         end copy position in either array is out of bounds
   */
  static native void arraycopy(Object src, int srcStart,
                               Object dest, int destStart, int len);

  /**
   * Get a hash code computed by the VM for the Object. This hash code will
   * be the same as Object's hashCode() method.  It is usually some
   * convolution of the pointer to the Object internal to the VM.  It
   * follows standard hash code rules, in that it will remain the same for a
   * given Object for the lifetime of that Object.
   *
   * @param o the Object to get the hash code for
   * @return the VM-dependent hash code for this Object
   */
  static native int identityHashCode(Object o);

  /**
   * Set {@link System#in} to a new InputStream.
   *
   * @param in the new InputStream
   * @see #setIn(InputStream)
   */
  static native void setIn(InputStream in);

  /**
   * Set {@link System#out} to a new PrintStream.
   *
   * @param out the new PrintStream
   * @see #setOut(PrintStream)
   */
  static native void setOut(PrintStream out);

  /**
   * Set {@link System#err} to a new PrintStream.
   *
   * @param err the new PrintStream
   * @see #setErr(PrintStream)
   */
  static native void setErr(PrintStream err);

  /**
   * Get the current time, measured in the number of milliseconds from the
   * beginning of Jan. 1, 1970. This is gathered from the system clock, with
   * any attendant incorrectness (it may be timezone dependent).
   *
   * @return the current time
   * @see java.util.Date
   */
  static native long currentTimeMillis();

  /**
   * <p>
   * Returns the current value of a nanosecond-precise system timer.
   * The value of the timer is an offset relative to some arbitrary fixed
   * time, which may be in the future (making the value negative).  This
   * method is useful for timing events where nanosecond precision is
   * required.  This is achieved by calling this method before and after the
   * event, and taking the difference betweent the two times:
   * </p>
   * <p>
   * <code>long startTime = System.nanoTime();</code><br />
   * <code>... <emph>event code</emph> ...</code><br />
   * <code>long endTime = System.nanoTime();</code><br />
   * <code>long duration = endTime - startTime;</code><br />
   * </p>
   * <p>
   * Note that the value is only nanosecond-precise, and not accurate; there
   * is no guarantee that the difference between two values is really a
   * nanosecond.  Also, the value is prone to overflow if the offset
   * exceeds 2^63.
   * </p>
   *
   * @return the time of a system timer in nanoseconds.
   * @since 1.5
   */
  static native long nanoTime();

  /**
   * Returns a list of 'name=value' pairs representing the current environment
   * variables.
   *
   * @return a list of 'name=value' pairs.
   */
  static native List environ();

  /**
   * Helper method which creates the standard input stream.
   * VM implementors may choose to construct these streams differently.
   * This method can also return null if the stream is created somewhere
   * else in the VM startup sequence.
   */
  static InputStream makeStandardInputStream()
  {
    return new BufferedInputStream(new FileInputStream(FileDescriptor.in));
  }

  /**
   * Helper method which creates the standard output stream.
   * VM implementors may choose to construct these streams differently.
   * This method can also return null if the stream is created somewhere
   * else in the VM startup sequence.
   */
  static PrintStream makeStandardOutputStream()
  {
    return new PrintStream(new BufferedOutputStream(new FileOutputStream(FileDescriptor.out)), true);
  }

  /**
   * Helper method which creates the standard error stream.
   * VM implementors may choose to construct these streams differently.
   * This method can also return null if the stream is created somewhere
   * else in the VM startup sequence.
   */
  static PrintStream makeStandardErrorStream()
  {
    return new PrintStream(new BufferedOutputStream(new FileOutputStream(FileDescriptor.err)), true);
  }

  /**
   * Gets the value of an environment variable.
   * Always returning null is a valid (but not very useful) implementation.
   *
   * @param name The name of the environment variable (will not be null).
   * @return The string value of the variable or null when the
   *         environment variable is not defined.
   */
  static native String getenv(String name);
}