/*

 * Copyright (c) 1994, 2011, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code 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

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package java.lang;

import java.io.*;

import java.util.Properties;

import java.util.PropertyPermission;

import java.util.StringTokenizer;

import java.security.AccessController;

import java.security.PrivilegedAction;

import java.security.AllPermission;

import java.nio.channels.Channel;

import java.nio.channels.spi.SelectorProvider;

import sun.nio.ch.Interruptible;

import sun.reflect.Reflection;

import sun.security.util.SecurityConstants;

import sun.reflect.annotation.AnnotationType;

/**

 * The <code>System</code> class contains several useful class fields

 * and methods. It cannot be instantiated.

 *

 * <p>Among the facilities provided by the <code>System</code> class

 * are standard input, standard output, and error output streams;

 * access to externally defined properties and environment

 * variables; a means of loading files and libraries; and a utility

 * method for quickly copying a portion of an array.

 *

 * @author  unascribed

 * @since   JDK1.0

 */

public final class System {

    /* register the natives via the static initializer.

     *

     * VM will invoke the initializeSystemClass method to complete

     * the initialization for this class separated from clinit.

     * Note that to use properties set by the VM, see the constraints

     * described in the initializeSystemClass method.

     */

    private static native void registerNatives();

    static {

        registerNatives();

    }

    /** Don't let anyone instantiate this class */

    private System() {

    }

    /**

     * The "standard" input stream. This stream is already

     * open and ready to supply input data. Typically this stream

     * corresponds to keyboard input or another input source specified by

     * the host environment or user.

     */

    public final static InputStream in = null;

    /**

     * The "standard" output stream. This stream is already

     * open and ready to accept output data. Typically this stream

     * corresponds to display output or another output destination

     * specified by the host environment or user.

     * <p>

     * For simple stand-alone Java applications, a typical way to write

     * a line of output data is:

     * <blockquote><pre>

     *     System.out.println(data)

     * </pre></blockquote>

     * <p>

     * See the <code>println</code> methods in class <code>PrintStream</code>.

     *

     * @see     java.io.PrintStream#println()

     * @see     java.io.PrintStream#println(boolean)

     * @see     java.io.PrintStream#println(char)

     * @see     java.io.PrintStream#println(char[])

     * @see     java.io.PrintStream#println(double)

     * @see     java.io.PrintStream#println(float)

     * @see     java.io.PrintStream#println(int)

     * @see     java.io.PrintStream#println(long)

     * @see     java.io.PrintStream#println(java.lang.Object)

     * @see     java.io.PrintStream#println(java.lang.String)

     */

    public final static PrintStream out = null;

    /**

     * The "standard" error output stream. This stream is already

     * open and ready to accept output data.

     * <p>

     * Typically this stream corresponds to display output or another

     * output destination specified by the host environment or user. By

     * convention, this output stream is used to display error messages

     * or other information that should come to the immediate attention

     * of a user even if the principal output stream, the value of the

     * variable <code>out</code>, has been redirected to a file or other

     * destination that is typically not continuously monitored.

     */

    public final static PrintStream err = null;

    /* The security manager for the system.

     */

    private static volatile SecurityManager security = null;

    /**

     * Reassigns the "standard" input stream.

     *

     * <p>First, if there is a security manager, its <code>checkPermission</code>

     * method is called with a <code>RuntimePermission("setIO")</code> permission

     *  to see if it's ok to reassign the "standard" input stream.

     * <p>

     *

     * @param in the new standard input stream.

     *

     * @throws SecurityException

     *        if a security manager exists and its

     *        <code>checkPermission</code> method doesn't allow

     *        reassigning of the standard input stream.

     *

     * @see SecurityManager#checkPermission

     * @see java.lang.RuntimePermission

     *

     * @since   JDK1.1

     */

    public static void setIn(InputStream in) {

        checkIO();

        setIn0(in);

    }

    /**

     * Reassigns the "standard" output stream.

     *

     * <p>First, if there is a security manager, its <code>checkPermission</code>

     * method is called with a <code>RuntimePermission("setIO")</code> permission

     *  to see if it's ok to reassign the "standard" output stream.

     *

     * @param out the new standard output stream

     *

     * @throws SecurityException

     *        if a security manager exists and its

     *        <code>checkPermission</code> method doesn't allow

     *        reassigning of the standard output stream.

     *

     * @see SecurityManager#checkPermission

     * @see java.lang.RuntimePermission

     *

     * @since   JDK1.1

     */

    public static void setOut(PrintStream out) {

        checkIO();

        setOut0(out);

    }

    /**

     * Reassigns the "standard" error output stream.

     *

     * <p>First, if there is a security manager, its <code>checkPermission</code>

     * method is called with a <code>RuntimePermission("setIO")</code> permission

     *  to see if it's ok to reassign the "standard" error output stream.

     *

     * @param err the new standard error output stream.

     *

     * @throws SecurityException

     *        if a security manager exists and its

     *        <code>checkPermission</code> method doesn't allow

     *        reassigning of the standard error output stream.

     *

     * @see SecurityManager#checkPermission

     * @see java.lang.RuntimePermission

     *

     * @since   JDK1.1

     */

    public static void setErr(PrintStream err) {

        checkIO();

        setErr0(err);

    }

    private static volatile Console cons = null;

    /**

     * Returns the unique {@link java.io.Console Console} object associated

     * with the current Java virtual machine, if any.

     *

     * @return  The system console, if any, otherwise <tt>null</tt>.

     *

     * @since   1.6

     */

     public static Console console() {

         if (cons == null) {

             synchronized (System.class) {

                 cons = sun.misc.SharedSecrets.getJavaIOAccess().console();

             }

         }

         return cons;

     }

    /**

     * Returns the channel inherited from the entity that created this

     * Java virtual machine.

     *

     * <p> This method returns the channel obtained by invoking the

     * {@link java.nio.channels.spi.SelectorProvider#inheritedChannel

     * inheritedChannel} method of the system-wide default

     * {@link java.nio.channels.spi.SelectorProvider} object. </p>

     *

     * <p> In addition to the network-oriented channels described in

     * {@link java.nio.channels.spi.SelectorProvider#inheritedChannel

     * inheritedChannel}, this method may return other kinds of

     * channels in the future.

     *

     * @return  The inherited channel, if any, otherwise <tt>null</tt>.

     *

     * @throws  IOException

     *          If an I/O error occurs

     *

     * @throws  SecurityException

     *          If a security manager is present and it does not

     *          permit access to the channel.

     *

     * @since 1.5

     */

    public static Channel inheritedChannel() throws IOException {

        return SelectorProvider.provider().inheritedChannel();

    }

    private static void checkIO() {

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            sm.checkPermission(new RuntimePermission("setIO"));

        }

    }

    private static native void setIn0(InputStream in);

    private static native void setOut0(PrintStream out);

    private static native void setErr0(PrintStream err);

    /**

     * Sets the System security.

     *

     * <p> If there is a security manager already installed, this method first

     * calls the security manager's <code>checkPermission</code> method

     * with a <code>RuntimePermission("setSecurityManager")</code>

     * permission to ensure it's ok to replace the existing

     * security manager.

     * This may result in throwing a <code>SecurityException</code>.

     *

     * <p> Otherwise, the argument is established as the current

     * security manager. If the argument is <code>null</code> and no

     * security manager has been established, then no action is taken and

     * the method simply returns.

     *

     * @param      s   the security manager.

     * @exception  SecurityException  if the security manager has already

     *             been set and its <code>checkPermission</code> method

     *             doesn't allow it to be replaced.

     * @see #getSecurityManager

     * @see SecurityManager#checkPermission

     * @see java.lang.RuntimePermission

     */

    public static

    void setSecurityManager(final SecurityManager s) {

        try {

            s.checkPackageAccess("java.lang");

        } catch (Exception e) {

            // no-op

        }

        setSecurityManager0(s);

    }

    private static synchronized

    void setSecurityManager0(final SecurityManager s) {

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            // ask the currently installed security manager if we

            // can replace it.

            sm.checkPermission(new RuntimePermission

                                     ("setSecurityManager"));

        }

        if ((s != null) && (s.getClass().getClassLoader() != null)) {

            // New security manager class is not on bootstrap classpath.

            // Cause policy to get initialized before we install the new

            // security manager, in order to prevent infinite loops when

            // trying to initialize the policy (which usually involves

            // accessing some security and/or system properties, which in turn

            // calls the installed security manager's checkPermission method

            // which will loop infinitely if there is a non-system class

            // (in this case: the new security manager class) on the stack).

            AccessController.doPrivileged(new PrivilegedAction<Object>() {

                public Object run() {

                    s.getClass().getProtectionDomain().implies

                        (SecurityConstants.ALL_PERMISSION);

                    return null;

                }

            });

        }

        security = s;

    }

    /**

     * Gets the system security interface.

     *

     * @return  if a security manager has already been established for the

     *          current application, then that security manager is returned;

     *          otherwise, <code>null</code> is returned.

     * @see     #setSecurityManager

     */

    public static SecurityManager getSecurityManager() {

        return security;

    }

    /**

     * Returns the current time in milliseconds.  Note that

     * while the unit of time of the return value is a millisecond,

     * the granularity of the value depends on the underlying

     * operating system and may be larger.  For example, many

     * operating systems measure time in units of tens of

     * milliseconds.

     *

     * <p> See the description of the class <code>Date</code> for

     * a discussion of slight discrepancies that may arise between

     * "computer time" and coordinated universal time (UTC).

     *

     * @return  the difference, measured in milliseconds, between

     *          the current time and midnight, January 1, 1970 UTC.

     * @see     java.util.Date

     */

    public static native long currentTimeMillis();

    /**

     * Returns the current value of the running Java Virtual Machine's

     * high-resolution time source, in nanoseconds.

     *

     * <p>This method can only be used to measure elapsed time and is

     * not related to any other notion of system or wall-clock time.

     * The value returned represents nanoseconds since some fixed but

     * arbitrary <i>origin</i> time (perhaps in the future, so values

     * may be negative).  The same origin is used by all invocations of

     * this method in an instance of a Java virtual machine; other

     * virtual machine instances are likely to use a different origin.

     *

     * <p>This method provides nanosecond precision, but not necessarily

     * nanosecond resolution (that is, how frequently the value changes)

     * - no guarantees are made except that the resolution is at least as

     * good as that of {@link #currentTimeMillis()}.

     *

     * <p>Differences in successive calls that span greater than

     * approximately 292 years (2<sup>63</sup> nanoseconds) will not

     * correctly compute elapsed time due to numerical overflow.

     *

     * <p>The values returned by this method become meaningful only when

     * the difference between two such values, obtained within the same

     * instance of a Java virtual machine, is computed.

     *

     * <p> For example, to measure how long some code takes to execute:

     *  <pre> {@code

     * long startTime = System.nanoTime();

     * // ... the code being measured ...

     * long estimatedTime = System.nanoTime() - startTime;}</pre>

     *

     * <p>To compare two nanoTime values

     *  <pre> {@code

     * long t0 = System.nanoTime();

     * ...

     * long t1 = System.nanoTime();}</pre>

     *

     * one should use {@code t1 - t0 < 0}, not {@code t1 < t0},

     * because of the possibility of numerical overflow.

     *

     * @return the current value of the running Java Virtual Machine's

     *         high-resolution time source, in nanoseconds

     * @since 1.5

     */

    public static native long nanoTime();

    /**

     * Copies an array from the specified source array, beginning at the

     * specified position, to the specified position of the destination array.

     * A subsequence of array components are copied from the source

     * array referenced by <code>src</code> to the destination array

     * referenced by <code>dest</code>. The number of components copied is

     * equal to the <code>length</code> argument. The components at

     * positions <code>srcPos</code> through

     * <code>srcPos+length-1</code> in the source array are copied into

     * positions <code>destPos</code> through

     * <code>destPos+length-1</code>, respectively, of the destination

     * array.

     * <p>

     * If the <code>src</code> and <code>dest</code> arguments refer to the

     * same array object, then the copying is performed as if the

     * components at positions <code>srcPos</code> through

     * <code>srcPos+length-1</code> were first copied to a temporary

     * array with <code>length</code> components and then the contents of

     * the temporary array were copied into positions

     * <code>destPos</code> through <code>destPos+length-1</code> of the

     * destination array.

     * <p>

     * If <code>dest</code> is <code>null</code>, then a

     * <code>NullPointerException</code> is thrown.

     * <p>

     * If <code>src</code> is <code>null</code>, then a

     * <code>NullPointerException</code> is thrown and the destination

     * array is not modified.

     * <p>

     * Otherwise, if any of the following is true, an

     * <code>ArrayStoreException</code> is thrown and the destination is

     * not modified:

     * <ul>

     * <li>The <code>src</code> argument refers to an object that is not an

     *     array.

     * <li>The <code>dest</code> argument refers to an object that is not an

     *     array.

     * <li>The <code>src</code> argument and <code>dest</code> argument refer

     *     to arrays whose component types are different primitive types.

     * <li>The <code>src</code> argument refers to an array with a primitive

     *    component type and the <code>dest</code> argument refers to an array

     *     with a reference component type.

     * <li>The <code>src</code> argument refers to an array with a reference

     *    component type and the <code>dest</code> argument refers to an array

     *     with a primitive component type.

     * </ul>

     * <p>

     * Otherwise, if any of the following is true, an

     * <code>IndexOutOfBoundsException</code> is

     * thrown and the destination is not modified:

     * <ul>

     * <li>The <code>srcPos</code> argument is negative.

     * <li>The <code>destPos</code> argument is negative.

     * <li>The <code>length</code> argument is negative.

     * <li><code>srcPos+length</code> is greater than

     *     <code>src.length</code>, the length of the source array.

     * <li><code>destPos+length</code> is greater than

     *     <code>dest.length</code>, the length of the destination array.

     * </ul>

     * <p>

     * Otherwise, if any actual component of the source array from

     * position <code>srcPos</code> through

     * <code>srcPos+length-1</code> cannot be converted to the component

     * type of the destination array by assignment conversion, an

     * <code>ArrayStoreException</code> is thrown. In this case, let

     * <b><i>k</i></b> be the smallest nonnegative integer less than

     * length such that <code>src[srcPos+</code><i>k</i><code>]</code>

     * cannot be converted to the component type of the destination

     * array; when the exception is thrown, source array components from

     * positions <code>srcPos</code> through

     * <code>srcPos+</code><i>k</i><code>-1</code>

     * will already have been copied to destination array positions

     * <code>destPos</code> through

     * <code>destPos+</code><i>k</I><code>-1</code> and no other

     * positions of the destination array will have been modified.

     * (Because of the restrictions already itemized, this

     * paragraph effectively applies only to the situation where both

     * arrays have component types that are reference types.)

     *

     * @param      src      the source array.

     * @param      srcPos   starting position in the source array.

     * @param      dest     the destination array.

     * @param      destPos  starting position in the destination data.

     * @param      length   the number of array elements to be copied.

     * @exception  IndexOutOfBoundsException  if copying would cause

     *               access of data outside array bounds.

     * @exception  ArrayStoreException  if an element in the <code>src</code>

     *               array could not be stored into the <code>dest</code> array

     *               because of a type mismatch.

     * @exception  NullPointerException if either <code>src</code> or

     *               <code>dest</code> is <code>null</code>.

     */

    public static native void arraycopy(Object src,  int  srcPos,

                                        Object dest, int destPos,

                                        int length);

    /**

     * Returns the same hash code for the given object as

     * would be returned by the default method hashCode(),

     * whether or not the given object's class overrides

     * hashCode().

     * The hash code for the null reference is zero.

     *

     * @param x object for which the hashCode is to be calculated

     * @return  the hashCode

     * @since   JDK1.1

     */

    public static native int identityHashCode(Object x);

    /**

     * System properties. The following properties are guaranteed to be defined:

     * <dl>

     * <dt>java.version         <dd>Java version number

     * <dt>java.vendor          <dd>Java vendor specific string

     * <dt>java.vendor.url      <dd>Java vendor URL

     * <dt>java.home            <dd>Java installation directory

     * <dt>java.class.version   <dd>Java class version number

     * <dt>java.class.path      <dd>Java classpath

     * <dt>os.name              <dd>Operating System Name

     * <dt>os.arch              <dd>Operating System Architecture

     * <dt>os.version           <dd>Operating System Version

     * <dt>file.separator       <dd>File separator ("/" on Unix)

     * <dt>path.separator       <dd>Path separator (":" on Unix)

     * <dt>line.separator       <dd>Line separator ("\n" on Unix)

     * <dt>user.name            <dd>User account name

     * <dt>user.home            <dd>User home directory

     * <dt>user.dir             <dd>User's current working directory

     * </dl>

     */

    private static Properties props;

    private static native Properties initProperties(Properties props);

    /**

     * Determines the current system properties.

     * <p>

     * First, if there is a security manager, its

     * <code>checkPropertiesAccess</code> method is called with no

     * arguments. This may result in a security exception.

     * <p>

     * The current set of system properties for use by the

     * {@link #getProperty(String)} method is returned as a

     * <code>Properties</code> object. If there is no current set of

     * system properties, a set of system properties is first created and

     * initialized. This set of system properties always includes values

     * for the following keys:

     * <table summary="Shows property keys and associated values">

     * <tr><th>Key</th>

     *     <th>Description of Associated Value</th></tr>

     * <tr><td><code>java.version</code></td>

     *     <td>Java Runtime Environment version</td></tr>

     * <tr><td><code>java.vendor</code></td>

     *     <td>Java Runtime Environment vendor</td></tr

     * <tr><td><code>java.vendor.url</code></td>

     *     <td>Java vendor URL</td></tr>

     * <tr><td><code>java.home</code></td>

     *     <td>Java installation directory</td></tr>

     * <tr><td><code>java.vm.specification.version</code></td>

     *     <td>Java Virtual Machine specification version</td></tr>

     * <tr><td><code>java.vm.specification.vendor</code></td>

     *     <td>Java Virtual Machine specification vendor</td></tr>

     * <tr><td><code>java.vm.specification.name</code></td>

     *     <td>Java Virtual Machine specification name</td></tr>

     * <tr><td><code>java.vm.version</code></td>

     *     <td>Java Virtual Machine implementation version</td></tr>

     * <tr><td><code>java.vm.vendor</code></td>

     *     <td>Java Virtual Machine implementation vendor</td></tr>

     * <tr><td><code>java.vm.name</code></td>

     *     <td>Java Virtual Machine implementation name</td></tr>

     * <tr><td><code>java.specification.version</code></td>

     *     <td>Java Runtime Environment specification  version</td></tr>

     * <tr><td><code>java.specification.vendor</code></td>

     *     <td>Java Runtime Environment specification  vendor</td></tr>

     * <tr><td><code>java.specification.name</code></td>

     *     <td>Java Runtime Environment specification  name</td></tr>

     * <tr><td><code>java.class.version</code></td>

     *     <td>Java class format version number</td></tr>

     * <tr><td><code>java.class.path</code></td>

     *     <td>Java class path</td></tr>

     * <tr><td><code>java.library.path</code></td>

     *     <td>List of paths to search when loading libraries</td></tr>

     * <tr><td><code>java.io.tmpdir</code></td>

     *     <td>Default temp file path</td></tr>

     * <tr><td><code>java.compiler</code></td>

     *     <td>Name of JIT compiler to use</td></tr>

     * <tr><td><code>java.ext.dirs</code></td>

     *     <td>Path of extension directory or directories</td></tr>

     * <tr><td><code>os.name</code></td>

     *     <td>Operating system name</td></tr>

     * <tr><td><code>os.arch</code></td>

     *     <td>Operating system architecture</td></tr>

     * <tr><td><code>os.version</code></td>

     *     <td>Operating system version</td></tr>

     * <tr><td><code>file.separator</code></td>

     *     <td>File separator ("/" on UNIX)</td></tr>

     * <tr><td><code>path.separator</code></td>

     *     <td>Path separator (":" on UNIX)</td></tr>

     * <tr><td><code>line.separator</code></td>

     *     <td>Line separator ("\n" on UNIX)</td></tr>

     * <tr><td><code>user.name</code></td>

     *     <td>User's account name</td></tr>

     * <tr><td><code>user.home</code></td>

     *     <td>User's home directory</td></tr>

     * <tr><td><code>user.dir</code></td>

     *     <td>User's current working directory</td></tr>

     * </table>

     * <p>

     * Multiple paths in a system property value are separated by the path

     * separator character of the platform.

     * <p>

     * Note that even if the security manager does not permit the

     * <code>getProperties</code> operation, it may choose to permit the

     * {@link #getProperty(String)} operation.

     *

     * @return     the system properties

     * @exception  SecurityException  if a security manager exists and its

     *             <code>checkPropertiesAccess</code> method doesn't allow access

     *              to the system properties.

     * @see        #setProperties

     * @see        java.lang.SecurityException

     * @see        java.lang.SecurityManager#checkPropertiesAccess()

     * @see        java.util.Properties

     */

    public static Properties getProperties() {

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            sm.checkPropertiesAccess();

        }

        return props;

    }

    /**

     * Returns the system-dependent line separator string.  It always

     * returns the same value - the initial value of the {@linkplain

     * #getProperty(String) system property} {@code line.separator}.

     *

     * <p>On UNIX systems, it returns {@code "\n"}; on Microsoft

     * Windows systems it returns {@code "\r\n"}.

     */

    public static String lineSeparator() {

        return lineSeparator;

    }

    private static String lineSeparator;

    /**

     * Sets the system properties to the <code>Properties</code>

     * argument.

     * <p>

     * First, if there is a security manager, its

     * <code>checkPropertiesAccess</code> method is called with no

     * arguments. This may result in a security exception.

     * <p>

     * The argument becomes the current set of system properties for use

     * by the {@link #getProperty(String)} method. If the argument is

     * <code>null</code>, then the current set of system properties is

     * forgotten.

     *

     * @param      props   the new system properties.

     * @exception  SecurityException  if a security manager exists and its

     *             <code>checkPropertiesAccess</code> method doesn't allow access

     *              to the system properties.

     * @see        #getProperties

     * @see        java.util.Properties

     * @see        java.lang.SecurityException

     * @see        java.lang.SecurityManager#checkPropertiesAccess()

     */

    public static void setProperties(Properties props) {

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            sm.checkPropertiesAccess();

        }

        if (props == null) {

            props = new Properties();

            initProperties(props);

        }

        System.props = props;

    }

    /**

     * Gets the system property indicated by the specified key.

     * <p>

     * First, if there is a security manager, its

     * <code>checkPropertyAccess</code> method is called with the key as

     * its argument. This may result in a SecurityException.

     * <p>

     * If there is no current set of system properties, a set of system

     * properties is first created and initialized in the same manner as

     * for the <code>getProperties</code> method.

     *

     * @param      key   the name of the system property.

     * @return     the string value of the system property,

     *             or <code>null</code> if there is no property with that key.

     *

     * @exception  SecurityException  if a security manager exists and its

     *             <code>checkPropertyAccess</code> method doesn't allow

     *              access to the specified system property.

     * @exception  NullPointerException if <code>key</code> is

     *             <code>null</code>.

     * @exception  IllegalArgumentException if <code>key</code> is empty.

     * @see        #setProperty

     * @see        java.lang.SecurityException

     * @see        java.lang.SecurityManager#checkPropertyAccess(java.lang.String)

     * @see        java.lang.System#getProperties()

     */

    public static String getProperty(String key) {

        checkKey(key);

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            sm.checkPropertyAccess(key);

        }

        return props.getProperty(key);

    }

    /**

     * Gets the system property indicated by the specified key.

     * <p>

     * First, if there is a security manager, its

     * <code>checkPropertyAccess</code> method is called with the

     * <code>key</code> as its argument.

     * <p>

     * If there is no current set of system properties, a set of system

     * properties is first created and initialized in the same manner as

     * for the <code>getProperties</code> method.

     *

     * @param      key   the name of the system property.

     * @param      def   a default value.

     * @return     the string value of the system property,

     *             or the default value if there is no property with that key.

     *

     * @exception  SecurityException  if a security manager exists and its

     *             <code>checkPropertyAccess</code> method doesn't allow

     *             access to the specified system property.

     * @exception  NullPointerException if <code>key</code> is

     *             <code>null</code>.

     * @exception  IllegalArgumentException if <code>key</code> is empty.

     * @see        #setProperty

     * @see        java.lang.SecurityManager#checkPropertyAccess(java.lang.String)

     * @see        java.lang.System#getProperties()

     */

    public static String getProperty(String key, String def) {

        checkKey(key);

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            sm.checkPropertyAccess(key);

        }

        return props.getProperty(key, def);

    }

    /**

     * Sets the system property indicated by the specified key.

     * <p>

     * First, if a security manager exists, its

     * <code>SecurityManager.checkPermission</code> method

     * is called with a <code>PropertyPermission(key, "write")</code>

     * permission. This may result in a SecurityException being thrown.

     * If no exception is thrown, the specified property is set to the given

     * value.

     * <p>

     *

     * @param      key   the name of the system property.

     * @param      value the value of the system property.

     * @return     the previous value of the system property,

     *             or <code>null</code> if it did not have one.

     *

     * @exception  SecurityException  if a security manager exists and its

     *             <code>checkPermission</code> method doesn't allow

     *             setting of the specified property.

     * @exception  NullPointerException if <code>key</code> or

     *             <code>value</code> is <code>null</code>.

     * @exception  IllegalArgumentException if <code>key</code> is empty.

     * @see        #getProperty

     * @see        java.lang.System#getProperty(java.lang.String)

     * @see        java.lang.System#getProperty(java.lang.String, java.lang.String)

     * @see        java.util.PropertyPermission

     * @see        SecurityManager#checkPermission

     * @since      1.2

     */

    public static String setProperty(String key, String value) {

        checkKey(key);

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            sm.checkPermission(new PropertyPermission(key,

                SecurityConstants.PROPERTY_WRITE_ACTION));

        }

        return (String) props.setProperty(key, value);

    }

    /**

     * Removes the system property indicated by the specified key.

     * <p>

     * First, if a security manager exists, its

     * <code>SecurityManager.checkPermission</code> method

     * is called with a <code>PropertyPermission(key, "write")</code>

     * permission. This may result in a SecurityException being thrown.

     * If no exception is thrown, the specified property is removed.

     * <p>

     *

     * @param      key   the name of the system property to be removed.

     * @return     the previous string value of the system property,

     *             or <code>null</code> if there was no property with that key.

     *

     * @exception  SecurityException  if a security manager exists and its

     *             <code>checkPropertyAccess</code> method doesn't allow

     *              access to the specified system property.

     * @exception  NullPointerException if <code>key</code> is

     *             <code>null</code>.

     * @exception  IllegalArgumentException if <code>key</code> is empty.

     * @see        #getProperty

     * @see        #setProperty

     * @see        java.util.Properties

     * @see        java.lang.SecurityException

     * @see        java.lang.SecurityManager#checkPropertiesAccess()

     * @since 1.5

     */

    public static String clearProperty(String key) {

        checkKey(key);

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            sm.checkPermission(new PropertyPermission(key, "write"));

        }

        return (String) props.remove(key);

    }

    private static void checkKey(String key) {

        if (key == null) {

            throw new NullPointerException("key can't be null");

        }

        if (key.equals("")) {

            throw new IllegalArgumentException("key can't be empty");

        }

    }

    /**

     * Gets the value of the specified environment variable. An

     * environment variable is a system-dependent external named

     * value.

     *

     * <p>If a security manager exists, its

     * {@link SecurityManager#checkPermission checkPermission}

     * method is called with a

     * <code>{@link RuntimePermission}("getenv."+name)</code>

     * permission.  This may result in a {@link SecurityException}

     * being thrown.  If no exception is thrown the value of the

     * variable <code>name</code> is returned.

     *

     * <p><a name="EnvironmentVSSystemProperties"><i>System

     * properties</i> and <i>environment variables</i></a> are both

     * conceptually mappings between names and values.  Both

     * mechanisms can be used to pass user-defined information to a

     * Java process.  Environment variables have a more global effect,

     * because they are visible to all descendants of the process

     * which defines them, not just the immediate Java subprocess.

     * They can have subtly different semantics, such as case

     * insensitivity, on different operating systems.  For these

     * reasons, environment variables are more likely to have

     * unintended side effects.  It is best to use system properties

     * where possible.  Environment variables should be used when a

     * global effect is desired, or when an external system interface

     * requires an environment variable (such as <code>PATH</code>).

     *

     * <p>On UNIX systems the alphabetic case of <code>name</code> is

     * typically significant, while on Microsoft Windows systems it is

     * typically not.  For example, the expression

     * <code>System.getenv("FOO").equals(System.getenv("foo"))</code>

     * is likely to be true on Microsoft Windows.

     *

     * @param  name the name of the environment variable

     * @return the string value of the variable, or <code>null</code>

     *         if the variable is not defined in the system environment

     * @throws NullPointerException if <code>name</code> is <code>null</code>

     * @throws SecurityException

     *         if a security manager exists and its

     *         {@link SecurityManager#checkPermission checkPermission}

     *         method doesn't allow access to the environment variable

     *         <code>name</code>

     * @see    #getenv()

     * @see    ProcessBuilder#environment()

     */

    public static String getenv(String name) {

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            sm.checkPermission(new RuntimePermission("getenv."+name));

        }

        return ProcessEnvironment.getenv(name);

    }

    /**

     * Returns an unmodifiable string map view of the current system environment.

     * The environment is a system-dependent mapping from names to

     * values which is passed from parent to child processes.

     *

     * <p>If the system does not support environment variables, an

     * empty map is returned.

     *

     * <p>The returned map will never contain null keys or values.

     * Attempting to query the presence of a null key or value will

     * throw a {@link NullPointerException}.  Attempting to query

     * the presence of a key or value which is not of type

     * {@link String} will throw a {@link ClassCastException}.

     *

     * <p>The returned map and its collection views may not obey the

     * general contract of the {@link Object#equals} and

     * {@link Object#hashCode} methods.

     *

     * <p>The returned map is typically case-sensitive on all platforms.

     *

     * <p>If a security manager exists, its

     * {@link SecurityManager#checkPermission checkPermission}

     * method is called with a

     * <code>{@link RuntimePermission}("getenv.*")</code>

     * permission.  This may result in a {@link SecurityException} being

     * thrown.

     *

     * <p>When passing information to a Java subprocess,

     * <a href=#EnvironmentVSSystemProperties>system properties</a>

     * are generally preferred over environment variables.

     *

     * @return the environment as a map of variable names to values

     * @throws SecurityException

     *         if a security manager exists and its

     *         {@link SecurityManager#checkPermission checkPermission}

     *         method doesn't allow access to the process environment

     * @see    #getenv(String)

     * @see    ProcessBuilder#environment()

     * @since  1.5

     */

    public static java.util.Map<String,String> getenv() {

        SecurityManager sm = getSecurityManager();

        if (sm != null) {

            sm.checkPermission(new RuntimePermission("getenv.*"));

        }

        return ProcessEnvironment.getenv();

    }

    /**

     * Terminates the currently running Java Virtual Machine. The

     * argument serves as a status code; by convention, a nonzero status

     * code indicates abnormal termination.

     * <p>

     * This method calls the <code>exit</code> method in class

     * <code>Runtime</code>. This method never returns normally.

     * <p>

     * The call <code>System.exit(n)</code> is effectively equivalent to

     * the call:

     * <blockquote><pre>

     * Runtime.getRuntime().exit(n)

     * </pre></blockquote>

     *

     * @param      status   exit status.

     * @throws  SecurityException

     *        if a security manager exists and its <code>checkExit</code>

     *        method doesn't allow exit with the specified status.

     * @see        java.lang.Runtime#exit(int)

     */

    public static void exit(int status) {

        Runtime.getRuntime().exit(status);

    }

    /**

     * Runs the garbage collector.

     * <p>

     * Calling the <code>gc</code> method suggests that the Java Virtual

     * Machine expend effort toward recycling unused objects in order to

     * make the memory they currently occupy available for quick reuse.

     * When control returns from the method call, the Java Virtual

     * Machine has made a best effort to reclaim space from all discarded

     * objects.

     * <p>

     * The call <code>System.gc()</code> is effectively equivalent to the

     * call:

     * <blockquote><pre>

     * Runtime.getRuntime().gc()

     * </pre></blockquote>

     *

     * @see     java.lang.Runtime#gc()

     */

    public static void gc() {

        Runtime.getRuntime().gc();

    }

    /**

     * Runs the finalization methods of any objects pending finalization.

     * <p>

     * Calling this method suggests that the Java Virtual Machine expend

     * effort toward running the <code>finalize</code> methods of objects

     * that have been found to be discarded but whose <code>finalize</code>

     * methods have not yet been run. When control returns from the

     * method call, the Java Virtual Machine has made a best effort to

     * complete all outstanding finalizations.

     * <p>