javaee中的System介绍--->Properties

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/*
 * Copyright (c) 1994, 2014, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */
package java.lang;

import java.io.*;
import java.security.AccessControlContext;
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.CallerSensitive;
import sun.reflect.Reflection;
import sun.security.util.SecurityConstants;
import sun.reflect.annotation.AnnotationType;

/**
 * The System class contains several useful class fields
 * and methods. It cannot be instantiated.
 *
 * 
Among the facilities provided by the System 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.
     * 

     * For simple stand-alone Java applications, a typical way to write
     * a line of output data is:
     * 
     *     System.out.println(data)
     * 
     * 

     * See the println methods in class PrintStream.
     *
     * @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.
     * 

     * 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 out, 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.
     *
     * 
First, if there is a security manager, its checkPermission
     * method is called with a RuntimePermission("setIO") permission
     *  to see if it's ok to reassign the "standard" input stream.
     * 

     *
     * @param in the new standard input stream.
     *
     * @throws SecurityException
     *        if a security manager exists and its
     *        checkPermission 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.
     *
     * 
First, if there is a security manager, its checkPermission
     * method is called with a RuntimePermission("setIO") 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
     *        checkPermission 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.
     *
     * 
First, if there is a security manager, its checkPermission
     * method is called with a RuntimePermission("setIO") 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
     *        checkPermission 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 null.
     *
     * @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.
     *
     * 
 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. 
     *
     * 
 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 null.
     *
     * @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.
     *
     * 
 If there is a security manager already installed, this method first
     * calls the security manager's checkPermission method
     * with a RuntimePermission("setSecurityManager")
     * permission to ensure it's ok to replace the existing
     * security manager.
     * This may result in throwing a SecurityException.
     *
     * 
 Otherwise, the argument is established as the current
     * security manager. If the argument is null 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 checkPermission 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