java获取系统属性System.getProperty

旧城等待， 2022-05-15 09:00 208阅读 0赞

在java中System.getProperty可以获取系统的属性，这个大家都知道。但是能获取到那些属性呢？

今天看了一下源码，如下：

/*
 * @(#)System.java	1.158 06/03/13
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
 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.net.InetAddressCachePolicy;
 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.
 *
 * 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
 * @version 1.158, 03/13/06
 * @since JDK1.0
 */
 public final class System {
 
 /* First thing---register the natives */
 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 = nullInputStream();
 
 /**
 * 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:
 * <blockquote><pre>
 * System.out.println(data)
 * </pre></blockquote>
 * 
 * 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 = nullPrintStream();
 
 /**
 * 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 <code>out</code>, has been redirected to a file or other
 * destination that is typically not continuously monitored.
 */
 public final static PrintStream err = nullPrintStream();
 
 /* 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 <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.
 * 
 *
 * @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.
 *
 * 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.
 *
 * 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.
 *
 * 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 <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.
 *
 * 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>.
 *
 * 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() {
 		public Object run() {
 		 s.getClass().getProtectionDomain().implies
 			(SecurityConstants.ALL_PERMISSION);
 		 return null;
 		}
 	 });
 	}
 
 	security = s;
 	InetAddressCachePolicy.setIfNotSet(InetAddressCachePolicy.FOREVER);
 }
 
 /**
 * 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.
 *
 * 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 most precise available system
 * timer, in nanoseconds.
 *
 * 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 time (perhaps in the future, so values may be
 * negative). This method provides nanosecond precision, but not
 * necessarily nanosecond accuracy. No guarantees are made about
 * how frequently values change. Differences in successive calls
 * that span greater than approximately 292 years (263
 * nanoseconds) will not accurately compute elapsed time due to
 * numerical overflow.
 *
 * For example, to measure how long some code takes to execute:
 * <pre>
 * long startTime = System.nanoTime();
 * // ... the code being measured ...
 * long estimatedTime = System.nanoTime() - startTime;
 * </pre>
 * 
 * @return The current value of the system timer, 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.
 * 
 * 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.
 * 
 * If <code>dest</code> is <code>null</code>, then a
 * <code>NullPointerException</code> is thrown.
 * 
 * If <code>src</code> is <code>null</code>, then a
 * <code>NullPointerException</code> is thrown and the destination
 * array is not modified.
 * 
 * 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>
 * 
 * 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>
 * 
 * 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
 * k be the smallest nonnegative integer less than
 * length such that <code>src[srcPos+</code>k<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>k<code>-1</code>
 * will already have been copied to destination array positions
 * <code>destPos</code> through
 * <code>destPos+</code>k<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.
 * 
 * First, if there is a security manager, its
 * <code>checkPropertiesAccess</code> method is called with no
 * arguments. This may result in a security exception.
 * 
 * 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>
 * 
 * Multiple paths in a system property value are separated by the path
 * separator character of the platform.
 * 
 * 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;
 }
 
 /**
 * Sets the system properties to the <code>Properties</code>
 * argument.
 * 
 * First, if there is a security manager, its
 * <code>checkPropertiesAccess</code> method is called with no
 * arguments. This may result in a security exception.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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.
 * 
 * First, if there is a security manager, its
 * <code>checkPropertyAccess</code> method is called with the
 * <code>key</code> as its argument.
 * 
 * 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.
 * 
 * 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.
 * 
 *
 * @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. 
 * 
 * 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.
 * 
 *
 * @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.
 *
 * 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.
 *
 * <a name="EnvironmentVSSystemProperties">System
 * properties and environment variables</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>).
 *
 * 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.
 *
 * If the system does not support environment variables, an
 * empty map is returned.
 *
 * 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}.
 *
 * The returned map and its collection views may not obey the
 * general contract of the {@link Object#equals} and
 * {@link Object#hashCode} methods.
 *
 * The returned map is typically case-sensitive on all platforms.
 *
 * 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.
 *
 * 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.
 * 
 * This method calls the <code>exit</code> method in class
 * <code>Runtime</code>. This method never returns normally.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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.
 * 
 * The call <code>System.runFinalization()</code> is effectively
 * equivalent to the call:
 * <blockquote><pre>
 * Runtime.getRuntime().runFinalization()
 * </pre></blockquote>
 *
 * @see java.lang.Runtime#runFinalization()
 */
 public static void runFinalization() {
 	Runtime.getRuntime().runFinalization();
 }
 
 /**
 * Enable or disable finalization on exit; doing so specifies that the
 * finalizers of all objects that have finalizers that have not yet been
 * automatically invoked are to be run before the Java runtime exits.
 * By default, finalization on exit is disabled.
 *
 * If there is a security manager,
 * its <code>checkExit</code> method is first called
 * with 0 as its argument to ensure the exit is allowed.
 * This could result in a SecurityException.
 *
 * @deprecated This method is inherently unsafe. It may result in
 * 	 finalizers being called on live objects while other threads are
 * concurrently manipulating those objects, resulting in erratic
 *	 behavior or deadlock.
 * @param value indicating enabling or disabling of finalization
 * @throws SecurityException
 * if a security manager exists and its <code>checkExit</code>
 * method doesn't allow the exit.
 *
 * @see java.lang.Runtime#exit(int)
 * @see java.lang.Runtime#gc()
 * @see java.lang.SecurityManager#checkExit(int)
 * @since JDK1.1
 */
 @Deprecated
 public static void runFinalizersOnExit(boolean value) {
 	Runtime.getRuntime().runFinalizersOnExit(value);
 }
 
 /**
 * Loads a code file with the specified filename from the local file
 * system as a dynamic library. The filename
 * argument must be a complete path name.
 * 
 * The call <code>System.load(name)</code> is effectively equivalent
 * to the call:
 * <blockquote><pre>
 * Runtime.getRuntime().load(name)
 * </pre></blockquote>
 *
 * @param filename the file to load.
 * @exception SecurityException if a security manager exists and its
 * <code>checkLink</code> method doesn't allow
 * loading of the specified dynamic library
 * @exception UnsatisfiedLinkError if the file does not exist.
 * @exception NullPointerException if <code>filename</code> is
 * <code>null</code>
 * @see java.lang.Runtime#load(java.lang.String)
 * @see java.lang.SecurityManager#checkLink(java.lang.String)
 */
 public static void load(String filename) {
 	Runtime.getRuntime().load0(getCallerClass(), filename);
 }
 
 /**
 * Loads the system library specified by the <code>libname</code>
 * argument. The manner in which a library name is mapped to the
 * actual system library is system dependent.
 * 
 * The call <code>System.loadLibrary(name)</code> is effectively
 * equivalent to the call
 * <blockquote><pre>
 * Runtime.getRuntime().loadLibrary(name)
 * </pre></blockquote>
 *
 * @param libname the name of the library.
 * @exception SecurityException if a security manager exists and its
 * <code>checkLink</code> method doesn't allow
 * loading of the specified dynamic library
 * @exception UnsatisfiedLinkError if the library does not exist.
 * @exception NullPointerException if <code>libname</code> is
 * <code>null</code>
 * @see java.lang.Runtime#loadLibrary(java.lang.String)
 * @see java.lang.SecurityManager#checkLink(java.lang.String)
 */
 public static void loadLibrary(String libname) {
 	Runtime.getRuntime().loadLibrary0(getCallerClass(), libname);
 }
 
 /**
 * Maps a library name into a platform-specific string representing
 * a native library.
 *
 * @param libname the name of the library.
 * @return a platform-dependent native library name.
 * @exception NullPointerException if <code>libname</code> is
 * <code>null</code>
 * @see java.lang.System#loadLibrary(java.lang.String)
 * @see java.lang.ClassLoader#findLibrary(java.lang.String)
 * @since 1.2
 */
 public static native String mapLibraryName(String libname);
 
 /**
 * The following two methods exist because in, out, and err must be
 * initialized to null. The compiler, however, cannot be permitted to
 * inline access to them, since they are later set to more sensible values
 * by initializeSystemClass().
 */
 private static InputStream nullInputStream() throws NullPointerException {
 	if (currentTimeMillis() > 0) {
 	 return null;
 	}
 	throw new NullPointerException();
 }
 
 private static PrintStream nullPrintStream() throws NullPointerException {
 	if (currentTimeMillis() > 0) {
 	 return null;
 	}
 	throw new NullPointerException();
 }
 
 /**
 * Initialize the system class. Called after thread initialization.
 */
 private static void initializeSystemClass() {
 	props = new Properties();
 	initProperties(props);
 	sun.misc.Version.init();
 	FileInputStream fdIn = new FileInputStream(FileDescriptor.in);
 	FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
 	FileOutputStream fdErr = new FileOutputStream(FileDescriptor.err);
 	setIn0(new BufferedInputStream(fdIn));
 	setOut0(new PrintStream(new BufferedOutputStream(fdOut, 128), true));
 	setErr0(new PrintStream(new BufferedOutputStream(fdErr, 128), true));
 
 	// Load the zip library now in order to keep java.util.zip.ZipFile
 	// from trying to use itself to load this library later.
 	loadLibrary("zip");
 
 	// Setup Java signal handlers for HUP, TERM, and INT (where available).
 Terminator.setup();
 
 	// The order in with the hooks are added here is important as it
 	// determines the order in which they are run. 
 // (1)Console restore hook needs to be called first.
 // (2)Application hooks must be run before calling deleteOnExitHook.
 	Shutdown.add(sun.misc.SharedSecrets.getJavaIOAccess().consoleRestoreHook());
 	Shutdown.add(ApplicationShutdownHooks.hook());
 	Shutdown.add(sun.misc.SharedSecrets.getJavaIODeleteOnExitAccess());
 
 // Initialize any miscellenous operating system settings that need to be
 // set for the class libraries. Currently this is no-op everywhere except
 // for Windows where the process-wide error mode is set before the java.io
 // classes are used.
 sun.misc.VM.initializeOSEnvironment();
 
 	// Set the maximum amount of direct memory. This value is controlled
 	// by the vm option -XX:MaxDirectMemorySize=<size>. This method acts
 	// as an initializer only if it is called before sun.misc.VM.booted().
 	sun.misc.VM.maxDirectMemory();
 
 	// Set a boolean to determine whether ClassLoader.loadClass accepts
 	// array syntax. This value is controlled by the system property
 	// "sun.lang.ClassLoader.allowArraySyntax". This method acts as
 	// an initializer only if it is called before sun.misc.VM.booted().
 	sun.misc.VM.allowArraySyntax();
 
 	// Subsystems that are invoked during initialization can invoke
 	// sun.misc.VM.isBooted() in order to avoid doing things that should
 	// wait until the application class loader has been set up.
 	sun.misc.VM.booted();
 
 // The main thread is not added to its thread group in the same
 // way as other threads; we must do it ourselves here.
 Thread current = Thread.currentThread();
 current.getThreadGroup().add(current);
 
 // Allow privileged classes outside of java.lang
 sun.misc.SharedSecrets.setJavaLangAccess(new sun.misc.JavaLangAccess(){
 public sun.reflect.ConstantPool getConstantPool(Class klass) {
 return klass.getConstantPool();
 }
 public void setAnnotationType(Class klass, AnnotationType type) {
 klass.setAnnotationType(type);
 }
 public AnnotationType getAnnotationType(Class klass) {
 return klass.getAnnotationType();
 }
 public <E extends Enum<E>>
 		 E[] getEnumConstantsShared(Class<E> klass) {
 return klass.getEnumConstantsShared();
 }
 public void blockedOn(Thread t, Interruptible b) {
 t.blockedOn(b);
 }
 });
 }
 
 /* returns the class of the caller. */
 static Class getCallerClass() {
 // NOTE use of more generic Reflection.getCallerClass()
 return Reflection.getCallerClass(3);
 }
 }

自己写了个测试类，看看输出

package test;
    
    public class TTTest {
    
    	/**
    	 * @param args
    	 */
    	public static void main(String[] args) {
    		// TODO Auto-generated method stub
    		System.out.println(System.getProperty("os.name"));
    		System.out.println(System.getProperty("os.arch"));
    		System.out.println(System.getProperty("os.version"));
    		System.out.println("-----------------------------------------------------");
    		
    		System.out.println(System.getProperty("Java Runtime Environment version"));
    		System.out.println(System.getProperty("Java Runtime Environment vendor"));
    		System.out.println(System.getProperty("java.vendor.url"));
    		System.out.println(System.getProperty("java.home"));
    		System.out.println(System.getProperty("java.vm.specification.version"));
    		System.out.println(System.getProperty("java.class.version"));
    		
    		System.out.println(System.getProperty("Java class path"));
    		System.out.println(System.getProperty("java.version"));
    		System.out.println("-----------------------------------------------------");
    		
    	}
    
    }

打印：

Windows Vista  
x86  
6.1  
\-----------------------------------------------------  
null  
null  
[http://java.sun.com/][http_java.sun.com]  
C:\\Program Files\\Java\\jdk1.6.0\_06\\jre  
1.0  
50.0  
null  
1.6.0\_06  
\-----------------------------------------------------

System.getProperty能获取的属性值源码里说的很清楚，

比如

/\*\* 
     \* 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> 
     \*/

如果英语水平不高就去google翻译一下。

这个方法不是所有属性都能获取的，根据我自己的测试，这个方法获取的值也不一定就是正确的。

比如实际打印我的操作系统是Windows Vista！

这个不太靠谱，有图为证：

![1353572038_3160.jpg][]

所以这个Windows Vista是怎么打出来的，是在是不靠谱。

程序就是程序必须经过实际检验，测试才可以使用。如果你要使用这个方法获取操作系统名称，那你要悠着点。

补充：为了测试我的系统是不是真的win7，我使用cmd命令看了一下。

输入ver

打印：

Microsoft Windows \[版本 6.1.7601\]

去百度百科查了一下有这样一句话：