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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 <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 ma
1ab5c
nager 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>
* 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.
*
* <p>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.
* <p>
* 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)
*/
@CallerSensitive
public static void load(String filename) {
Runtime.getRuntime().load0(Reflection.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.
* <p>
* 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)
*/
@CallerSensitive
public static void loadLibrary(String libname) {
Runtime.getRuntime().loadLibrary0(Reflection.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);

/**
* Initialize the system class.  Called after thread initialization.
*/
private static void initializeSystemClass() {

// VM might invoke JNU_NewStringPlatform() to set those encoding
// sensitive properties (user.home, user.name, boot.class.path, etc.)
// during "props" initialization, in which it may need access, via
// System.getProperty(), to the related system encoding property that
// have been initialized (put into "props") at early stage of the
// initialization. So make sure the "props" is available at the
// very beginning of the initialization and all system properties to
// be put into it directly.
props = new Properties();
initProperties(props);  // initialized by the VM

// There are certain system configurations that may be controlled by
// VM options such as the maximum amount of direct memory and
// Integer cache size used to support the object identity semantics
// of autoboxing.  Typically, the library will obtain these values
// from the properties set by the VM.  If the properties are for
// internal implementation use only, these properties should be
// removed from the system properties.
//
// See java.lang.Integer.IntegerCache and the
// sun.misc.VM.saveAndRemoveProperties method for example.
//
// Save a private copy of the system properties object that
// can only be accessed by the internal implementation.  Remove
// certain system properties that are not intended for public access.
sun.misc.VM.saveAndRemoveProperties(props);

lineSeparator = props.getProperty("line.separator");
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();

// 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();

// 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);

// register shared secrets
setJavaLangAccess();

// 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.
// IMPORTANT: Ensure that this remains the last initialization action!
sun.misc.VM.booted();
}

private static void setJavaLangAccess() {
// 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 boolean casAnnotationType(Class<?> klass, AnnotationType oldType, AnnotationType newType) {
return klass.casAnnotationType(oldType, newType);
}
public AnnotationType getAnnotationType(Class klass) {
return klass.getAnnotationType();
}
public byte[] getRawClassAnnotations(Class<?> klass) {
return klass.getRawAnnotations();
}
public <E extends Enum<E>>
E[] getEnumConstantsShared(Class<E> klass) {
return klass.getEnumConstantsShared();
}
public void blockedOn(Thread t, Interruptible b) {
t.blockedOn(b);
}
public void registerShutdownHook(int slot, boolean registerShutdownInProgress, Runnable hook) {
Shutdown.add(slot, registerShutdownInProgress, hook);
}
public int getStackTraceDepth(Throwable t) {
return t.getStackTraceDepth();
}
public StackTraceElement getStackTraceElement(Throwable t, int i) {
return t.getStackTraceElement(i);
}
public int getStringHash32(String string) {
return string.hash32();
}
public Thread newThreadWithAcc(Runnable target, AccessControlContext acc) {
return new Thread(target, acc);
}
public void invokeFinalize(Object o) throws Throwable {
o.finalize();
}
});
}
}


System类代表系统,系统级的很多属性和控制方法都放置在该类的内部,本文重点介绍System中的properties属性。

成员定义的变量:

private static Properties props;

获取Properties的方式:
Properties properties = System.getProperties();

在tomcat中运行,Systsem也会加重Tomcat中的一些属性以及方法,测试demo如下:
package cn.xdf.manager.listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import java.util.Properties;
import java.util.Set;

/**
* Created by Administrator on 2017/5/18.
*/
public class MyFirstListener implements ServletContextListener {

@Override
public void contextInitialized(ServletContextEvent servletContextEvent) {
//String property = System.getProperty("catalina.home");
Properties properties = System.getProperties();

Set<Object> keys = properties.keySet();
for (Object key : keys) {
if(key instanceof String) {
System.out.println("key :" + key + " and value :" + System.getProperty((String)key));
}
}

// System.out.println("aaaaaaaaaaaaaaa " + property);
}

@Override
public void contextDestroyed(ServletContextEvent servletContextEvent) {

}
}

<listener>
<listener-class>cn.xdf.manager.listener.MyFirstListener</listener-class>
</listener>


项目启动时候,会对servlet的上下文进行监听,测试的结果如下:
key:79个

properties size:79个

key :sun.boot.class.path and value :E:\Program Files (x86)\Java\jdk1.7.0_80\jre\lib\resources.jar;E:\Program Files (x86)\Java\jdk1.7.0_80\jre\lib\rt.jar;E:\Program Files (x86)\Java\jdk1.7.0_80\jre\lib\sunrsasign.jar;E:\Program
Files (x86)\Java\jdk1.7.0_80\jre\lib\jsse.jar;E:\Program Files (x86)\Java\jdk1.7.0_80\jre\lib\jce.jar;E:\Program Files (x86)\Java\jdk1.7.0_80\jre\lib\charsets.jar;E:\Program Files (x86)\Java\jdk1.7.0_80\jre\lib\jfr.jar;E:\Program Files (x86)\Java\jdk1.7.0_80\jre\classes

key :java.util.logging.config.file and value :C:\Users\Administrator\.IntelliJIdea14\system\tomcat\Unnamed_xdfmanager_3\conf\logging.properties
key :sun.desktop and value :windows
key :com.sun.management.jmxremote and value :
key :java.vm.specification.vendor and value :Oracle Corporation
key :java.runtime.version and value :1.7.0_80-b15
key :user.name and value :Administrator
key :shared.loader and value :
key :tomcat.util.buf.StringCache.byte.enabled and value :true
key :java.naming.factory.initial and value :org.apache.naming.java.javaURLContextFactory
key :user.language and value :zh
key :sun.boot.library.path and value :E:\Program Files (x86)\Java\jdk1.7.0_80\jre\bin
key :com.sun.management.jmxremote.port and value :1099
key :jdk.tls.ephemeralDHKeySize and value :2048
key :java.version and value :1.7.0_80
key :java.util.logging.manager and value :org.apache.juli.ClassLoaderLogManager
key :user.timezone and value :Asia/Shanghai
key :sun.arch.data.model and value :64
key :java.util.concurrent.ForkJoinPool.common.threadFactory and value :org.apache.catalina.startup.SafeForkJoinWorkerThreadFactory
key :java.endorsed.dirs and value :F:\apache-tomcat-7.0.77\endorsed
key :java.rmi.server.randomIDs and value :true
key :sun.cpu.isalist and value :amd64
key :sun.jnu.encoding and value :GBK
key :file.encoding.pkg and value :sun.io
key :package.access and value :sun.,org.apache.catalina.,org.apache.coyote.,org.apache.jasper.,org.apache.naming.resources.,org.apache.tomcat.
key :file.separator and value :\
key :java.specification.name and value :Java Platform API Specification
key :java.class.version and value :51.0
key :user.country and value :CN
key :java.home and value :E:\Program Files (x86)\Java\jdk1.7.0_80\jre
key :java.vm.info and value :mixed mode
key :os.version and value :6.1
key :com.sun.management.jmxremote.ssl and value :false
key :path.separator and value :;
key :java.vm.version and value :24.80-b11
key :user.variant and value :
key :java.awt.printerjob and value :sun.awt.windows.WPrinterJob
key :tomcat.util.scan.DefaultJarScanner.jarsToSkip and value :bootstrap.jar,commons-daemon.jar,tomcat-juli.jar,annotations-api.jar,el-api.jar,jsp-api.jar,servlet-api.jar,websocket-api.jar,catalina.jar,catalina-ant.jar,catalina-ha.jar,catalina-tribes.jar,jasper.jar,jasper-el.jar,ecj-*.jar,tomcat-api.jar,tomcat-util.jar,tomcat-coyote.jar,tomcat-dbcp.jar,tomcat-jni.jar,tomcat-spdy.jar,tomcat-i18n-en.jar,tomcat-i18n-es.jar,tomcat-i18n-fr.jar,tomcat-i18n-ja.jar,tomcat-juli-adapters.jar,catalina-jmx-remote.jar,catalina-ws.jar,tomcat-jdbc.jar,tools.jar,commons-beanutils*.jar,commons-codec*.jar,commons-collections*.jar,commons-dbcp*.jar,commons-digester*.jar,commons-fileupload*.jar,commons-httpclient*.jar,commons-io*.jar,commons-lang*.jar,commons-logging*.jar,commons-math*.jar,commons-pool*.jar,jstl.jar,taglibs-standard-spec-*.jar,geronimo-spec-jaxrpc*.jar,wsdl4j*.jar,ant.jar,ant-junit*.jar,aspectj*.jar,jmx.jar,h2*.jar,hibernate*.jar,httpclient*.jar,jmx-tools.jar,jta*.jar,log4j.jar,log4j-1*.jar,mail*.jar,slf4j*.jar,xercesImpl.jar,xmlParserAPIs.jar,xml-apis.jar,junit.jar,junit-*.jar,hamcrest*.jar,org.hamcrest*.jar,ant-launcher.jar,cobertura-*.jar,asm-*.jar,dom4j-*.jar,icu4j-*.jar,jaxen-*.jar,jdom-*.jar,jetty-*.jar,oro-*.jar,servlet-api-*.jar,tagsoup-*.jar,xmlParserAPIs-*.jar,xom-*.jar
key :sun.io.unicode.encoding and value :UnicodeLittle
key :awt.toolkit and value :sun.awt.windows.WToolkit
key :com.sun.management.jmxremote.authenticate and value :false
key :package.definition and value :sun.,java.,org.apache.catalina.,org.apache.coyote.,org.apache.jasper.,org.apache.naming.,org.apache.tomcat.
key :user.script and value :
key :java.naming.factory.url.pkgs and value :org.apache.naming
key :user.home and value :C:\Users\Administrator
key :org.apache.catalina.startup.ContextConfig.jarsToSkip and value :
key :java.specification.vendor and value :Oracle Corporation
key :java.library.path and value :E:\Program Files (x86)\Java\jdk1.7.0_80\bin;C:\Windows\Sun\Java\bin;C:\Windows\system32;C:\Windows;E:\apache-maven-3.5.0/bin;E:\Program Files (x86)\Java\jdk1.7.0_80/bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\ATI Technologies\ATI.ACE\Core-Static;E:\Program Files (x86)\TortoiseSVN\bin;E:\Program Files (x86)\MySQL\MySQL Server 5.5\bin;.
key :java.vendor.url and value :http://java.oracle.com/
key :org.apache.catalina.startup.TldConfig.jarsToSkip and value :tomcat7-websocket.jar
key :java.vm.vendor and value :Oracle Corporation
key :common.loader and value :F:/apache-tomcat-7.0.77/lib,F:/apache-tomcat-7.0.77/lib/*.jar,${catalina.home}/lib,${catalina.home}/lib/*.jar
key :java.runtime.name and value :Java(TM) SE Runtime Environment
key :sun.java.command and value :org.apache.catalina.startup.Bootstrap start
key :java.class.path and value :F:\apache-tomcat-7.0.77\bin\bootstrap.jar;F:\apache-tomcat-7.0.77\bin\tomcat-juli.jar
key :java.vm.specification.name and value :Java Virtual Machine Specification
key :java.vm.specification.version and value :1.7
key :catalina.home and value :F:\apache-tomcat-7.0.77
key :sun.cpu.endian and value :little
key :sun.os.patch.level and value :Service Pack 1
key :java.io.tmpdir and value :F:\apache-tomcat-7.0.77\temp
key :java.vendor.url.bug and value :http://bugreport.sun.com/bugreport/
key :server.loader and value :
key :java.rmi.server.hostname and value :127.0.0.1
key :os.arch and value :amd64
key :java.awt.graphicsenv and value :sun.awt.Win32GraphicsEnvironment
key :java.ext.dirs and value :E:\Program Files (x86)\Java\jdk1.7.0_80\jre\lib\ext;C:\Windows\Sun\Java\lib\ext
key :user.dir and value :F:\apache-tomcat-7.0.77\bin
key :line.separator and value :

key :java.vm.name and value :Java HotSpot(TM) 64-Bit Server VM
key :file.encoding and value :GBK
key :java.specification.version and value :1.7

有些东西我们肯定会用到,例如设置log4j打印日志的的时候,我们会将日志文件指定一个位置,此时通过System中的properties就可以很好的就进行动态指定了,例如:

key :catalina.home and value :F:\apache-tomcat-7.0.77根据key值为satalina.home,得到的值为我tomcat服务器所在的位置。

作用之一:设置log4J的日志文件所在位置

无论是用log4j还是logback,都需要指定日志的输出文件所在的问题:

### set log levels ###
log4j.rootLogger = debug,stdout,D

### to console ###
log4j.appender.stdout = org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target = System.out
log4j.appender.stdout.layout = org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern = %d{ABSOLUTE} %5p %c{1}:%L - %m%n

log4j.logger.com.ibatis=DEBUG
log4j.logger.com.ibatis.common.jdbc.SimpleDataSource=DEBUG
log4j.logger.com.ibatis.common.jdbc.ScriptRunner=DEBUG
log4j.logger.com.ibatis.sqlmap.engine.impl.SqlMapClientDelegate=DEBUG
log4j.logger.org.mybatis=DEBUG
log4j.logger.java.sql.Connection=DEBUG
log4j.logger.java.sql.Statement=DEBUG
log4j.logger.java.sql.PreparedStatement=DEBUG
log4j.logger.java.sql.ResultSet=DEBUG

### to file ###
log4j.appender.D = org.apache.log4j.DailyRollingFileAppender
log4j.appender.D.File = ${catalina.home}/logs/springMVC.log
log4j.appender.D.Append = true
log4j.appender.D.Threshold = DEBUG
log4j.appender.D.layout = org.apache.log4j.PatternLayout
log4j.appender.D.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} [ %t:%r ] - [ %p ] %m%n

### exception to another file ###
#log4j.appender.D = org.apache.log4j.DailyRllingFileAppender
#file nameo
#log4j.appender.D.File = F://logs/error.log
#log4j.appender.D.Append = true
#level ERROR
#log4j.appender.D.Threshold = ERROR
#log4j.appender.D.layout = org.apache.log4j.PatternLayout
#log4j.appender.D.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} [ %t:%r ] - [ %p ] %m%n
配置的文件中的${catalina.home}/logs/springMVC.log,此时就讲springMVC.log文件放在了服务器tomcat所在的路径下的logs中了!
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