javaee中的System介绍--->Properties

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/* * Copyright (c) 1994, 2014, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */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 manager exists and its     *        <code>checkPermission</code> method doesn't allow     *        reassigning of the standard output stream.     *     * @see SecurityManager#checkPermission     * @see java.lang.RuntimePermission     *     * @since   JDK1.1     */    public static void setOut(PrintStream out) {        checkIO();        setOut0(out);    }    /**     * Reassigns the "standard" error output stream.     *     * <p>First, if there is a security manager, its <code>checkPermission</code>     * method is called with a <code>RuntimePermission("setIO")</code> permission     *  to see if it's ok to reassign the "standard" error output stream.     *     * @param err the new standard error output stream.     *     * @throws SecurityException     *        if a security manager exists and its     *        <code>checkPermission</code> method doesn't allow     *        reassigning of the standard error output stream.     *     * @see SecurityManager#checkPermission     * @see java.lang.RuntimePermission     *     * @since   JDK1.1     */    public static void setErr(PrintStream err) {        checkIO();        setErr0(err);    }    private static volatile Console cons = null;    /**     * Returns the unique {@link java.io.Console Console} object associated     * with the current Java virtual machine, if any.     *     * @return  The system console, if any, otherwise <tt>null</tt>.     *     * @since   1.6     */     public static Console console() {         if (cons == null) {             synchronized (System.class) {                 cons = sun.misc.SharedSecrets.getJavaIOAccess().console();             }         }         return cons;     }    /**     * Returns the channel inherited from the entity that created this     * Java virtual machine.     *     * <p> This method returns the channel obtained by invoking the     * {@link java.nio.channels.spi.SelectorProvider#inheritedChannel     * inheritedChannel} method of the system-wide default     * {@link java.nio.channels.spi.SelectorProvider} object. </p>     *     * <p> In addition to the network-oriented channels described in     * {@link java.nio.channels.spi.SelectorProvider#inheritedChannel     * inheritedChannel}, this method may return other kinds of     * channels in the future.     *     * @return  The inherited channel, if any, otherwise <tt>null</tt>.     *     * @throws  IOException     *          If an I/O error occurs     *     * @throws  SecurityException     *          If a security manager is present and it does not     *          permit access to the channel.     *     * @since 1.5     */    public static Channel inheritedChannel() throws IOException {        return SelectorProvider.provider().inheritedChannel();    }    private static void checkIO() {        SecurityManager sm = getSecurityManager();        if (sm != null) {            sm.checkPermission(new RuntimePermission("setIO"));        }    }    private static native void setIn0(InputStream in);    private static native void setOut0(PrintStream out);    private static native void setErr0(PrintStream err);    /**     * Sets the System security.     *     * <p> If there is a security manager already installed, this method first     * calls the security manager's <code>checkPermission</code> method     * with a <code>RuntimePermission("setSecurityManager")</code>     * permission to ensure it's ok to replace the existing     * security manager.     * This may result in throwing a <code>SecurityException</code>.     *     * <p> Otherwise, the argument is established as the current     * security manager. If the argument is <code>null</code> and no     * security manager has been established, then no action is taken and     * the method simply returns.     *     * @param      s   the security manager.     * @exception  SecurityException  if the security manager has already     *             been set and its <code>checkPermission</code> method     *             doesn't allow it to be replaced.     * @see #getSecurityManager     * @see SecurityManager#checkPermission     * @see java.lang.RuntimePermission     */    public static    void setSecurityManager(final SecurityManager s) {        try {            s.checkPackageAccess("java.lang");        } catch (Exception e) {            // no-op        }        setSecurityManager0(s);    }    private static synchronized    void setSecurityManager0(final SecurityManager s) {        SecurityManager sm = getSecurityManager();        if (sm != null) {            // ask the currently installed security manager if we            // can replace it.            sm.checkPermission(new RuntimePermission                                     ("setSecurityManager"));        }        if ((s != null) && (s.getClass().getClassLoader() != null)) {            // New security manager class is not on bootstrap classpath.            // Cause policy to get initialized before we install the new            // security manager, in order to prevent infinite loops when            // trying to initialize the policy (which usually involves            // accessing some security and/or system properties, which in turn            // calls the installed security manager's checkPermission method            // which will loop infinitely if there is a non-system class            // (in this case: the new security manager class) on the stack).            AccessController.doPrivileged(new PrivilegedAction<Object>() {                public Object run() {                    s.getClass().getProtectionDomain().implies                        (SecurityConstants.ALL_PERMISSION);                    return null;                }            });        }        security = s;    }    /**     * Gets the system security interface.     *     * @return  if a security manager has already been established for the     *          current application, then that security manager is returned;     *          otherwise, <code>null</code> is returned.     * @see     #setSecurityManager     */    public static SecurityManager getSecurityManager() {        return security;    }    /**     * Returns the current time in milliseconds.  Note that     * while the unit of time of the return value is a millisecond,     * the granularity of the value depends on the underlying     * operating system and may be larger.  For example, many     * operating systems measure time in units of tens of     * milliseconds.     *     * <p> See the description of the class <code>Date</code> for     * a discussion of slight discrepancies that may arise between     * "computer time" and coordinated universal time (UTC).     *     * @return  the difference, measured in milliseconds, between     *          the current time and midnight, January 1, 1970 UTC.     * @see     java.util.Date     */    public static native long currentTimeMillis();    /**     * Returns the current value of the running Java Virtual Machine's     * high-resolution time source, in nanoseconds.     *     * <p>This method can only be used to measure elapsed time and is     * not related to any other notion of system or wall-clock time.     * The value returned represents nanoseconds since some fixed but     * arbitrary <i>origin</i> time (perhaps in the future, so values     * may be negative).  The same origin is used by all invocations of     * this method in an instance of a Java virtual machine; other     * virtual machine instances are likely to use a different origin.     *     * <p>This method provides nanosecond precision, but not necessarily     * nanosecond resolution (that is, how frequently the value changes)     * - no guarantees are made except that the resolution is at least as     * good as that of {@link #currentTimeMillis()}.     *     * <p>Differences in successive calls that span greater than     * approximately 292 years (2<sup>63</sup> nanoseconds) will not     * correctly compute elapsed time due to numerical overflow.     *     * <p>The values returned by this method become meaningful only when     * the difference between two such values, obtained within the same     * instance of a Java virtual machine, is computed.     *     * <p> For example, to measure how long some code takes to execute:     *  <pre> {@code     * long startTime = System.nanoTime();     * // ... the code being measured ...     * long estimatedTime = System.nanoTime() - startTime;}</pre>     *     * <p>To compare two nanoTime values     *  <pre> {@code     * long t0 = System.nanoTime();     * ...     * long t1 = System.nanoTime();}</pre>     *     * one should use {@code t1 - t0 < 0}, not {@code t1 < t0},     * because of the possibility of numerical overflow.     *     * @return the current value of the running Java Virtual Machine's     *         high-resolution time source, in nanoseconds     * @since 1.5     */    public static native long nanoTime();    /**     * Copies an array from the specified source array, beginning at the     * specified position, to the specified position of the destination array.     * A subsequence of array components are copied from the source     * array referenced by <code>src</code> to the destination array     * referenced by <code>dest</code>. The number of components copied is     * equal to the <code>length</code> argument. The components at     * positions <code>srcPos</code> through     * <code>srcPos+length-1</code> in the source array are copied into     * positions <code>destPos</code> through     * <code>destPos+length-1</code>, respectively, of the destination     * array.     * <p>     * If the <code>src</code> and <code>dest</code> arguments refer to the     * same array object, then the copying is performed as if the     * components at positions <code>srcPos</code> through     * <code>srcPos+length-1</code> were first copied to a temporary     * array with <code>length</code> components and then the contents of     * the temporary array were copied into positions     * <code>destPos</code> through <code>destPos+length-1</code> of the     * destination array.     * <p>     * If <code>dest</code> is <code>null</code>, then a     * <code>NullPointerException</code> is thrown.     * <p>     * If <code>src</code> is <code>null</code>, then a     * <code>NullPointerException</code> is thrown and the destination     * array is not modified.     * <p>     * Otherwise, if any of the following is true, an     * <code>ArrayStoreException</code> is thrown and the destination is     * not modified:     * <ul>     * <li>The <code>src</code> argument refers to an object that is not an     *     array.     * <li>The <code>dest</code> argument refers to an object that is not an     *     array.     * <li>The <code>src</code> argument and <code>dest</code> argument refer     *     to arrays whose component types are different primitive types.     * <li>The <code>src</code> argument refers to an array with a primitive     *    component type and the <code>dest</code> argument refers to an array     *     with a reference component type.     * <li>The <code>src</code> argument refers to an array with a reference     *    component type and the <code>dest</code> argument refers to an array     *     with a primitive component type.     * </ul>     * <p>     * Otherwise, if any of the following is true, an     * <code>IndexOutOfBoundsException</code> is     * thrown and the destination is not modified:     * <ul>     * <li>The <code>srcPos</code> argument is negative.     * <li>The <code>destPos</code> argument is negative.     * <li>The <code>length</code> argument is negative.     * <li><code>srcPos+length</code> is greater than     *     <code>src.length</code>, the length of the source array.     * <li><code>destPos+length</code> is greater than     *     <code>dest.length</code>, the length of the destination array.     * </ul>     * <p>     * Otherwise, if any actual component of the source array from     * position <code>srcPos</code> through     * <code>srcPos+length-1</code> cannot be converted to the component     * type of the destination array by assignment conversion, an     * <code>ArrayStoreException</code> is thrown. In this case, let     * <b><i>k</i></b> be the smallest nonnegative integer less than     * length such that <code>src[srcPos+</code><i>k</i><code>]</code>     * cannot be converted to the component type of the destination     * array; when the exception is thrown, source array components from     * positions <code>srcPos</code> through     * <code>srcPos+</code><i>k</i><code>-1</code>     * will already have been copied to destination array positions     * <code>destPos</code> through     * <code>destPos+</code><i>k</I><code>-1</code> and no other     * positions of the destination array will have been modified.     * (Because of the restrictions already itemized, this     * paragraph effectively applies only to the situation where both     * arrays have component types that are reference types.)     *     * @param      src      the source array.     * @param      srcPos   starting position in the source array.     * @param      dest     the destination array.     * @param      destPos  starting position in the destination data.     * @param      length   the number of array elements to be copied.     * @exception  IndexOutOfBoundsException  if copying would cause     *               access of data outside array bounds.     * @exception  ArrayStoreException  if an element in the <code>src</code>     *               array could not be stored into the <code>dest</code> array     *               because of a type mismatch.     * @exception  NullPointerException if either <code>src</code> or     *               <code>dest</code> is <code>null</code>.     */    public static native void arraycopy(Object src,  int  srcPos,                                        Object dest, int destPos,                                        int length);    /**     * Returns the same hash code for the given object as     * would be returned by the default method hashCode(),     * whether or not the given object's class overrides     * hashCode().     * The hash code for the null reference is zero.     *     * @param x object for which the hashCode is to be calculated     * @return  the hashCode     * @since   JDK1.1     */    public static native int identityHashCode(Object x);    /**     * System properties. The following properties are guaranteed to be defined:     * <dl>     * <dt>java.version         <dd>Java version number     * <dt>java.vendor          <dd>Java vendor specific string     * <dt>java.vendor.url      <dd>Java vendor URL     * <dt>java.home            <dd>Java installation directory     * <dt>java.class.version   <dd>Java class version number     * <dt>java.class.path      <dd>Java classpath     * <dt>os.name              <dd>Operating System Name     * <dt>os.arch              <dd>Operating System Architecture     * <dt>os.version           <dd>Operating System Version     * <dt>file.separator       <dd>File separator ("/" on Unix)     * <dt>path.separator       <dd>Path separator (":" on Unix)     * <dt>line.separator       <dd>Line separator ("\n" on Unix)     * <dt>user.name            <dd>User account name     * <dt>user.home            <dd>User home directory     * <dt>user.dir             <dd>User's current working directory     * </dl>     */    private static Properties props;    private static native Properties initProperties(Properties props);    /**     * Determines the current system properties.     * <p>     * First, if there is a security manager, its     * <code>checkPropertiesAccess</code> method is called with no     * arguments. This may result in a security exception.     * <p>     * The current set of system properties for use by the     * {@link #getProperty(String)} method is returned as a     * <code>Properties</code> object. If there is no current set of     * system properties, a set of system properties is first created and     * initialized. This set of system properties always includes values     * for the following keys:     * <table summary="Shows property keys and associated values">     * <tr><th>Key</th>     *     <th>Description of Associated Value</th></tr>     * <tr><td><code>java.version</code></td>     *     <td>Java Runtime Environment version</td></tr>     * <tr><td><code>java.vendor</code></td>     *     <td>Java Runtime Environment vendor</td></tr     * <tr><td><code>java.vendor.url</code></td>     *     <td>Java vendor URL</td></tr>     * <tr><td><code>java.home</code></td>     *     <td>Java installation directory</td></tr>     * <tr><td><code>java.vm.specification.version</code></td>     *     <td>Java Virtual Machine specification version</td></tr>     * <tr><td><code>java.vm.specification.vendor</code></td>     *     <td>Java Virtual Machine specification vendor</td></tr>     * <tr><td><code>java.vm.specification.name</code></td>     *     <td>Java Virtual Machine specification name</td></tr>     * <tr><td><code>java.vm.version</code></td>     *     <td>Java Virtual Machine implementation version</td></tr>     * <tr><td><code>java.vm.vendor</code></td>     *     <td>Java Virtual Machine implementation vendor</td></tr>     * <tr><td><code>java.vm.name</code></td>     *     <td>Java Virtual Machine implementation name</td></tr>     * <tr><td><code>java.specification.version</code></td>     *     <td>Java Runtime Environment specification  version</td></tr>     * <tr><td><code>java.specification.vendor</code></td>     *     <td>Java Runtime Environment specification  vendor</td></tr>     * <tr><td><code>java.specification.name</code></td>     *     <td>Java Runtime Environment specification  name</td></tr>     * <tr><td><code>java.class.version</code></td>     *     <td>Java class format version number</td></tr>     * <tr><td><code>java.class.path</code></td>     *     <td>Java class path</td></tr>     * <tr><td><code>java.library.path</code></td>     *     <td>List of paths to search when loading libraries</td></tr>     * <tr><td><code>java.io.tmpdir</code></td>     *     <td>Default temp file path</td></tr>     * <tr><td><code>java.compiler</code></td>     *     <td>Name of JIT compiler to use</td></tr>     * <tr><td><code>java.ext.dirs</code></td>     *     <td>Path of extension directory or directories</td></tr>     * <tr><td><code>os.name</code></td>     *     <td>Operating system name</td></tr>     * <tr><td><code>os.arch</code></td>     *     <td>Operating system architecture</td></tr>     * <tr><td><code>os.version</code></td>     *     <td>Operating system version</td></tr>     * <tr><td><code>file.separator</code></td>     *     <td>File separator ("/" on UNIX)</td></tr>     * <tr><td><code>path.separator</code></td>     *     <td>Path separator (":" on UNIX)</td></tr>     * <tr><td><code>line.separator</code></td>     *     <td>Line separator ("\n" on UNIX)</td></tr>     * <tr><td><code>user.name</code></td>     *     <td>User's account name</td></tr>     * <tr><td><code>user.home</code></td>     *     <td>User's home directory</td></tr>     * <tr><td><code>user.dir</code></td>     *     <td>User's current working directory</td></tr>     * </table>     * <p>     * Multiple paths in a system property value are separated by the path     * separator character of the platform.     * <p>     * Note that even if the security manager does not permit the     * <code>getProperties</code> operation, it may choose to permit the     * {@link #getProperty(String)} operation.     *     * @return     the system properties     * @exception  SecurityException  if a security manager exists and its     *             <code>checkPropertiesAccess</code> method doesn't allow access     *              to the system properties.     * @see        #setProperties     * @see        java.lang.SecurityException     * @see        java.lang.SecurityManager#checkPropertiesAccess()     * @see        java.util.Properties     */    public static Properties getProperties() {        SecurityManager sm = getSecurityManager();        if (sm != null) {            sm.checkPropertiesAccess();        }        return props;    }    /**     * Returns the system-dependent line separator string.  It always     * returns the same value - the initial value of the {@linkplain     * #getProperty(String) system property} {@code line.separator}.     *     * <p>On UNIX systems, it returns {@code "\n"}; on Microsoft     * Windows systems it returns {@code "\r\n"}.     */    public static String lineSeparator() {        return lineSeparator;    }    private static String lineSeparator;    /**     * Sets the system properties to the <code>Properties</code>     * argument.     * <p>     * First, if there is a security manager, its     * <code>checkPropertiesAccess</code> method is called with no     * arguments. This may result in a security exception.     * <p>     * The argument becomes the current set of system properties for use     * by the {@link #getProperty(String)} method. If the argument is     * <code>null</code>, then the current set of system properties is     * forgotten.     *     * @param      props   the new system properties.     * @exception  SecurityException  if a security manager exists and its     *             <code>checkPropertiesAccess</code> method doesn't allow access     *              to the system properties.     * @see        #getProperties     * @see        java.util.Properties     * @see        java.lang.SecurityException     * @see        java.lang.SecurityManager#checkPropertiesAccess()     */    public static void setProperties(Properties props) {        SecurityManager sm = getSecurityManager();        if (sm != null) {            sm.checkPropertiesAccess();        }        if (props == null) {            props = new Properties();            initProperties(props);        }        System.props = props;    }    /**     * Gets the system property indicated by the specified key.     * <p>     * First, if there is a security manager, its     * <code>checkPropertyAccess</code> method is called with the key as     * its argument. This may result in a SecurityException.     * <p>     * If there is no current set of system properties, a set of system     * properties is first created and initialized in the same manner as     * for the <code>getProperties</code> method.     *     * @param      key   the name of the system property.     * @return     the string value of the system property,     *             or <code>null</code> if there is no property with that key.     *     * @exception  SecurityException  if a security manager exists and its     *             <code>checkPropertyAccess</code> method doesn't allow     *              access to the specified system property.     * @exception  NullPointerException if <code>key</code> is     *             <code>null</code>.     * @exception  IllegalArgumentException if <code>key</code> is empty.     * @see        #setProperty     * @see        java.lang.SecurityException     * @see        java.lang.SecurityManager#checkPropertyAccess(java.lang.String)     * @see        java.lang.System#getProperties()     */    public static String getProperty(String key) {        checkKey(key);        SecurityManager sm = getSecurityManager();        if (sm != null) {            sm.checkPropertyAccess(key);        }        return props.getProperty(key);    }    /**     * Gets the system property indicated by the specified key.     * <p>     * First, if there is a security manager, its     * <code>checkPropertyAccess</code> method is called with the     * <code>key</code> as its argument.     * <p>     * If there is no current set of system properties, a set of system     * properties is first created and initialized in the same manner as     * for the <code>getProperties</code> method.     *     * @param      key   the name of the system property.     * @param      def   a default value.     * @return     the string value of the system property,     *             or the default value if there is no property with that key.     *     * @exception  SecurityException  if a security manager exists and its     *             <code>checkPropertyAccess</code> method doesn't allow     *             access to the specified system property.     * @exception  NullPointerException if <code>key</code> is     *             <code>null</code>.     * @exception  IllegalArgumentException if <code>key</code> is empty.     * @see        #setProperty     * @see        java.lang.SecurityManager#checkPropertyAccess(java.lang.String)     * @see        java.lang.System#getProperties()     */    public static String getProperty(String key, String def) {        checkKey(key);        SecurityManager sm = getSecurityManager();        if (sm != null) {            sm.checkPropertyAccess(key);        }        return props.getProperty(key, def);    }    /**     * Sets the system property indicated by the specified key.     * <p>     * First, if a security manager exists, its     * <code>SecurityManager.checkPermission</code> method     * is called with a <code>PropertyPermission(key, "write")</code>     * permission. This may result in a SecurityException being thrown.     * If no exception is thrown, the specified property is set to the given     * value.     * <p>     *     * @param      key   the name of the system property.     * @param      value the value of the system property.     * @return     the previous value of the system property,     *             or <code>null</code> if it did not have one.     *     * @exception  SecurityException  if a security manager exists and its     *             <code>checkPermission</code> method doesn't allow     *             setting of the specified property.     * @exception  NullPointerException if <code>key</code> or     *             <code>value</code> is <code>null</code>.     * @exception  IllegalArgumentException if <code>key</code> is empty.     * @see        #getProperty     * @see        java.lang.System#getProperty(java.lang.String)     * @see        java.lang.System#getProperty(java.lang.String, java.lang.String)     * @see        java.util.PropertyPermission     * @see        SecurityManager#checkPermission     * @since      1.2     */    public static String setProperty(String key, String value) {        checkKey(key);        SecurityManager sm = getSecurityManager();        if (sm != null) {            sm.checkPermission(new PropertyPermission(key,                SecurityConstants.PROPERTY_WRITE_ACTION));        }        return (String) props.setProperty(key, value);    }    /**     * Removes the system property indicated by the specified key.     * <p>     * First, if a security manager exists, its     * <code>SecurityManager.checkPermission</code> method     * is called with a <code>PropertyPermission(key, "write")</code>     * permission. This may result in a SecurityException being thrown.     * If no exception is thrown, the specified property is removed.     * <p>     *     * @param      key   the name of the system property to be removed.     * @return     the previous string value of the system property,     *             or <code>null</code> if there was no property with that key.     *     * @exception  SecurityException  if a security manager exists and its     *             <code>checkPropertyAccess</code> method doesn't allow     *              access to the specified system property.     * @exception  NullPointerException if <code>key</code> is     *             <code>null</code>.     * @exception  IllegalArgumentException if <code>key</code> is empty.     * @see        #getProperty     * @see        #setProperty     * @see        java.util.Properties     * @see        java.lang.SecurityException     * @see        java.lang.SecurityManager#checkPropertiesAccess()     * @since 1.5     */    public static String clearProperty(String key) {        checkKey(key);        SecurityManager sm = getSecurityManager();        if (sm != null) {            sm.checkPermission(new PropertyPermission(key, "write"));        }        return (String) props.remove(key);    }    private static void checkKey(String key) {        if (key == null) {            throw new NullPointerException("key can't be null");        }        if (key.equals("")) {            throw new IllegalArgumentException("key can't be empty");        }    }    /**     * Gets the value of the specified environment variable. An     * environment variable is a system-dependent external named     * value.     *     * <p>If a security manager exists, its     * {@link SecurityManager#checkPermission checkPermission}     * method is called with a     * <code>{@link RuntimePermission}("getenv."+name)</code>     * permission.  This may result in a {@link SecurityException}     * being thrown.  If no exception is thrown the value of the     * variable <code>name</code> is returned.     *     * <p><a name="EnvironmentVSSystemProperties"><i>System     * properties</i> and <i>environment variables</i></a> are both     * conceptually mappings between names and values.  Both     * mechanisms can be used to pass user-defined information to a     * Java process.  Environment variables have a more global effect,     * because they are visible to all descendants of the process     * which defines them, not just the immediate Java subprocess.     * They can have subtly different semantics, such as case     * insensitivity, on different operating systems.  For these     * reasons, environment variables are more likely to have     * unintended side effects.  It is best to use system properties     * where possible.  Environment variables should be used when a     * global effect is desired, or when an external system interface     * requires an environment variable (such as <code>PATH</code>).     *     * <p>On UNIX systems the alphabetic case of <code>name</code> is     * typically significant, while on Microsoft Windows systems it is     * typically not.  For example, the expression     * <code>System.getenv("FOO").equals(System.getenv("foo"))</code>     * is likely to be true on Microsoft Windows.     *     * @param  name the name of the environment variable     * @return the string value of the variable, or <code>null</code>     *         if the variable is not defined in the system environment     * @throws NullPointerException if <code>name</code> is <code>null</code>     * @throws SecurityException     *         if a security manager exists and its     *         {@link SecurityManager#checkPermission checkPermission}     *         method doesn't allow access to the environment variable     *         <code>name</code>     * @see    #getenv()     * @see    ProcessBuilder#environment()     */    public static String getenv(String name) {        SecurityManager sm = getSecurityManager();        if (sm != null) {            sm.checkPermission(new RuntimePermission("getenv."+name));        }        return ProcessEnvironment.getenv(name);    }    /**     * Returns an unmodifiable string map view of the current system environment.     * The environment is a system-dependent mapping from names to     * values which is passed from parent to child processes.     *     * <p>If the system does not support environment variables, an     * empty map is returned.     *     * <p>The returned map will never contain null keys or values.     * Attempting to query the presence of a null key or value will     * throw a {@link NullPointerException}.  Attempting to query     * the presence of a key or value which is not of type     * {@link String} will throw a {@link ClassCastException}.     *     * <p>The returned map and its collection views may not obey the     * general contract of the {@link Object#equals} and     * {@link Object#hashCode} methods.     *     * <p>The returned map is typically case-sensitive on all platforms.     *     * <p>If a security manager exists, its     * {@link SecurityManager#checkPermission checkPermission}     * method is called with a     * <code>{@link RuntimePermission}("getenv.*")</code>     * permission.  This may result in a {@link SecurityException} being     * thrown.     *     * <p>When passing information to a Java subprocess,     * <a href=#EnvironmentVSSystemProperties>system properties</a>     * are generally preferred over environment variables.     *     * @return the environment as a map of variable names to values     * @throws SecurityException     *         if a security manager exists and its     *         {@link SecurityManager#checkPermission checkPermission}     *         method doesn't allow access to the process environment     * @see    #getenv(String)     * @see    ProcessBuilder#environment()     * @since  1.5     */    public static java.util.Map<String,String> getenv() {        SecurityManager sm = getSecurityManager();        if (sm != null) {            sm.checkPermission(new RuntimePermission("getenv.*"));        }        return ProcessEnvironment.getenv();    }    /**     * Terminates the currently running Java Virtual Machine. The     * argument serves as a status code; by convention, a nonzero status     * code indicates abnormal termination.     * <p>     * This method calls the <code>exit</code> method in class     * <code>Runtime</code>. This method never returns normally.     * <p>     * The call <code>System.exit(n)</code> is effectively equivalent to     * the call:     * <blockquote><pre>     * Runtime.getRuntime().exit(n)     * </pre></blockquote>     *     * @param      status   exit status.     * @throws  SecurityException     *        if a security manager exists and its <code>checkExit</code>     *        method doesn't allow exit with the specified status.     * @see        java.lang.Runtime#exit(int)     */    public static void exit(int status) {        Runtime.getRuntime().exit(status);    }    /**     * Runs the garbage collector.     * <p>     * Calling the <code>gc</code> method suggests that the Java Virtual     * Machine expend effort toward recycling unused objects in order to     * make the memory they currently occupy available for quick reuse.     * When control returns from the method call, the Java Virtual     * Machine has made a best effort to reclaim space from all discarded     * objects.     * <p>     * The call <code>System.gc()</code> is effectively equivalent to the     * call:     * <blockquote><pre>     * Runtime.getRuntime().gc()     * </pre></blockquote>     *     * @see     java.lang.Runtime#gc()     */    public static void gc() {        Runtime.getRuntime().gc();    }    /**     * Runs the finalization methods of any objects pending finalization.     * <p>     * Calling this method suggests that the Java Virtual Machine expend     * effort toward running the <code>finalize</code> methods of objects     * that have been found to be discarded but whose <code>finalize</code>     * methods have not yet been run. When control returns from the     * method call, the Java Virtual Machine has made a best effort to     * complete all outstanding finalizations.     * <p>     * 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.propertieskey :sun.desktop and value  :windowskey :com.sun.management.jmxremote and value  :key :java.vm.specification.vendor and value  :Oracle Corporationkey :java.runtime.version and value  :1.7.0_80-b15key :user.name and value  :Administratorkey :shared.loader and value  :key :tomcat.util.buf.StringCache.byte.enabled and value  :truekey :java.naming.factory.initial and value  :org.apache.naming.java.javaURLContextFactorykey :user.language and value  :zhkey :sun.boot.library.path and value  :E:\Program Files (x86)\Java\jdk1.7.0_80\jre\binkey :com.sun.management.jmxremote.port and value  :1099key :jdk.tls.ephemeralDHKeySize and value  :2048key :java.version and value  :1.7.0_80key :java.util.logging.manager and value  :org.apache.juli.ClassLoaderLogManagerkey :user.timezone and value  :Asia/Shanghaikey :sun.arch.data.model and value  :64key :java.util.concurrent.ForkJoinPool.common.threadFactory and value  :org.apache.catalina.startup.SafeForkJoinWorkerThreadFactorykey :java.endorsed.dirs and value  :F:\apache-tomcat-7.0.77\endorsedkey :java.rmi.server.randomIDs and value  :truekey :sun.cpu.isalist and value  :amd64key :sun.jnu.encoding and value  :GBKkey :file.encoding.pkg and value  :sun.iokey :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 Specificationkey :java.class.version and value  :51.0key :user.country and value  :CNkey :java.home and value  :E:\Program Files (x86)\Java\jdk1.7.0_80\jrekey :java.vm.info and value  :mixed modekey :os.version and value  :6.1key :com.sun.management.jmxremote.ssl and value  :falsekey :path.separator and value  :;key :java.vm.version and value  :24.80-b11key :user.variant and value  :key :java.awt.printerjob and value  :sun.awt.windows.WPrinterJobkey :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-*.jarkey :sun.io.unicode.encoding and value  :UnicodeLittlekey :awt.toolkit and value  :sun.awt.windows.WToolkitkey :com.sun.management.jmxremote.authenticate and value  :falsekey :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.namingkey :user.home and value  :C:\Users\Administratorkey :org.apache.catalina.startup.ContextConfig.jarsToSkip and value  :key :java.specification.vendor and value  :Oracle Corporationkey :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.jarkey :java.vm.vendor and value  :Oracle Corporationkey :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/*.jarkey :java.runtime.name and value  :Java(TM) SE Runtime Environmentkey :sun.java.command and value  :org.apache.catalina.startup.Bootstrap startkey :java.class.path and value  :F:\apache-tomcat-7.0.77\bin\bootstrap.jar;F:\apache-tomcat-7.0.77\bin\tomcat-juli.jarkey :java.vm.specification.name and value  :Java Virtual Machine Specificationkey :java.vm.specification.version and value  :1.7key :catalina.home and value  :F:\apache-tomcat-7.0.77key :sun.cpu.endian and value  :littlekey :sun.os.patch.level and value  :Service Pack 1key :java.io.tmpdir and value  :F:\apache-tomcat-7.0.77\tempkey :java.vendor.url.bug and value  :http://bugreport.sun.com/bugreport/key :server.loader and value  :key :java.rmi.server.hostname and value  :127.0.0.1key :os.arch and value  :amd64key :java.awt.graphicsenv and value  :sun.awt.Win32GraphicsEnvironmentkey :java.ext.dirs and value  :E:\Program Files (x86)\Java\jdk1.7.0_80\jre\lib\ext;C:\Windows\Sun\Java\lib\extkey :user.dir and value  :F:\apache-tomcat-7.0.77\binkey :line.separator and value  :key :java.vm.name and value  :Java HotSpot(TM) 64-Bit Server VMkey :file.encoding and value  :GBKkey :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.ConsoleAppenderlog4j.appender.stdout.Target = System.outlog4j.appender.stdout.layout = org.apache.log4j.PatternLayoutlog4j.appender.stdout.layout.ConversionPattern =  %d{ABSOLUTE} %5p %c{1}:%L - %m%nlog4j.logger.com.ibatis=DEBUGlog4j.logger.com.ibatis.common.jdbc.SimpleDataSource=DEBUGlog4j.logger.com.ibatis.common.jdbc.ScriptRunner=DEBUGlog4j.logger.com.ibatis.sqlmap.engine.impl.SqlMapClientDelegate=DEBUGlog4j.logger.org.mybatis=DEBUGlog4j.logger.java.sql.Connection=DEBUGlog4j.logger.java.sql.Statement=DEBUGlog4j.logger.java.sql.PreparedStatement=DEBUGlog4j.logger.java.sql.ResultSet=DEBUG### to file ###log4j.appender.D = org.apache.log4j.DailyRollingFileAppenderlog4j.appender.D.File = ${catalina.home}/logs/springMVC.loglog4j.appender.D.Append = truelog4j.appender.D.Threshold = DEBUG log4j.appender.D.layout = org.apache.log4j.PatternLayoutlog4j.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!