System 源码阅读

System

• 属性说明

/**
 *  System 类包含了几个有用的字段和方法，并且不能被实例化。
 *
 * @author  unascribed
 * @since   1.0
 */
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() {
    }

    /**
     *  标准输入流，默认为键盘
     */
    public static final InputStream in = null;

    /**
     *  标识输出流，默认为控制台
     */
    public static final PrintStream out = null;

    /**
     *  标准错误流，默认为控制台
     */
    public static final PrintStream err = null;

    /**
     *  此系统的安全管理器
     */
    private static volatile SecurityManager security;

    /**
     *  控制台
     */
    private static volatile Console cons;

    /**
     *  系统属性值
     *
     * The following properties are guaranteed to be defined:
     * <dl>
     * <dt>java.version         <dd>Java version number
     * <dt>java.version.date    <dd>Java version date
     * <dt>java.vendor          <dd>Java vendor specific string
     * <dt>java.vendor.url      <dd>Java vendor URL
     * <dt>java.vendor.version  <dd>Java vendor version
     * <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 ("
" 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;

• 输入输出流及控制台

    /**
     *  重设标准输入流
     *
     * @since   1.1
     */
    public static void setIn(InputStream in) {
        checkIO();
        setIn0(in);
    }

    /**
     *  重设标准输出流
     *
     * @param out   PrintStream 实例
     * @since   1.1
     */
    public static void setOut(PrintStream out) {
        checkIO();
        setOut0(out);
    }

    /**
     *  重设标准错误流
     *
     * @param err   PrintStream 实例
     * @since   1.1
     */
    public static void setErr(PrintStream err) {
        checkIO();
        setErr0(err);
    }

    private static native void setIn0(InputStream in);
    private static native void setOut0(PrintStream out);
    private static native void setErr0(PrintStream err);

• 读取系统属性和环境变量的值

    /**
     *  读取当前系统时序性
     *
     * <tr><th scope="row"><code>java.version</code></th>
     *     <td>Java Runtime Environment version, which may be interpreted
     *     as a {@link Runtime.Version}</td></tr>
     * <tr><th scope="row"><code>java.version.date</code></th>
     *     <td>Java Runtime Environment version date, in ISO-8601 YYYY-MM-DD
     *     format, which may be interpreted as a {@link
     *     java.time.LocalDate}</td></tr>
     * <tr><th scope="row"><code>java.vendor</code></th>
     *     <td>Java Runtime Environment vendor</td></tr>
     * <tr><th scope="row"><code>java.vendor.url</code></th>
     *     <td>Java vendor URL</td></tr>
     * <tr><th scope="row"><code>java.vendor.version</code></th>
     *     <td>Java vendor version</td></tr>
     * <tr><th scope="row"><code>java.home</code></th>
     *     <td>Java installation directory</td></tr>
     * <tr><th scope="row"><code>java.vm.specification.version</code></th>
     *     <td>Java Virtual Machine specification version which may be
     *     interpreted as a {@link Runtime.Version}</td></tr>
     * <tr><th scope="row"><code>java.vm.specification.vendor</code></th>
     *     <td>Java Virtual Machine specification vendor</td></tr>
     * <tr><th scope="row"><code>java.vm.specification.name</code></th>
     *     <td>Java Virtual Machine specification name</td></tr>
     * <tr><th scope="row"><code>java.vm.version</code></th>
     *     <td>Java Virtual Machine implementation version which may be
     *     interpreted as a {@link Runtime.Version}</td></tr>
     * <tr><th scope="row"><code>java.vm.vendor</code></th>
     *     <td>Java Virtual Machine implementation vendor</td></tr>
     * <tr><th scope="row"><code>java.vm.name</code></th>
     *     <td>Java Virtual Machine implementation name</td></tr>
     * <tr><th scope="row"><code>java.specification.version</code></th>
     *     <td>Java Runtime Environment specification version which may be
     *     interpreted as a {@link Runtime.Version}</td></tr>
     * <tr><th scope="row"><code>java.specification.vendor</code></th>
     *     <td>Java Runtime Environment specification  vendor</td></tr>
     * <tr><th scope="row"><code>java.specification.name</code></th>
     *     <td>Java Runtime Environment specification  name</td></tr>
     * <tr><th scope="row"><code>java.class.version</code></th>
     *     <td>Java class format version number</td></tr>
     * <tr><th scope="row"><code>java.class.path</code></th>
     *     <td>Java class path</td></tr>
     * <tr><th scope="row"><code>java.library.path</code></th>
     *     <td>List of paths to search when loading libraries</td></tr>
     * <tr><th scope="row"><code>java.io.tmpdir</code></th>
     *     <td>Default temp file path</td></tr>
     * <tr><th scope="row"><code>java.compiler</code></th>
     *     <td>Name of JIT compiler to use</td></tr>
     * <tr><th scope="row"><code>os.name</code></th>
     *     <td>Operating system name</td></tr>
     * <tr><th scope="row"><code>os.arch</code></th>
     *     <td>Operating system architecture</td></tr>
     * <tr><th scope="row"><code>os.version</code></th>
     *     <td>Operating system version</td></tr>
     * <tr><th scope="row"><code>file.separator</code></th>
     *     <td>File separator ("/" on UNIX)</td></tr>
     * <tr><th scope="row"><code>path.separator</code></th>
     *     <td>Path separator (":" on UNIX)</td></tr>
     * <tr><th scope="row"><code>line.separator</code></th>
     *     <td>Line separator ("
" on UNIX)</td></tr>
     * <tr><th scope="row"><code>user.name</code></th>
     *     <td>User's account name</td></tr>
     * <tr><th scope="row"><code>user.home</code></th>
     *     <td>User's home directory</td></tr>
     * <tr><th scope="row"><code>user.dir</code></th>
     *     <td>User's current working directory</td></tr>
     * <p>
     *
     * @implNote In addition to the standard system properties, the system
     * properties may include the following keys:
     * <table class="striped">
     * <caption style="display:none">Shows property keys and associated values</caption>
     * <thead>
     * <tr><th scope="col">Key</th>
     *     <th scope="col">Description of Associated Value</th></tr>
     * </thead>
     * <tbody>
     * <tr><th scope="row">{@code jdk.module.path}</th>
     *     <td>The application module path</td></tr>
     * <tr><th scope="row">{@code jdk.module.upgrade.path}</th>
     *     <td>The upgrade module path</td></tr>
     * <tr><th scope="row">{@code jdk.module.main}</th>
     *     <td>The module name of the initial/main module</td></tr>
     * <tr><th scope="row">{@code jdk.module.main.class}</th>
     *     <td>The main class name of the initial module</td></tr>
     * </tbody>
     * </table>
     */
    public static Properties getProperties() {
        final SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertiesAccess();
        }

        return props;
    }

    /**
     *  读取指定名称的系统属性
     *
     * @param      key  系统属性的名称
     */
    public static String getProperty(String key) {
        checkKey(key);
        final SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertyAccess(key);
        }

        return props.getProperty(key);
    }

    /**
     *  读取指定名称 key 的系统属性值，如果不存在，则使用默认值
     *
     * @param      key  系统属性的名称
     * @param      def  默认值
     */
    public static String getProperty(String key, String def) {
        checkKey(key);
        final SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertyAccess(key);
        }

        return props.getProperty(key, def);
    }

    /**
     *  获取当前系统的环境变量映射
     *
     * @since  1.5
     */
    public static java.util.Map<String,String> getenv() {
        final SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("getenv.*"));
        }

        return ProcessEnvironment.getenv();
    }

    /**
     *  读取指定名称的环境变量
     *
     * @param  name 环境变量名称
     */
    public static String getenv(String name) {
        final SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("getenv."+name));
        }

        return ProcessEnvironment.getenv(name);
    }

• 其他常用方法

    /**
     *  将源数组的子数组拷贝到目标数组中
     *
     * @param      src      源数组
     * @param      srcPos   源数组起始索引，包括   
     * @param      dest     目标数组
     * @param      destPos  目标数组起始索引，包括
     * @param      length   从源数组拷贝的元素个数
     */
    @HotSpotIntrinsicCandidate
    public static native void arraycopy(Object src,  int  srcPos,
            Object dest, int destPos,
            int length);

    /**
     *  返回当前 JVM 关联的控制台
     *
     * @since   1.6
     */
    public static Console console() {
        Console c;
        if ((c = cons) == null) {
            synchronized (System.class) {
                if ((c = cons) == null) {
                    cons = c = SharedSecrets.getJavaIOAccess().console();
                }
            }
        }
        return c;
    }

    /**
     *  1970-01-01 UTC 午夜到此刻之间的毫秒数
     */
    @HotSpotIntrinsicCandidate
    public static native long currentTimeMillis();

    /**
     *  返回当前虚拟机高分辨率的时间源，以纳秒为单位，只能用于计算时间差
     *
     * @since 1.5
     */
    @HotSpotIntrinsicCandidate
    public static native long nanoTime();

    /**
     *  主动终止当前虚拟机，非 0 的退出状态表示异常终止。
     *
     * @param      status   退出状态值
     */
    public static void exit(int status) {
        Runtime.getRuntime().exit(status);
    }

    /**
     *  主动运行垃圾收集器，不建议调用
     */
    public static void gc() {
        Runtime.getRuntime().gc();
    }

    /**
     *  计算指定对象的哈希值，null 对象为 0
     *
     * @param x 待计算哈希值的目标对象
     * @since   1.1
     */
    @HotSpotIntrinsicCandidate
    public static native int identityHashCode(Object x);