System
/**
* System 類包含了幾個有用的字段和方法，並且不能被實例化。
*
* @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() {
}
/**
* 標準輸入流，默認爲鍵盤
*/
public static final InputStream in = null;
/**
* 標識輸出流，默認爲控制檯
*/
public static final PrintStream out = null;
/**
* 標準錯誤流，默認爲控制檯
*/
public static final PrintStream err = null;
/**
* 此係統的安全管理器
*/
private static volatile SecurityManager security;
/**
* 控制檯
*/
private static volatile Console cons;
/**
* 系統屬性值
*
* 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 ("\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;
/**
* 重設標準輸入流
*
* @since 1.1
*/
public static void setIn(InputStream in) {
checkIO();
setIn0(in);
}
/**
* 重設標準輸出流
*
* @param out PrintStream 實例
* @since 1.1
*/
public static void setOut(PrintStream out) {
checkIO();
setOut0(out);
}
/**
* 重設標準錯誤流
*
* @param err PrintStream 實例
* @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);
/**
* 讀取當前系統時序性
*
* <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 ("\n" 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;
}
/**
* 讀取指定名稱的系統屬性
*
* @param key 系統屬性的名稱
*/
public static String getProperty(String key) {
checkKey(key);
final SecurityManager sm = getSecurityManager();
if (sm != null) {
sm.checkPropertyAccess(key);
}
return props.getProperty(key);
}
/**
* 讀取指定名稱 key 的系統屬性值，如果不存在，則使用默認值
*
* @param key 系統屬性的名稱
* @param def 默認值
*/
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);
}
/**
* 獲取當前系統的環境變量映射
*
* @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();
}
/**
* 讀取指定名稱的環境變量
*
* @param name 環境變量名稱
*/
public static String getenv(String name) {
final SecurityManager sm = getSecurityManager();
if (sm != null) {
sm.checkPermission(new RuntimePermission("getenv."+name));
}
return ProcessEnvironment.getenv(name);
}
/**
* 將源數組的子數組拷貝到目標數組中
*
* @param src 源數組
* @param srcPos 源數組起始索引，包括
* @param dest 目標數組
* @param destPos 目標數組起始索引，包括
* @param length 從源數組拷貝的元素個數
*/
@HotSpotIntrinsicCandidate
public static native void arraycopy(Object src, int srcPos,
Object dest, int destPos,
int length);
/**
* 返回當前 JVM 關聯的控制檯
*
* @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 午夜到此刻之間的毫秒數
*/
@HotSpotIntrinsicCandidate
public static native long currentTimeMillis();
/**
* 返回當前虛擬機高分辨率的時間源，以納秒爲單位，只能用於計算時間差
*
* @since 1.5
*/
@HotSpotIntrinsicCandidate
public static native long nanoTime();
/**
* 主動終止當前虛擬機，非 0 的退出狀態表示異常終止。
*
* @param status 退出狀態值
*/
public static void exit(int status) {
Runtime.getRuntime().exit(status);
}
/**
* 主動運行垃圾收集器，不建議調用
*/
public static void gc() {
Runtime.getRuntime().gc();
}
/**
* 計算指定對象的哈希值，null 對象爲 0
*
* @param x 待計算哈希值的目標對象
* @since 1.1
*/
@HotSpotIntrinsicCandidate
public static native int identityHashCode(Object x);
