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System 类

• 所在包：java.lang.System
• 构造方法为私有方法，因此不能被实例化
• 类名被 final 修饰，因此不能被继承：public final class System {

System.in 标准输入流

/**
 * 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;

读取 System.in

用 BufferedReader 封装：

public static void main(String[] args) throws Exception {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    String s = br.readLine();
    System.out.println(s);
}

System.out 标准输出流

 * 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;

System.in 与 System.out 何时被构造？

System 类中有方法 initializeSystemClass()，在线程初始化后调用该方法，用于构造 System 类的相关属性，包括 in 和 out：

/**
 * 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(newPrintStream(fdOut, props.getProperty("sun.stdout.encoding")));
    setErr0(newPrintStream(fdErr, props.getProperty("sun.stderr.encoding")));

    // 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();
}