public class HelloWorld {
 
    /**
     * 
     * @param args
     */
 
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("Hello World");
    }
 
}

$ javac HelloWorld.java 
$ java HelloWorld       
Hello World

功能就是在控制台打印 Hello World 字符串， 我们这里深入每一个步骤，探讨是如何将字符串打印到控制台的。

我们刚才的程序运行在 Ubuntu Linux 的控制台 bash 程序里。 javac 是编译器， java 运行虚拟机，载入 HelloWorld.class 执行 HelloWorld 类的 main 函数，这部分是属于虚拟机的内容，暂时不做考虑，我们主要从代码调用的步骤分析。

从代码逻辑上，我们很容易看出是 System 类的 out 成员 println 函数执行了打印操作：

public final class System {

    /**
     * Default input stream.
     */
    public static final InputStream in;

    /**
     * Default output stream.
     */
    public static final PrintStream out;

    /**
     * Default error output stream.
     */
    public static final PrintStream err;

    private static final String lineSeparator;
    private static Properties systemProperties;

    static {
        err = new PrintStream(new FileOutputStream(FileDescriptor.err));
        out = new PrintStream(new FileOutputStream(FileDescriptor.out));
        in = new BufferedInputStream(new FileInputStream(FileDescriptor.in));
        lineSeparator = System.getProperty("line.separator");
    }

out 是个 PrintStream

    /**
     * Prints a string followed by a newline. The string is converted to an array of bytes using
     * the encoding chosen during the construction of this stream. The bytes are
     * then written to the target stream with {@code write(int)}.
     *
     * <p>If an I/O error occurs, this stream's error state is set to {@code true}.
     *
     * @param str
     *            the string to print to the target stream.
     * @see #write(int)
     */
    public synchronized void println(String str) {
        print(str);
        newline();
    }

具体的工作交给了 print ，  我们在这个类里面可以看到 newline 只是打印了两个特殊字符，换行。

    public synchronized void print(String str) {
        if (out == null) {
            setError();
            return;
        }
        if (str == null) {
            print("null");
            return;
        }

        try {
            if (encoding == null) {
                write(str.getBytes());
            } else {
                write(str.getBytes(encoding));
            }
        } catch (IOException e) {
            setError();
        }
    }

进一步的工作由 write 函数完成：

    public synchronized void write(int oneByte) {
        if (out == null) {
            setError();
            return;
        }
        try {
            out.write(oneByte);
            int b = oneByte & 0xFF;
            // 0x0A is ASCII newline, 0x15 is EBCDIC newline.
            boolean isNewline = b == 0x0A || b == 0x15;
            if (autoFlush && isNewline) {
                flush();
            }
        } catch (IOException e) {
            setError();
        }
    }

又来了个 out 这个，out 是 来自 OutputStream 的实现类 public class FileOutputStream extends OutputStream 

public class FileOutputStream extends OutputStream {

    private FileDescriptor fd;
    private final boolean shouldClose;

    /** The unique file channel. Lazily initialized because it's rarely needed. */
    private FileChannel channel;

    /** File access mode */
    private final int mode;

    private final CloseGuard guard = CloseGuard.get();

FileDescriptor

public final class FileDescriptor {

    /**
     * Corresponds to {@code stdin}.
     */
    public static final FileDescriptor in = new FileDescriptor();

    /**
     * Corresponds to {@code stdout}.
     */
    public static final FileDescriptor out = new FileDescriptor();

    /**
     * Corresponds to {@code stderr}.
     */
    public static final FileDescriptor err = new FileDescriptor();

    /**
     * The Unix file descriptor backing this FileDescriptor.
     * A value of -1 indicates that this FileDescriptor is invalid.
     */
    private int descriptor = -1;

    static {
        in.descriptor = STDIN_FILENO;
        out.descriptor = STDOUT_FILENO;
        err.descriptor = STDERR_FILENO;
    }

这里来到 STDOUT_FILENO， 该变量定义在： 在<stdio.h>。
STDIN_FILENO等是文件描述符，是非负整数，一般定义为0, 1, 2，属于没有buffer的I/O，直接调用系统调用，在<unistd.h>。

    @Override
    public void write(byte[] buffer, int byteOffset, int byteCount) throws IOException {
        IoBridge.write(fd, buffer, byteOffset, byteCount);
    }

    @Override
    public void write(int oneByte) throws IOException {
        write(new 

最后是调用 os 的 write