Real hello world in java

Real “Hello World” We all know how to write Hello World in java, but what should it be when we dive into implementation of System class?

System.out.println("Hello World!");

The VM will invoke the initPhase1 method to complete the initialization of this class separate from clinit.

java.lang.System

public final class System {
  private static native void registerNatives();
  static {
      registerNatives();
  }
}

We jump to initPhase1, it creates a PrintStream object, then triggers setOut0(PrintStream out) method.

private static void initPhase1() {
  /// ...
  FileInputStream fdIn = new FileInputStream(FileDescriptor.in);
  FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
  FileOutputStream fdErr = new FileOutputStream(FileDescriptor.err);
  initialIn = new BufferedInputStream(fdIn);
  setIn0(initialIn);
  // stdout/err.encoding are set when the VM is associated with the terminal,
  // thus they are equivalent to Console.charset(), otherwise the encodings
  // of those properties default to native.encoding
  setOut0(newPrintStream(fdOut, props.getProperty("stdout.encoding")));
  setErr0(newPrintStream(fdErr, props.getProperty("stderr.encoding")));
  // ...
}

setOut0 is a native method, but newPrintStream returns a PrintStream, which has the println() method, so that’s clear enough for us to understand what happens.

private static PrintStream newPrintStream(OutputStream out, String enc) {
  if (enc != null) {
      return new PrintStream(new BufferedOutputStream(out, 128), true,
                              Charset.forName(enc, UTF_8.INSTANCE));
  }
  return new PrintStream(new BufferedOutputStream(out, 128), true);
}

println() prints to a new line, and it has multi-thread safety because of synchronized keyword.

public class PrintStream extends FilterOutputStream implements Appendable, Closeable {

    public void println(boolean x) {
        if (getClass() == PrintStream.class) {
            writeln(String.valueOf(x));
        } else {
            synchronized (this) {
                print(x);
                newLine();
            }
        }
    }
}

Finally, setOut0 puts the PrintStream to the class variable of System, which is System.out:

public static final PrintStream out = null;

That’s it, we can make HelloWorld more complex by writing like:

public class HelloWorld {
  public static void main(String[] args) {
    FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
    PrintStream out = new PrintStream(new BufferedOutputStream(fdOut, 128), true);
    out.println("Hello World!");
  }
}

So cool~

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