console.writeline and System.out.println

问题:

What exactly is the technical difference between console.writeline and System.out.println? I know that System.out.println writes to standard output but is this not the same thing as the console?

I do not fully understand the documentation for console.writeline.

回答1:

Here are the primary differences between using System.out/.err/.in and System.console():

• System.console() returns null if your application is not run in a terminal (though you can handle this in your application)
• System.console() provides methods for reading password without echoing characters
• System.out and System.err use the default platform encoding, while the Console class output methods use the console encoding

This latter behaviour may not be immediately obvious, but code like this can demonstrate the difference:

public class ConsoleDemo {
  public static void main(String[] args) {
    String[] data = { "\u250C\u2500\u2500\u2500\u2500\u2500\u2510", 
        "\u2502Hello\u2502",
        "\u2514\u2500\u2500\u2500\u2500\u2500\u2518" };
    for (String s : data) {
      System.out.println(s);
    }
    for (String s : data) {
      System.console().writer().println(s);
    }
  }
}

On my Windows XP which has a system encoding of windows-1252 and a default console encoding of IBM850, this code will write:

???????
?Hello?
???????
┌─────┐
│Hello│
└─────┘

Note that this behaviour depends on the console encoding being set to a different encoding to the system encoding. This is the default behaviour on Windows for a bunch of historical reasons.

回答2:

They're essentially the same, if your program is run from an interactive prompt and you haven't redirected stdin or stdout:

public class ConsoleTest {
    public static void main(String[] args) {
        System.out.println("Console is: " + System.console());
    }
}

results in:

$ java ConsoleTest
Console is: java.io.Console@2747ee05
$ java ConsoleTest </dev/null
Console is: null
$ java ConsoleTest | cat
Console is: null

The reason Console exists is to provide features that are useful in the specific case that you're being run from an interactive command line:

• secure password entry (hard to do cross-platform)
• synchronisation (multiple threads can prompt for input and Console will queue them up nicely, whereas if you used System.in/out then all of the prompts would appear simultaneously).

Notice above that redirecting even one of the streams results in System.console() returning null; another irritation is that there's often no Console object available when spawned from another program such as Eclipse or Maven.

回答3:

First I am afraid your question contains a little mistake. There is not method writeline in class Console. Instead class Console provides method writer() that returns PrintWriter. This print writer has println().

Now what is the difference between

System.console().writer().println("hello from console");

and

System.out.println("hello system out");

If you run your application from command line I think there is no difference. But if console is unavailable System.console() returns null while System.out still exists. This may happen if you invoke your application and perform redirect of STDOUT to file.

Here is an example I have just implemented.

import java.io.Console;


public class TestConsole {
    public static void main(String[] args) {
        Console console = System.console();
        System.out.println("console=" + console);
        console.writer().println("hello from console");
    }
}

When I ran the application from command prompt I got the following:

$ java TestConsole
console=java.io.Console@93dcd
hello from console

but when I redirected the STDOUT to file...

$ java TestConsole >/tmp/test
Exception in thread "main" java.lang.NullPointerException
        at TestConsole.main(TestConsole.java:8)

Line 8 is console.writer().println().

Here is the content of /tmp/test

console=null

I hope my explanations help.

回答4:

There's no Console.writeline in Java. Its in .NET.

Console and standard out are not same. If you read the Javadoc page you mentioned, you will see that an application can have access to a console only if it is invoked from the command line and the output is not redirected like this

java -jar MyApp.jar > MyApp.log

Other such cases are covered in SimonJ's answer, though he missed out on the point that there's no Console.writeline.