为什么 Java 中的 System.out.println() 会打印到控制台？

Question

I read several posts online explaining what System.out.println() is in Java. Most of them go like this:

• System is a final class in the java.lang package.
• out is a public static object inside the System class of type PrintStream .
• println() prints a line of text to the output stream.

My question is when we do System.out.println() in our code, why does it end up writing to the console? This article explains how we can make it write to a file by calling System.setOut() . So my question translates to where is System.setOut() called to redirect its output to the console?

I checked System.setOut() 's source . It makes a call to setOut0() which is a native method. This method is directly called inside the initializeSystemClass() method by passing it fdOut which is a FileOutputStream defined here . I did not find a console output stream passed to setOut0() anywhere, nor did I find a call to the non-native setOut() done anywhere. Is it done somewhere else outside the System class by the JVM while starting execution? If so, can someone point me to it?

Answer 1

My doubt is when we do System.out.println() in our code, why it ends up in writing to console?

In any POSIX compliant shell, each process gets three "standard" streams when the shell starts it:

• The "standard input" stream is for reading input.
• The "standard output" stream is for writing ordinary output.
• The "standard error" stream is for writing error output.

(The same idea is also used in many non-POSIX compliant shells as well.)

For an interactive POSIX shell, the default is for these streams to read from and write to the shell's "console"... which could be a physical console, but is more likely to be a "terminal emulator" on the user's (ultimate) desktop machine. (Details vary.)

A POSIX shell allows you to redirect the standard streams in various ways; eg

$ some-command < file     # read stdin from 'file'
$ some-command > file     # write stdout to 'file'
$ some-command 2> file    # write stderr to 'file'
$ some-command << EOF     # read stdin from a 'here' document
  lines of input
  ...
  EOF
$ some-command | another  # connect stdout for one command to
                          # stdin for the next one in a pipeline

and so on. If you do this, one or more of the standard streams is NOT connected to the console.

Further reading:

• "What are stdin, stdout and stderr on Linux?"
• "Standard Streams"

---

So how does this relate to the question?

When a Java program start, the System.in/out/err streams are connected to the standard input / output / error streams specified by the parent process; typically a shell.

In the case of System.out , that could be the console (however you define that) or it could be a file, or another program or... /dev/null . But where the output goes is determined by how the JVM was launched.

So, the literal answer is "because that is what the parent process has told the Java program to do".

---

How internally shell communicates with jvm to set standard input / output in both Windows and Linux?

This is what happens with Linux, UNIX, Mac OSX and similar. (I don't know for Windows... but I imagine it is similar.)

Suppose that the shell is going to run aaa > bbb.txt .

1. The parent shell forks a child process... sharing the parent shell's address space.
2. The child process closes file descriptor 1 (the standard output file descriptor)
3. The child process opens "bbb.txt" for writing on file descriptor 1.
4. The child process execs the "aaa" command, and it becomes the "aaa" command process. The file descriptors 0, 1, and 2 are preserved by the exec call.
5. The "aaa" command starts...

When the "aaa" command starts, it finds that file descriptors 0 and 2 (stdin and stderr) refer to the same "file" as the parent shell. File descriptor 1 (stdout) refers to "bbb.txt".

The same thing happens when "aaa" is the java command.

Answer 2

It doesn't need to. We can redirect to somewhere else. Here is the code to re-direct into the file:

PrintStream output = new PrintStream(new File("output.txt"));
System.setOut(output);
System.out.println("This will be written to file");

By default, the console is the standard output stream (System.in) in Java.

Answer 3

System.out.println does not print to the console, it prints to the standard output stream ( System.out is Java's name of the standard output stream). The standard output stream is usually the console, but it doesn't have to be. The Java runtime just wraps the standard output stream of the operating system in a nice Java object.

A non-interactive program often uses a few standard input and output channels: it reads input from the standard input stream, does some operations on it, and produces output on the standard output stream. The standard output stream can be the console, but it can also be piped to the standard input stream of another program or to a file. In the end, the operating system running the programming decides what the standard output stream output to.

For example, in Unix terminals you can do something like:

java -jar your.program.jar > output.txt

and store the output of your program in a text file, or

java -jar your.program.jar | grep hello

to only display the lines of the output which contain 'hello'. Only if you don't specify another destination, the standard output stream writes to the console.