Execute non-Java instructions from Java

Classes and methods to use

java.lang.Runtime#exec

The handling of the result is carried out using the returned java.lang.Process.

Code example

Process proc = Runtime.getRuntime().exec( "java --version" );

Execution example

jshell> Process proc = Runtime.getRuntime().exec( "java --version" );
proc ==> Process[pid=19916, exitValue="not exited"]

jshell> proc.exitValue();
$2 ==> 0

Input / output

Note that the method name is the viewpoint from the side that executed Process # exec **.

Output from the execution process

• Process#getInputStream
  → Get InputStream to get the contents output to the standard output by the process executed by Process # exec
  
  

import java.io.*;

Process proc = Runtime.getRuntime().exec( "java Hello" );
String line = null;

try ( var buf = new BufferedReader( new InputStreamReader( proc.getInputStream() ) ) ) {
  while( ( line = buf.readLine() ) != null ) System.out.println( line );
}

# Hello.Java contents
public class Hello {
    public static void main( String[] args ) {
        System.out.println( "Hello, world!!" );
    }
}

Result

Hello, world!!

• Process#getErrorStream
  →
  to get the contents output to the standard error output by the process executed by Process # exec. Get InputStream

import java.io.*;

Process proc = Runtime.getRuntime().exec( "javac NotExist.java" );
String line = null;

try ( var buf = new BufferedReader( new InputStreamReader( proc.getErrorStream() ) ) ) {
  while( ( line = buf.readLine() ) != null ) System.out.println( line );
}

Result

error:File not found: NotExist.java
how to use: javac <options> <source files>
For a list of available options--Use help

Input to the execution process

• Process#getOutputStream
  → Get OutputStream to pass information to standard input of process executed by Process # exec
  

import java.io.*;

Process proc = Runtime.getRuntime().exec( "java -cp . SampleCode" );

try ( var buf = new BufferedWriter( new OutputStreamWriter( proc.getOutputStream() ) ) ) {
     buf.write( "3" );
     buf.newLine();
     buf.write( "5" );
     buf.newLine();
}

//Display the output contents from the acquired process
try ( var buf = new BufferedReader( new InputStreamReader( proc.getInputStream() ) ) ) {
   while( ( line = buf.readLine() ) != null ) System.out.println( line );
}

// SampleCode.Java contents
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class SampleCode {
	public static void main( String[] args ) throws Exception{
		try ( var buf = new BufferedReader(
				new InputStreamReader( System.in )
				) ) {
			int x = Integer.parseInt( buf.readLine() );
			int y = Integer.parseInt( buf.readLine() );

			System.out.println( x + "*" + y + "=" + x*y );
		}
	}
}

Result

3*5=15

Other

--Execute Process # waitFor to wait for the process to complete. --The exit code of the process can be obtained with Process # exitValue.
However, if you call exitValue when the process is not terminated, an exception (java.lang.IllegalThreadStateException) will occur. (See below)

Exception in thread "main" java.lang.IllegalThreadStateException: process has not exited
  at java.base/java.lang.ProcessImpl.exitValue(ProcessImpl.java:478)