How to debug JShell in Java 9?

JShell is a REPL tool that allows snippets of code to be run without placing them in classes. This tool provides a way to evaluate declarations, statements, and expressions in Java and no need to create the main() method to test some parts of the code.

The command "/debug" can be used to display debugging information for the JShell tool implementation. Once we type the "/debug" command, the debugging mode is on. After enabling the debug mode and type something like a simple addition, or a simple string, then it prints as below.

Example-1

jshell> /debug
| Debugging on

jshell> 5+3
Compiling: 5+3
Kind: EXPRESSION_STATEMENT -- 5 + 3;
compileAndLoad [Unit($1)]
++setCompilationInfo() Snippet:VariableKey($1)#11-5+3
package REPL;
import java.io.*;import java.math.*;import java.net.*;import java.nio.file.*;import java.util.*;
import java.util.concurrent.*;import java.util.function.*;import java.util.prefs.*;
import java.util.regex.*;import java.util.stream.*;class $JShell
$11 {
public static
int $1;
public static Object do_it$() throws Throwable {
return $1 = 5+3;
}
}

-- diags: []
setStatus() Snippet:VariableKey($1)#11-5+3 - status: VALID
compileAndLoad ins = [Unit($1)] -- legit = [Unit($1)]
Compiler generating class REPL.$JShell$11
compileAndLoad [Unit($1)] -- deps: [] success: true
recordCompilation: Snippet:VariableKey($1)#11-5+3 -- status VALID, unresolved []

$1 ==> 8

Example-2

jshell> /debug
| Debugging on

jshell> String s = "Adithya"
Compiling: String s = "Adithya";
Kind: VARIABLE -- String s = "Adithya"
compileAndLoad [Unit(s)]
++setCompilationInfo() Snippet:VariableKey(s)#12-String s = "Adithya";
package REPL;
import java.io.*;import java.math.*;import java.net.*;import java.nio.file.*;import java.util.*;
import java.util.concurrent.*;import java.util.function.*;import java.util.prefs.*;
import java.util.regex.*;import java.util.stream.*;import static REPL.$JShell$11.$1;
class $JShell$12 {
public static String s;
public static Object do_it$() throws Throwable {
String s_ =
"Adithya";
return s = s_;
}
}

-- diags: []
setStatus() Snippet:VariableKey(s)#12-String s = "Adithya"; - status: VALID
compileAndLoad ins = [Unit(s)] -- legit = [Unit(s)]
Compiler generating class REPL.$JShell$12
compileAndLoad [Unit(s)] -- deps: [] success: true
recordCompilation: Snippet:VariableKey(s)#12-String s = "Adithya"; -- status VALID, unresolved []
s ==> "Adithya"

If we want to "off" the debugging mode, then again type "/debug" command for the same session.

jshell> /debug
| Debugging off