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