Getting Started with Jshell (Java Shell Tool)

Contents

• What is REPL?
• Jshell (Java Shell Tool)
• Install Jshell 
• Advantages of Jshell
• Using Jshell 

What is REPL?

REPL full form stands for read-evaluate-print-loop.  REPL is an interactive shell which performs the read-evaluate-print-loop operations that means it reads the line, evaluates it and prints the output. 

Jshell (Java Shell Tool)

Jshell or Java shell lets us execute the java code from the shell to perform REPL Operations. With Jshell, no need of all the boiler plate code to test snippets of code and evaluate results.  Just write the code interactively and execute them without delay. 

Advantages of  Jshell

The usual java program development process involves following steps

• Open editor and write the program
• Save the file,  compile and fix errors if any.
• Run the program, check for runtime errors and fix them.
• Repeat the process. 

If you have added any new functionality or used a new library, it would take more time for testing. Jshell saves the time and boosts productivity by helping us to try out the code and explore the options as you develop the program. We can evaluate the individual statements, methods, different APIs from different libraries while we develop our program. More importantly, it doesn't replace IDE but as an extra tool to try out the code.

Installing Jshell

There is no separate installation process for Jshell utility. It comes as part of JDK 9 and above.  To install required version of JDK, refer oracle download page and oracle archives page. 

To start Jshell, enter jshell on the command line. 

❯ jshell                                                                                                          
|  Welcome to JShell -- Version 16.0.1
|  For an introduction type: /help intro

jshell> 

 JDK 9 or greater versions must be installed on your system. If multiple versions of JDK exist in your system,  set the JAVA_HOME path to JDK 9 or greater version to use the jshell or invoke them directly by specifying the fullpath.

❯ $JAVA_HOME/bin/jshell                                                                                                           
|  Welcome to JShell -- Version 16.0.1
|  For an introduction type: /help intro

jshell> 

Using Jshell

• Print Statements

• Using Variables

                We can declare variables and use them anywhere in the entire jshell session.  Note that, semicolon at the end is optional.

 

• Expressions

          When an expression is entered that doesn't have a named variable, a scratch variable is created so that the value can be referenced later. A scratch variable start with $ symbol.

We can evaluate compound expressions as well line below. 

• Defining and invoking methods

        We can define the methods as well and invoke them as shown below. 

The below example shows the continuation prompt (...>) that is used when a snippet requires more than one line of input to complete and also we can change the definition of any variable, method, class anytime. 

• Forward References

        JShell accepts method definitions that reference methods, variables, or classes that are not yet defined.  But invocation can be done, only when all the references are defined.

• Exceptions

• Auto Completion with Tab for Snippets

        When you enter snippets, use the Tab key to automatically complete the item. If the item can’t be determined from what was entered, then possible options are provided.

Jshell Commands

Jshell provides some meta commands which are not related to evaluating the snippets. These commands start with forward-slash(/). 

• /import

       By default, 10 packages are imported and can also be imported any package by using import statement.

• /vars

      It is used to show all the variables. 

• /methods

      It is used to show all the methods. 

• /lists

      It is used to show the source code written so far.

• /history

        It shows snippets executed so far. A history of snippets and commands is maintained across JShell sessions. This history provides you with access to items that you entered in the current and previous sessions.

• Tab Completion for commands

            Similar to snippet completion, when you enter commands and command options, use the Tab key to automatically complete the command or option. If the completion can’t be determined from what was entered, then possible choices are provided.

• Search

        Search the history of Jshell  snippets executed is a unique feature of JShell.

jshell> <Ctrl+R>
((reverse-i-search)`': 

Searching & Macros Shortcuts

Ctrl+R	Searches backward through history
Ctrl+S	Searches forwards through history
Ctrl+X (	Starts a macro definition
Ctrl+X )	Finishes a macro definition
Ctrl+X e	Executes a macro

• External Editor

If we have declared a lengthy function or class inside Jshell, we can edit it in the external editor with /edit command which is alternative to command prompt.

jshell> /edit printMyWebsite

jshell> 

Scripts

A Jshell script is a sequence of al code snippets and Jshell commands in a file. We generally use scripts to set up your JShell session with import statements and code that you want available during the session.

jshell> /save mysnippet.jsh

jshell> /exit
|  Goodbye

❯ ls mysnippet.jsh                                                                                                
mysnippet.jsh

Exiting Jshell

/exit command is used to exit Jshell. 

jshell> /exit
|  Goodbye

Jshell is a valuable addition to Java toolbox. Start using in your day to day development work and comment your experience.