How to use JShell: An Command Line tool introduced in JDK 9

Many of you might be aware that Oracle released JDK 9 officially on 21st September, 2017. After JDK 5 and JDK 8, JDK 9 is supposed to be the most effective release from application development perspective. JDK 9 has introduced a lot of new functionality, language level features and API enhancements for developers. This article is about one of command line tool named JShell introduced in JDK 9. Lets learn about it in short.

What is JShell?

  1. JShell was introduced in JDK 9 as part of Java Enhancement Proposal (JEP) 222.
  2.  Generally, JDK tools and their commands enable developers to handle development tasks such as compiling and running a program, packaging source files into a Java Archive (JAR) file, applying security policies to a JAR file, troubleshooting and monitoring application, watching JVM statics, etc. These all tools are available in JDK as executable binaries. Examples: javac, javap, javah, javadoc, java, jar, jlink, jmod, jdeps, jshell (language shell), wsimport, etc.
  3. The JShell command line tool will provide a way to interactively evaluate declarations, statements and expressions of the Java programming language within the JShell state. The JShell state includes an evolving code and execution state. To facilitate rapid investigation and coding, statements and expressions need not occur within a method, and variables and method need not occur within a class.
  4. JShell is a REPL (Read-Evaluate-Print-Loop). It is ideal both for learning the Java language and exploring unfamiliar code (include new Java APIs). A Read-Eval-Print Loop (REPL) is an interactive programming tool which loops, continually reading user input, evaluating the input, and printing the value of the input or a description of the state change the input caused. Scala, Ruby, JavaScript and Python all have REPLs and all allow small initial programs. JShell adds REPL functionality to the Java platform as part of JDK 9.
  5. Prior to JDK 9, If developer wanted to test something, he had to write a compulsory Test class. There was no way to test functionality in isolation of a class. One of the reasons Oracle developed JShell was to handle such cases.

Starting a JShell console

Once you have JDK 9 installed on your machine and set up all environment variables (like JAVA_HOME, PATH etc.), you can start a JShell console by typing jshell in cmd.

To Start JShell:

 jshell 

To Start JShell in Verbose mode:

 jshell -v 

To Exit JShell console:

 /exit 

So now we know how to start JShell console. Let’s look at some examples to help us better understand JShell.

Examples

Example 1

JShell accepts Java statements, variable, method, and class definitions, imports and expressions. We refer to these pieces of Java code as snippets.

Defining variables :

 int a=10; 

Printing variables :

 System.out.println("value of a is ": +a) ;

Defining & calling Methods:

 void printHello(String name){System.out.println("Hello "+name)}; 
 printHello("Jshell");
 String concate(String s1, String s2){ return s1+s2;};
 concate("hello","jshell");

Creating a Custom Type:

 public class Employee{};

Import classes:

 import java.io.FileReader;

So we can see, whatever we do in a typical IDE, same can be done in JShell in a quick amout of time.

Example 2

Lets define a method which helps to calculate area of a  Circle.

 public long calculateArea(int radius){ return PI* square(radius)};

If we execute this snippet in Jshell, It won’t give any compilation error saying PI and square(int) not defined. Error will come only when we try to call this method. This concept is called forward Reference in JShell.

With that in mind, let’s define PI and square(int r) before calling calculateArea(int r) to avoid compilation error.

 double PI=3.14; 
 public int square(int n){return n*n;}

And now call the method:

 calculateArea(5);

Example 3

JShell has a number of commands for controlling the environment and displaying information. They are distinguished from snippets by a leading slash (/). You can get information on the current variables, methods and types with the /vars, /methods, and /types .You can get a list of entered snippets with the /list command. To see a list of imports , just use /imports.  To know more about a command , simply use /help commandName.

 /vars
 /methods
 /types
 /imports
 /list
 /help /list
 /help /vars
 /help /exit

Example 4

Searching Mechanism

Search through history is a powerful tool. Press Ctrl-r followed by a string you wish to search for in your history in JShell. The search will proceed backward, starting from you most recent entries and include previous sessions of the jshell tool.

Example 5

Feedback Modes

A feedback mode is a named user interaction configuration in JShell. There are built-in modes and the user can create custom modes. The built-in modes cannot be modified, but a copy can be made as the base of a user defined mode. There are four built-in modes, in descending order of verbosity: verbose, normal, concise and silent.  To set a feedback mode, use:

/set feedback {modeName}
 /set feedback verbose
 /set feedback silent

I will let developers try these modes and try to understand how they are different in terms of Verbosity.

Hope this article will help you in getting started with JShell as part of JDK 9.

Thanks!

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