In Java development, JShell has become an indispensable tool for developers looking to streamline their coding process. Introduced in Java 9 as the official Java REPL (Read-Eval-Print Loop), JShell enables interactive programming by allowing developers to execute Java code snippets, evaluate expressions, and test new concepts on-the-fly without the need for a full-fledged project setup. This powerful feature significantly improves the efficiency and productivity of Java developers.
This article is intended for experienced Java developers who are already familiar with the basics of JShell and want to explore its advanced usage and customization options. By the end of this article, you’ll not only gain a deeper understanding of JShell’s capabilities but also learn how to tailor it to your specific needs, ultimately elevating your Java development experience. Let’s dive into the world of JShell and unveil its advanced features, customizations, and best practices.
JShell Features
JShell is packed with a variety of features that make it a powerful tool for Java developers. In this section, we’ll cover some of the most useful features that contribute to JShell’s ease of use and efficiency.
Tab completion and code suggestions
One of the standout features of JShell is its tab completion and code suggestions. This functionality allows you to type partial code fragments, press the ‘Tab’ key, and JShell will automatically suggest potential completions. This feature not only saves time but also helps avoid typing errors. Additionally, JShell’s code suggestions include relevant methods, variables, and class names from the current context, making it easier to discover and use available APIs.
History and navigation
JShell keeps track of your command history, allowing you to easily navigate through previously executed code snippets and commands using the up and down arrow keys. This feature is particularly useful for quickly re-executing or modifying previously entered code. You can also use the ‘/history’ command to display a complete list of your input history, which can be helpful for reviewing past actions or exporting your work.
Command-line options and editor integration
JShell provides various command-line options that enable you to customize its behavior and tailor it to your preferences. Some of these options include setting the initial classpath, enabling or disabling feedback, and launching JShell with a specific JDK version. Moreover, JShell can be integrated with popular Java Integrated Development Environments (IDEs) like IntelliJ IDEA and Eclipse, allowing you to use JShell’s interactive features directly within your favorite development environment.
Error reporting and recovery
JShell is designed to handle errors gracefully and provide meaningful feedback to help you quickly identify and fix issues. When you encounter a syntax error or runtime exception, JShell displays a detailed error message with relevant information to help you diagnose the problem. Additionally, JShell’s error recovery mechanism allows you to fix the problematic code snippet and automatically updates any dependent code snippets, saving you the trouble of manually correcting all related errors. This feature is particularly useful in an interactive environment where code snippets are often built incrementally and depend on each other.
Advanced JShell Usage
In this section, we’ll delve into more advanced JShell features and explore how you can use them to further enhance your interactive programming experience.
Methods and lambda expressions
JShell allows you to define methods and lambda expressions, enabling you to create reusable code snippets and modularize your interactive code. You can define methods in JShell just like you would in a regular Java source file. Here’s an example:
jshell> int sum(int a, int b) { return a + b; }
| created method sum(int,int)
Code language: Java (java)Similarly, you can create lambda expressions to represent anonymous functions:
jshell> Function<Integer, Integer> square = x -> x * x;
| created variable square : Function<Integer, Integer> = $Lambda$16/0x000000080012bc40@3f8e7a01Code language: Java (java)Working with imports and classes
JShell automatically imports several commonly used Java packages, such as java.io, java.util, and java.math. However, you can also import additional packages or specific classes as needed:
jshell> import java.time.LocalDateTime;Code language: Java (java)You can also define your own classes within JShell, allowing you to create more complex and structured code:
jshell> class Person {
...> private String name;
...> private int age;
...>
...> Person(String name, int age) {
...> this.name = name;
...> this.age = age;
...> }
...>
...> String getName() { return name; }
...> int getAge() { return age; }
...> }
| created class PersonCode language: Java (java)Evaluating expressions and statements
JShell can evaluate both expressions and statements, allowing you to perform calculations, create variables, or execute methods on the fly. Here are some examples:
jshell> int result = sum(5, 3);
| created variable result : int = 8
jshell> square.apply(4);
| Expression value is: 16
| assigned to temporary variable $1 of type IntegerManaging code snippets and their dependencies
As you work with JShell, you may end up with numerous code snippets, some of which depend on others. JShell provides several built-in commands to help you manage these snippets and their dependencies:
/list: Lists all code snippets in the current session./edit <id>: Opens an editor to modify a specific code snippet./drop <id>: Removes a code snippet and its dependencies./save <filename>: Saves your code snippets to a file.
When you modify or remove a code snippet, JShell automatically updates any dependent snippets, ensuring that your code remains consistent.
For example, suppose you have two snippets that depend on each other:
jshell> String greet(String name) { return "Hello, " + name + "!"; }
| created method greet(String)
jshell> String greeting = greet("John");
| created variable greeting : String = "Hello, John!"Code language: Java (java)If you modify the greet method, JShell will automatically update the greeting variable:
jshell> String greet(String name) { return "Hi, " + name + "!"; }
| replaced method greet(String)
| update modified variable greeting : String = "Hi, John!"Code language: Java (java)Customization and Configuration
JShell offers several customization and configuration options to help you create a tailored interactive programming environment. In this section, we’ll explore some of these options and how they can be used to enhance your JShell experience.
JShell startup configuration
You can configure JShell to load custom settings or code snippets automatically when it starts up. To do this, create a startup script file containing JShell commands, code snippets, or imports, and then pass the file to JShell using the --startup command-line option:
jshell --startup my_startup.jshCode language: Java (java)For example, you might create a my_startup.jsh file with the following contents:
import java.time.LocalDateTime;
import java.util.stream.Collectors;
System.out.println("Welcome to JShell!");Code language: Java (java)Customizing feedback mode
JShell provides several built-in feedback modes that control the amount and format of the information displayed when you execute code snippets. You can switch between these modes using the /set command:
jshell> /set feedback [verbose|normal|concise|silent]Code language: Java (java)Alternatively, you can create a custom feedback mode by modifying an existing mode or defining a new one. To do this, use the /set command with a mode parameter:
jshell> /set mode my_custom_mode normal
jshell> /set prompt my_custom_mode "\u001B[32mjs> \u001B[0m"
jshell> /set feedback my_custom_modeCode language: Java (java)This example creates a custom mode based on the normal mode, but with a green-colored prompt.
Defining custom commands and aliases
JShell allows you to define custom commands and aliases using the /alias command. This can be useful for creating shortcuts to frequently used actions or adding new functionality to JShell. Here’s an example of defining a custom alias:
jshell> /alias now /eval LocalDateTime.now()
jshell> /now
| Expression value is: 2023-04-17T18:31:40.817697
| assigned to temporary variable $1 of type LocalDateTimeCode language: Java (java)In this example, we created a custom alias now that evaluates the current date and time using the LocalDateTime.now() method.
Using external libraries and custom classpath
JShell allows you to use external libraries and custom classpath settings to extend its functionality. To use an external library, download the JAR file and add it to JShell’s classpath using the --class-path command-line option:
jshell --class-path path/to/your/library.jarCode language: Java (java)Alternatively, you can add the JAR file to the classpath during an active JShell session using the /env command:
jshell> /env --class-path path/to/your/library.jar
Code language: Java (java)Once the library is added to the classpath, you can import and use its classes and methods just like you would in a regular Java project:
jshell> import com.example.MyLibraryClass;
jshell> MyLibraryClass.doSomething();
Code language: Java (java)JShell API
The JShell API provides a programmatic interface for interacting with JShell, allowing you to embed JShell functionality within your Java applications. In this section, we’ll explore the JShell API and demonstrate how to use it to create custom JShell instances, evaluate code snippets, and implement event handlers and feedback providers.
Overview of JShell API and its capabilities
The JShell API, available in the jdk.jshell package, allows you to create and manage JShell instances, execute code snippets, and manage the evaluation state programmatically. Some of the key classes in the API include:
JShell: Represents an instance of JShell.Snippet: Represents a code snippet, such as a method, variable, or expression.SnippetEvent: Represents an event related to a code snippet, such as creation, modification, or deletion.ExecutionControl: Controls the execution of code snippets in a JShell instance.
Creating a custom JShell instance
To create a custom JShell instance, you can use the JShell class’s create() method. This method returns a JShell.Builder object that you can use to configure the JShell instance:
import jdk.jshell.JShell;
public class JShellExample {
public static void main(String[] args) {
JShell jshell = JShell.create();
// Use the JShell instance
}
}
Code language: Java (java)Evaluating code snippets programmatically
To evaluate code snippets programmatically, use the eval() method of the JShell class. This method takes a string containing the code snippet and returns a list of SnippetEvent objects representing the results:
import jdk.jshell.JShell;
import jdk.jshell.SnippetEvent;
public class JShellExample {
public static void main(String[] args) {
JShell jshell = JShell.create();
String code = "int x = 5; int y = 3; x + y;";
List<SnippetEvent> events = jshell.eval(code);
for (SnippetEvent event : events) {
System.out.println("Snippet: " + event.snippet().source());
System.out.println("Value: " + event.value());
System.out.println("Status: " + event.status());
System.out.println("---");
}
}
}
Code language: Java (java)Implementing a custom event handler and feedback provider
You can customize the behavior of a JShell instance by implementing event handlers and feedback providers. Event handlers are used to respond to events related to code snippets, while feedback providers control the feedback displayed during the evaluation process. To implement a custom event handler, create a class that extends jdk.jshell.spi.ExecutionControl and override its methods:
import jdk.jshell.spi.ExecutionControl;
import jdk.jshell.spi.ExecutionEnv;
public class MyExecutionControl extends ExecutionControl {
private final ExecutionControl defaultControl;
public MyExecutionControl(ExecutionEnv env) {
this.defaultControl = ExecutionControl.generateDefault(env);
}
// Override methods to customize behavior
}
Code language: Java (java)To implement a custom feedback provider, create a class that implements jdk.jshell.spi.ExecutionControlProvider:
import jdk.jshell.spi.ExecutionControlProvider;
import jdk.jshell.spi.ExecutionEnv;
public class MyExecutionControlProvider implements ExecutionControlProvider {
@Override
public String name() {
return "MyExecutionControlProvider";
}
@Override
public ExecutionControl generate(ExecutionEnv env) {
return new MyExecutionControl(env);
}
}
Code language: Java (java)JShell Use Cases and Practical Examples
JShell is a versatile tool that can be used in various scenarios to enhance the Java development process. In this section, we’ll discuss some practical use cases for JShell and provide examples to demonstrate its capabilities.
Rapid prototyping and experimentation
JShell is ideal for quickly testing ideas, experimenting with new APIs, and prototyping code snippets without the overhead of setting up a complete Java project. You can use JShell to test individual methods, classes, or even full algorithms to determine their correctness and efficiency.
For example, suppose you want to test the performance of a sorting algorithm:
jshell> import java.util.Arrays;
jshell> int[] data = {5, 1, 9, 3, 7};
jshell> long start = System.nanoTime();
jshell> Arrays.sort(data);
jshell> long end = System.nanoTime();
jshell> System.out.println("Sorting time: " + (end - start) + " ns");
Code language: Java (java)Debugging and diagnosing issues
JShell can be used to reproduce and diagnose issues or bugs in your code. By isolating problematic code snippets in JShell, you can quickly identify the cause of the issue and test potential solutions. This can be particularly helpful for debugging complex applications with multiple dependencies.
For example, suppose you encounter a NullPointerException when calling a method:
jshell> String formatName(String firstName, String lastName) {
...> return firstName.trim() + " " + lastName.trim();
...> }
| created method formatName(String,String)
jshell> String result = formatName(null, "Doe");
| java.lang.NullPointerException thrown
Code language: Java (java)You can use JShell to test different approaches for handling null values:
jshell> String formatName(String firstName, String lastName) {
...> return (firstName == null ? "" : firstName.trim()) + " " + (lastName == null ? "" : lastName.trim());
...> }
| replaced method formatName(String,String)
jshell> String result = formatName(null, "Doe");
| created variable result : String = " Doe"
Code language: Java (java)Creating interactive tutorials and documentation
JShell can be used to create interactive tutorials, demonstrations, and documentation for Java libraries and APIs. By embedding JShell instances in your documentation or tutorial materials, you can provide users with live, executable examples that help illustrate concepts and reinforce learning.
For example, you can create an interactive tutorial for a custom math library:
jshell> /save tutorial.jsh
jshell> /open tutorial.jsh
jshell> System.out.println("Welcome to the Math Library Tutorial!");
jshell> System.out.println("Example: Calculate the square of a number.");
jshell> int square(int x) { return x * x; }
jshell> System.out.println("Square of 4: " + square(4));
Code language: Java (java)Automating tasks and scripting
JShell can be used as a scripting tool for automating tasks and creating custom scripts in Java. By combining JShell with the Java standard library and external libraries, you can create powerful scripts to automate tasks such as file manipulation, data processing, or system administration.
For example, you can create a script to process a CSV file and generate a summary report:
jshell> /env --class-path path/to/csv-library.jar
jshell> import java.nio.file.*;
jshell> import java.io.*;
jshell> import com.opencsv.*;
jshell> void processCSV(String inputFilename, String outputFilename) {
...> try (CSVReader reader = new CSVReader(new FileReader(inputFilename));
...> BufferedWriter writer = Files.newBufferedWriter(Paths.get(outputFilename))) {
...> String[] nextLine;
...> int lineCount = 0;
...> while ((nextLine = reader.readNext()) != null) {
...> lineCount++;
...> }
...> writer.write("Total lines: " + lineCount);
...> } catch (IOException e) {
...> e.printStackTrace();
...> }
...> }
jshell> processCSV("input.csv", "report.txt");
Code language: Java (java)In this example, we created a script to read data from a CSV file, count the number of lines, and write the result to a summary report file. The script leverages an external CSV library and the Java standard library for file handling. This showcases JShell’s capabilities in automating tasks and creating custom scripts in a Java environment.
Tips and Tricks for Efficient JShell Usage
To make the most of JShell, it’s essential to know some tips and tricks that can help you use the tool more efficiently. In this section, we’ll discuss some useful techniques for enhancing your JShell experience.
Keyboard shortcuts and navigation tips
JShell provides a variety of keyboard shortcuts and navigation tips to help you work more efficiently. Some of the most useful shortcuts include:
- Tab: Autocomplete a command or code snippet.
- Shift + Tab: Display a list of possible completions.
- Up/Down Arrow: Navigate through the command history.
- Ctrl + C: Cancel the current command or code snippet.
Additionally, you can use the /history command to view your command history and the /list command to view the code snippets you’ve entered during the current session.
Code snippet organization and management
JShell provides several commands for organizing and managing your code snippets:
/list: List all code snippets entered during the current session./save <filename>: Save all code snippets to a file./open <filename>: Load code snippets from a file./drop <snippet-id>: Remove a specific code snippet.
By using these commands, you can easily manage your code snippets, save them for future use, and load them as needed.
Leveraging Java language features in JShell
JShell supports most Java language features, which means you can use features such as lambdas, streams, and optional values to make your code more concise and expressive:
jshell> List<String> names = List.of("Alice", "Bob", "Charlie");
jshell> names.stream().filter(n -> n.startsWith("A")).forEach(System.out::println);
Alice
Code language: Java (java)In this example, we used Java streams and a lambda expression to filter and print the names starting with “A”.
Integrating JShell with popular Java IDEs
Many popular Java IDEs, such as IntelliJ IDEA and Eclipse, offer built-in support or plugins for JShell. By integrating JShell with your favorite IDE, you can take advantage of the IDE’s advanced features, such as code completion, syntax highlighting, and debugging, while working with JShell.
To integrate JShell with IntelliJ IDEA, follow these steps:
- Open IntelliJ IDEA and create or open a Java project.
- Go to the “Tools” menu, and click on “JShell Console.”
- A new JShell console will open, allowing you to use JShell directly within the IDE.
For Eclipse, you can use the “JShell for Eclipse” plugin, which can be installed from the Eclipse Marketplace.
These tips and tricks will help you work more efficiently with JShell and make the most of its powerful features.