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Checkstyle is a development tool to help programmers write Java code that adheres to a coding standard. By default it supports the Google Java Style Guide and Sun Code Conventions, but is highly configurable. It can be invoked with an ANT task and a command line program.

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Quick Overview

Checkstyle is a development tool to help programmers write Java code that adheres to a coding standard. It automates the process of checking Java code to spare humans of this boring (but important) task. This makes it ideal for projects that want to enforce a coding standard.

Pros

  • Highly configurable, allowing teams to define and enforce custom coding standards
  • Integrates well with various build tools and IDEs (e.g., Maven, Gradle, Eclipse, IntelliJ IDEA)
  • Supports a wide range of checks, from basic formatting to complex design patterns
  • Active community and regular updates

Cons

  • Can be overwhelming to configure due to the large number of available checks
  • May produce false positives in some cases, requiring fine-tuning of rules
  • Learning curve for creating custom checks
  • Performance can be impacted when running on large codebases

Code Examples

  1. Basic usage with Maven:
<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-checkstyle-plugin</artifactId>
  <version>3.1.2</version>
  <configuration>
    <configLocation>checkstyle.xml</configLocation>
  </configuration>
</plugin>
  1. Configuring a specific check in checkstyle.xml:
<module name="Checker">
  <module name="TreeWalker">
    <module name="MethodLength">
      <property name="tokens" value="METHOD_DEF"/>
      <property name="max" value="50"/>
    </module>
  </module>
</module>
  1. Suppressing a check for a specific line:
public class MyClass {
    @SuppressWarnings("checkstyle:magicnumber")
    private static final int MAX_SIZE = 100;
}

Getting Started

  1. Add Checkstyle to your project (e.g., using Maven):
<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-checkstyle-plugin</artifactId>
  <version>3.1.2</version>
  <executions>
    <execution>
      <id>validate</id>
      <phase>validate</phase>
      <goals>
        <goal>check</goal>
      </goals>
    </execution>
  </executions>
</plugin>
  1. Create a checkstyle.xml configuration file in your project root.
  2. Run Checkstyle with: mvn checkstyle:check

Competitor Comparisons

Style guides for Google-originated open-source projects

Pros of styleguide

  • Provides style guides for multiple languages (Java, Python, JavaScript, etc.)
  • Offers configuration files for various IDEs and tools
  • Maintained by Google, benefiting from their expertise and best practices

Cons of styleguide

  • Less automated enforcement compared to Checkstyle
  • Primarily documentation-based, requiring manual adherence
  • Limited customization options for specific project needs

Code Comparison

styleguide (Java):

class MyClass {
  @Override public void method() {
    if (condition()) {
      try {
        something();
      } catch (ProblemException e) {
        recover();
      }
    }
  }
}

Checkstyle (XML configuration):

<module name="Checker">
  <module name="TreeWalker">
    <module name="LeftCurly"/>
    <module name="RightCurly"/>
    <module name="WhitespaceAround"/>
  </module>
</module>

Summary

styleguide offers comprehensive style guides for multiple languages, backed by Google's expertise. However, it relies more on manual adherence and offers less automation compared to Checkstyle. Checkstyle provides more granular control and automated enforcement of coding standards through its XML-based configuration, making it easier to maintain consistent code style across large projects.

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A Ruby static code analyzer and formatter, based on the community Ruby style guide.

Pros of RuboCop

  • Specifically designed for Ruby, offering more Ruby-centric rules and best practices
  • Includes auto-correction features for many style issues
  • Highly configurable with easy-to-use YAML configuration files

Cons of RuboCop

  • Limited to Ruby language, whereas Checkstyle supports multiple languages
  • Can be slower on large codebases compared to Checkstyle
  • Less extensive documentation and community support compared to Checkstyle

Code Comparison

RuboCop configuration example:

AllCops:
  NewCops: enable
  SuggestExtensions: false

Style/StringLiterals:
  EnforcedStyle: double_quotes

Checkstyle configuration example:

<module name="Checker">
  <module name="TreeWalker">
    <module name="AvoidStarImport"/>
    <module name="ConstantName"/>
  </module>
</module>

Both tools allow for customization of rules and coding standards, but RuboCop's YAML configuration is generally considered more user-friendly compared to Checkstyle's XML format.

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Pros of Pylint

  • Supports Python-specific features and best practices
  • Highly customizable with extensive configuration options
  • Integrates well with popular Python IDEs and development tools

Cons of Pylint

  • Can be slower than Checkstyle, especially on large codebases
  • May produce more false positives, requiring careful configuration
  • Learning curve can be steeper for new users

Code Comparison

Pylint configuration example:

[MASTER]
ignore=CVS
ignore-patterns=
persistent=yes
load-plugins=
jobs=1
unsafe-load-any-extension=no
extension-pkg-whitelist=

Checkstyle configuration example:

<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
    "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
    "https://checkstyle.org/dtds/configuration_1_3.dtd">
<module name="Checker">
    <module name="TreeWalker">
        <module name="AvoidStarImport"/>
    </module>
</module>

Both tools offer extensive configuration options, but Pylint's configuration is typically done in Python-style INI files, while Checkstyle uses XML. Pylint focuses on Python-specific rules and conventions, whereas Checkstyle is designed for Java and other languages that compile to Java bytecode.

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Find and fix problems in your JavaScript code.

Pros of ESLint

  • Highly configurable with a wide range of built-in rules and plugins
  • Supports modern JavaScript features and frameworks like React
  • Faster performance, especially for large codebases

Cons of ESLint

  • Limited to JavaScript and related technologies
  • Steeper learning curve for complex configurations
  • Less comprehensive documentation compared to Checkstyle

Code Comparison

Checkstyle (XML configuration)

<module name="Checker">
  <module name="TreeWalker">
    <module name="MethodLength">
      <property name="max" value="50"/>
    </module>
  </module>
</module>

ESLint (JavaScript configuration)

module.exports = {
  rules: {
    'max-lines-per-function': ['error', 50]
  }
};

Both tools aim to enforce coding standards, but they differ in their target languages and configuration approaches. Checkstyle focuses on Java and uses XML for configuration, while ESLint is designed for JavaScript and uses JavaScript objects for configuration. ESLint offers more flexibility and easier integration with modern JavaScript ecosystems, but Checkstyle provides a more comprehensive set of rules for Java development. The choice between the two depends on the primary programming language and specific project requirements.

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Static code analysis for Kotlin

Pros of detekt

  • Specifically designed for Kotlin, offering better support for Kotlin-specific language features
  • Provides more advanced static code analysis, including complexity metrics and code smell detection
  • Integrates well with Gradle and other Kotlin-centric build tools

Cons of detekt

  • Limited to Kotlin projects, whereas Checkstyle supports multiple languages
  • Smaller community and ecosystem compared to Checkstyle's mature and widely-adopted platform
  • May require more configuration and customization for specific project needs

Code Comparison

detekt:

val config = DetektConfig(
    rules = setOf(
        NoEmptyClassBody(),
        MaxLineLength(maxLineLength = 120)
    )
)

Checkstyle:

<module name="Checker">
  <module name="TreeWalker">
    <module name="EmptyBlock"/>
    <module name="LineLength">
      <property name="max" value="120"/>
    </module>
  </module>
</module>

Both tools allow for customizable rule sets, but detekt's configuration is typically done in Kotlin, while Checkstyle uses XML. detekt's approach may be more intuitive for Kotlin developers, while Checkstyle's XML configuration is language-agnostic and widely supported across different development environments.

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Prettier is an opinionated code formatter.

Pros of Prettier

  • Language-agnostic, supporting multiple languages beyond Java
  • Opinionated formatting with minimal configuration options
  • Integrates well with modern JavaScript ecosystems and build tools

Cons of Prettier

  • Less granular control over specific formatting rules
  • May not adhere to some established coding standards in certain languages
  • Limited customization options compared to Checkstyle's extensive rule set

Code Comparison

Prettier (JavaScript):

function example(longArgument1,
                 longArgument2) {
  const result = longArgument1 + longArgument2;
  return result;
}

Checkstyle (Java):

public void example(String longArgument1,
    String longArgument2) {
    String result = longArgument1 + longArgument2;
    return result;
}

Prettier automatically formats code with minimal configuration, while Checkstyle requires more detailed rule setup but offers greater control. Prettier focuses on consistent, opinionated formatting across multiple languages, whereas Checkstyle specializes in Java with extensive customization options. The code examples demonstrate how each tool handles formatting, with Prettier emphasizing simplicity and Checkstyle allowing for more specific indentation rules.

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README

Checkstyle - Java Code Quality Tool


Checkstyle is a tool that ensures adherence to a code standard or a set of best practices.

The latest release version can be found at GitHub releases or at Maven repo.

Each-commit builds of maven artifacts can be found at Maven Snapshot repository.

Documentation is available in HTML format, see https://checkstyle.org/checks.html .

Table of Contents

Quick Start

$ cat config.xml
<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
          "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
          "https://checkstyle.org/dtds/configuration_1_3.dtd">
<module name="Checker">
  <module name="TreeWalker">
    <module name="FallThrough"/>
  </module>
</module>

$ cat Test.java
class Test {
  public void foo() {
    int i = 0;
    while (i >= 0) {
      switch (i) {
        case 1:
        case 2:
          i++;
        case 3: // violation 'fall from previous branch of the switch'
          i++;
      }
    }
  }
}

$ java -jar checkstyle-10.18.1-all.jar -c config.xml Test.java
Starting audit...
[ERROR] Test.java:9:9: Fall through from previous branch of switch statement [FallThrough]
Audit done.
Checkstyle ends with 1 errors.

Contributing

Thanks for your interest in contributing to CheckStyle! Please see the Contribution Guidelines for information on how to contribute to the project. This includes creating issues, submitting pull requests, and setting up your development environment.

Build Instructions

Please see the CheckStyle Documentation for information on how to build the project.

Feedback and Support

  • Visit our Discussions Page, where you can ask questions and discuss the project with other users and contributors. This is our preferred method of communication for topics like usage and configuration questions, debugging, and other feedback.
  • Stack Overflow is another place to ask questions about Checkstyle usage.
  • If you are interested in contributing to the project, you can join our Discord Contributors Chat with invite link.
  • Our Google Groups Forum is a mailing list for discussion and support; however, we may be slow to respond there.

Javadoc

Take a look at our javadoc to see our API documentation.

Sponsor Checkstyle

Checkstyle is an open-source project that is developed and maintained by volunteers. If you find Checkstyle useful, please consider sponsoring the project. Your support helps us to maintain and improve Checkstyle.

Licensing

Checkstyle is licensed under the GNU LGPL v2.1 License. Checkstyle uses libraries: