Descartes: A Mutation Engine for PIT

What is Descartes?

Descartes evaluates the capability of your test suite to detect bugs using extreme mutation testing. It is able to find your worst tested methods.

Descartes is a mutation engine plugin for PIT which implements extreme mutation operators as proposed in the paper Will my tests tell me if I break this code?.

Quick start with Maven

To use Descartes in a Maven project, add the following plugin configuration to your pom.xml:

<plugin>
  <groupId>org.pitest</groupId>
  <artifactId>pitest-maven</artifactId>
  <version>1.7.0</version>
  <configuration>
    <mutationEngine>descartes</mutationEngine>
  </configuration>
  <dependencies>
    <dependency>
      <groupId>eu.stamp-project</groupId>
      <artifactId>descartes</artifactId>
      <version>1.3.2</version>
    </dependency>
  </dependencies>
</plugin>

Then, execute the regular mutation coverage goal in the root folder of the project under test:

cd my-project-under-test
mvn clean package org.pitest:pitest-maven:mutationCoverage

All options from PIT can be used. For more details check the PIT documentation.

Check "Running Descartes on your project" for additional Descartes configuration options.

For Gradle support check Using Gradle.

For quick configuration snippets showing how to use other testing frameworks such as JUnit 5 or TestNG and how to change Descartes' behavior check How to....

If you use a multi-module project take a look at PitMP.

Table of contents

• How does Descartes work?
• Descartes Output
• Running Descartes on your project
  • Using Maven
    • Specifying operators
    • Configuring stop methods
    • Configuring reports
  • Using Gradle
  • Running from the command line
  • Installing and building from source
• Releases
• More...

How does Descartes work?

Mutation testing

Mutation testing allows you to verify if your test suite can detect possible bugs. The technique works by introducing small changes or faults into the original program. These modified versions are called mutants. A good test suite should be able to detect or kill a mutant. That is, at least one test case should fail when the test suite is executed with the mutant. Read more. Traditional mutation testing works at the instruction level, e.g., replacing ">" by "<=", so the number of generated mutants is huge, as the time required to check the entire test suite. That's why the authors of Will my tests tell me if I break this code? proposed an Extreme Mutation strategy, which works at the method level.

Extreme Mutation Testing

In Extreme Mutation testing, the whole logic of a method under test is eliminated. All statements in a void method are removed. In other case, the body is replaced by a single return statement. This approach generates fewer mutants. Code from the authors can be found in their GitHub repository.

The goal of Descartes is to bring an effective implementation of this kind of mutation operator into the world of PIT and check its performance in real world projects.

Mutation operators

The goal of extreme mutation operators is to replace the body of a method by one simple return instruction or just remove all instructions if is possible. Descartes supports the following mutation operators:

void mutation operator

This operator accepts a void method and removes all the instructions on its body. For example, with the following class as input:

class A {

  int field = 3;

  public void Method(int inc) {
    field += 3;
  }

}

the mutation operator will generate:

class A {

  int field = 3;

  public void Method(int inc) { }

}

null mutation operator

This operator accepts a method with a reference return type and replaces all instructions with return null. For example, using the following class as input:

class A {
    public A clone() {
        return new A();
    }
}

this operator will generate:

class A {
    public A clone() {
        return null;
    }
}

empty mutation operator

This is a special operator which targets methods that return arrays. It replaces the entire body with a return statement that produces an empty array of the corresponding type. For example, the following class:

class A {
  public int[] getRange(int count) {
    int[] result = new int[count];
    for(int i=0; i < count; i++) {
      result[i] = i;
    }
    return result;
  }
}

will become:

class A {
  public int[] getRange(int count) {
    return new int[0];
  }
}

Constant mutation operator

This operator accepts any method with primitive or String return type. It replaces the method body with a single instruction returning a defined constant. For example, if the integer constant 3 is specified, then for the following class:

class A {
    int field;

    public int getAbsField() {
        if(field >= 0)
            return field;
        return -field;
    }
}

this operator will generate:

class A {
    int field;

    public int getAbsField() {
        return 3;
    }
}

new mutation operator

New in version 1.2.6

This operator accepts any method whose return type has a constructor with no parameters and belongs to a java package. It replaces the code of the method by a single instruction returning a new instance.

For example:

class A {
    int field;
    
    public ArrayList range(int end) {
        ArrayList l = new ArrayList();
        for(int i = 0; i < size; i++) {
            A a = new A();
            a.field = i;
            l.add(a);
        }
        return l;
    }
}  

is transformed to:

class A {
    int field;
    
    public List range(int end) {
        return new ArrayList();
    }
}  

This operator handles the following special cases:

Return Type	Replacement
Collection	ArrayList
Iterable	ArrayList
List	ArrayList
Queue	LinkedList
Set	HashSet
Map	HashMap

This means that if a method returns an instance of Collection the code of the mutated method will be return new ArrayList();.

This operator is not enabled by default.

optional mutation operator

New in version 1.2.6

This operator accepts any method whose return type is java.util.Optional. It replaces the code of the method by a single instruction returning an empty instance.

For example:

class A {
    int field;
    
    public Optional<Integer> getOptional() {
        return Optional.of(field);
    }
}  

is transformed to:

class A {
    int field;
    
   public Optional<Integer> getOptional() {
           return Optional.empty();
   }
}  

This operator is not enabled by default.

argument mutation operator

New in version 1.3

This operator replaces the body of a method by returning the value of the first parameter that has the same type as the return type of the method.

For example:

class A {
    public int m(int x, int y) {
        return x + 2 * y;
    }
}

is transformed to:

class A {
    public int m(int x) {
        return x;
    }
}

This operator is not enabled by default.

this mutation operator

New in version 1.3

Replaces the body of a method by return this; if applicable. The goal of this operator is to perform better transformations targeting fluent APIs.

For example:

class A {

    int value = 0;
    public A addOne() {
        value += 1;
        return this;
    }
}

is transformed to:

class A {

    int value = 0;
    public A addOne() {
        return this;
    }
}

This operator is not enabled by default.

Stop Methods

Descartes avoids some methods that are generally not interesting and may introduce false positives such as simple getters, simple setters, empty void methods or methods returning constant values, delegation patterns as well as deprecated and compiler generated methods. Those methods are automatically detected by inspecting their code. A complete list of examples can be found here. The exclusion of stop methods can be configured. For more details see section: "Running Descartes on your project".

Do not use null in methods annotated with @NotNull

New in version 1.2.6

Descartes will avoid using the null operators in methods annotated with @NotNull. This increases its compatibility with Kotlin sources. This feature can be configured. See "Running Descartes on your project" for more details.

Skip methods using @DoNotMutate

New in version 1.3

Descartes skips all method that are annotated with any annotation whose name is DoNotMutate. For example, in the following fragment of code, the method m will not be mutated.

class A {
    @DoNotMutate
    public void m() {...}
}

All methods in a class annotated with @DoNotMutate will be avoided as well. For example, in the following fragment of code, the method m will not be mutated:

@DoNotMutate
class A {
    public void m() {...}
}

The DoNotMutate annotation may specify which operators should be considered. For example:

class A {
    @DoNotMutate(operators = "false")
    public boolean m() { return true; }
}

will instruct Descartes not to use the false mutation operator to mutate m.

When specifying operators, a method annotation takes precedence over class annotations. That, is the @DoNotMutate of a method overrides the same annotation in the class For example:

@DoNotMutate(operators = "true")
class A {

    public boolean n() { return false; }

    @DoNotMutate(operators = "false")
    public boolean m() { return true; }
}

will not mutate method n with true, as instructed in the class annotation. On the other hand, m will be mutated by true but not by false.

Descartes includes a definition of DoNotMutate. However, when the tool inspects the annotations of a class or method it matches only the simple name of the annotation class and ignores the package. So, any DoNotMutate annotation will be considered. In this way a project does not need to add Descartes as a dependency, it can declare its own DoNotMutate and use it.

This feature is also configurable. See "Running Descartes on your project" for more details.

Descartes Output

PIT reporting extensions work with Descartes and include XML, CSV and HTML formats. The HTML format is rather convenient since it also shows the line coverage. Descartes also provides three new reporting extensions:

• a general reporting extension supporting JSON files. It works also with Gregor, the default mutation engine for PIT. To use just set JSON as report format for PIT.
• a reporting extension designed for Descartes that generates a JSON file with information about pseudo and partially tested methods. To use just set METHOD as report format for PIT.
• Descartes can generate a human-readable report containing only the list of methods with testing issues by using the ISSUES format.

Examples of these reporting extensions for Apache Commons-CLI can be checked here.

For more details on how to use and configure these reporting extensions please check section: "Running Descartes on your project". For a more detailed description of the formats see Output Formats.

Running Descartes on your project

Stable releases of Descartes are available from Maven Central. Descartes is a plugin for PIT, so they have to be used together. PIT integrates with majors test and build tools such as Maven, Ant and Gradle.

Using Maven

The minimum configuration to use Descartes in a Maven project is the following:

<plugin>
  <groupId>org.pitest</groupId>
  <artifactId>pitest-maven</artifactId>
  <version>1.7.0</version>
  <configuration>
    <mutationEngine>descartes</mutationEngine>
  </configuration>
  <dependencies>
    <dependency>
      <groupId>eu.stamp-project</groupId>
      <artifactId>descartes</artifactId>
      <version>1.3.2</version>
    </dependency>
  </dependencies>
</plugin>

This actually configures PIT for the project, adds Descartes as a dependency of PIT and sets Descartes as the mutation engine to use. Later, one need to run the mutation coverage goal provided by the PIT Maven plugin as follows:

cd my-project-under-test
mvn clean package # ensures clean state
mvn org.pitest:pitest-maven:mutationCoverage

Specifying operators

The operators must be specified in the pom.xml file. Each operator identifier should be added to the mutators element inside the configuration element. void and null operators are identified by void and null respectively. The values for the constant mutation operator can be specified using the regular literal notation used in a Java program. For example true, 1, 2L, 3.0f, 4.0, 'a', "literal", represent boolean, int, long, float, double, char, string constants. Negative values and binary, octal and hexadecimal bases for integer constants are supported as stated by the language specification. In order to specify a byte or short value, a cast-like notation can be used: (short) -1, (byte)0x1A.

The following configuration:

<mutators>
    <mutator>void</mutator>
    <mutator>4</mutator>
    <mutator>"some string"</mutator>
    <mutator>false</mutator>
</mutators>

will instruct the tool to use the void operator. The constant operator will replace the body of every method returning int with return 4; and will use "some string" and false for every string and boolean method.

By default, Descartes uses the following operators:

<mutators>
  <mutator>void</mutator>
  <mutator>null</mutator>
  <mutator>true</mutator>
  <mutator>false</mutator>
  <mutator>empty</mutator>
  <mutator>0</mutator>
  <mutator>1</mutator>
  <mutator>(byte)0</mutator>
  <mutator>(byte)1</mutator>
  <mutator>(short)0</mutator>
  <mutator>(short)1</mutator>
  <mutator>0L</mutator>
  <mutator>1L</mutator>
  <mutator>0.0</mutator>
  <mutator>1.0</mutator>
  <mutator>0.0f</mutator>
  <mutator>1.0f</mutator>
  <mutator>'\40'</mutator>
  <mutator>'A'</mutator>
  <mutator>""</mutator>
  <mutator>"A"</mutator>
</mutators>

All the goals defined by the pitest-maven plugin work with Descartes. For more information check the Maven plugin's web page.

Configuring stop methods

To configure the stop methods under consideration Descartes provides a STOP_METHODS feature. This feature is enabled by default. The parameter exclude can be used to prevent certain methods to be treated as stop methods and bring them back to the analysis. This parameter can take any of the following values:

exclude	Method description	Example
empty	void methods with no instruction.	public void m() {}
enum	Methods generated by the compiler to support enum types (values and valueOf).
to_string	toString methods.
hash_code	hashCode methods.
deprecated	Methods annotated with @Deprecated or belonging to a class with the same annotation.	@Deprecated public void m() {...}
synthetic	Methods generated by the compiler.
getter	Simple getters.	public int getAge() { return this.age; }
setter	Simple setters. Includes also fluent simple setters.	public void setX(int x) { this.x = x; } and public A setX(int x){ this.x = x; return this; }
constant	Methods returning a literal constant.	public double getPI() { return 3.14; }
delegate	Methods implementing simple delegation.	public int sum(int[] a, int i, int j) {return this.adder(a, i, j); }
clinit	Static class initializers.
return_this	Methods that only return this.	public A m() { return this; }
return_param	Methods that only return the value of a real parameter	public int m(int x, int y) { return y; }
kotlin_setter	Setters generated for data classes in Kotlin (New in version 2.1.6)

So, for example, if we don't want to exclude deprecated methods and mutate them the following snippet should be added under the configuration element:

<features>
  <feature>
  <!-- This will allow descartes to mutate deprecated methods -->
    +STOP_METHODS(except[deprecated])
  </feature>
</features>

More than one group can be excluded at the same time:

<features>
  <feature>
  <!-- This will allow descartes to mutate toString and enum generated methods -->
    +STOP_METHODS(except[to_string] except[enum])
  </feature>
</features>

The feature can be completely disabled:

<features>
  <feature>
  <!--No method is considered as a stop method and therefore all of them will be mutated -->
    -STOP_METHODS()
  </feature>
</features>

Configuring reports

As said before, there are several reporting options provided by Descartes:

• JSON for a general mutation testing report using that file format. It can be used with Gregor.
• METHODS that produces a methods.json file with the list of all methods analyzed and categorized according to the mutation testing result.
• ISSUES a human-readable report containing only the methods with testing issues.

They can be configured and combined as regular PIT report formats:

<plugin>
  <groupId>org.pitest</groupId>
  <artifactId>pitest-maven</artifactId>
  <version>1.7.0</version>
  <configuration>
    <outputFormats>
      <value>JSON</value>
      <value>METHODS</value>
      <value>ISSUES</value>
    </outputFormats>
    <mutationEngine>descartes</mutationEngine>
  </configuration>
    <dependencies>
    <dependency>
      <groupId>eu.stamp-project</groupId>
      <artifactId>descartes</artifactId>
      <version>1.3.2</version>
    </dependency>
  </dependencies>
</plugin>

Other filters

From version 1.2.6 Descartes includes AVOID_NULL as a feature preventing the null operator to mutate a method marked with @NotNull. The feature is active by default.

It can be removed by adding the following configuration:

<features>
  <feature>
    -AVOID_NULL()
  </feature>
</features>

From version 1.3 Descartes includes DO_NOT_MUTATE as a feature preventing the mutation of method and classes annotated with @DoNotMutate. The feature is active by default.

It can be removed by adding the following configuration:

<features>
  <feature>
    -DO_NOT_MUTATE()
  </feature>
</features>

From version 1.3 Descartes includes SKIP_SHORT as a feature to skip methods shorter than a given number of lines. The feature is not active by default. To activate add the following configuration:

<features>
  <feature>
    +SKIP_SHORT()
  </feature>
</features>

The number of lines can be configured as follows:

+SKIP_SHORT(lines[3])

The line threshold is equal to 5 by default.

Using Gradle

Follow the instructions to set up PIT for a project that uses Gradle. Add Descartes as a dependency of the pitest task

pitest 'eu.stamp-project:descartes:1.3.2'

then specify descartes in the mutationEngine option inside the plugin configuration.

An example of the final configuration could be:

plugins {
    id 'java'
    id 'info.solidsoft.pitest' version '1.5.1'
}

// Other configurations

repositories {
    mavenCentral()
    mavenLocal()
}

dependencies {
    testImplementation group: 'junit', name: 'junit', version: '4.13.1'
    pitest 'eu.stamp-project:descartes:1.3.2'
}

pitest {
  mutationEngine = "descartes"
  pitestVersion = "1.7.0"
}

The pitestVersion property has to be specified to avoid version issues with the default version shipped with the Gradle plugin. At last, run the pitest task for the project under test:

gradle pitest

Specifying operators

To specify a custom selection of mutation operators, use the operators property in the pitest block as follows:

mutators = [ '1.2', 'true', 'optional', '"a"' ]

This mutates methods returning double values with 1.2, boolean methods with true, methods returning Optional and methods returning String.

See the Maven section for more details on the operator identifiers.

Configuring stop methods

As explained in the Maven section, Descartes skips some methods that are usually of no interest like simple getters and setters. However, these methods can be reincorporated to the analysis using the STOP_METHODS feature. The following removes all stop methods but allows mutating simple setters:

features = ['+STOP_METHODS(except[setter])']

To allow all stop methods do:

features = ['-STOP_METHODS()']

Configuring reports

Descartes custom output formats can be configured as follows:

outputFormats = ['ISSUES', 'METHODS', 'JSON']

Other features

Descartes includes AVOID_NULL and DO_NOT_MUTATE features as explained previously. These are active by default. To disable this features it is enough to add the following configuration to the pitest block:

features = ['-AVOID_NULL()']

features = ['-DO_NOT_MUTATE()']

Running from the command line

Descartes can be used when invoking PIT from the command line. To do this, follow the instructions to run PIT, include Descartes in the classpath specification and add --mutationEngine=descartes.

Installing and building from source

In a terminal, clone the repository:

git clone https://github.com/STAMP-project/pitest-descartes.git

switch to the cloned folder:

cd  pitest-descartes

install Descartes using the regular Apache Maven commands:

mvn install

After installing the package, PIT should be able to find the Descartes mutation engine.

More...

Performance

You can check here a comparison between Descartes and Gregor, the default mutation engine for PITest, considering the number of mutants both engines created for the same projects, and the total execution time.

External Links

Articles mentioning Descartes:

• A Comprehensive Study of Pseudo-tested Methods (Empirical Software Engineering, doi:10.1007/s10664-018-9653-2)
• Descartes: a PITest engine to detect pseudo-tested methods - Tool Demonstration (Proceedings of ASE, doi:10.1145/3238147.3240474)
• Mutation testing with PIT and Descartes
• Controlling Test Quality

License

Descartes is published under LGPL-3.0 (see LICENSE.md for further details).

Contributing

Issues, pull requests and other contributions are welcome.

Funding

Until December 2019 Descartes was partially funded by research project STAMP (European Commission - H2020)