PMD provides a simple test framework for unit testing rules
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Introduction

Good rules have tests. At least a positive test case - a code example, that triggers the rule and reports a violation - and a negative test case - a code example, that doesn’t trigger the rule - should be created. Of course, the more tests, the better the rule is verified. If the rule is more complex or defines properties, with which the behavior can be modified, then these different cases can also be tested.

And if there is a bug fix for a rule, be it a false positive or a false negative case, should be accompanied with an additional test case, so that the bug is not accidentally reintroduced later on.

How it works

PMD’s built-in rules are organized in rulesets, such as “java-basic”. Each ruleset has a single test class, which executes all the test cases for all rules in this ruleset. The actual test cases are stored in separate XML files, for each rule a separate file is used.

The test class subclasses net.sourceforge.pmd.testframework.SimpleAggregatorTst, which provides the seamless integration with JUnit. You basically tell the framework, which rules should be tested and it searches the test code on its own.

The test code (see below Test XML Reference) describes the test case completely with the expected behavior like number of expected rule violations, where the violations are expected, and so on.

When you are running the test class in your IDE (e.g. Eclipse or IntelliJ IDEA) you can also select a single test case and just execute this one.

Where to place the test code

The SimpleAggregatorTst class searches the XML file, that describes the test cases for a certain rule using the following convention: The XML file is a test resource, so it is searched in the tree under src/test/resources.

The sub package xml of the test class’s package should contain a file with the same name as the rule’s name which is under test.

For example, to test the “Java Error Prone Category”, the fully qualified test class is:

net.sourceforge.pmd.lang.java.rule.errorprone.ErrorProneRulesTest

The test code for the rule “AvoidBranchingStatementAsLastInLoop” can be found in the file:

src/test/resources/net/sourceforge/pmd/lang/java/rule/errorprone/xml/AvoidBranchingStatementAsLastInLoop.xml

In general, the class name and file name pattern for the test class and data is this:

net.sourceforge.pmd.lang.<Language Terse Name>.rule.<Category Name>.<Category Name>RulesTest
src/test/resources/net/sourceforge/pmd/lang/<Language Terse Name>/rule/<Category Name>/xml/<Rule Name>.xml

Simple example

Test Class: ErrorProneRulesTest

This is a stripped down example for the Java Error Prone Category:

package net.sourceforge.pmd.lang.java.rule.errorprone;

import net.sourceforge.pmd.testframework.SimpleAggregatorTst;

public class ErrorProneRulesTest extends SimpleAggregatorTst {

    private static final String RULESET = "category/java/errorprone.xml";

    @Override
    public void setUp() {
        addRule(RULESET, "AvoidBranchingStatementAsLastInLoop");
        addRule(RULESET, "AvoidDecimalLiteralsInBigDecimalConstructor");
    }
}

This test class overrides the method setUp in order to register test cases for the two rules. If there are more rules, just add additional addRule(...) calls.

Test Data: AvoidBranchingStatementAsLastInLoop.xml

This is a stripped down example which just contains two test cases.

<?xml version="1.0" encoding="UTF-8"?>
<test-data
    xmlns="http://pmd.sourceforge.net/rule-tests"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://pmd.sourceforge.net/rule-tests http://pmd.sourceforge.net/rule-tests_1_0_0.xsd">
    <test-code>
        <description>ok: no violations</description>
        <expected-problems>0</expected-problems>
        <code><![CDATA[
public class Good {
    public void foo(boolean b) {
        for (int i = 0; i < 10;) {
            if (b) {
                return true;
            }
        }
    }
}
            ]]></code>
    </test-code>

    <test-code>
        <description>violations: return:for</description>
        <expected-problems>1</expected-problems>
        <expected-linenumbers>4</expected-linenumbers>
        <code><![CDATA[
public class Bad {
    public void bar() {
        for (int i = 0; i < 10;) {
            return true;
        }
    }
}
            ]]></code>
    </test-code>
</test-data>

Each test case is in an own <test-code> element. The first defines 0 expected problems, means this code doesn’t trigger the rule. The second test case expects 1 problem. Since the rule violations also report the exact AST node, you can verify the line number, too.

Test XML Reference

The root element is <test-data>. It can contain one or more <test-code> and <code-fragment> elements. Each <test-code> element defines a single test case. <code-fragment> elements are used to share code snippets between different test cases.

<test-code> attributes

The <test-code> elements understands three optional attributes:

  • reinitializeRule: By default, it’s true, so each test case starts with a fresh instantiated rule. Set it to false to reproduce cases, where the previous run has influences.

  • regressionTest: By default, it’s true. Set it to false, to ignore and skip a test case.

  • useAuxClasspath: By default, it’s true. Set it to false to reproduce issues which only appear without type resolution.

<test-code> children

  • <description>: Short description of the test case. This will be the JUnit test name in the report. If applicable, this description should contain a reference to the bug number, this test case reproduces.

  • <rule-property>: Optional rule properties, if the rule is configurable. Just add multiple elements, to set multiple properties for one test case. For an example, see below.

  • <expected-problems>: The the raw number of expected rule violations, that this rule is expected to report. For false-positive test cases, this is always “0”. For false-negative test cases, it can be any positive number.

  • <expected-linenumbers>: Optional element. It’s a comma separated list of line numbers. If there are rule violations reported, then this allows you to assert the line numbers. Useful if multiple violations should be detected and to be sure that false positives and negatives don’t erase each other.

  • <expected-messages>: Optional element, with <message> elements as children. Can be used to validate the correct error message, e.g. if the error message references a variable name.

  • <code>: Either the <code> element or the <code-ref> element is required. It provides the actual code snippet on which the rule is executed. The code itself is usually wrapped in a “CDATA” section, so that no further XML escapes (entity references such as &lt;) are necessary.

  • <code-ref id=...>: Alternative to <code>. References a <code-fragment> defined earlier in the file. This allows you to share the same code snippet with several test cases. The attribute id must match the id of the references code fragment.

  • <source-type>: Optional element that specifies the source code language. This defines the parser that is used for parsing the code snippet. If not given, java is used as default.

<code-fragment>

The code fragment has just one required attribute: id. This is used to reference it via a <code-ref> element inside a <test-code>. Similar like the <code> element, the content of <code-fragment> is usually wrapped in a “CDATA” section, so that no further XML escapes (entity references such as &lt;) are necessary.

Complete XML example

<?xml version="1.0" encoding="UTF-8"?>
<test-data>
    <test-code reinitializeRule="true" regressionTest="true" useAuxClasspath="true">
        <description>Just a description, will be used as the test name for JUnit in the reports</description>
        <rule-property name="somePropName">propValue</rule-property>    <!-- optional -->
        <expected-problems>2</expected-problems>
        <expected-linenumbers>5,14</expected-linenumbers>               <!-- optional -->
        <expected-messages>                                             <!-- optional -->
            <message>Violation message 1</message>
            <message>Violation message 2</message>
        </expected-messages>
        <code><![CDATA[
    public class ConsistentReturn {
        public Boolean foo() {
        }
    }
         ]]></code>
            <source-type>apex</source-type>                             <!-- optional -->
        </test-code>

        <code-fragment id="codeSnippet1"><![CDATA[
    public class ConsistentReturn {
        public Boolean foo() {
    }
    }
        ]]></code-fragment>
        <test-code>
            <description>test case using a code fragment</description>
            <expected-problems>0</expected-problems>
            <code-ref id="codeSnippet1"/>
        </test-code>
    </test-data>

Using the test framework externally

It is also possible to use the test framework for custom rules developed outside the PMD source base. Therefore you just need to reference the dependency net.sourceforge.pmd:pmd-test.

For maven, you can use this snippet:

<dependency>
    <groupId>net.sourceforge.pmd</groupId>
    <artifactId>pmd-test</artifactId>
    <version>6.0.0</version>
    <scope>test</scope>
</dependency>

Then proceed as described earlier: create your test class, create your test cases and run the unit test.

How the test framework is implemented

The framework uses a custom JUnit test runner under the hood, among a couple of utility classes:

  • SimpleAggregatorTst: This is the base class for the test classes and defines the custom JUnit test runner. It itself is a subclass of RuleTst.

  • RuleTst: contains the logic to parse the XML files and provide a list of TestDescriptors. Each test descriptor describes a single test case. It also contains the logic to execute such a test descriptor and assert the results.

  • PMDTestRunner: A custom JUnit test runner, that combines two separate test runners: The custom RuleTestRunner and the standard JUnit4 test runner. This combination allows you to add additional standard unit test methods annotated with @Test to your test class.

    Note: Since the test class is executed through two test runners, it is actually instantiated twice. Be aware of this, if you do any initialization in the constructor. Also, the static hooks @BeforeClass and @AfterClass will be executed twice.

  • RuleTestRunner: This test runner executes the test descriptors with the help of RuleTst.