check for junit4 on classpath before loading plugin

pitest-maven-verification/src/test/java/org/pitest/PitMojoIT.java

@@ -580,4 +580,15 @@ public void shouldNotNullPointerWhenEnumInitializerNotCalled() throws IOExceptio
     assertThat(actual).isNotEmpty();
   }
 
+  @Test
+  public void shouldFailCleanlyWhenTestPluginMissing() throws IOException, VerificationException {
+    File testDir = prepare("/pit-missing-test-plugin");
+    verifier.executeGoal("test");
+    verifier.executeGoal("org.pitest:pitest-maven:mutationCoverage");
+
+    String actual = readResults(testDir);
+    assertThat(actual).isNotEmpty();
+  }
+
+
 }

pitest-maven-verification/src/test/resources/pit-missing-test-plugin/pom.xml

@@ -0,0 +1,41 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<groupId>org.example</groupId>
+	<artifactId>junit-categories-check</artifactId>
+	<packaging>jar</packaging>
+	<version>1.0-SNAPSHOT</version>
+	<name>missing-test-plugin</name>
+	<dependencies>
+<dependency>
+    <groupId>org.junit.jupiter</groupId>
+    <artifactId>junit-jupiter-engine</artifactId>
+    <version>5.8.1</version>
+    <scope>test</scope>
+</dependency>
+	</dependencies>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-compiler-plugin</artifactId>
+				<version>3.10.1</version>
+			</plugin>
+			<plugin>
+				<groupId>org.pitest</groupId>
+				<artifactId>pitest-maven</artifactId>
+				<version>${pit.version}</version>
+				<configuration>
+	                <exportLineCoverage>true</exportLineCoverage>
+				    <outputFormats><value>XML</value></outputFormats>
+				    <timestampedReports>false</timestampedReports>
+                    <targetTests><param>com.example.*</param></targetTests>
+                    <targetClasses><param>com.example*</param></targetClasses>
+					<verbose>true</verbose>
+					<features>+CLASSLIMIT(limit[6])</features>
+				</configuration>
+			</plugin>
+		</plugins>
+	</build>
+
+</project>

pitest-maven-verification/src/test/resources/pit-missing-test-plugin/src/main/java/com/example/Covered.java

@@ -0,0 +1,13 @@
+package com.example;
+
+public class Covered {
+
+  public static int someCode(int i) {
+    if ( i == 0 ) {
+      return 1;
+    }
+
+    return 1;
+  }
+
+}

pitest-maven-verification/src/test/resources/pit-missing-test-plugin/src/main/java/com/example/NotCovered.java

@@ -0,0 +1,17 @@
+package com.example;
+
+public class NotCovered {
+
+  public static int someCode(int i) {
+    if ( i == 0 ) {
+      return 1;
+    }
+
+    if ( i == 2 ) {
+      return 42;
+    }
+
+    return i;
+  }
+
+}

pitest-maven-verification/src/test/resources/pit-missing-test-plugin/src/test/java/com/example/ATest.java

@@ -0,0 +1,21 @@
+package com.example;
+
+import java.util.Arrays;
+import java.util.Collection;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ATest {
+
+  @Test
+  public void aTest() {
+    assertEquals(1, NotCovered.someCode(0));
+  }
+
+  @Test
+  public void anotherTest() {
+    assertEquals(42, NotCovered.someCode(2));
+  }
+
+}

pitest-maven-verification/src/test/resources/pit-missing-test-plugin/src/test/java/com/example/AnotherTest.java

@@ -0,0 +1,21 @@
+package com.example;
+
+import java.util.Arrays;
+import java.util.Collection;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class AnotherTest {
+
+  @Test
+  public void aTest() {
+    assertEquals(1, Covered.someCode(0));
+  }
+
+  @Test
+  public void anotherTest() {
+    assertEquals(1, Covered.someCode(1));
+  }
+
+}

pitest/src/main/java/org/pitest/junit/JUnitTestPlugin.java

@@ -35,7 +35,22 @@ public String description() {
   public Configuration createTestFrameworkConfiguration(TestGroupConfig config,
       ClassByteArraySource source, Collection<String> excludedRunners, Collection<String> includedTestMethods) {
     Objects.requireNonNull(config);
-    return new JUnitCompatibleConfiguration(config, excludedRunners, includedTestMethods);
+
+    if (junit4IsPresent()) {
+      return new JUnitCompatibleConfiguration(config, excludedRunners, includedTestMethods);
+    }
+    return new NullConfiguration();
+  }
+
+  // the plugin performs junit version checks later, but these don't get to run
+  // if junit is completely absent and cannot be loaded.
+  private boolean junit4IsPresent() {
+    try {
+      Class.forName("org.junit.runner.manipulation.Filter");
+      return true;
+    } catch (ClassNotFoundException ex) {
+      return false;
+    }
   }
 
   @Override

pitest/src/main/java/org/pitest/junit/NullConfiguration.java

@@ -0,0 +1,26 @@
+package org.pitest.junit;
+
+import org.pitest.help.PitHelpError;
+import org.pitest.testapi.Configuration;
+import org.pitest.testapi.TestSuiteFinder;
+import org.pitest.testapi.TestUnitFinder;
+
+import java.util.Collections;
+import java.util.Optional;
+
+public class NullConfiguration implements Configuration {
+    @Override
+    public TestUnitFinder testUnitFinder() {
+        return c -> Collections.emptyList();
+    }
+
+    @Override
+    public TestSuiteFinder testSuiteFinder() {
+        return c -> Collections.emptyList();
+    }
+
+    @Override
+    public Optional<PitHelpError> verifyEnvironment() {
+        return Optional.empty();
+    }
+}

pitest/src/test/java/org/pitest/junit/NullConfigurationTest.java

@@ -0,0 +1,25 @@
+package org.pitest.junit;
+
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class NullConfigurationTest {
+
+    NullConfiguration underTest = new NullConfiguration();
+
+    @Test
+    public void findsNoTests() {
+        assertThat(underTest.testUnitFinder().findTestUnits(this.getClass())).isEmpty();
+    }
+
+    @Test
+    public void findsNoSuites() {
+        assertThat(underTest.testSuiteFinder().apply(this.getClass())).isEmpty();
+    }
+
+    @Test
+    public void passesVerification() {
+        assertThat(underTest.verifyEnvironment()).isEmpty();
+    }
+}