[SUREFIRE-2075] Only set thread count if specified in configuration

surefire-its/src/test/java/org/apache/maven/surefire/its/CheckTestNg740ParallelIT.java

@@ -35,4 +35,12 @@ public void withTestNG740AndParallelSet()
             .executeTest()
             .assertTestSuiteResults( 2, 0, 0, 0 );
     }
+
+    @Test
+    public void withTestNG740AndParallelSetWithoutThreadCount()
+    {
+        unpack( "testng-740-parallel-without-threadcount" )
+            .executeTest()
+            .assertTestSuiteResults( 2, 0, 0, 0 );
+    }
 }

surefire-its/src/test/resources/testng-740-parallel-without-threadcount/pom.xml

@@ -0,0 +1,73 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Licensed to the Apache Software Foundation (ASF) under one
+  ~ or more contributor license agreements.  See the NOTICE file
+  ~ distributed with this work for additional information
+  ~ regarding copyright ownership.  The ASF licenses this file
+  ~ to you under the Apache License, Version 2.0 (the
+  ~ "License"); you may not use this file except in compliance
+  ~ with the License.  You may obtain a copy of the License at
+  ~
+  ~     http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing,
+  ~ software distributed under the License is distributed on an
+  ~ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  ~ KIND, either express or implied.  See the License for the
+  ~ specific language governing permissions and limitations
+  ~ under the License.
+  -->
+
+<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/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.apache.maven.plugins.surefire</groupId>
+    <artifactId>testng-740-parallel</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <name>TestNG 7.4.0 parallel test</name>
+
+    <properties>
+        <surefire.testng.verbose>0</surefire.testng.verbose>
+        <argLine/>
+        <jacoco.agent/>
+        <maven.compiler.source>1.7</maven.compiler.source>
+        <maven.compiler.target>1.7</maven.compiler.target>
+    </properties>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>${surefire.version}</version>
+                <configuration>
+                    <argLine>${argLine} ${jacoco.agent}</argLine>
+                    <parallel>methods</parallel>
+                    <properties>
+                        <property>
+                            <name>surefire.testng.verbose</name>
+                            <value>${surefire.testng.verbose}</value>
+                        </property>
+                    </properties>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+    <dependencies>
+        <dependency>
+            <groupId>org.testng</groupId>
+            <artifactId>testng</artifactId>
+            <version>7.4.0</version>
+            <exclusions>
+                <exclusion>
+                    <!-- NOTE: Deliberaty excluding junit to enforce TestNG only tests, cf. SUREFIRE-642 -->
+                    <groupId>junit</groupId>
+                    <artifactId>junit</artifactId>
+                </exclusion>
+            </exclusions>
+        </dependency>
+    </dependencies>
+
+</project>

surefire-its/src/test/resources/testng-740-parallel-without-threadcount/src/test/java/testng740/TestNG740ParallelTest.java

@@ -0,0 +1,35 @@
+package testng.simple;
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import org.testng.annotations.Test;
+
+
+public class TestNG740ParallelTest {
+    @Test
+    public void testOne() {
+
+    }
+
+    @Test
+    public void testTwo() {
+
+    }
+}

surefire-providers/surefire-testng/src/main/java/org/apache/maven/surefire/testng/conf/TestNGMapConfigurator.java

@@ -28,7 +28,6 @@
 import org.testng.TestNG;
 import org.testng.xml.XmlSuite;
 
-import static java.lang.Integer.parseInt;
 import static org.apache.maven.surefire.api.booter.ProviderParameterNames.PARALLEL_PROP;
 import static org.apache.maven.surefire.api.booter.ProviderParameterNames.THREADCOUNT_PROP;
 import static org.apache.maven.surefire.testng.conf.AbstractDirectConfigurator.loadListenerClasses;
@@ -68,9 +67,18 @@ public void configure( XmlSuite suite, Map<String, String> options )
     protected void configureThreadCount( XmlSuite suite, Map<String, String> options )
         throws TestSetFailedException
     {
-        String threadCountAsString = options.get( THREADCOUNT_PROP );
-        int threadCount = threadCountAsString == null ? 1 : parseInt( threadCountAsString );
-        suite.setThreadCount( threadCount );
+        String threadCount = options.get( THREADCOUNT_PROP );
+        if ( threadCount != null )
+        {
+            try
+            {
+                suite.setThreadCount( Integer.parseInt( threadCount ) );
+            }
+            catch ( NumberFormatException e )
+            {
+                throw new TestSetFailedException( "Non-integer TestNG [threadcount] setting: " + threadCount, e );
+            }
+        }
     }
 
     protected void configureParallel( XmlSuite suite, Map<String, String> options )