Fixes #57 using un-modifyable lists & judicious synchronization

pom.xml

@@ -11,7 +11,7 @@
   <groupId>org.scalatest</groupId>
   <artifactId>scalatest-maven-plugin</artifactId>
   <packaging>maven-plugin</packaging>
-  <version>2.0.0</version>
+  <version>2.0.1-SNAPSHOT</version>
   <name>ScalaTest Maven Plugin</name>
   <description>Integrates ScalaTest into Maven</description>
 

src/main/java/org/scalatest/tools/maven/AbstractScalaTestMojo.java

@@ -6,7 +6,21 @@
 import org.apache.maven.project.MavenProject;
 import org.codehaus.plexus.util.cli.*;
 
-import java.io.File;
+import static java.util.Collections.unmodifiableList;
+import static org.scalatest.tools.maven.MojoUtils.*;
+
+import java.io.*;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Map;
+
+import static java.util.Collections.singletonList;
+
+import java.net.MalformedURLException;
+import java.net.URLClassLoader;
+import java.net.URL;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.net.MalformedURLException;
@@ -388,7 +402,8 @@ private List<String> testClasspathElements() {
 
     // This is the configuration parameters shared by all concrete Mojo subclasses
     List<String> sharedConfiguration() {
-        return new ArrayList<String>() {{
+        return unmodifiableList(
+            new ArrayList<String>() {{
             addAll(runpath());
             addAll(config());
             addAll(tagsToInclude());
@@ -404,15 +419,15 @@ List<String> sharedConfiguration() {
             addAll(testsFiles());
             addAll(junitClasses());
             addAll(spanScaleFactor());
-        }};
+        }});
     }
 
     private List<String> config() {
         List<String> c = new ArrayList<String>();
         for(String pair : splitOnComma(config)){
             c.add("-D"+pair);
         }
-        return c;
+        return unmodifiableList(c);
     }
 
     private List<String> runpath() {
@@ -431,7 +446,7 @@ private List<String> tagsToExclude() {
     }
 
     private List<String> parallel() {
-        return parallel ? singletonList("-P") : Collections.<String>emptyList();
+        return parallel ? unmodifiableList(singletonList("-P")) : Collections.<String>emptyList();
     }
 
     //
@@ -459,7 +474,7 @@ private List<String> suites() {
                 }
             }
         }
-        return list;
+        return unmodifiableList(list);
     }
 
     //
@@ -526,7 +541,7 @@ private List<String> tests() {
         for (String test: splitOnComma(tests)) {
             addTest(list, test);
         }
-        return list;
+        return unmodifiableList(list);
     }
 
     private List<String> spanScaleFactor() {
@@ -535,7 +550,7 @@ private List<String> spanScaleFactor() {
             list.add("-F");
             list.add(spanScaleFactor + "");
         }
-        return list;
+        return unmodifiableList(list);
     }
 
     private List<String> suffixes() {
@@ -567,7 +582,7 @@ private List<String> testsFiles() {
                 list.add(param);
             }
         }
-        return list;
+        return unmodifiableList(list);
     }
 
     private List<String> junitClasses() {

src/main/java/org/scalatest/tools/maven/MojoUtils.java

@@ -1,11 +1,14 @@
 package org.scalatest.tools.maven;
 
+import java.util.Collections;
 import java.util.List;
 import java.util.ArrayList;
 import java.io.File;
 
 import static org.apache.commons.lang3.StringUtils.isEmpty;
 
+import static java.util.Collections.unmodifiableList;
+
 /**
  * Provides internal utilities for the Mojo's operations.
  * 
@@ -47,7 +50,7 @@ public String f(String in){
     }
 
     // sideeffect!
-    private static void createIfNotExists(File dir) {
+    private static synchronized void createIfNotExists(File dir) {
         if(!dir.exists() && !dir.mkdirs()){
             throw new IllegalStateException("Cannot create directory " + dir);
         }
@@ -70,7 +73,7 @@ static List<String> compoundArg(String name, String...strings){
             }
             list.add(a);
         }
-        return list;
+        return unmodifiableList(list);
     }
 
     static List<String> suiteArg(String name, String commaSeparated) {
@@ -79,7 +82,7 @@ static List<String> suiteArg(String name, String commaSeparated) {
             list.add(name);
             list.add(param);
         }
-        return list;
+        return unmodifiableList(list);
     }
 
     static List<String> reporterArg(String name, String commaSeparated, F map) {
@@ -94,7 +97,7 @@ static List<String> reporterArg(String name, String commaSeparated, F map) {
                 r.add(map.f(split[1]));
             }
         }
-        return r;
+        return unmodifiableList(r);
     }
 
     //
@@ -105,13 +108,13 @@ static List<String> reporterArg(String name, String commaSeparated, F map) {
     static List<String> splitOnComma(String cs) {
         List<String> args = new ArrayList<String>();
         if (cs == null) {
-            return args;
+            return unmodifiableList(args);
         } else {
             String[] split = cs.split("(?<!\\\\),");
             for (String arg : split) {
                 args.add(arg.trim().replaceAll("\\\\,", ","));
             }
-            return args;
+            return unmodifiableList(args);
         }
     }
 

src/main/java/org/scalatest/tools/maven/TestMojo.java

@@ -7,6 +7,7 @@
 import java.io.File;
 import java.util.Collections;
 import static java.util.Collections.singletonList;
+import static java.util.Collections.unmodifiableList;
 import java.util.List;
 import java.util.ArrayList;
 
@@ -21,6 +22,7 @@
  * @author Bill Venners
  * @phase test
  * @goal test
+ * @threadSafe
  */
 public class TestMojo extends AbstractScalaTestMojo {
 
@@ -124,15 +126,15 @@ String[] configuration() {
     // These private methods create the relevant portion of the command line
     // to pass to Runner based on the corresponding Maven configuration parameter.
     private List<String> stdout() {
-        return singletonList(stdout == null ? "-o" : "-o" + stdout);
+        return unmodifiableList(singletonList(stdout == null ? "-o" : "-o" + stdout));
     }
 
     private List<String> stderr() {
-        return stderr == null ? Collections.<String>emptyList() : singletonList("-e" + stderr);
+        return stderr == null ? Collections.<String>emptyList() : unmodifiableList(singletonList("-e" + stderr));
     }
 
     private List<String> filereports() {
-        return reporterArg("-f", filereports, fileRelativeTo(reportsDirectory));
+        return unmodifiableList(reporterArg("-f", filereports, fileRelativeTo(reportsDirectory)));
     }
 
     private List<String> htmlreporters() {
@@ -151,7 +153,7 @@ private List<String> htmlreporters() {
                 }
             }
         }
-        return r;
+        return unmodifiableList(r);
     }
 
     private List<String> reporters() {