Make the Mojo threadsafe

Fixes #161

Signed-off-by: Gary O'Neall <gary@sourceauditor.com>

src/main/java/org/spdx/maven/CreateSpdxMojo.java

@@ -98,7 +98,8 @@
  */
 @Mojo( name = "createSPDX",
        defaultPhase = LifecyclePhase.VERIFY,
-       requiresDependencyResolution = ResolutionScope.TEST )
+       requiresDependencyResolution = ResolutionScope.TEST,
+       threadSafe = true )
 public class CreateSpdxMojo extends AbstractMojo
 {
     public static final String INCLUDE_ALL = "**/*";

src/main/java/org/spdx/maven/utils/MavenToSpdxLicenseMapper.java

@@ -61,7 +61,8 @@ public class MavenToSpdxLicenseMapper
     private static final String LISTED_LICENSE_JSON_URL = SPDX_LICENSE_URL_PREFIX + "licenses.json";
     private static final String LISTED_LICENSE_JSON_PATH = "resources/licenses.json";
 
-    static MavenToSpdxLicenseMapper instance;
+    static volatile MavenToSpdxLicenseMapper instance;
+    private static Object instanceMutex = new Object();
     private Map<String, String> urlStringToSpdxLicenseId;
 
     private MavenToSpdxLicenseMapper() throws LicenseMapperException
@@ -101,11 +102,19 @@ private MavenToSpdxLicenseMapper() throws LicenseMapperException
 
     public static MavenToSpdxLicenseMapper getInstance() throws LicenseMapperException
     {
-        if ( instance == null )
+        MavenToSpdxLicenseMapper result = instance;
+        if ( result == null )
         {
-            instance = new MavenToSpdxLicenseMapper();
+            synchronized ( instanceMutex )
+            {
+                result = instance;
+                if ( result == null )
+                {
+                    instance = result = new MavenToSpdxLicenseMapper();
+                }
+            }
         }
-        return instance;
+        return result;
     }
 
     /**