Fix #6260 - make plugin thread safe by preventing parallelism. This naive approach should be replaced in the future and allow parallel runs to achieve faster build times but this change will at least allow users to run maven with threads and other plugins and modules will run in parallel. This approach is also suggested by Apache Maven as a workaround: https://cwiki.apache.org/confluence/display/MAVEN/Parallel+builds+in+Maven+3

Author: MosheElisha

modules/swagger-codegen-maven-plugin/src/main/java/io/swagger/codegen/plugin/CodeGenMojo.java

@@ -50,7 +50,7 @@
 /**
  * Goal which generates client/server code from a swagger json/yaml definition.
  */
-@Mojo(name = "generate", defaultPhase = LifecyclePhase.GENERATE_SOURCES)
+@Mojo(name = "generate", defaultPhase = LifecyclePhase.GENERATE_SOURCES, threadSafe = true)
 public class CodeGenMojo extends AbstractMojo {
 
     @Parameter(name = "verbose", required = false, defaultValue = "false")
@@ -315,6 +315,13 @@ public class CodeGenMojo extends AbstractMojo {
 
     @Override
     public void execute() throws MojoExecutionException {
+        // Using the naive approach for achieving thread safety
+        synchronized (CodeGenMojo.class) {
+            execute_();
+        }
+    }
+
+    protected void execute_() throws MojoExecutionException {
 
         if (skip) {
             getLog().info("Code generation is skipped.");