pure: Store variable values in a ThreadLocal map

To ensure that variable values are not overwritten when different XML
are validated on separate threads, store the variable values in aborts
ThreadLocal map.

Fixes issue #182.

ph-schematron-pure/src/main/java/com/helger/schematron/pure/xpath/XPathLetVariableResolver.java

@@ -30,7 +30,14 @@
 @NotThreadSafe
 public class XPathLetVariableResolver implements XPathVariableResolver
 {
-  private final ICommonsMap <QName, Object> m_aVariables = new CommonsHashMap <> ();
+  private final ThreadLocal<ICommonsMap <QName, Object>> m_aVariables = new ThreadLocal<> () {
+    @Override
+    protected ICommonsMap <QName, Object> initialValue()
+    {
+      return new CommonsHashMap <> ();
+    }
+  };
+
   private final XPathVariableResolver m_aDelegatedResolver;
 
   public XPathLetVariableResolver (@Nullable final XPathVariableResolver aResolver)
@@ -41,7 +48,7 @@ public XPathLetVariableResolver (@Nullable final XPathVariableResolver aResolver
   public void setVariableValue (@Nonnull final QName aVariableName, @Nullable final Object aValue)
   {
     ValueEnforcer.notNull (aVariableName, "VariableName");
-    m_aVariables.put (aVariableName, aValue);
+    m_aVariables.get().put (aVariableName, aValue);
   }
 
   /**
@@ -53,7 +60,7 @@ public void setVariableValue (@Nonnull final QName aVariableName, @Nullable fina
   public void removeVariable (@Nullable final QName aVariableName)
   {
     if (aVariableName != null)
-      m_aVariables.remove (aVariableName);
+      m_aVariables.get().remove (aVariableName);
   }
 
   @Override
@@ -62,7 +69,7 @@ public Object resolveVariable (@Nullable final QName aVariableName)
     if (aVariableName != null)
     {
       // 1. variables
-      final Object result = m_aVariables.get (aVariableName);
+      final Object result = m_aVariables.get().get (aVariableName);
       if (result != null)
         return result;