diff --git a/runtime/Java/src/org/antlr/v4/runtime/atn/ATNConfigSet.java b/runtime/Java/src/org/antlr/v4/runtime/atn/ATNConfigSet.java
index 0e57bae5e..2ebff1129 100755
--- a/runtime/Java/src/org/antlr/v4/runtime/atn/ATNConfigSet.java
+++ b/runtime/Java/src/org/antlr/v4/runtime/atn/ATNConfigSet.java
@@ -470,6 +470,10 @@ public class ATNConfigSet implements Set<ATNConfig> {
 		configLookup.clear();
 	}
 
+	public boolean isReadonly() {
+		return readonly;
+	}
+
 	public void setReadonly(boolean readonly) {
 		this.readonly = readonly;
 		configLookup = null; // can't mod, no need for lookup cache
diff --git a/runtime/Java/src/org/antlr/v4/runtime/atn/ParserATNSimulator.java b/runtime/Java/src/org/antlr/v4/runtime/atn/ParserATNSimulator.java
index 2489b05d1..098ed860d 100755
--- a/runtime/Java/src/org/antlr/v4/runtime/atn/ParserATNSimulator.java
+++ b/runtime/Java/src/org/antlr/v4/runtime/atn/ParserATNSimulator.java
@@ -1572,8 +1572,10 @@ public class ParserATNSimulator extends ATNSimulator {
 			if ( existing!=null ) return existing;
 
 			D.stateNumber = dfa.states.size();
-			D.configs.optimizeConfigs(this);
-			D.configs.setReadonly(true);
+			if (!D.configs.isReadonly()) {
+				D.configs.optimizeConfigs(this);
+				D.configs.setReadonly(true);
+			}
 			dfa.states.put(D, D);
 			if ( debug ) System.out.println("adding new DFA state: "+D);
 			return D;