back to core sync around creating DFA in decisionToDFA. not thread-safe for dfa update.

runtime/Java/src/org/antlr/v4/runtime/atn/LexerATNSimulator.java

@@ -162,10 +162,6 @@ public class LexerATNSimulator extends ATNSimulator {
 		int mark = input.mark();
 		traceBeginMatch(input, mode);
 		try {
-			synchronized (decisionToDFA[mode]) {
-				// Synchronizing on the mode DFA causes a lot of contention
-				// between shared lexers in multiple threads, but is the
-				// simplest safety measure. We can start with it.
 			if ( decisionToDFA[mode].s0==null ) {
 				return matchATN(input);
 			}
@@ -173,7 +169,6 @@ public class LexerATNSimulator extends ATNSimulator {
 				return execDFA(input, decisionToDFA[mode].s0);
 			}
 		}
-		}
 		finally {
 			traceEndMatch();
 			input.release(mark);

runtime/Java/src/org/antlr/v4/runtime/atn/ParserATNSimulator.java

@@ -196,18 +196,10 @@ public class ParserATNSimulator<Symbol extends Token> extends ATNSimulator {
 				}
 			}
 			// Now we are certain to have a specific decision's DFA
-			// Synchronize on the DFA so that nobody can read or write
-			// to it while we updated during ATN simulation
-			synchronized (decisionToDFA[decision]) {
 			return predictATN(dfa, input, outerContext);
 		}
-		}
 
 		// We can start with an existing DFA
-		synchronized (decisionToDFA[decision]) {
-			// Only enter the DFA simulation if nobody else is playing with it.
-			// This blocks multiple readonly simulations of the same DFA but that's
-			// unlikely to happen a lot
 		int m = input.mark();
 		int index = input.index();
 		try {
@@ -219,7 +211,6 @@ public class ParserATNSimulator<Symbol extends Token> extends ATNSimulator {
 			input.release(m);
 		}
 	}
-	}
 
 	public int predictATN(@NotNull DFA dfa, @NotNull TokenStream input,
 						  @Nullable ParserRuleContext<?> outerContext)

runtime/Java/src/org/antlr/v4/runtime/atn/PredictionContextCache.java

@@ -36,7 +36,7 @@ public class PredictionContextCache {
 		return cache.get(ctx);
 	}
 
-	public synchronized int size() {
+	public int size() {
 		return cache.size();
 	}
 }