Does String concatenation get optimized to use existing StringBuilders?

I have the following code:

StringBuilder str = new StringBuilder("foo");
for(Field f : fields){
    str.append("|" + f);
}
str.append("|" + bar);
String result = str.toString();

I know compiler will optimize string concatenation "|" + f and replace it with StringBuilder. However will a new StringBuilder be created or the existing str will be used in Java 8? How about Java 9?