Simplify String concatenation

[stsypanov]

In some cases we can skip creation of StringBuilder or use invokedynamic-based concatenation.

[sbrannen]

@jhoeller, I've assigned this to you for consideration.

[izeye]

use invokedynamic-based concatenation

@stsypanov This doesn’t seem to be true unless the Spring Framework itself is compiled against Java 9+. Am I missing something?

[stsypanov]

Correct, as mentioned in https://openjdk.java.net/jeps/280 the byte code with indified concatenation is produced by javac of JDK 9. However as of current version (8) javac will produce new StringBuilder().append() implicitly, so explicit declaration of StringBuilder is not necessary.