When was Javac StringBuilder/StringBuffer optimization introduced?

Question

I know that Javac compiler is able to transform String concatenation + using StringBuilder/StringBuffer, and I'm curious to know starting from which version this change was introduced?

I'm using this sample code:

public class Main {
  public static void main(String[] args) {
      String a = args[0];
      String s = "a";
      s = s + a;
      s = s + "b";
      s = s + "c";
      s = s + "d";
      s = s + "e";
      System.out.println(s);
  }
}

So far I've tried with javac 1.8.0_121, javac 1.6.0_20, javac 1.5.0_22 and java 1.4.2_19.

Here is a sample of the bytecode I see using javap -c from 1.4.2_19:

6:  astore_2
7:  new #3; //class StringBuffer
10: dup
11: invokespecial   #4; //Method java/lang/StringBuffer."<init>":()V
14: aload_2
15: invokevirtual   #5; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
18: aload_1
19: invokevirtual   #5; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
22: invokevirtual   #6; //Method java/lang/StringBuffer.toString:()Ljava/lang/String;

All 4 versions seems to be using the StringBuilder/StringBuffer optimization, so I'm curious to know starting from which Javac version this change was introduced?

Answer 1

Here's a quote from the language specification from version 1:

An implementation may choose to perform conversion and concatenation in one step to avoid creating and then discarding an intermediate String object. To increase the performance of repeated string concatenation, a Java compiler may use the StringBuffer class (§20.13) or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression.

Back at the time, they had StringBuffer instead of StringBuilder.

Also a quote from StringBuffer of JDK1.0.2:

This Class is a growable buffer for characters. It is mainly used to create Strings. The compiler uses it to implement the "+" operator.