java何时引入Javac StringBuilder/StringBuffer优化？

Question

我知道Javac编译器能够使用StringBuilder/StringBuffer转换字符串连接+，我很想知道这个更改是从哪个版本开始的

我正在使用以下示例代码：

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);
  }
}

到目前为止，我已经尝试了javac 1.8.0_121、javac 1.6.0_20、javac 1.5.0_22和java 1.4.2_19

下面是我使用1.4.2_19中的javap -c看到的字节码示例：

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;

所有4个版本似乎都在使用StringBuilder/StringBuffer优化，所以我很想知道从哪个Javac版本开始引入这个更改

Answer 1

以下是来自language specification from version 1的一段话：

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.

当时，他们用StringBuffer代替了StringBuilder

也引用了JDK1的^{}。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.

Answer 2

这并没有回答问题，但我只想补充一点，在jdk-9中，这个StringBuilder::append是允许的策略之一，但不是默认策略

private enum Strategy {
   /**
    * Bytecode generator, calling into {@link java.lang.StringBuilder}.
    */
    BC_SB,

    /**
     * Bytecode generator, calling into {@link java.lang.StringBuilder};
     * but trying to estimate the required storage.
     */
    BC_SB_SIZED,

    /**
     * Bytecode generator, calling into {@link java.lang.StringBuilder};
     * but computing the required storage exactly.
     */
     BC_SB_SIZED_EXACT,

   /**
    * MethodHandle-based generator, that in the end calls into {@link java.lang.StringBuilder}.
    * This strategy also tries to estimate the required storage.
    */
    MH_SB_SIZED,

    /**
     * MethodHandle-based generator, that in the end calls into {@link java.lang.StringBuilder}.
     * This strategy also estimate the required storage exactly.
     */
    MH_SB_SIZED_EXACT,

    /**
     * MethodHandle-based generator, that constructs its own byte[] array from
     * the arguments. It computes the required storage exactly.
     */
     MH_INLINE_SIZED_EXACT
}

它实际上是用于字符串连接的invokedynamic字节码，因此它的实现现在是特定于JRE的，而不是编译器的。默认策略btw是：MH_INLINE_SIZED_EXACT

Answer 3

我查阅了Java语言规范第一版（1996年起）。不容易找到，但是here it is。即使在那时，也有关于串联优化的文章：

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.

该规范与StringBuffer相关，但是StringBuilder（当前JLS措辞所指）可能被认为性能更好，因为它的方法不同步

然而，这并不意味着人们应该依赖于优化，因为总是。例如，循环中的字符串连接不会得到优化

Answer 4

一些答案中已经给出了JLS。我只想强调一点，StringBuffer（https://docs.oracle.com/javase/8/docs/api/java/lang/StringBuffer.html）从1.0开始就存在了

StringBuilder（https://docs.oracle.com/javase/8/docs/api/java/lang/StringBuilder.html）的版本是1.5。请参见相应javadocs的since:部分