java编译器对字符串“+”连接的优化

下面一段源代码：

使用JD-GUI反编译如下：

PS: 它在干什么，为了方便看么？

使用IntelliJ IDEA的内置反编译工具：

PS: 原样反编译

使用jdk自带工具javap反编译字节码如下：

可以看到 + 连接符被StringBuilder代替

换用Luyten看下：

java

字节码

规范是这么说的：http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.18.1

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 or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression.

For primitive types, an implementation may also optimize away the creation of a wrapper object by converting directly from a primitive type to a string.