javac生成的类文件是否始终相同? [英] Is the class file generated by javac always the same?

问题描述

当前，我们正在为一个大型项目(大约2000个源文件)重新编写所有构建系统的脚本，并且一直在讨论对文件进行二进制比较，以确保所有内容正确无误，从而导致以下问题:是否保证javac的输出在各个编译中相同还是可以更改?

Currently we are in the process of re-scripting all of our build system for a large project (around 2000 source files) and there has been talk of doing a binary comparison on the files to ensure that everything is correct which leads to the following question: Is the output of javac guaranteed to be same across compilations or could it be subject to change?

Another question implied that the constant pool could have a different order, but assuming we are able to control for the order of the files going into the javac call is there still a potential for differences? We are using Ant and Maven as part of the build if that may influence things as well.

推荐答案

字节码绝对不保证是相同的；一方面，允许编译器执行不影响任何保证行为的优化. Java语言规范甚至在一些地方提到了编译器可能执行的优化.例如，对于字符串串联运算符+，它指出:

The bytecode is absolutely not guaranteed to be the same; for one thing, compilers are allowed to perform optimizations that don't affect any guaranteed behaviors. The Java Language Specification even mentions, in a few places, optimizations that a compiler might perform; for example, of the string concatenation operator +, it notes that:

实现可以选择一步执行转换和连接，以避免创建然后丢弃中间的String对象.为了提高重复字符串连接的性能，Java编译器可以使用StringBuffer类或类似的技术来减少通过对表达式求值而创建的中间String对象的数量.

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.

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