Java 的 switch 在底层是如何工作的?

问题描述

Java 的 switch 语句在底层是如何工作的?它如何将正在使用的变量的值与案例部分中给出的值进行比较?它是使用 == 还是 .equals()，还是完全是别的东西?

How does Java's switch statement work under the hood? How does it compare the value of the variable being used, to those given in the case parts? Does it use == or .equals(), or is it something else entirely?

我主要对 1.7 之前的版本感兴趣.

I'm mainly interested in the pre 1.7 versions.

推荐答案

两者都没有. 它使用 lookupswitch JVM指令，本质上是查表.看一下下面例子的字节码:

Neither. it uses the lookupswitch JVM instruction, which is essentially a table lookup. Take a look at the bytecode of the following example:

public static void main(String... args) {
  switch (1) {
  case 1:
    break;
  case 2:
    break;
  }
}

public static void main(java.lang.String[]);
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   iconst_1
   1:   lookupswitch{ //2
                1: 28;
                2: 31;
                default: 31 }
   28:  goto    31
   31:  return