jdk11-jvm-自增++

结论

java语言层面而言，前++和后++的性能及开销没有区别

java代码

class Demo{

    public void pre00(){
        int num=0;
        ++num;
    }

    public void post00(){
        int num=0;
        num++;
    }

    public void pre01(int num){
        ++num;
    }

    public void post01(int num){
        num++;
    }

    public int pre02(){
        int num=0;
        return ++num;
    }

    public int post02(){
        int num=0;
        return num++;
    }

    public int pre03(int num){
        return ++num;
    }

    public int post03(int num){
        return num++;
    }
}

反编译字节码

Compiled from "Demo.java"
class Demo {
  Demo();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return
    LineNumberTable:
      line 1: 0

  public void pre00();
    Code:
       0: iconst_0 // 0入栈
       1: istore_1 // 栈顶元素0放到局部变量表脚标1
       2: iinc 1, 1 // 局部变量表脚标1元素自增1
       5: return
    LineNumberTable:
      line 4: 0
      line 5: 2
      line 6: 5

  public void post00();
    Code:
       0: iconst_0 // 0入栈
       1: istore_1 // 栈顶元素0放到局部变量表脚标1
       2: iinc          1, 1 // 局部变量表脚标1元素自增1
       5: return
    LineNumberTable:
      line 9: 0
      line 10: 2
      line 11: 5

  public void pre01(int);
    Code:
       0: iinc          1, 1 // 局部变量脚标1元素自增1 也就是方法形参num
       3: return
    LineNumberTable:
      line 14: 0
      line 15: 3

  public void post01(int);
    Code:
       0: iinc          1, 1 // 局部变量脚标1元素自增1 也就是方法形参num
       3: return
    LineNumberTable:
      line 18: 0
      line 19: 3

  public int pre02();
    Code:
       0: iconst_0 // 0入栈
       1: istore_1 // 栈顶元素放到局部变量表脚标1处
       2: iinc          1, 1 // 局部变量表脚标1处元素自增1
       5: iload_1 // 将局部变量表脚标1处元素放到栈顶
       6: ireturn // 栈顶元素返回
    LineNumberTable:
      line 22: 0
      line 23: 2

  public int post02();
    Code:
       0: iconst_0 // 0入栈
       1: istore_1 // 栈顶元素放到局部变量表脚标1处
       2: iload_1 // 将局部变量表脚标1处元素放到栈顶
       3: iinc          1, 1 // 局部变量表脚标1处元素自增1
       6: ireturn // 栈顶元素返回
    LineNumberTable:
      line 27: 0
      line 28: 2

  public int pre03(int);
    Code:
       0: iinc          1, 1 //  局部变量表脚标1处元素自增1
       3: iload_1 // 将局部变量表脚标1处元素放到栈顶
       4: ireturn // 栈顶元素返回
    LineNumberTable:
      line 32: 0

  public int post03(int);
    Code:
       0: iload_1 // 将局部变量表脚标1处元素放到栈顶
       1: iinc          1, 1 // 局部变量表脚标1处元素自增1
       4: ireturn // 栈顶元素返回
    LineNumberTable:
      line 36: 0
}

图示