Java switch 为什么比 for 循环快？

测试代码
for (int j = 0; j < 30; j++) {
long current = System.currentTimeMillis();
for (long i = 0; i < 1000000000; i++) {
getPriority(10);
}
long first = System.currentTimeMillis();
for (long i = 0; i < 1000000000; i++) {
getPriority1(10);
}
long second = System.currentTimeMillis();
System.out.println("first= " + (first - current) + " second= " + (second - first));
}

原方法
private static int getPriority(int cameraType) {
switch (cameraType) {
case 4:
return CameraType.CAMERA_1.getPriority();
case 5:
return CameraType.CAMERA_2.getPriority();
case 6:
return CameraType.CAMERA_3.getPriority();
case 7:
return CameraType.CAMERA_4.getPriority();
case 8:
return CameraType.CAMERA_5.getPriority();
case 9:
return CameraType.CAMERA_6.getPriority();
case 10:
return CameraType.CAMERA_7.getPriority();
default:
return -1;
}
}

private static int getPriority1(int cameraType) {
for (CameraType cameraType1 : CameraType.values()) {
if (cameraType1.getCameraType() == cameraType) {
return cameraType1.getPriority();
}
}
return -1;
}

所用枚举类型
public enum CameraType {
CAMERA_1("相机 1", 4, 0), CAMERA_2("相机 2", 5, 2), CAMERA_3("相机 3", 6, 3), CAMERA_4("相机 4", 7, 4),
CAMERA_5("相机 5", 8, 5), CAMERA_6("相机 6", 9, 6), CAMERA_7("相机 7", 10, 1);
private final String mCameraName;
private final int mCameraType;
private final int mPriority;

CameraType(String name, int cameraType, int priority) {
mCameraName = name;
mCameraType = cameraType;
mPriority = priority;
}

public String getCameraName() {
return mCameraName;
}

public int getCameraType() {
return mCameraType;
}

public int getPriority() {
return mPriority;
}
}

结果
first= 500 second= 8683
first= 536 second= 6801
first= 520 second= 7329
first= 639 second= 7556
first= 562 second= 7279
first= 592 second= 7458

个人感觉，switch 不还是从最开始 case 匹配到最终符合的 case 然后返回的吗？那不就跟 for 循环的匹配办法一样吗！为什么两者速度差一个数量级

lmshl

2022-08-03 15:38:00 +08:00

不考虑 JIT 介入的前提下：

# for of
1. 循环 Array 会带来检查数组边界的开销，每次访问元素都要 check condition
2. getCameraType 是函数，函数需要跳转过去，再跳转回来
3. 发生了额外的三次内存访问，a) 堆获取枚举静态数组, b) 根据数组下标访问枚举对象引用, c) 根据对象引用访问堆内存地址取属性

# switch
而 switch 只有简简单单的 compare 和 jump ，高下立判

不是很懂 jvm ，仅根据操作系统印象胡诌几句