前后端分离，后端使用Gin，POST的接口老是OPTIONS返回404，用postman测试接口正常，最后发现，跨域中间键一定要在你路由组（group）之前全局使用。

关键代码如下：

• router.go

package Router
import (
    "github.com/gin-gonic/gin"
    "gin/Controllers"
    "gin/Middlewares"
)
func InitRouter() {
    router := gin.Default()
    // 要在路由组之前全局使用「跨域中间件」, 否则OPTIONS会返回404
    router.Use(Middlewares.Cors())
    v1 := router.Group("v1")
    {
        v1.GET("/jsontest", Controllers.JsonTest)
        v1.POST("/jsonposttest", Controllers.JsonPostTest)
    }
    
    router.Run(":8080")
}

• corsMIddleware.go

package Middlewares
import (
    "github.com/gin-gonic/gin"
    "net/http"
)
func Cors() gin.HandlerFunc {
    return func(c *gin.Context) {
        method := c.Request.Method
        origin := c.Request.Header.Get("Origin")
        if origin != "" {
            c.Header("Access-Control-Allow-Origin", origin)
            c.Header("Access-Control-Allow-Methods", "POST, GET, OPTIONS, PUT, DELETE, UPDATE")
            c.Header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept, Authorization")
            c.Header("Access-Control-Expose-Headers", "Content-Length, Access-Control-Allow-Origin, Access-Control-Allow-Headers, Cache-Control, Content-Language, Content-Type")
            c.Header("Access-Control-Allow-Credentials", "false")
            c.Set("content-type", "application/json")
        }
        if method == "OPTIONS" {
            c.AbortWithStatus(http.StatusNoContent)
        }
        c.Next()
    }
}