gin/docs/how-to-build-an-effective-middleware.md
2018-11-05 09:13:17 +08:00

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# How to build one effective middleware?
## Consitituent part
The middleware has two parts:
- part one is what is executed once, when you initialize your middleware. That's where you set up all the global objects, logicals etc. Everything that happens one per application lifetime.
- part two is what executes on every request. For example, a database middleware you simply inject your "global" database object into the context. Once it's inside the context, you can retrieve it from within other middlewares and your handler function.
```go
func funcName(params string) gin.HandlerFunc {
// <---
// This is part one
// --->
// The follow code is an example
if err := check(params); err != nil {
panic(err)
}
return func(c *gin.Context) {
// <---
// This is part two
// --->
// The follow code is an example
c.Set("TestVar", params)
c.Next()
}
}
```
## Execution process
Firstly, we have the follow example code:
```go
func main() {
router := gin.Default()
router.Use(globalMiddleware())
router.GET("/rest/n/api/*some", mid1(), mid2(), handler)
router.Run()
}
func globalMiddleware() gin.HandlerFunc {
fmt.Println("globalMiddleware...1")
return func(c *gin.Context) {
fmt.Println("globalMiddleware...2")
c.Next()
fmt.Println("globalMiddleware...3")
}
}
func handler(c *gin.Context) {
fmt.Println("exec handler.")
}
func mid1() gin.HandlerFunc {
fmt.Println("mid1...1")
return func(c *gin.Context) {
fmt.Println("mid1...2")
c.Next()
fmt.Println("mid1...3")
}
}
func mid2() gin.HandlerFunc {
fmt.Println("mid2...1")
return func(c *gin.Context) {
fmt.Println("mid2...2")
c.Next()
fmt.Println("mid2...3")
}
}
```
According to [Consitituent part](#consitituent-part) said, when we run the gin process, **part one** will execute firstly and will print the follow information:
```go
globalMiddleware...1
mid1...1
mid2...1
```
And init order are:
```go
globalMiddleware...1
|
v
mid1...1
|
v
mid2...1
```
When we curl one request `curl -v localhost:8080/rest/n/api/some`, **part two** will execute their middleware and output the following information:
```go
globalMiddleware...2
mid1...2
mid2...2
exec handler.
mid2...3
mid1...3
globalMiddleware...3
```
In other words, run order are:
```go
globalMiddleware...2
|
v
mid1...2
|
v
mid2...2
|
v
exec handler.
|
v
mid2...3
|
v
mid1...3
|
v
globalMiddleware...3
```