Is the controller a single instance or multiple instances? How to ensure the safety of concurrency

answer ：

controller The default is singleton , Don't use non static member variables , Otherwise, there will be data logic confusion . Because of the singleton, it's not thread safe .

Let's do a simple verification ：

package com.riemann.springbootdemo.controller;

import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

/**
 * @author riemann
 * @date 2019/07/29 22:56
 */
@Controller
public class ScopeTestController {

    private int num = 0;

    @RequestMapping("/testScope")
    public void testScope() {
        System.out.println(++num);
    }

    @RequestMapping("/testScope2")
    public void testScope2() {
        System.out.println(++num);
    }

}

Let's first visit http://localhost:8080/testScope, The answer is 1;
Then we will visit http://localhost:8080/testScope2, The answer is 2.

Different values you get , This is thread unsafe .

Next we'll give controller There are many cases of increasing effect  @Scope("prototype")

package com.riemann.springbootdemo.controller;

import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

/**
 * @author riemann
 * @date 2019/07/29 22:56
 */
@Controller
@Scope("prototype")
public class ScopeTestController {

    private int num = 0;

    @RequestMapping("/testScope")
    public void testScope() {
        System.out.println(++num);
    }

    @RequestMapping("/testScope2")
    public void testScope2() {
        System.out.println(++num);
    }

}

We are still the first to visit http://localhost:8080/testScope, The answer is 1;
Then we will visit http://localhost:8080/testScope2, The answer is still 1.

I believe it's not hard to find out ：

Singleton is not safe , Will cause the attribute to be reused .

Solution

1、 Not in controller Define member variables in .2、 In case you have to define a non static member variable , By way of annotation @Scope(“prototype”), Set it to multi instance mode .3、 stay Controller Use in ThreadLocal Variable

Additional explanation

spring bean The scope is as follows 5 individual ：

• singleton: The singleton pattern , When spring establish applicationContext Container time ,spring Will want to initialize all instances of the scope , add lazy-init You can avoid preprocessing ;

• prototype： Archetypal model , Each time through getBean Get the bean It will create a new instance , After creation spring It will no longer be managed ;

  （ The following is the web Only under the project ）

• request： To make web Everyone should understand request It's the domain of , That is, each request generates a new instance , and prototype The difference is that after creation , The next management ,spring Still monitoring ;

• session: Every conversation , ditto ;

• global session: Overall web Domain , Be similar to servlet Medium application.