Spring Controller单例与线程安全那些事儿

目录

• 单例（singleton）作用域
• 原型（Prototype）作用域
• 多个HTTP请求在Spring控制器内部串行还是并行执行方法？
• 实现单例模式并模拟大量并发请求，验证线程安全
• 附录：Spring Bean作用域

单例（singleton）作用域

每个添加@RestController或@Controller的控制器，默认是单例(singleton)，这也是Spring Bean的默认作用域。

下面代码示例参考了Building a RESTful Web Service，该教程搭建基于Spring Boot的web项目，源代码可参考spring-guides/gs-rest-service

GreetingController.java代码如下：

package com.example.controller;

import java.util.concurrent.atomic.AtomicLong;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class GreetingController {

    private static final String template = "Hello, %s!";
    private final AtomicLong counter = new AtomicLong();

    @GetMapping("/greeting")
    public Greeting greeting(@RequestParam(value = "name", defaultValue = "World") String name) {
        Greeting greet =  new Greeting(counter.incrementAndGet(), String.format(template, name));
        System.out.println("id=" + greet.getId() + ", instance=" + this);
        return greet;
    }
}

我们使用HTTP基准工具wrk来生成大量HTTP请求。在终端输入如下命令来测试：

wrk -t12 -c400 -d10s http://127.0.0.1:8080/greeting

在服务端的标准输出中，可以看到类似日志。

id=162440, instance=com.example.controller.GreetingController@368b1b03
id=162439, instance=com.example.controller.GreetingController@368b1b03
id=162438, instance=com.example.controller.GreetingController@368b1b03
id=162441, instance=com.example.controller.GreetingController@368b1b03
id=162442, instance=com.example.controller.GreetingController@368b1b03
id=162443, instance=com.example.controller.GreetingController@368b1b03
id=162444, instance=com.example.controller.GreetingController@368b1b03
id=162445, instance=com.example.controller.GreetingController@368b1b03
id=162446, instance=com.example.controller.GreetingController@368b1b03

日志中所有GreetingController实例的地址都是一样的，说明多个请求对同一个 GreetingController 实例进行处理，并且它的AtomicLong类型的counter字段正按预期在每次调用时递增。

原型（Prototype）作用域

如果我们在@RestController注解上方增加@Scope("prototype")注解，使bean作用域变成原型作用域，其它内容保持不变。

...

@Scope("prototype")
@RestController
public class GreetingController {
    ...
}

服务端的标准输出日志如下，说明改成原型作用域后，每次请求都会创建新的bean，所以返回的id始终是1，bean实例地址也不同。

id=1, instance=com.example.controller.GreetingController@2437b9b6
id=1, instance=com.example.controller.GreetingController@c35e3b8
id=1, instance=com.example.controller.GreetingController@6ea455db
id=1, instance=com.example.controller.GreetingController@3fa9d3a4
id=1, instance=com.example.controller.GreetingController@3cb58b3

多个HTTP请求在Spring控制器内部串行还是并行执行方法？

如果我们在greeting()方法中增加休眠时间，来看下每个http请求是否会串行调用控制器里面的方法。

@RestController
public class GreetingController {

    private static final String template = "Hello, %s!";
    private final AtomicLong counter = new AtomicLong();

    @GetMapping("/greeting")
    public Greeting greeting(@RequestParam(value = "name", defaultValue = "World") String name) throws InterruptedException {
        Thread.sleep(1000); // 休眠1s
        Greeting greet =  new Greeting(counter.incrementAndGet(), String.format(template, name));
        System.out.println("id=" + greet.getId() + ", instance=" + this);
        return greet;
    }
}

还是使用wrk来创建大量请求，可以看出即使服务端的方法休眠1秒，导致每个请求的平均延迟达到1.18s，但每秒能处理的请求仍达到166个，证明HTTP请求在Spring MVC内部是并发调用控制器的方法，而不是串行。

wrk -t12 -c400 -d10s http://127.0.0.1:8080/greeting

Running 10s test @ http://127.0.0.1:8080/greeting
  12 threads and 400 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     1.18s   296.41ms   1.89s    85.22%
    Req/Sec    37.85     38.04   153.00     80.00%
  1664 requests in 10.02s, 262.17KB read
  Socket errors: connect 155, read 234, write 0, timeout 0
Requests/sec:    166.08
Transfer/sec:     26.17KB

实现单例模式并模拟大量并发请求，验证线程安全

单例类的定义：Singleton.java

package com.demo.designpattern;

import java.util.concurrent.atomic.AtomicInteger;

public class Singleton {

    private volatile static Singleton singleton;
    private int counter = 0;
    private AtomicInteger atomicInteger = new AtomicInteger(0);

    private Singleton() {
    }

    public static Singleton getSingleton() {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }

    public int getUnsafeNext() {
        return ++counter;
    }

    public int getUnsafeCounter() {
        return counter;
    }

    public int getSafeNext() {
        return atomicInteger.incrementAndGet();
    }

    public int getSafeCounter() {
        return atomicInteger.get();
    }

}

测试单例类并创建大量请求并发调用：SingletonTest.java

package com.demo.designpattern;

import java.util.*;
import java.util.concurrent.*;

public class SingletonTest {

    public static void main(String[] args) {
        // 定义可返回计算结果的非线程安全的Callback实例
        Callable<Integer> unsafeCallableTask = () -> Singleton.getSingleton().getUnsafeNext();
        runTask(unsafeCallableTask);
        // unsafe counter may less than 1000, i.e. 984
        System.out.println("current counter = " + Singleton.getSingleton().getUnsafeCounter());

        // 定义可返回计算结果的线程安全的Callback实例（基于AtomicInteger）
        Callable<Integer> safeCallableTask = () -> Singleton.getSingleton().getSafeNext();
        runTask(safeCallableTask);
        // safe counter should be 1000
        System.out.println("current counter = " + Singleton.getSingleton().getSafeCounter());

    }

    public static void runTask(Callable<Integer> callableTask) {
        int cores = Runtime.getRuntime().availableProcessors();
        ExecutorService threadPool = Executors.newFixedThreadPool(cores);
        List<Callable<Integer>> callableTasks = new ArrayList<>();
        for (int i = 0; i < 1000; i++) {
            callableTasks.add(callableTask);
        }
        Map<Integer, Integer> frequency = new HashMap<>();
        try {
            List<Future<Integer>> futures = threadPool.invokeAll(callableTasks);
            for (Future<Integer> future : futures) {
                frequency.put(future.get(), frequency.getOrDefault(future.get(), 0) + 1);
                //System.out.printf("counter=%s
", future.get());
            }
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }
        threadPool.shutdown();
    }
}

附录：Spring Bean作用域