Java中的Mutex--这看起来像是一个正确的实现吗？

问题描述

这不是一个非常严肃的问题，更像是在思考：JavaScript的await关键字应该允许在普通的"并发语言"中使用感觉非常像互斥体的东西。

function Mutex() {
    var self = this; // still unsure about how "this" is captured
    var mtx = new Promise(t => t()); // fulfilled promise ≡ unlocked mutex
    this.lock = async function() {
        await mtx;
        mtx = new Promise(t => {
            self.unlock = () => t();
        });
    }
}
// Lock
await mutex.lock();
// Unlock
mutex.unlock();

这是正确的实现吗(除了正确的错误处理)？和…我可以拥有C++-RAII样式的锁保护吗？

推荐答案

您的实现允许与请求锁一样多的使用者获得锁；对lock的每次调用都等待一个承诺：

function Mutex() {
    var self = this; // still unsure about how "this" is captured
    var mtx = new Promise(t => t()); // fulfilled promise ≡ unlocked mutex
    this.lock = async function() {
        await mtx;
        mtx = new Promise(t => {
            self.unlock = () => t();
        });
    }
}

const mutex = new Mutex();

(async () => {
  await Promise.resolve();
  await mutex.lock();
  console.log("A got the lock");
})();
(async () => {
  await Promise.resolve();
  await mutex.lock();
  console.log("B got the lock");
})();

您需要实现承诺队列，为每个锁定请求创建一个新的承诺队列。

附注：

• new Promise(t => t())可以更简单、更贴切地写成Promise.resolve()：-)
• 如果使用类似的箭头函数，则不需要self；箭头函数关闭创建它们的this(与关闭变量完全相同)
• 将unlock作为锁承诺的解析值可能是有意义的，因此只有获得锁的代码才能释放它

类似以下内容：

function Mutex() {
    let current = Promise.resolve();
    this.lock = () => {
        let _resolve;
        const p = new Promise(resolve => {
            _resolve = () => resolve();
        });
        // Caller gets a promise that resolves when the current outstanding
        // lock resolves
        const rv = current.then(() => _resolve);
        // Don't allow the next request until the new promise is done
        current = p;
        // Return the new promise
        return rv;
    };
}

现场示例：

"use strict";
function Mutex() {
    let current = Promise.resolve();
    this.lock = () => {
        let _resolve;
        const p = new Promise(resolve => {
            _resolve = () => resolve();
        });
        // Caller gets a promise that resolves when the current outstanding
        // lock resolves
        const rv = current.then(() => _resolve);
        // Don't allow the next request until the new promise is done
        current = p;
        // Return the new promise
        return rv;
    };
}

const rand = max => Math.floor(Math.random() * max);

const delay = (ms, value) => new Promise(resolve => setTimeout(resolve, ms, value));

const mutex = new Mutex();

function go(name) {
    (async () => {
        console.log(name + " random initial delay");
        await delay(rand(50));
        console.log(name + " requesting lock");
        const unlock = await mutex.lock();
        console.log(name + " got lock");
        await delay(rand(1000));
        console.log(name + " releasing lock");
        unlock();
    })();
}
go("A");
go("B");
go("C");
go("D");

.as-console-wrapper {
  max-height: 100% !important;
}