java線程池核心線程不被摧毀的原理及分析

更新時(shí)間：2025年05月07日 11:18:04 作者：春風(fēng)十里不及你

這篇文章主要介紹了java線程池核心線程不被摧毀的原理及分析,具有很好的參考價(jià)值,希望對(duì)大家有所幫助,如有錯(cuò)誤或未考慮完全的地方,望不吝賜教

為什么要用線程池？

減少了創(chuàng)建和銷毀線程的次數(shù)，每個(gè)工作線程都可以被重復(fù)利用
可以根據(jù)系統(tǒng)的承受能力，調(diào)整線程池中工作線線程的數(shù)目，防止因線程過(guò)多消耗內(nèi)存，也避免了因線程過(guò)少，浪費(fèi)系統(tǒng)資源

如何做到每個(gè)工作線程都可以被重復(fù)利用呢？

先看下線程池的工作原理：

原理如上圖，線程池有七個(gè)核心參數(shù)

corePoolSize 線程池核心線程數(shù)
maximumPoolSize 線程池最大線程數(shù)量
keepAliveTime 空閑線程存活時(shí)間
unit 空閑線程存活時(shí)間單位
workQueue 工作隊(duì)列
threadFactory 線程工廠
handler 拒絕策略

線程池之所以能做到重復(fù)利用，是因?yàn)榫€程池的核心線程不會(huì)被摧毀，執(zhí)行完任務(wù)后會(huì)重復(fù)利用

線程池是如何保持核心線程不被摧毀呢?

首先看先線程池是如何處理任務(wù)的，如下圖

下面我們看下核心部分源碼：

當(dāng)有一個(gè)任務(wù)添加進(jìn)來(lái)時(shí)，線程池會(huì)創(chuàng)建一個(gè)Worker，Worker是實(shí)現(xiàn)Runnable方法的，所以Worker執(zhí)行時(shí)會(huì)調(diào)用run方法，run方法會(huì)接著調(diào)用runWoker方法
主要看這一行代碼，調(diào)用getTask方法獲取任務(wù)并且執(zhí)行 while (task != null || (task = getTask()) != null)

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            // 主要看這一行代碼，調(diào)用getTask方法獲取任務(wù)并且執(zhí)行
            while (task != null || (task = getTask()) != null) {
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        task.run();
                    } catch (RuntimeException x) {
                        thrown = x; throw x;
                    } catch (Error x) {
                        thrown = x; throw x;
                    } catch (Throwable x) {
                        thrown = x; throw new Error(x);
                    } finally {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

看下getTask方法是如何實(shí)現(xiàn)的

private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            int wc = workerCountOf(c);

            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }