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    Java進階(二)當我們說線程安全時,到底在說什么

    原創文章,轉載請務必將下面這段話置于文章開頭處(保留超鏈接)。
    本文轉發自技術世界原文鏈接 http://www.luozeyang.com/java/thread_safe/

    多線程編程中的三個核心概念

    原子性

    這一點,跟數據庫事務的原子性概念差不多,即一個操作(有可能包含有多個子操作)要么全部執行(生效),要么全部都不執行(都不生效)。

    關于原子性,一個非常經典的例子就是銀行轉賬問題:比如A和B同時向C轉賬10萬元。如果轉賬操作不具有原子性,A在向C轉賬時,讀取了C的余額為20萬,然后加上轉賬的10萬,計算出此時應該有30萬,但還未來及將30萬寫回C的賬戶,此時B的轉賬請求過來了,B發現C的余額為20萬,然后將其加10萬并寫回。然后A的轉賬操作繼續——將30萬寫回C的余額。這種情況下C的最終余額為30萬,而非預期的40萬。

    可見性

    可見性是指,當多個線程并發訪問共享變量時,一個線程對共享變量的修改,其它線程能夠立即看到。可見性問題是好多人忽略或者理解錯誤的一點。

    CPU從主內存中讀數據的效率相對來說不高,現在主流的計算機中,都有幾級緩存。每個線程讀取共享變量時,都會將該變量加載進其對應CPU的高速緩存里,修改該變量后,CPU會立即更新該緩存,但并不一定會立即將其寫回主內存(實際上寫回主內存的時間不可預期)。此時其它線程(尤其是不在同一個CPU上執行的線程)訪問該變量時,從主內存中讀到的就是舊的數據,而非第一個線程更新后的數據。

    這一點是操作系統或者說是硬件層面的機制,所以很多應用開發人員經常會忽略。

    順序性

    順序性指的是,程序執行的順序按照代碼的先后順序執行。

    以下面這段代碼為例

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    boolean started = false; // 語句1
    long counter = 0L; // 語句2
    counter = 1; // 語句3
    started = true; // 語句4

    從代碼順序上看,上面四條語句應該依次執行,但實際上JVM真正在執行這段代碼時,并不保證它們一定完全按照此順序執行。

    處理器為了提高程序整體的執行效率,可能會對代碼進行優化,其中的一項優化方式就是調整代碼順序,按照更高效的順序執行代碼。

    講到這里,有人要著急了——什么,CPU不按照我的代碼順序執行代碼,那怎么保證得到我們想要的效果呢?實際上,大家大可放心,CPU雖然并不保證完全按照代碼順序執行,但它會保證程序最終的執行結果和代碼順序執行時的結果一致。

    Java如何解決多線程并發問題

    Java如何保證原子性

    鎖和同步

    常用的保證Java操作原子性的工具是鎖和同步方法(或者同步代碼塊)。使用鎖,可以保證同一時間只有一個線程能拿到鎖,也就保證了同一時間只有一個線程能執行申請鎖和釋放鎖之間的代碼。

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    public void testLock () {
    lock.lock();
    try{
    int j = i;
    i = j + 1;
    } finally {
    lock.unlock();
    }
    }

    與鎖類似的是同步方法或者同步代碼塊。使用非靜態同步方法時,鎖住的是當前實例;使用靜態同步方法時,鎖住的是該類的Class對象;使用靜態代碼塊時,鎖住的是synchronized關鍵字后面括號內的對象。下面是同步代碼塊示例

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    public void testLock () {
    synchronized (anyObject){
    int j = i;
    i = j + 1;
    }
    }

    無論使用鎖還是synchronized,本質都是一樣,通過鎖來實現資源的排它性,從而實際目標代碼段同一時間只會被一個線程執行,進而保證了目標代碼段的原子性。這是一種以犧牲性能為代價的方法。

    CAS(compare and swap)

    基礎類型變量自增(i++)是一種常被新手誤以為是原子操作而實際不是的操作。Java中提供了對應的原子操作類來實現該操作,并保證原子性,其本質是利用了CPU級別的CAS指令。由于是CPU級別的指令,其開銷比需要操作系統參與的鎖的開銷小。AtomicInteger使用方法如下。

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    AtomicInteger atomicInteger = new AtomicInteger();
    for(int b = 0; b < numThreads; b++) {
    new Thread(() -> {
    for(int a = 0; a < iteration; a++) {
    atomicInteger.incrementAndGet();
    }
    }).start();
    }

    Java如何保證可見性

    Java提供了volatile關鍵字來保證可見性。當使用volatile修飾某個變量時,它會保證對該變量的修改會立即被更新到內存中,并且將其它緩存中對該變量的緩存設置成無效,因此其它線程需要讀取該值時必須從主內存中讀取,從而得到最新的值。

    Java如何保證順序性

    上文講過編譯器和處理器對指令進行重新排序時,會保證重新排序后的執行結果和代碼順序執行的結果一致,所以重新排序過程并不會影響單線程程序的執行,卻可能影響多線程程序并發執行的正確性。

    Java中可通過volatile在一定程序上保證順序性,另外還可以通過synchronized和鎖來保證順序性。

    synchronized和鎖保證順序性的原理和保證原子性一樣,都是通過保證同一時間只會有一個線程執行目標代碼段來實現的。

    除了從應用層面保證目標代碼段執行的順序性外,JVM還通過被稱為happens-before原則隱式地保證順序性。兩個操作的執行順序只要可以通過happens-before推導出來,則JVM會保證其順序性,反之JVM對其順序性不作任何保證,可對其進行任意必要的重新排序以獲取高效率。

    happens-before原則(先行發生原則)

    • 傳遞規則:如果操作1在操作2前面,而操作2在操作3前面,則操作1肯定會在操作3前發生。該規則說明了happens-before原則具有傳遞性
    • 鎖定規則:一個unlock操作肯定會在后面對同一個鎖的lock操作前發生。這個很好理解,鎖只有被釋放了才會被再次獲取
    • volatile變量規則:對一個被volatile修飾的寫操作先發生于后面對該變量的讀操作
    • 程序次序規則:一個線程內,按照代碼順序執行
    • 線程啟動規則:Thread對象的start()方法先發生于此線程的其它動作
    • 線程終結原則:線程的終止檢測后發生于線程中其它的所有操作
    • 線程中斷規則: 對線程interrupt()方法的調用先發生于對該中斷異常的獲取
    • 對象終結規則:一個對象構造先于它的finalize發生

    volatile適用場景

    volatile適用于不需要保證原子性,但卻需要保證可見性的場景。一種典型的使用場景是用它修飾用于停止線程的狀態標記。如下所示

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    boolean isRunning = false;

    public void start () {
    new Thread( () -> {
    while(isRunning) {
    someOperation();
    }
    }).start();
    }

    public void stop () {
    isRunning = false;
    }

    在這種實現方式下,即使其它線程通過調用stop()方法將isRunning設置為false,循環也不一定會立即結束。可以通過volatile關鍵字,保證while循環及時得到isRunning最新的狀態從而及時停止循環,結束線程。

    線程安全十萬個為什么

    問:平時項目中使用鎖和synchronized比較多,而很少使用volatile,難道就沒有保證可見性?
    答:鎖和synchronized即可以保證原子性,也可以保證可見性。都是通過保證同一時間只有一個線程執行目標代碼段來實現的。


    問:鎖和synchronized為何能保證可見性?
    答:根據JDK 7的Java doc中對concurrent包的說明,一個線程的寫結果保證對另外線程的讀操作可見,只要該寫操作可以由happen-before原則推斷出在讀操作之前發生。

    The results of a write by one thread are guaranteed to be visible to a read by another thread only if the write operation happens-before the read operation. The synchronized and volatile constructs, as well as the Thread.start() and Thread.join() methods, can form happens-before relationships.

    問:既然鎖和synchronized即可保證原子性也可保證可見性,為何還需要volatile?
    答:synchronized和鎖需要通過操作系統來仲裁誰獲得鎖,開銷比較高,而volatile開銷小很多。因此在只需要保證可見性的條件下,使用volatile的性能要比使用鎖和synchronized高得多。

    問:既然鎖和synchronized可以保證原子性,為什么還需要AtomicInteger這種的類來保證原子操作?
    答:鎖和synchronized需要通過操作系統來仲裁誰獲得鎖,開銷比較高,而AtomicInteger是通過CPU級的CAS操作來保證原子性,開銷比較小。所以使用AtomicInteger的目的還是為了提高性能。

    問:還有沒有別的辦法保證線程安全
    答:有。盡可能避免引起非線程安全的條件——共享變量。如果能從設計上避免共享變量的使用,即可避免非線程安全的發生,也就無須通過鎖或者synchronized以及volatile解決原子性、可見性和順序性的問題。

    問:synchronized即可修飾非靜態方式,也可修飾靜態方法,還可修飾代碼塊,有何區別
    答:synchronized修飾非靜態同步方法時,鎖住的是當前實例;synchronized修飾靜態同步方法時,鎖住的是該類的Class對象;synchronized修飾靜態代碼塊時,鎖住的是synchronized關鍵字后面括號內的對象。

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