JDK源碼——java.util.concurrent(六)

測試代碼:
https://github.com/kevindai007/springboot_houseSearch/tree/master/src/test/java/com/kevindai/juc

CyclicBarrier

咱們首先通過一個demo來了解CyclicBarrier的用法和特點

public class CyclicBarrierTest {
    public static void main(String[] args) {
        final CyclicBarrier cyclicBarrier = new CyclicBarrier(10);


        for (int i = 0; i < 11; i++) {
            Runnable run = new Runnable() {
                @Override
                public void run() {
                    System.out.println("線程開始" + Thread.currentThread().getName());
                    try {
                        TimeUnit.SECONDS.sleep(3);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    try {
                        cyclicBarrier.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } catch (BrokenBarrierException e) {
                        e.printStackTrace();
                    }
                    System.out.println("線程開始啓動!"  + Thread.currentThread().getName());
                }
            };

            Thread thread = new Thread(run,"Thread" + i);
            thread.start();
        }
    }
}

這裏能看到CyclicBarrier會讓調用await()的線程等待,把CyclicBarrier的資源獲取完之後,所有的線程一起運行.

下面咱們一起看看其源碼.先看看構造函數

    public CyclicBarrier(int parties) {
        this(parties, null);
    }
    //傳入可獲取的資源數及資源被獲取完時執行的命令
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

再來看看await()方法

    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            //CyclicBarrier可重複使用,用於做判斷是否在同一個條件中
            final Generation g = generation;
            //爲true表示已經被打破,拋異常
            if (g.broken)
                throw new BrokenBarrierException();
            //如果線程被中斷,那麼打破屏障,喚醒屏障前的其他等待線程，拋出異常  
            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }
            //剩餘資源數減一
            int index = --count;
            //如果最後一個資源被獲取,那麼執行barrierCommand,然後喚醒所有線程
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    //喚醒屏障前其他等待線程,重置count和new generation
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            //如果還有資源可以被其他線程獲取,那麼自旋等待
            for (;;) {
                try {
                    if (!timed)
                        trip.await();
                    else if (nanos > 0L)
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    //如果await的線程被中斷，檢查下generation
                    if (g == generation && ! g.broken) {
                        //處於當前generation並且屏障沒有被打破,那就打破屏障
                        breakBarrier();
                        throw ie;
                    } else {
                        Thread.currentThread().interrupt();
                    }
                }
                if (g.broken)
                    throw new BrokenBarrierException();

                if (g != generation)
                    return index;

                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }

    //當資源被減完時調用此方法,讓所有等待線程繼續執行,重置count,設置一個新的Generation
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }
    //打破屏障
    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }

CyclicBarrier的主要方法到這裏就分析完了,主要是用了ReentrantLock+Condition+int count組成,沒用 AQS;
注意與CountDownLatch的區別:

• CountDownLatch是等待所有線程運行完成之後,然後去運行另外一個(或一組)線程;而CyclicBarrier則是一組線程相互等待,當所有線程準備完畢之後,這組線程一起執行,且可以在等待完成後執行一個屏障命令
• CountDownLatch只能使用一次,而CyclicBarrier正常結束後調用nextGeneration初始化可以重複使用

ConcurrentHashMap

說到ConcurrentHashMap不得不先說說HashMap,還好原來分析過HashMap的源碼,大家看這裏,咱們直接看是看源碼吧(這個不做demo了,這就是一個線程安全的HashMap用法也基本相似)

---

昨天電腦上裝了換了JDK8,然後發現JDK8中ConcurrenHashMap的代碼真的是複雜到家了,果斷換成JDK7來研究

---

先看看看一些重要的屬性

    //默認初始大小
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    //負載因子
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    //segment的個數
    static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    //最大容量
    static final int MAXIMUM_CAPACITY = 1 << 30;

    //segment中table的最小容量
    static final int MIN_SEGMENT_TABLE_CAPACITY = 2;

    //最大segent數量
    static final int MAX_SEGMENTS = 1 << 16; 

    static final int RETRIES_BEFORE_LOCK = 2;
        final int segmentMask;

    final int segmentShift;

    final Segment<K,V>[] segments;

這是主要字段,基本能夠理解,下面咱們從構造方法開始開看ConcurrentHashMap的具體流程

    public ConcurrentHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    public ConcurrentHashMap(int initialCapacity,
                             float loadFactor, int concurrencyLevel) {
        //參數校驗
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        //設置最大segment數量
        if (concurrencyLevel > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;
        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;//segment數量
        while (ssize < concurrencyLevel) {
            ++sshift;
            //ssize向左位移一位
            ssize <<= 1;
        }
        this.segmentShift = 32 - sshift;//segment的偏移量
        this.segmentMask = ssize - 1;//segment掩碼值
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
        int cap = MIN_SEGMENT_TABLE_CAPACITY;//2,segment大小
        while (cap < c)//這裏保證每個segment的大小爲2的倍數
            cap <<= 1;
        Segment<K,V> s0 =
            new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
                             (HashEntry<K,V>[])new HashEntry[cap]);
        Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];//用ssize初始化segments數組  
        UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
        this.segments = ss;
    }

構造方法中主要進行了參數校驗,確認了segment的數量和大小,並初始化S0,下面看看put方法

    public V put(K key, V value) {
        Segment<K,V> s;
        //ConcurrentHashMap value不能爲Null
        if (value == null)
            throw new NullPointerException();
        //取key的hashcode再來一次hash,2次hash打撒分佈,避免衝突
        int hash = hash(key);
        //計算要存入的segment的下標 
        int j = (hash >>> segmentShift) & segmentMask;
        if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            //只初始化了s0，這裏確保segment存在
            s = ensureSegment(j);
        return s.put(key, hash, value, false);//掉segment的put
    }

    //因爲只初始化了S0,所以要保證當存放位置不爲S0時segment不爲空
    private Segment<K,V> ensureSegment(int k) {
        final Segment<K,V>[] ss = this.segments;
        long u = (k << SSHIFT) + SBASE; //計算偏移量
        Segment<K,V> seg;
        if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
            Segment<K,V> proto = ss[0]; //s0不爲空null,所以一些參數直接從s0獲取
            int cap = proto.table.length;
            float lf = proto.loadFactor;
            int threshold = (int)(cap * lf);
             //構造segment裏面的table
            HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
            if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
                == null) { // recheck
                Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
                //自旋+cas保證存儲位置一定設置成功
                while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
                       == null) {
                    if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
                        break;
                }
            }
        }
        return seg;
    }

這時咱們來看看Segment是怎麼實現的

static final class Segment<K,V> extends ReentrantLock implements Serializable {
    //segment中的table
    transient volatile HashEntry<K,V>[] table;
    //鏈表長度,即元素數量
    transient int count;
    //修改次數
    transient int modCount;
    //極限值,當table中包含的HashEntry元素的個數超過此值時,觸發table的再散列
    transient int threshold;
    //加載因子
    final float loadFactor;
    Segment(float lf, int threshold, HashEntry<K,V>[] tab) {
            this.loadFactor = lf;
            this.threshold = threshold;
            this.table = tab;
    }
        //Concurrent的put操作,其實就是找到相應的Sement然後調用此put方法
        final V put(K key, int hash, V value, boolean onlyIfAbsent) {
            //首先嚐試加鎖,加鎖失敗則調用scanAndLockForPut自旋加鎖
            HashEntry<K,V> node = tryLock() ? null :
                scanAndLockForPut(key, hash, value);
            V oldValue;
            try {
                HashEntry<K,V>[] tab = table;
                //在table中查找key對應的位置
                int index = (tab.length - 1) & hash;
                //獲取第一個節點
                HashEntry<K,V> first = entryAt(tab, index);
                for (HashEntry<K,V> e = first;;) {
                    //節點存在就檢查是否存在相同的key,如果存在則覆蓋值
                    if (e != null) {
                        K k;
                        if ((k = e.key) == key ||
                            (e.hash == hash && key.equals(k))) {
                            oldValue = e.value;
                            if (!onlyIfAbsent) {
                                e.value = value;
                                ++modCount;
                            }
                            break;
                        }
                        e = e.next;
                    }
                    else {//不存在就新建一個
                        if (node != null)
                            node.setNext(first);
                        else
                            node = new HashEntry<K,V>(hash, key, value, first);
                        int c = count + 1;
                        //超過長度則rehash
                        if (c > threshold && tab.length < MAXIMUM_CAPACITY)
                            rehash(node);
                        else
                            setEntryAt(tab, index, node);
                        ++modCount;
                        count = c;
                        oldValue = null;
                        break;
                    }
                }
            } finally {
                unlock();
            }
            return oldValue;
        }
        private void rehash(HashEntry<K,V> node) {          
        HashEntry<K,V>[] oldTable = table;  
        int oldCapacity = oldTable.length;  
        int newCapacity = oldCapacity << 1; //新table大小  
        threshold = (int)(newCapacity * loadFactor); //新的極限值  
        HashEntry<K,V>[] newTable =  
            (HashEntry<K,V>[]) new HashEntry[newCapacity]; //創建新的table數組  
        int sizeMask = newCapacity - 1; //計算具體位置時用，跟hashmap計算方式一樣  
        for (int i = 0; i < oldCapacity ; i++) { //循環oldtable  
            HashEntry<K,V> e = oldTable[i];  
            if (e != null) {  
                HashEntry<K,V> next = e.next;  
                int idx = e.hash & sizeMask;   
                if (next == null)   //  只有一個節點，直接移過去  
                    newTable[idx] = e;  
                else { // 節點重用  
                    HashEntry<K,V> lastRun = e;  
                    int lastIdx = idx;  
                    //下面2個for循環的邏輯是lastRun，last從next節點往後移，最後lastRun指向最後一個轉移到新table的index不變的節點  
                    //比較亂，畫圖走幾遍，意思就是說假如原來的table[1]有10個節點，然後不停計算節點在newtable的位置，很可能從第四個節點的時候開始，  
                    //後面的所有節點在newtable中的存儲位置都一樣了，那麼我newtable只要把第4個節點直接放過去就行，然後從鏈表頭開始處理其他節點，  
                    //就不用把所有節點都新建一遍了  
                    for (HashEntry<K,V> last = next;  
                         last != null;  
                         last = last.next) {  
                        int k = last.hash & sizeMask;  
                        if (k != lastIdx) {  
                            lastIdx = k;  
                            lastRun = last;  
                        }  
                    }  
                    newTable[lastIdx] = lastRun; //直接lastRun設置到newtable  
                    // 複製其他節點  
                    for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {  
                        V v = p.value;  
                        int h = p.hash;  
                        int k = h & sizeMask;  
                        HashEntry<K,V> n = newTable[k];  
                        newTable[k] = new HashEntry<K,V>(h, p.key, v, n);  
                    }  
                }  
            }  
        }  
        int nodeIndex = node.hash & sizeMask; // 把新節點加入到newtable  
        node.setNext(newTable[nodeIndex]);  
        newTable[nodeIndex] = node;  
        table = newTable;  
    }  

    /** 
     * 自旋嘗試加鎖，不成功掃描對應位置的鏈表，如果鏈表中key不存在就創建一個node，達到最大次數後就阻塞加鎖，如果key存在返回的null 
     * 處理過程中其他線程改變了鏈表結構，那就重頭再來 
     */  
    private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {  
        HashEntry<K,V> first = entryForHash(this, hash);  
        HashEntry<K,V> e = first;  
        HashEntry<K,V> node = null;  
        int retries = -1; // negative while locating node  
        while (!tryLock()) {  
            HashEntry<K,V> f; // to recheck first below  
            if (retries < 0) {  
                if (e == null) { //基本就是查找key不存在就創建一個,存在就trylock一直到次數限制，再不行就阻塞加鎖  
                    if (node == null)   
                        node = new HashEntry<K,V>(hash, key, value, null);  
                    retries = 0;  
                }  
                else if (key.equals(e.key))  
                    retries = 0;  
                else  
                    e = e.next;  
            }  
            else if (++retries > MAX_SCAN_RETRIES) { //超過最大嘗試次數，那麼就lock阻塞，單核1，多核64  
                lock();  
                break;  
            }  
            else if ((retries & 1) == 0 &&  
                     (f = entryForHash(this, hash)) != first) { //隔一次檢查一遍嘗試的時候發現鏈表的首節點變化了，也就是有別的線程操作了，那就重來  
                e = first = f; // re-traverse if entry changed  
                retries = -1;  
            }  
        }  
        return node;  
    } 
}

可以看到Segment繼承了ReentrantLock,因此在put方法中能保證線程安全.通過Segment中比較重要的方法基本就是這些,但其中還有很多看不懂的地方,會繼續努力的