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ConcurrentHashMap 源码分析

时间:2015-07-11 15:13:05      阅读:146      评论:0      收藏:0      [点我收藏+]

标签:源码   concurrenthashmap   

 CocurrentHashMap 作用

        HashTable通过对整张表加锁的方式实现并发hash查找与储存,CocurrentHashMapt通过Segment的方式可以实现相同的功能,不过效率更加高,在jdk1.6的时候,CocuentHashMap有弱一致性的问题,不过在jdk1.7的时候,这个问题已经修复了。所以是并发安全性还是性能都是非常高的。接下来我尝试基于jdk1.7的源码去分析CocurrentHashMap。

cocurrentHashMap 初始化预处理

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long SBASE;
    private static final int SSHIFT;
    private static final long TBASE;
    private static final int TSHIFT;

    static {
        int ss, ts;
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> tc = HashEntry[].class;
            Class<?> sc = Segment[].class;
            TBASE = UNSAFE.arrayBaseOffset(tc);
            SBASE = UNSAFE.arrayBaseOffset(sc);
            ts = UNSAFE.arrayIndexScale(tc);
            ss = UNSAFE.arrayIndexScale(sc);
        } catch (Exception e) {
            throw new Error(e);
        }
        if ((ss & (ss-1)) != 0 || (ts & (ts-1)) != 0)
            throw new Error("data type scale not a power of two");
        SSHIFT = 31 - Integer.numberOfLeadingZeros(ss);
        TSHIFT = 31 - Integer.numberOfLeadingZeros(ts);
    }
代码解析:首先获取Unsafe提供cas操作,java底层多线程并发都是通过cas完成的,不过cas操作对于高精度的并发还是存在一定问题。【至于这个问题,以后再分析】。UNSAFE.arrayBaseOffset(tc)和UNSAFE.arrayBaseOffset(sc)这两个都是用于计算HashEntry和Segment实体对象相对于数组对象的内存偏移值。这是cas操作必须要获取的值。
注释:
//获取数组中第一个元素的偏移量(get offset of a first element in the array)  
public native int arrayBaseOffset(java.lang.Class aClass);  
//获取数组中一个元素的大小(get size of an element in the array)  
public native int arrayIndexScale(java.lang.Class aClass); 


cocurrentHashMap 初始化

    public ConcurrentHashMap(int initialCapacity,
                             float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        if (concurrencyLevel > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;
        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
        while (ssize < concurrencyLevel) {
            ++sshift;
            ssize <<= 1;
        }
        this.segmentShift = 32 - sshift;
        this.segmentMask = ssize - 1;
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        while (cap < c)
            cap <<= 1;
        // create segments and segments[0]
        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];
        UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
        this.segments = ss;
    }

代码解析:以上代码主要的作用是初始化Segment[]数组对象以及Segment对象。解析来分析最重要的put和get方法。

   put方法

  <span style="font-size:18px;">  public V put(K key, V value) {
        Segment<K,V> s;
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        int j = (hash >>> segmentShift) & segmentMask;
        if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            s = ensureSegment(j);
        return s.put(key, hash, value, false);
    }</span>

代码解析:大体的意思是先计算出key的hash值,然后利用这个hash值得到Segment对象。然后Segment对象执行put方法。这样就完成了put操作。由于这个过程非常重要,我们肯定想要知道它是如何处理并发以及 内部实现。

ensureSegment

    private Segment<K,V> ensureSegment(int k) {
        final Segment<K,V>[] ss = this.segments;
        long u = (k << SSHIFT) + SBASE; // raw offset
        Segment<K,V> seg;
        if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
            Segment<K,V> proto = ss[0]; // use segment 0 as prototype
            int cap = proto.table.length;
            float lf = proto.loadFactor;
            int threshold = (int)(cap * lf);
            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);
                while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
                       == null) {
                    if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
                        break;
                }
            }
        }
        return seg;
    }

首先计算出偏移量,然后利用UnSafe去获取对象。在这里有可能大家对这个偏移值的获取有点疑惑,在这里我也分析一下这个偏移量获取既long u=(k<<SSHIFT)+SBASE; 大家有可能会提出为什么会这样计算呢。一般人都会这样计算偏移值:设k为index,size为对象的大小,_offset为第一个元素的偏移值,那偏移值的大小应该为offset=index*size+offset。没错,理论上这么来说是对的。但是在java内存模型中内存遵循8字节对齐。所以在java内存模型中你这样计算是错误的。有前面的初始化可以知道:SSHIFT为对象大小对应的二进制数对应的位数。所以k<<SSHIFT也就实现了8字节对齐。

segment->put

        final V put(K key, int hash, V value, boolean onlyIfAbsent) {
            HashEntry<K,V> node = tryLock() ? null :
                scanAndLockForPut(key, hash, value);
            V oldValue;
            try {
                HashEntry<K,V>[] tab = table;
                int index = (tab.length - 1) & hash;
                HashEntry<K,V> first = entryAt(tab, index);
                for (HashEntry<K,V> e = first;;) {
                    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 {
                        if (node != null)
                            node.setNext(first);
                        else
                            node = new HashEntry<K,V>(hash, key, value, first);
                        int c = count + 1;
                        if (c > threshold && tab.length < MAXIMUM_CAPACITY)
                            rehash(node);
                        else
                            setEntryAt(tab, index, node);
                        ++modCount;
                        count = c;
                        oldValue = null;
                        break;
                    }
                }
            } finally {
                unlock();
            }
            return oldValue;
        }
以下画一个流程图:

技术分享


  get()

    public V get(Object key) {
        Segment<K,V> s; // manually integrate access methods to reduce overhead
        HashEntry<K,V>[] tab;
        int h = hash(key.hashCode());
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null &&
            (tab = s.table) != null) {
            for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile
                     (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE);
                 e != null; e = e.next) {
                K k;
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    return e.value;
            }
        }
        return null;
    }

原理很简单先定位到segment,然后定位到实体。并且通过getObjectVolatie保证能够读到最新的数据。

总结:concurrentHashMap实现涉及到很多多线程的知识和java内存模型这方面的知识,如果没有足够的能力,介意不要模仿,但是我们可以学习它的思想以及是如何实现的。


 

版权声明:本文为博主原创文章,未经博主允许不得转载。

ConcurrentHashMap 源码分析

标签:源码   concurrenthashmap   

原文地址:http://blog.csdn.net/asking1233/article/details/46840919

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