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+## CAS与volatile关键字回顾
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+
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+### volatile
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+ 一旦一个共享变量(类的成员变量、类的静态成员变量)被volatile修饰之后,那么就具备了两层语义:
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+1) 保证了不同线程对这个变量进行操作时的可见性,即一个线程修改了某个变量的值,这新值对其他线程来说是立即可见的。
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+2) 禁止进行指令重排序。
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+
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+### CAS
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+
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+CAS机制当中使用了3个基本操作数:内存地址V,旧的预期值A,要修改的新值B。更新一个变量的时候,只有当变量的预期值A和内存地址V当中的实际值相同时,才会将内存地址V对应的值修改为B。
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+
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+## hashtable底层实现
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+
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+底层是synchronized锁实现,put和get方法不能同时实现。有可能阻塞线程。并且如果有多个线程执行put操作,会使得操作变成单线程。
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+
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+## 实现原理
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+
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+
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+将一个大的ConcurrentHashMap集合,拆分成n多个不同的小的hashtable,在每个小的hashtable中都有自己独立的table数组。
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+
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+每一个segment对象就是一个hashtable,存放的时候会计算两次index值
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+
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+hashtable的个数即segment的个数不会新增,只会扩容自己独立的table
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+
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+## 核心参数分析
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+
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+1. static final int DEFAULT_INITIAL_CAPACITY = 16;
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+segment集合的默认大小
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+2. static final int DEFAULT_CONCURRENCY_LEVEL = 16;
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+并发的数量,分成segment[16]
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+3. static final float DEFAULT_LOAD_FACTOR = 0.75f;
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+segment的加载因子,就是table的加载因子
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+
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+## 无参构造源码分析
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+
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+```
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+ public ConcurrentHashMap(int initialCapacity,
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+ float loadFactor, int concurrencyLevel) {
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+ if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
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+ throw new IllegalArgumentException();
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+ // 并发级别的最大值
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+ if (concurrencyLevel > MAX_SEGMENTS)
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+ concurrencyLevel = MAX_SEGMENTS;
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+ // Find power-of-two sizes best matching arguments
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+ // 计算出ssize2的平法的个数
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+ int sshift = 0;
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+ // segment数组容量
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+ int ssize = 1;
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+ while (ssize < concurrencyLevel) {
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+ ++sshift;
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+ ssize <<= 1;
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+ }
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+ // sshift=4 ssize=16
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+ // 计算index 右移动28位置
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+ this.segmentShift = 32 - sshift; // 28
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+ // 与运算均匀分布
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+ this.segmentMask = ssize - 1; // 15
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+ if (initialCapacity > MAXIMUM_CAPACITY)
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+ initialCapacity = MAXIMUM_CAPACITY;
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+ // c实际上就是hashtable的初始化容量
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+ int c = initialCapacity / ssize; // 1
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+ if (c * ssize < initialCapacity)
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+ ++c;
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+ // hashtable默认容量为2
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+ int cap = MIN_SEGMENT_TABLE_CAPACITY;
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+ while (cap < c)
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+ cap <<= 1;
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+ // create segments and segments[0]
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+ // 创建一个segment s0对象,放入下标为0的位置。默认大小为2,默认扩容大小为1,加载因子为0,75f
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+ // 为什么在构造函数初始化s0? 方便后期其他key落到不同的segment中,能够知道加载因子,和默认容量一些基本参数,就是相当于提供了一个模板
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+ Segment<K,V> s0 =
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+ new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
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+ (HashEntry<K,V>[])new HashEntry[cap]);
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+ Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
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+ // ss[0] = s0
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+ UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
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+ this.segments = ss;
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+ }
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+```
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+
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+## 1.7put源码解析
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+
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+```
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+ @SuppressWarnings("unchecked")
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+ public V put(K key, V value) {
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+ Segment<K,V> s;
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+ if (value == null)
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+ throw new NullPointerException();
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+ // 获取key的hash值
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+ int hash = hash(key);
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+ // 右移动28位
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+ /**
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+ * this.segmentShift = 32 - sshift; // 28
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+ * this.segmentMask = ssize - 1; // 15
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+ */
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+ int j = (hash >>> segmentShift) & segmentMask;
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+ // s = Segment[j] == null
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+ if ((s = (Segment<K,V>)UNSAFE.getObject
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+ (segments, (j << SSHIFT) + SBASE)) == null)
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+ // 如果为null,则创建一个segment对象
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+ s = ensureSegment(j);
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+ return s.put(key, hash, value, false);
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+ }
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+```
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+
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+
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+## ensureSegment函数解析
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+```
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+ @SuppressWarnings("unchecked")
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+ private Segment<K,V> ensureSegment(int k) {
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+ final Segment<K,V>[] ss = this.segments;
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+ long u = (k << SSHIFT) + SBASE; // raw offset
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+ Segment<K,V> seg;
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+ // 强制读取主内存的数据
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+ // 查询segment是否为空?,如果为空,拿到模板
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+ if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
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+ Segment<K,V> proto = ss[0]; // use segment 0 as prototype
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+ int cap = proto.table.length;
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+ float lf = proto.loadFactor;
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+ int threshold = (int)(cap * lf);
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+ HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
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+ if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
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+ == null) { // recheck
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+ Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
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+ while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
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+ == null) {
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+ if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
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+ break;
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+ }
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+ }
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+ }
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+ return seg;
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+ }
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+```
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+为什么会执行3次这个判断? 就是为了线程安全
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+seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null
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+
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+第一次判断==null
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+获取s0默认参数信息,创建table对象HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
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+第二个判断==null
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+创建Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);对象
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+第三次判断==null
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+通过CAS操作赋值让segment[index] = ss
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+
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+## 深度解析segment的put方法
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+
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+解析``` return s.put(key, hash, value, false);```这一句话
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+
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+```
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+final V put(K key, int hash, V value, boolean onlyIfAbsent) {
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+ HashEntry<K,V> node = tryLock() ? null :
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+ scanAndLockForPut(key, hash, value);
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+ V oldValue;
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+ try {
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+ HashEntry<K,V>[] tab = table;
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+ // 计算hashtable的下标
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+ int index = (tab.length - 1) & hash;
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+ HashEntry<K,V> first = entryAt(tab, index);
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+ for (HashEntry<K,V> e = first;;) {
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+ if (e != null) {
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+ K k;
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+ // 是链表,找到要存放的位置
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+ if ((k = e.key) == key ||
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+ (e.hash == hash && key.equals(k))) {
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+ oldValue = e.value;
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+ if (!onlyIfAbsent) {
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+ e.value = value;
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+ ++modCount;
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+ }
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+ break;
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+ }
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+ e = e.next;
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+ }
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+ else {
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+ // node节点为null
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+ if (node != null)
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+ node.setNext(first);
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+ else
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+ // 头插法
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+ node = new HashEntry<K,V>(hash, key, value, first);
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+ // 判断是否扩容
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+ int c = count + 1;
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+ if (c > threshold && tab.length < MAXIMUM_CAPACITY)
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+ rehash(node);
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+ else
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+ // 赋值
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+ setEntryAt(tab, index, node);
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+ ++modCount;
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+ count = c;
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+ oldValue = null;
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+ break;
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+ }
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+ }
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+ } finally {
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+ unlock();
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+ }
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+ return oldValue;
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+}
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+```
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+
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+误区:ConcurrentHashMap不会造成阻塞?
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+如果有16个线程,16个key,正好落到不同segment位置的情况不会阻塞
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+但是相同的key会导致线程阻塞
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+
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+因为put方法底层使用了lock锁,调用trylock方法(该方法常用在自旋操作中),他与lock方法不同的地方是没有获取到锁的地方不会阻塞,返回false
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+
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+## put方法没有抢到锁
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+
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+```
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+private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
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+ // 找到链表的位置
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+ HashEntry<K,V> first = entryForHash(this, hash);
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+ HashEntry<K,V> e = first;
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+ HashEntry<K,V> node = null;
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+ int retries = -1; // negative while locating node
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+ while (!tryLock()) {
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+ HashEntry<K,V> f; // to recheck first below
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+ if (retries < 0) {
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+ // 假设当前e不为空
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+ if (e == null) {
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+ if (node == null) // speculatively create node
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+ node = new HashEntry<K,V>(hash, key, value, null);
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+ retries = 0;
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+ }
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+ else if (key.equals(e.key))
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+ retries = 0;
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+ else
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+ e = e.next;
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+ }
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+ // 重试获取所得状态,如果超过最大次数,则阻塞等待
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+ else if (++retries > MAX_SCAN_RETRIES) {
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+ lock();
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+ break;
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+ }
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+ // 重新拿到它的链表
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+ // 需要注意这行代码,不单单是保证线程的时刻更新,也将死循环的时候最大限度利用CPU资源,形成cache,为后续的插入操作做准备
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+ else if ((retries & 1) == 0 &&
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+ (f = entryForHash(this, hash)) != first) {
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+ e = first = f; // re-traverse if entry changed
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+ retries = -1;
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+ }
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+ }
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+ return node;
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+}
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+```
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+
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+## ConcurrentHashMap底层实现原理
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+
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+1. 有多个不同的segment对象组成
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+2. lock锁
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+3. UNSAFE查询内存最新的数据
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+4. 使用cas做修改
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