JavaHashtable机制深入了解

@Test
    public void test() {
        Hashtable<String, String> table=new Hashtable<>();
        Hashtable<String, String> table1=new Hashtable<>(16);
        Hashtable<String, String> table2=new Hashtable<>(16, 0.75f);
        table.put("T1", "1");
        table.put("T2", "2");
        System.out.println(table);
        // 报空指针异常
        table.put(null, "3");
    }

// 内部采用Entry数组存储键值对数据，Entry实际为单向链表的表头
private transient Entry<?,?>[] table;
// HashTable里键值对个数
private transient int count;
// 扩容阈值，当超过这个值时，进行扩容操作，计算方式为：数组容量*加载因子
private int threshold;
// 加载因子
private float loadFactor;
// 修改次数，用于快速失败机制
private transient int modCount = 0;

private static class Entry<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Entry<K,V> next;
    protected Entry(int hash, K key, V value, Entry<K,V> next) {
        this.hash = hash;
        this.key =  key;
        this.value = value;
        this.next = next;
    }
    ......
}

// 设置指定容量和加载因子，初始化HashTable
public Hashtable(int initialCapacity, float loadFactor) {
    // 非法参数校验
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal Capacity: "+
                                          initialCapacity);
    // 非法参数校验
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal Load: "+loadFactor);
    if (initialCapacity==0)
        // 容量最小为1
        initialCapacity = 1;
    this.loadFactor = loadFactor;
    // 初始化数组
    table = new Entry<?,?>[initialCapacity];
    // 初始扩容阈值
    threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}
// 设置指定容量初始HashTable，加载因子为0.75
public Hashtable(int initialCapacity) {
    this(initialCapacity, 0.75f);
}
// 手动指定数组初始容量为11，加载因子为0.75
public Hashtable() {
    this(11, 0.75f);
}

// 方法synchronized修饰，线程安全
public synchronized V put(K key, V value) {
    // 如果value为空，直接空指针
    if (value == null) {
        throw new NullPointerException();
    }
    // Makes sure the key is not already in the hashtable.
    Entry<?,?> tab[] = table;
    // 得到key的哈希值
    int hash = key.hashCode();
    // 得到该key存在到数组中的下标
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    // 得到该下标对应的Entry
    Entry<K,V> entry = (Entry<K,V>)tab[index];
    // 如果该下标的Entry不为null，则进行链表遍历
    for(; entry != null ; entry = entry.next) {
        // 遍历链表，如果存在key相等的节点，则替换这个节点的值，并返回旧值
        if ((entry.hash == hash) && entry.key.equals(key)) {
            V old = entry.value;
            entry.value = value;
            return old;
        }
    }
    // 如果数组下标对应的节点为空，或者遍历链表后发现没有和该key相等的节点，则执行插入操作
    addEntry(hash, key, value, index);
    return null;
}
private void addEntry(int hash, K key, V value, int index) {
    // 修改次数+1
    modCount++;
    Entry<?,?> tab[] = table;
    // 判断是否需要扩容
    if (count >= threshold) {
        // 如果count大于等于扩容阈值，则进行扩容
        rehash();
        tab = table;
        // 扩容后，重新计算该key在扩容后table里的下标
        hash = key.hashCode();
        index = (hash & 0x7FFFFFFF) % tab.length;
    }
    // Creates the new entry.
    @SuppressWarnings("unchecked")
    // 采用头插的方式插入，index位置的节点为新节点的next节点
    // 新节点取代inde位置节点
    Entry<K,V> e = (Entry<K,V>) tab[index];
    tab[index] = new Entry<>(hash, key, value, e);
    // count+1
    count++;
}

protected void rehash() {
    // 暂存旧的table和容量
    int oldCapacity = table.length;
    Entry<?,?>[] oldMap = table;
    // 新容量为旧容量的2n+1倍
    int newCapacity = (oldCapacity << 1) + 1;
    // 判断新容量是否超过最大容量
    if (newCapacity - MAX_ARRAY_SIZE > 0) {
        // 如果旧容量已经是最大容量大话，就不扩容了
        if (oldCapacity == MAX_ARRAY_SIZE)
            // Keep running with MAX_ARRAY_SIZE buckets
            return;
        // 新容量最大值只能是MAX_ARRAY_SIZE
        newCapacity = MAX_ARRAY_SIZE;
    }
    // 用新容量创建一个新Entry数组
    Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];
    // 模数+1
    modCount++;
    // 重新计算下次扩容阈值
    threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    // 将新Entry数组赋值给table
    table = newMap;
    // 遍历数组和链表，进行新table赋值操作
    for (int i = oldCapacity ; i-- > 0 ;) {
        for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
            Entry<K,V> e = old;
            old = old.next;
 
            int index = (e.hash & 0x7FFFFFFF) % newCapacity;
            e.next = (Entry<K,V>)newMap[index];
            newMap[index] = e;
        }
    }
}

public synchronized V remove(Object key) {
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    // 获取key对应的index
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    // 遍历链表，如果找到key相等的节点，则改变前继和后继节点的关系，并删除相应引用，让GC回收
    Entry<K,V> e = (Entry<K,V>)tab[index];
    for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            modCount++;
            if (prev != null) {
                prev.next = e.next;
            } else {
                tab[index] = e.next;
            }
            count--;
            V oldValue = e.value;
            e.value = null;
            return oldValue;
        }
    }
    return null;
}

public synchronized V remove(Object key) {
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    // 获取key对应的index
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    // 遍历链表，如果找到key相等的节点，则改变前继和后继节点的关系，并删除相应引用，让GC回收
    Entry<K,V> e = (Entry<K,V>)tab[index];
    for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            modCount++;
            if (prev != null) {
                prev.next = e.next;
            } else {
                tab[index] = e.next;
            }
            count--;
            V oldValue = e.value;
            e.value = null;
            return oldValue;
        }
    }
    return null;
}