Hashtable源码详解

成员变量

private transient Entry[] table;//存储链表的数组

private transient int count;

private int threshold;

private float loadFactor;

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 next;
    //hashtable的核心链表结构
    protected Entry(int hash, K key, V value, Entry next) {
        this.hash = hash;
        this.key =  key;
        this.value = value;
        this.next = next;
    }
}

构造方法初始化

public Hashtable(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal Load: "+loadFactor);

    if (initialCapacity==0)
        initialCapacity = 1;
    this.loadFactor = loadFactor;
    table = new Entry[initialCapacity];//创建链表数组
    threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}

public Hashtable(int initialCapacity) {
    this(initialCapacity, 0.75f);
}
//初始值table的大小为11，负载因子0.75
public Hashtable() {
    this(11, 0.75f);
}

put方法

/*
当一个线程正在访问一个对象的 synchronized 实例方法，那么其他线程不能访问
该对象的其他synchronized方法，毕竟一个对象只有一把锁，当一个线程获取了该
对象的锁之后，其他线程无法获取该对象的锁，所以无法访问该对象的其他synchronized
实例方法，但是其他线程还是可以访问该实例对象的其他非synchronized方法
*/

//采用synchronized修饰方法，可以保证多个线程中只能有一个线程调用该方法
public synchronized V put(K key, V value) {
    // Make sure the value is not null
    if (value == null) {
        throw new NullPointerException();
    }

    // Makes sure the key is not already in the hashtable.
    Entry tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;//得到index的值
    @SuppressWarnings("unchecked")
    Entry entry = (Entry)tab[index];//获取该节点上的值
    for(; entry != null ; entry = entry.next) {
        //如果值不为空，则按照链表的方式来遍历，遇到相同的key值，进行替换
        if ((entry.hash == hash) && entry.key.equals(key)) {
            V old = entry.value;
            entry.value = value;
            return old;
        }
    }
    //index处的值为空，添加节点
    addEntry(hash, key, value, index);
    return null;
}
private void addEntry(int hash, K key, V value, int index) {
    modCount++;

    Entry tab[] = table;
    //判断是否需要扩容
    if (count >= threshold) {
        // Rehash the table if the threshold is exceeded
        rehash();//扩容，重新计算hash值

        tab = table;
        hash = key.hashCode();
        index = (hash & 0x7FFFFFFF) % tab.length;
    }

    // Creates the new entry.
    @SuppressWarnings("unchecked")
    Entry e = (Entry) tab[index];
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
}
protected void rehash() {
    int oldCapacity = table.length;
    Entry[] oldMap = table;

    // overflow-conscious code
    // 新的容量变为原来的2倍+1
    int newCapacity = (oldCapacity << 1) + 1;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        // 再大也大不了了啊
        if (oldCapacity == MAX_ARRAY_SIZE)
            // Keep running with MAX_ARRAY_SIZE buckets
            return;
        newCapacity = MAX_ARRAY_SIZE;
    }
    //创建新的链表数组
    Entry[] newMap = new Entry[newCapacity];

    modCount++;
    threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    table = newMap;
    //将之前的值放在新的数组中
    for (int i = oldCapacity ; i-- > 0 ;) {
        for (Entry old = (Entry)oldMap[i] ; old != null ; ) {
            Entry e = old;
            old = old.next;

            int index = (e.hash & 0x7FFFFFFF) % newCapacity;
            e.next = (Entry)newMap[index];
            newMap[index] = e;
        }
    }
}

get方法

//采用关键字synchronized修饰，调用该方法时是线程安全的
public synchronized V get(Object key) {
    Entry tab[] = table;
    int hash = key.hashCode();//得到key的hash值
    int index = (hash & 0x7FFFFFFF) % tab.length;//根据hash值找到数组下标
    for (Entry e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            //找到与key相同的节点，返回value
            return (V)e.value;
        }
    }
    return null;
}

remove方法

public synchronized V remove(Object key) {
    Entry tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;//获取下标
    @SuppressWarnings("unchecked")
    Entry e = (Entry)tab[index];
    for(Entry prev = null ; e != null ; prev = e, e = e.next) {
        //节点不为空的话，遍历链表，做删除节点的操作
        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;
        }
    }
    //下标处的节点为空的话返回null
    return null;
}