How to use ThreadLocal core method in java

1. get() method

(1) Get the currently used thread and find the threadLocalMap associated with the thread

(2) If threadLocalMap is empty, proceed Initialize a new one and return

(3) If threadLocalMap is not empty, search for Entry

based on threadlocal as the key. (4) If Entry is not empty, return the value corresponding to entry, otherwise execute the first Two

public T get() {
    // 获取当前线程
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    //若当前线程关联的ThreadLocal不为空则查询
    if (map != null) {
        //根据threadLocal查询对应的Entry
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}
 
 private T setInitialValue() {
     //默认返回null值
     T value = initialValue();
     Thread t = Thread.currentThread();
     ThreadLocalMap map = getMap(t);
     //如果当前调用线程关联的ThreadLocalMap为空则创建，否则设置值进去
     if (map != null)
         map.set(this, value);
     else
         //new ThreadLocalMap(this,value)
         createMap(t, value);
     return value;
 }
 
private Entry getEntry(ThreadLocal<?> key) {
    //根据key获取其在数组的下标位置
    int i = key.threadLocalHashCode & (table.length - 1);
    Entry e = table[i];
    if (e != null && e.get() == key)
        return e;
    else
        return getEntryAfterMiss(key, i, e);
}
 
private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
    Entry[] tab = table;
    int len = tab.length;
    //数组下标的Entry不为空且关联的threadlocal与查找的threadlocal不一致
    while (e != null) {
        ThreadLocal<?> k = e.get();
        //entry关联的threadlocal与查找的相等则直接返回
        if (k == key)
            return e;
        if (k == null)
            //关联的threadlocal为空，则触发清理key为null的Entry并重新进行rehash旧Entry数组的元素
            //threadLocalMap的hash冲突与hashMap的冲突处理方式不一致，hashMap使用的是链表地址法，
            //而threadLocalMap使用的开放地址法——线性探测，即顺序查找下一位置或者遍历全表，效率较低
            expungeStaleEntry(i);
        else
            //递增下标i的值进行下一轮的查找
            i = nextIndex(i, len);
        e = tab[i];
    }
    return null;
}

2, remove() method

(1) Get the currently used thread and find the threadLocalMap

## associated with the thread #(2) If it is not empty, delete the associated value in threadLocalMap, otherwise do nothing

//ThreadLocal
public void remove() {
    ThreadLocalMap m = getMap(Thread.currentThread());
    if (m != null)
        //删除当前threadLocal对象关联的Entry
        m.remove(this);
}
 
//ThreadLocalMap
private void remove(ThreadLocal<?> key) {
    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);
    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        if (e.get() == key) {
            e.clear();
            expungeStaleEntry(i);
            return;
        }
    }
}

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