TheadLocal底层原理超级图解,逐行讲解!!!(从0到1)

public class ThreadLocalExample {  

    // 创建一个ThreadLocal变量，用于存储每个线程的计数器, 初始值设为0
    private static final ThreadLocal<Integer> threadLocalCounter = ThreadLocal.withInitial(() -> 0);  

    public static void main(String[] args) {  
        // 创建多个线程并启动  
        for (int i = 0; i < 5; i++) {  
            new Thread(new CounterTask(i)).start();  
        }  
    }  

    // 内部静态类实现Runnable接口  
    private static class CounterTask implements Runnable {
        private final int threadId;  
        
        //对线程名进行赋值
        public CounterTask(int threadId) {  
            this.threadId = threadId;  
        }  

        @Override  
        public void run() {  
            // 每个线程增加自己的计数器值  
            for (int i = 0; i < 10; i++) {  
                int currentCount = threadLocalCounter.get();  
                threadLocalCounter.set(currentCount + 1);  
                System.out.println("Thread #" + threadId + " - Count: " + threadLocalCounter.get());  
            }  
        }  
    }  
}

 public T get() {
        //currentThread为本地方法, 通过其他语言实现, 作用为获取当前线程对象
        Thread t = Thread.currentThread();
        //返回之前在Threa类中声明的ThreadLocalMap类, 引用名为TthreadLocals, 可以看本文第二张图的184行
        ThreadLocalMap map = getMap(t);
        if (map != null) {
        /*这种写法想必大家很熟悉, 同样Entry为ThreaLocalMap的静态内部类,这个类下面再做详解,
            只需要知道表示单个键值就行了这里 */
            ThreadLocalMap.Entry e = map.getEntry(this);
            
            if (e != null) {
                @SuppressWarnings("unchecked")
                //获取值, 并返回
                T result = (T)e.value;
                return result;
            }
        }
        //如果没有设置值的话,默认值为设为null
        return setInitialValue();
    }

static class ThreadLocalMap {
///被忽略的代码
     
 private Entry[] table;
 
 //Entry静态内部类
         static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }
        
        
 
  
    private Entry getEntry(ThreadLocal<?> key) {
           // 算出位置i, 根据线程对象和当前键值对的长度来计算的
            int i = key.threadLocalHashCode & (table.length - 1);
            //获取到key对应位置的单个键值对
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
            //从i开始遍历后面的元素,找到对应键的元素, key相等就返回对应元素, 为null则进行探测式清理
                return getEntryAfterMiss(key, i, e);
        }
 private void set(ThreadLocal<?> key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            //获取这个键在hash中对应的位置
            int i = key.threadLocalHashCode & (len-1);
            
            //从i开始找,直到找到对应的位置
            for (Entry e = tab[i];
                //e!null表示发生了哈希冲突
                 e != null;
                 //从ThreadLocal的对象散列值开始每次往后移一位
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal<?> k = e.get();
                
                //如何要放的位置上的键和这个键相等说明是替换,进行值更新
                if (k == key) {
                    e.value = value;
                    return;
                }
                //如果碰到过期的key, 优先使用过期的key,节省空间
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
            //如果到这里说明key对应的hash位置上没有数据,且Entry中的key都在被时殷弘,
            tab[i] = new Entry(key, value);
            int sz = ++size;
             // 做一次启发式清理：
            // 条件一：!cleanSomeSlots(i, sz) 成立，说明启发式清理工作未清理到任何数据..
            // 条件二：sz >= threshold 成立，说明当前table内的entry已经达到扩容阈值了..会触发rehash操作。
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }
        
//被忽略的代码        
}