解析ThreadLocal的源码

ThreadLocal在多线程中为每一个线程提供了变量副本，这样子每一个线程都可以在自己的工作空间内使用该副本了。

ThreadLocal的四个常见方法：
 public T get() 
 private T setInitialValue()
 public void set(T value)
 public void remove()

ThreadLocal使用的数据结构是ThreadLocalMap（本质就是map），首先先看一下ThreadLocalMap的源码：

static class Entry extends WeakReference<ThreadLocal<?>> {
    //Entry类是弱引用类型的
            /** The value associated with this ThreadLocal. */
            Object value;
            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }
 private static final int INITIAL_CAPACITY = 16;//初始容量16
 private Entry[] table;
 private int size = 0;
  private int threshold;
  private void setThreshold(int len) {
            threshold = len * 2 / 3;//装载因子2/3
        }
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
           //构造函数
            table = new Entry[INITIAL_CAPACITY];//初始一个长度为INITIAL_CAPACITY的表
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);//计算元素存放的位置
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }
 private ThreadLocalMap(ThreadLocalMap parentMap) {
 //私有构造方法 
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];

            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
                    if (key != null) {
                        Object value = key.childValue(e.value);
                        Entry c = new Entry(key, value);
                        int h = key.threadLocalHashCode & (len - 1);
                        while (table[h] != null)
                            h = nextIndex(h, len);//用开放定址法 如果有冲突 则寻找下一个
                        table[h] = c;
                        size++;
                    }
                }
            }
        }

看完ThreadLocalMap的源代码，接下来看一下ThreadLocal几个常用方法。

 public T get() {
        Thread t = Thread.currentThread();//获取当前线程获取key
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();//获取值
    }

  private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

 public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

 public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }