ThreadLocal

功能

• 让每个线程使用一个独立的副本数据

源码

• class ThreadLocalMap {
      class Entry extends WeakReference<ThreadLocal<?>> {
          Object value;
  
          Entry(ThreadLocal<?> k, Object v) {
              super(k);// 关键是这行代码，调用父类的构造方法，才将k这个对象设置为弱引用对象
              value = v;
          }
  
          private void set(ThreadLocal<?> key, Object value) {
              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)]) {
                  ThreadLocal<?> k = e.get();
  
                  if (k == key) {
                      e.value = value;
                      return;
                  }
  
                  if (k == null) {
                      replaceStaleEntry(key, value, i);
                      return;
                  }
              }
              ......
          }
  
          private static int nextIndex(int i, int len) {
              return ((i + 1 < len) ? i + 1 : 0);
          }
      }
  }
  

• private int expungeStaleEntry(int staleSlot) {
      Entry[] tab = table;
      int len = tab.length;
  
      tab[staleSlot].value = null;
      tab[staleSlot] = null;
      size--;
      ......
  }
  

使用流程

先看下面这段代码

public class ThreadLocalRunner {
    private static ThreadLocal<Integer> threadLocal = new ThreadLocal<>();

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                threadLocal.set(0);
                for (int j = 1; j <= 100; j++) {
                    threadLocal.set(threadLocal.get() + j);
                }
                System.out.println("线程" + Thread.currentThread() + "的执行结果为：" + threadLocal.get());
            }).start();
        }
    }
}

这段代码的运行结果会是下面这样的

10 个线程都是用的同一个 threadLocal 对象，但是在使用 threadLocal.set(xx) 时会创建各自的 ThreadLocalMap 对象并且绑定在自己的线程对象里，代码如下

    // java.lang.ThreadLocal.java
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
  
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

当调用 threadLocal.get() 方法时也是使用自己线程的 ThreadLocalMap 对象获取值，代码：

    public T get() {
        Thread t = Thread.currentThread();
        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();
    }
  
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }