ThreadLocal的实现

基于Android 7.0分析

关键的几个点先列出来

public class ThreadLocal<T> {


  public T get(){
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if(map != null){
      ThreadLocalMap.Entry e  = map.getEntry(this);
      if(e!=null)
        return (T)e.value;
    }
    return setInitialValue();
  }

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

  ThreadLocalMap getMap(Thread t){
    return t.threadLocals;
  }

  void createMap(Thread t, T firstValue){
    t.threadLocals = new ThreadLocalMap(this, firstValue);
  }
}

可以看到核心了，ThreadLocal链接了Thread和ThreadLocalMap

public class Thread implements Runnable{

  ThreadLocal.ThreadLocalMap threadLocals = null;
}

所以，ThreadLocal保存的数据是存在线程自身的成员变量ThreadLocal.ThreadLocalMap中的

//ThreadLocal.java
static class ThreadLocalMap{
  static class Entry extends WeakReference<ThreadLocal>{
    Object value;
    Entry(ThreadLocal k, Object v){
      super(k);
      value = v;
    }
  }

  private Entry[] table;
  private Entry getEntry(ThreadLocal key){
    int i = key.threadLocalHashCode & (table.length -1);
    Entry e = table[I];
    if(e!=null && e.get() ==key){
      return e;
    }
  }

  private void set(ThreadLocal key, Object value){
    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (lend -1);
    
    for(Entry e = tab[I],e!=null;e=tab[I = nexIndex(I,len)]){
       ThreadLocal k = e.get();
        if(k==key){
          e.value = value;
          return;
        }
    }
    ....
    rehash();
  }
}

那我们是怎么使用的呢,我们会先实例化一个ThreadLocal对象，随后在线程调用set(T) or get() @ThreadLocal方法。
从上面可以看出，每个线程都有一个ThreadLocal.ThreadLocalMap对象，其中又有一个数组Entry[]，每次进行set/get的时候，会将这个ThreadLocal作为key，保存在线程的ThreadLocal.ThreadLocalMap中，保存的过程是通过将ThreadLocal进行hash取模保存在Entry[]中,如果发生了Hash冲突，就取下一个点位

private static int nextIndex(int I, int len){
  return ((I+1 > len)? I+1:0);
}

为了防止内存泄漏，Entry还继承自WeakReference<ThreadLocal>