java线程封闭之栈封闭和ThreadLocal

public T get() {
    // 当前线程
    Thread t = Thread.currentThread();
　　　// 拿到当前线程的threadLocalMap，即上图中的map引用
    ThreadLocalMap map = getMap(t);
    if (map != null) {
      // 拿到当前ThreadLocal为Key对应的Entry，里面做了防止内存泄漏的处理
      ThreadLocalMap.Entry e = map.getEntry(this);
      if (e != null) {
        @SuppressWarnings("unchecked")
        T result = (T)e.value;
        return result;
      }
    }
    // 如果为null设置默认值
    return setInitialValue();
  }

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;
  }

 // 默认null实现
 protected T initialValue() {
    return null;
 }

 // 这里main方法测试，所以用static修饰，会延长threadLocal的生命周期，有内存泄漏的风险，一般作为成员变量就足够了
 public static ThreadLocal<String> threadLocal = new ThreadLocal<String>(){
    @Override
    protected String initialValue() {
      return "init string from initialValue method";
    }
  };

  public static void main(String[] args) throws InterruptedException {
    // 未放入value直接调用get
    System.err.println("invoke get before any set：" + threadLocal.get());
    threadLocal.set("test");
    System.err.println("before thread start : " + threadLocal.get());
    new Thread(() -> {
      // 对相同的threadLocal对象放入值
      threadLocal.set("test in thread");
      System.err.println("In thread[" + Thread.currentThread().getName() + "] threadLocal value : " + threadLocal.get());
    }).start();
    TimeUnit.SECONDS.sleep(1);
    // 证明threadLocal中的value不在线程中共享
    System.err.println("after thread value : " + threadLocal.get());
  }