简介

这个类提供线程局部变量。这些变量与普通变量的不同之处在于，每个访问一个变量(通过其get或set方法)的线程都有自己的独立初始化的变量副本。ThreadLocal实例通常是类中的私有静态字段，希望将状态与线程关联(例如，用户ID或事务ID)。

使用

代码示例

public class ThreadLocalDemo {

    private static ThreadLocal<String> threadLocal = new ThreadLocal<>();

    public static void main(String[] args) {
        //主线程
        threadLocal.set("main thread.");
        System.out.println("Thread name：" + Thread.currentThread().getName() + "<-->" + threadLocal.get());
        //线程1
        new Thread(() -> {
            threadLocal.set("This is thread one.");
            System.out.println("Thread name：" + Thread.currentThread().getName() + "<-->" + threadLocal.get());
        }, "Thread one").start();
        //线程2
        new Thread(() -> {
            threadLocal.set("This is thread two.");
            System.out.println("Thread name：" + Thread.currentThread().getName() + "<-->" + threadLocal.get());
        }, "Thread two").start();
        //线程3
        new Thread(() -> {
            threadLocal.set("This is thread three.");
            System.out.println("Thread name：" + Thread.currentThread().getName() + "<-->" + threadLocal.get());
        }, "Thread three").start();
    }
}

代码运行

Thread name：main<-->main thread.
Thread name：Thread one<-->This is thread one.
Thread name：Thread two<-->This is thread two.
Thread name：Thread three<-->This is thread three.

Java实现过程

set方法实现过程

• 尝试获取或创建ThreadLocalMap

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

• 创建ThreadLocalMap并赋值个ThreadLocal私有变量

  /**
   * Create the map associated with a ThreadLocal. Overridden in
   * InheritableThreadLocal.
   *
   * @param t the current thread
   * @param firstValue value for the initial entry of the map
   */
  void createMap(Thread t, T firstValue) {
      t.threadLocals = new ThreadLocalMap(this, firstValue);
  }

• 获取一个ThreadLocalMap

  /**
   * Get the map associated with a ThreadLocal. Overridden in
   * InheritableThreadLocal.
   *
   * @param  t the current thread
   * @return the map
   */
  ThreadLocalMap getMap(Thread t) {
      return t.threadLocals;
  }

get方法实现过程

/**
   * Returns the value in the current thread's copy of this
   * thread-local variable.  If the variable has no value for the
   * current thread, it is first initialized to the value returned
   * by an invocation of the {@link #initialValue} method.
   *
   * @return the current thread's value of this thread-local
   */
  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();
  }

很明显，每一个Thread对象有一个私有的ThreadLocalMap对象。操作set数据时候，数据是设置到当前操作Thread上面的ThreadLocalMap，从而保证了数据的独立性。

ThreadLocalMap

ThreadLocalMap是一个定制的散列映射，仅适用于维护线程本地值。没有任何操作被导出到ThreadLocal类之外。这个类是包私有的，允许在Thread类中声明字段。为了帮助处理非常大的和长期存在的使用，散列表项使用WeakReferences作为键。但是，由于不使用引用队列，只有当表空间耗尽时，才会删除陈旧的表项。