ThreadLocal线程局部变量的实现

本是古典何须时尚 2024-03-25 21:33 53阅读 0赞

今天我们来谈一谈 Java 中存储线程局部变量的类 `ThreadLocal` 。

### ThreadLocal 介绍 ###

> This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its \{@code get\} or \{@code set\} method) has its own, independently initialized copy of the variable. \{@code ThreadLocal\} instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID)

上面这段是该类的注释：该提供线程局部变量。这些变量不同于它们正常的对应变量，因为每个通过 `get()、set()` 访问 `ThreadLocal` 变量的线程都有自己的、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)。

**简单来说它的作用是作为一个数据结构，可以为每个线程分别存储他们私有的数据**。我们可以暂时简单理解为下面这张图（实际上这个图是错的）

![format_png][]

后面我们会详细介绍它的设计原理。

#### 常用 API ####

<table> 
 <thead> 
  <tr> 
   <th>方法</th> 
   <th>作用</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>public ThreadLocal()</td> 
   <td>实例化对象</td> 
  </tr> 
  <tr> 
   <td>ThreadLocal.withInitial(Supplier&lt;? extends S&gt; supplier )</td> 
   <td>实例化对象并赋予它每个线程初始值</td> 
  </tr> 
  <tr> 
   <td>public void set(T value)</td> 
   <td>设置当前线程绑定的变量</td> 
  </tr> 
  <tr> 
   <td>public T get()</td> 
   <td>获取当前线程绑定的变量</td> 
  </tr> 
  <tr> 
   <td>public void remove()</td> 
   <td>移除当前线程绑定的变量</td> 
  </tr> 
 </tbody> 
</table>

### ThreadLocal 使用场景 ###

#### Spring 事务管理器 ####

在 Spring 事务实现中，`TransactionSynchronizationManager` 类中声明了多个 `ThreadLocal` 类型的成员变量用以将事务执行过程中各种上下文信息绑定到当前线程，包括当前事务连接对象、是否可读、事务名称、隔离级别等

public abstract class TransactionSynchronizationManager {
    
       private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<>("Transactional resources");
    
       private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations = new NamedThreadLocal<>("Transaction synchronizations");
    
       private static final ThreadLocal<String> currentTransactionName = new NamedThreadLocal<>("Current transaction name");
    
       private static final ThreadLocal<Boolean> currentTransactionReadOnly = new NamedThreadLocal<>("Current transaction read-only status");
    
       private static final ThreadLocal<Integer> currentTransactionIsolationLevel = new NamedThreadLocal<>("Current transaction isolation level");
    
       private static final ThreadLocal<Boolean> actualTransactionActive = new NamedThreadLocal<>("Actual transaction active");
       //.....

[format_png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/25/cde46901aeec480b9f939602b14b622e.png