ThreadLocal

junziln2022年1月12日约 2876 字大约 10 分钟

ThreadLocal

Thread类中，有个ThreadLocal.ThreadLocalMap 的成员变量。
ThreadLocalMap内部维护了Entry数组，每个Entry代表一个完整的对象，key是ThreadLocal本身，value是ThreadLocal的泛型对象值。

````

public class Thread implements Runnable {
    // ThreadLocal.ThreadLocalMap是Thread的属性
    ThreadLocal.ThreadLocalMap threadLocals = null;
}

static class ThreadLocalMap {
    
    static class Entry extends WeakReference<ThreadLocal<?>> {
        /** The value associated with this ThreadLocal. */
        Object value;
 
        Entry(ThreadLocal<?> k, Object v) {
            super(k);
            value = v;
        }
    }
    //Entry数组
    private Entry[] table;
    
    // ThreadLocalMap的构造器，ThreadLocal作为key
    ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
        table = new Entry[INITIAL_CAPACITY];
        int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
        table[i] = new Entry(firstKey, firstValue);
        size = 1;
        setThreshold(INITIAL_CAPACITY);
    }
}

 public void set(T value) {
        Thread t = Thread.currentThread(); //获取当前线程t
        ThreadLocalMap map = getMap(t);  //根据当前线程获取到ThreadLocalMap
        if (map != null)  //如果获取的ThreadLocalMap对象不为空
            map.set(this, value); //K，V设置到ThreadLocalMap中
        else
             createMap(t, value); //创建一个新的ThreadLocalMap
    }
   
     ThreadLocalMap getMap(Thread t) {
       return t.threadLocals; //返回Thread对象的ThreadLocalMap属性
    }
 
    void createMap(Thread t, T firstValue) { //调用ThreadLocalMap的构造函数
        t.threadLocals = new ThreadLocalMap(this, firstValue); this表示当前类ThreadLocal
    }
}

    public T get() {
        Thread t = Thread.currentThread();//获取当前线程t
        ThreadLocalMap map = getMap(t);//根据当前线程获取到ThreadLocalMap
        if (map != null) { //如果获取的ThreadLocalMap对象不为空
            //由this（即ThreadLoca对象）得到对应的Value，即ThreadLocal的泛型值
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value; 
                return result;
            }
        }
        return setInitialValue(); //初始化threadLocals成员变量的值
    }
    
     private T setInitialValue() {
        T value = initialValue(); //初始化value的值
        Thread t = Thread.currentThread(); 
        ThreadLocalMap map = getMap(t); //以当前线程为key，获取threadLocals成员变量，它是一个ThreadLocalMap
        if (map != null)
            map.set(this, value);  //K，V设置到ThreadLocalMap中
        else
            createMap(t, value); //实例化threadLocals成员变量
        return value;
    }

实现原理

Thread线程类有一个类型为ThreadLocal.ThreadLocalMap的实例变量threadLocals，即每个线程都有一个属于自己的ThreadLocalMap。
ThreadLocalMap内部维护着Entry数组，每个Entry代表一个完整的对象，key是ThreadLocal本身，value是ThreadLocal的泛型值。
并发多线程场景下，每个线程Thread，在往ThreadLocal里设置值的时候，都是往自己的ThreadLocalMap里存，读也是以某个ThreadLocal作为引用，在自己的map里找对应的key，从而可以实现了线程隔离。

public class TianLuoThreadLocalTest {
 
    private static final ThreadLocal<String> threadLocal1 = new ThreadLocal<>();
    private static final ThreadLocal<String> threadLocal2 = new ThreadLocal<>();
 
}

一个使用类，有两个共享变量，也就是说用了两个ThreadLocal成员变量的话。如果用线程id作为ThreadLocalMap的key，怎么区分哪个ThreadLocal成员变量呢？因此还是需要使用ThreadLocal作为Key来使用。每个ThreadLocal对象，都可以由threadLocalHashCode属性唯一区分的，每一个ThreadLocal对象都可以由这个对象的名字唯一区分（下面的例子）。看下ThreadLocal代码：

public class ThreadLocal<T> {
  private final int threadLocalHashCode = nextHashCode();
  
  private static int nextHashCode() {
    return nextHashCode.getAndAdd(HASH_INCREMENT);
  }
}

public class TianLuoThreadLocalTest {
 
    public static void main(String[] args) {
        Thread t = new Thread(new Runnable(){
            public void run(){
                ThreadLocal<TianLuoDTO> threadLocal1 = new ThreadLocal<>();
                threadLocal1.set(new TianLuoDTO("Hello 七度"));
                System.out.println(threadLocal1.get());
                ThreadLocal<TianLuoDTO> threadLocal2 = new ThreadLocal<>();
                threadLocal2.set(new TianLuoDTO("你好：ccc"));
                System.out.println(threadLocal2.get());
            }});
        t.start();
    }
 
}
// 运行结果
TianLuoDTO{name='Hello 七度'}
TianLuoDTO{name='你好：ccc'}

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贡献者

wangjialin

ThreadLocal

ThreadLocal

实现原理

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