java 中ThreadLocal實(shí)例分析

更新時(shí)間：2017年06月15日 08:38:53 投稿：lqh

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java 中ThreadLocal實(shí)例分析

從概念上理解，threadlocal使變量在多個(gè)線程中相互隔離實(shí)現(xiàn)線程安全，threadlocal包裝的變量最終都專屬于對應(yīng)的每個(gè)線程，線程之間相互獨(dú)立，用一個(gè)具體實(shí)現(xiàn)來說明：

public interface Consumer {
  int consume();
}
public class ComsumeThread implements Runnable {

  private Consumer consumer;

  public ComsumeThread(Consumer consumer) {
    this.consumer = consumer;
  }

  @Override
  public void run() {
    for(int i=0;i<10;i++){
      System.out.println(Thread.currentThread().getName()+" After Consume left:"+consumer.consume());
    }

  }
}
public class ConsumeClientA implements Consumer {

  private static int leftNum = 30;

  @Override
  public int consume() {
    int orgLeftNum = leftNum;
    Random random = new Random(System.currentTimeMillis());
    try {
      Thread.sleep(random.nextInt(3));
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    orgLeftNum = orgLeftNum -1;
    leftNum = orgLeftNum;
    return leftNum;
  }

  public static void main(String[] args){
    Consumer consumer = new ConsumeClientA();
    Thread thread1 = new Thread(new ComsumeThread(consumer));
    Thread thread2 = new Thread(new ComsumeThread(consumer));
    Thread thread3 = new Thread(new ComsumeThread(consumer));

    thread1.start();
    thread2.start();
    thread3.start();
  }
}

ConsumeClientA是在沒有做任何線程安全處理，結(jié)果如下：

Thread-2 After Consume left:29
Thread-1 After Consume left:29
Thread-3 After Consume left:29
Thread-2 After Consume left:28
Thread-1 After Consume left:28
Thread-3 After Consume left:28
Thread-2 After Consume left:27
Thread-1 After Consume left:27
Thread-2 After Consume left:26
Thread-3 After Consume left:27
Thread-1 After Consume left:25
Thread-2 After Consume left:25
Thread-3 After Consume left:25
Thread-1 After Consume left:24
Thread-2 After Consume left:24
Thread-3 After Consume left:24
Thread-1 After Consume left:23
Thread-2 After Consume left:23
Thread-3 After Consume left:23
Thread-1 After Consume left:22
Thread-2 After Consume left:22
Thread-3 After Consume left:22
Thread-1 After Consume left:21
Thread-2 After Consume left:21
Thread-3 After Consume left:21
Thread-1 After Consume left:20
Thread-2 After Consume left:20
Thread-3 After Consume left:20
Thread-1 After Consume left:19
Thread-3 After Consume left:18

增加threadlocal處理，每個(gè)線程相互獨(dú)立，實(shí)現(xiàn)如下：

public class ConsumeClientB implements Consumer {
  private ThreadLocal<Integer> leftNumThreadLocal = new ThreadLocal<Integer>(){
    @Override
    protected Integer initialValue() {
      return 30;
    }
  };

  @Override
  public int consume() {
    int orgLeftNum = leftNumThreadLocal.get();
    Random random = new Random(System.currentTimeMillis());
    try {
      Thread.sleep(random.nextInt(3));
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    orgLeftNum = orgLeftNum -1;
    leftNumThreadLocal.set(orgLeftNum);
    return leftNumThreadLocal.get();
  }

  public static void main(String[] args){
    Consumer consumer = new ConsumeClientB();
    Thread thread1 = new Thread(new ComsumeThread(consumer));
    Thread thread2 = new Thread(new ComsumeThread(consumer));
    Thread thread3 = new Thread(new ComsumeThread(consumer));

    thread1.start();
    thread2.start();
    thread3.start();
  }
}

運(yùn)行的結(jié)果如下：

Thread-1 After Consume left:29
Thread-3 After Consume left:29
Thread-2 After Consume left:29
Thread-1 After Consume left:28
Thread-3 After Consume left:28
Thread-2 After Consume left:28
Thread-1 After Consume left:27
Thread-3 After Consume left:27
Thread-2 After Consume left:27
Thread-1 After Consume left:26
Thread-3 After Consume left:26
Thread-2 After Consume left:26
Thread-1 After Consume left:25
Thread-3 After Consume left:25
Thread-2 After Consume left:25
Thread-1 After Consume left:24
Thread-3 After Consume left:24
Thread-2 After Consume left:24
Thread-1 After Consume left:23
Thread-3 After Consume left:23
Thread-2 After Consume left:23
Thread-1 After Consume left:22
Thread-3 After Consume left:22
Thread-2 After Consume left:22
Thread-1 After Consume left:21
Thread-3 After Consume left:21
Thread-2 After Consume left:21
Thread-1 After Consume left:20
Thread-3 After Consume left:20
Thread-2 After Consume left:20

每個(gè)線程擁有自己的獨(dú)立變量，相互隔離實(shí)現(xiàn)線程安全。

那ThreadLocal是怎樣實(shí)現(xiàn)這種線程隔離的線程安全的呢？

從ThreadLocal源碼可以看到，真正實(shí)現(xiàn)線程隔離，與線程掛鉤的，其實(shí)是ThreadLocal.ThreadLocalMap這個(gè)實(shí)現(xiàn)類，最明顯的體現(xiàn)就在于Thread類源碼的這樣一個(gè)變量申明說明了ThreadLocal.ThreadLocalMap與Thread的關(guān)系：

ThreadLocal.ThreadLocalMap threadLocals, inheritableThreadLocals;

Thread類是包含threadLocals對象的，ThreadLocal的具體實(shí)現(xiàn)就是根據(jù)提供的get,set等接口，對當(dāng)前thread的threadLocals變量進(jìn)行相關(guān)操作的，如get操作代碼如下：

  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();
  }

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

可以看到，getMap()方法就是從當(dāng)前thread獲取對應(yīng)的threadLocals變量，然后從這個(gè)ThreadLocal.ThreadLocalMap類型的threadLocals變量中獲取對應(yīng)線程中該ThreadLocal對象對應(yīng)的變量值。

set方法的操作也是一樣：

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

  }

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

static class Entry extends WeakReference<ThreadLocal> {
      Object value;

      Entry(ThreadLocal var1, Object var2) {
        super(var1);
        this.value = var2;
      }
    }

ThreadLocalMap中存的是內(nèi)部類Entry的數(shù)組,Entry是繼承WeakReference實(shí)現(xiàn)，WeakReference的好處是保存對象引用，而又不干擾該對象被GC回收，線程執(zhí)行完回收threadLocals變量時(shí)不會受到Entry封裝的變量的干擾。

而且ThreadLocalMap中的key是ThreadLocal，所以一個(gè)ThreadLocal對象只能在一個(gè)Thread對象中保存一個(gè)ThreadLocal的value。

綜上，很多人說ThreadLocal的實(shí)現(xiàn)是ThreadLocalMap中存Thread對象為key，變量為value的map結(jié)構(gòu)，其實(shí)是錯(cuò)誤的。

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