ThreadLocal

ThreadLocal入门

　　ThreadLocal 提供了线程本地的实例。它与普通变量的区别在于，每个使用该变量的线程都会初始化一个完全独立的实例副本。

　　ThreadLocal 变量通常被private static修饰。当一个线程结束时，它所使用的所有 ThreadLocal 相对的实例副本都可被回收。

　　ThreadLocal 适用于变量在线程间隔离而在方法或类间共享的场景。

使用示例：

public class ThreadLocalDemo {
  public static void main(String[] args) throws InterruptedException {
    int threads = 3;
    CountDownLatch countDownLatch = new CountDownLatch(threads);
    InnerClass innerClass = new InnerClass();
    for(int i = 1; i <= threads; i++) {
      new Thread(() -> {
        for(int j = 0; j < 4; j++) {
          innerClass.add(String.valueOf(j));
          innerClass.print();
        }
        innerClass.set("hello world");
        countDownLatch.countDown();
      }, "thread - " + i).start();
    }
    countDownLatch.await();
  }
  private static class InnerClass {
    public void add(String newStr) {
      StringBuilder str = Counter.counter.get();
      Counter.counter.set(str.append(newStr));
    }
    public void print() {
      System.out.printf("Thread name:%s , ThreadLocal hashcode:%s, Instance hashcode:%s, Value:%s
",
      Thread.currentThread().getName(),
      Counter.counter.hashCode(),
      Counter.counter.get().hashCode(),
      Counter.counter.get().toString());
    }
    public void set(String words) {
      Counter.counter.set(new StringBuilder(words));
      System.out.printf("Set, Thread name:%s , ThreadLocal hashcode:%s,  Instance hashcode:%s, Value:%s
",
      Thread.currentThread().getName(),
      Counter.counter.hashCode(),
      Counter.counter.get().hashCode(),
      Counter.counter.get().toString());
    }
  }
  private static class Counter {
    private static ThreadLocal<StringBuilder> counter = new ThreadLocal<StringBuilder>() {
      @Override
      protected StringBuilder initialValue() {
        return new StringBuilder();
      }
    };
  }
}

输出：

Thread name:thread - 1 , ThreadLocal hashcode:372282300, Instance hashcode:418873098, Value:0
Thread name:thread - 3 , ThreadLocal hashcode:372282300, Instance hashcode:1609588821, Value:0
Thread name:thread - 2 , ThreadLocal hashcode:372282300, Instance hashcode:1780437710, Value:0
Thread name:thread - 3 , ThreadLocal hashcode:372282300, Instance hashcode:1609588821, Value:01
Thread name:thread - 1 , ThreadLocal hashcode:372282300, Instance hashcode:418873098, Value:01
Thread name:thread - 3 , ThreadLocal hashcode:372282300, Instance hashcode:1609588821, Value:012
Thread name:thread - 3 , ThreadLocal hashcode:372282300, Instance hashcode:1609588821, Value:0123
Set, Thread name:thread - 3 , ThreadLocal hashcode:372282300,  Instance hashcode:1362597339, Value:hello world
Thread name:thread - 2 , ThreadLocal hashcode:372282300, Instance hashcode:1780437710, Value:01
Thread name:thread - 1 , ThreadLocal hashcode:372282300, Instance hashcode:418873098, Value:012
Thread name:thread - 2 , ThreadLocal hashcode:372282300, Instance hashcode:1780437710, Value:012
Thread name:thread - 1 , ThreadLocal hashcode:372282300, Instance hashcode:418873098, Value:0123
Thread name:thread - 2 , ThreadLocal hashcode:372282300, Instance hashcode:1780437710, Value:0123
Set, Thread name:thread - 1 , ThreadLocal hashcode:372282300,  Instance hashcode:482932940, Value:hello world
Set, Thread name:thread - 2 , ThreadLocal hashcode:372282300,  Instance hashcode:1691922941, Value:hello world

• 从第1-3行输出可见，每个线程通过 ThreadLocal 的 get() 方法拿到的是不同的 StringBuilder 实例
• 第1-3行输出表明，每个线程所访问到的是同一个 ThreadLocal 变量
• 对比第1行与第15行输出并结合第38行代码可知，使用 set(T t) 方法后，ThreadLocal 变量所指向的 StringBuilder 实例被替换

ThreadLocal原理

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();
}

　　2,3行取得当前线程，然后通过getMap(t)方法获取到一个map，map的类型为ThreadLocalMap。每一个线程都有一个ThreadLocalMap。

　　ThreadLocalMap的键是this，也就是调用get（）的ThreadLocal对象，值是实例。所以不同线程拿到的实例也是不同的。

　　

static class Entry extends WeakReference<ThreadLocal<?>> {
  /** The value associated with this ThreadLocal. */
  Object value;
  Entry(ThreadLocal<?> k, Object v) {
    super(k);
    value = v;
  }
}

　　与 HashMap 不同的是，ThreadLocalMap 的每个 Entry 都是一个对 键 的弱引用，这一点从super(k)可看出。另外，每个 Entry 都包含了一个对 值 的强引用。

　　使用弱引用的原因在于，当没有强引用指向 ThreadLocal 变量时，它可被回收，从而避免上文所述 ThreadLocal 不能被回收而造成的内存泄漏的问题。

　　但是，这里又可能出现另外一种内存泄漏的问题。ThreadLocalMap 维护 ThreadLocal 变量与具体实例的映射，当 ThreadLocal 变量被回收后，该映射的键变为 null，该 Entry 无法被移除。从而使得实例被该 Entry 引用而无法被回收造成内存泄漏。

适用场景：

• 每个线程需要有自己单独的实例
• 实例需要在多个方法中共享，但不希望被多线程共享

转自：http://www.jasongj.com/java/threadlocal/