Java学习笔记——ThreadLocal类的使用

Posted 2020-10-31 douglasdoudou

主要是对博客的一些自己的理解和补充  地址：http://www.cnblogs.com/-new/p/7604420.html

概述：

　　这里套用该博主的实验代码：

　　　

public class T {
    ThreadLocal<Long> longLocal = new ThreadLocal<Long>();
    public void set() {
        longLocal.set(Thread.currentThread().getId());
    }
    public long getLong() {
        return longLocal.get();
    }
    public static void main(String[] args) throws InterruptedException {
        final T test = new T();
        test.set();
        Thread thread1 = new Thread(){
            public void run() {
                test.set();
                System.out.println("线程一:"+test.getLong());
            };
        };
        thread1.start();
        thread1.join();
        System.out.println("main线程:"+test.getLong());
        System.out.println("没有发生值的覆盖，两个线程保存的值是不同的");
    }
}

　　1、基本分析

• 对于主线程和thread1两个线程来说，他们共享了T类的test这个对象，这个时候两者可以通过ThreadLocal类来分别保存一个test对象里面的值；也就是说：test对象不变，但是里面有一个变量是因使用test这个对象不同而不同的，longLocal变量就是这个变量
• 它的底层是使用一个ThreadLocalMap来实现的（每一个Thread里面都有这个对象），里面的Entry键名为某一个对象的ThreadLocal的实例，键值为具体的值，一个线程可以绑定多个ThreadLocal对象; 并且，Thread对象的成员ThreadLocalMap是懒加载的，用到的时候才加载：（注意：这里的threadLocals是线程t的一个成员变量ThreadLocalMap的实例）

   /**
       * 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);
      }

  这里是使用ThreadLocal对象的实例作为Entry的键名

  /**
           * Construct a new map initially containing (firstKey, firstValue).
           * ThreadLocalMaps are constructed lazily, so we only create
           * one when we have at least one entry to put in it.
           */
          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);
          }

  这里是set方法的源码，使用的是开地址法

   /**
           * Set the value associated with key.
           *
           * @param key the thread local object
           * @param value the value to be set
           */
          private void set(ThreadLocal<?> key, Object value) {
  
              // We don‘t use a fast path as with get() because it is at
              // least as common to use set() to create new entries as
              // it is to replace existing ones, in which case, a fast
              // path would fail more often than not.
  
              Entry[] tab = table;
              int len = tab.length;
              int i = key.threadLocalHashCode & (len-1);
  
              for (Entry e = tab[i];
                   e != null;
                   e = tab[i = nextIndex(i, len)]) {
                  ThreadLocal<?> k = e.get();
  
                  if (k == key) {
                      e.value = value;
                      return;
                  }
  
                  if (k == null) {
                      replaceStaleEntry(key, value, i);
                      return;
                  }
              }
  
              tab[i] = new Entry(key, value);
              int sz = ++size;
              if (!cleanSomeSlots(i, sz) && sz >= threshold)
                  rehash();
          }

   

  2、可能存在的问题：内存泄漏
   注意：由于ThreadLocal.ThreadLocalMap.Entry继承自弱连接，当强连接断了之后，下一次GC就可以释放键名ThreadLocal对象的内存。 但是，value：在ThreadLocal.ThreadLocalMap.Entry中使用的强连接，无法由程序自动GC，必须通过remove、set、get等方法触发GC

  /**
           * The entries in this hash map extend WeakReference, using
           * its main ref field as the key (which is always a
           * ThreadLocal object).  Note that null keys (i.e. entry.get()
           * == null) mean that the key is no longer referenced, so the
           * entry can be expunged from table.  Such entries are referred to
           * as "stale entries" in the code that follows.
           */
          static class Entry extends WeakReference<ThreadLocal<?>> {
              /** The value associated with this ThreadLocal. */
              Object value;
  
              Entry(ThreadLocal<?> k, Object v) {
                  super(k);
                  value = v;
              }
          }

   

  ps：第一次写博客，写的也只是自己的一些心得体会，有什么不足之处欢迎指出