探究ThreaLocal

朱雀 2021-01-26 04:53 548阅读 0赞

## 前言 ##

ThreadLocal提供一个（只有一个）线程的局部变量，为了确保多线程环境下，线程的安全性。其实可以这样理解，ThreadLocal其实就是一个普通类，它声明的对象有明确的作用范围，这个范围就是用ThreadLocal去声明对象的线程，当然每个线程可以有多个ThreadLocal变量，这些ThreadLocal变量被保存在ThreadLocalMap中，这个ThreadLocalMap其实相当于一个key-value结构的Map，结构大概是这样ThreadLocalMap<ThreadLocal,Object>。

## 例子 ##

/**
     * @author: ggp
     * @Date: 2019/7/2 20:56
     * @Description:
     */
    public class ThreadLocalTest {
        public static void main(String[] args) {
           ThreadLocal<Number> t = new ThreadLocal<>();
           Thread thread1 = new Thread(new Runnable() {
               @Override
               public void run() {
    
                  for(int i =0; i<100;i++){
                      if(null == t.get()){
                          t.set(new Number());
                      }
                      t.get().setVal(i);
                      System.out.println(Thread.currentThread().getName()+">>>>>>>>>>"+t.get().getVal());
                  }
               }
           });
           thread1.start();
           Thread thread2 = new Thread(new Runnable() {
               @Override
               public void run() {
                   for(int i=1000;i<1100;i++){
                       if(null == t.get()){
                           t.set(new Number());
                       }
                       t.get().setVal(i);
                       System.out.println(Thread.currentThread().getName()+">>>>>>>>>>"+t.get().getVal());
                   }
               }
           });
           thread2.start();
        }
    }
    class Number{
        int val = 2;
    
        public int getVal() {
            return val;
        }
    
        public void setVal(int val) {
            this.val = val;
        }
    }

## 运行结果 ##

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzNTQzNjM0_size_16_color_FFFFFF_t_70][]

可以看到两个线程互不干扰，确保了多线程之间的线程安全。

## 源码分析 ##

### threadLocal.get() ###

/**
         * Returns the value in the current thread's copy of this
         * thread-local variable.  If the variable has no value for the
         * current thread, it is first initialized to the value returned
         * by an invocation of the {@link #initialValue} method.
         *
         * @return the current thread's value of this thread-local
         */
        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();
        }
    
       /**
         * Get the map associated with a ThreadLocal. Overridden in
         * InheritableThreadLocal.
         *
         * @param  t the current thread
         * @return the map
         */
        ThreadLocalMap getMap(Thread t) {
            return t.threadLocals;
        }

首先先获取到当前线程t，然后拿到线程的局部变量表ThreadLocalMap

/* ThreadLocal values pertaining to this thread. This map is maintained
         * by the ThreadLocal class. */
        ThreadLocal.ThreadLocalMap threadLocals = null;

跟踪源码可以看出ThreadLocalMap是 ThreadLocal的一个静态内部类，而Entry是ThreadLocalMap的基本组成单位

static class ThreadLocalMap {
    
            /**
             * 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;
                }
            }

可以看到作者的注释，这个entry实现了弱引用的接口，而这个弱引用指向hashmap中的key值，所以当key值为null时，GC的时候就会回收key值，但是value并不是弱引用，所以不会被回收。entry没法被调用却也占着内存，就会造成内存泄露，直到当前线程结束，线程被回收，内存才会被完全释放。但是如果这个场景发生在线程池，那么内存泄露就会一直存在，直到应用结束。

但是，人家jdk早就有了解决方法。。。。。。。。。。

在entry的get和set方法中会做一个判断来清楚这些隐患

private Entry getEntry(ThreadLocal<?> key) {
                int i = key.threadLocalHashCode & (table.length - 1);
                Entry e = table[i];
                if (e != null && e.get() == key)
                    return e;
                else
                    return getEntryAfterMiss(key, i, e);
            }
    
             private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
                Entry[] tab = table;
                int len = tab.length;
    
                while (e != null) {
                    ThreadLocal<?> k = e.get();
                    if (k == key)
                        return e;
                    if (k == null)
                        expungeStaleEntry(i);
                    else
                        i = nextIndex(i, len);
                    e = tab[i];
                }
                return null;
            }
            private int expungeStaleEntry(int staleSlot) {
                Entry[] tab = table;
                int len = tab.length;
    
                // expunge entry at staleSlot
                tab[staleSlot].value = null;
                tab[staleSlot] = null;
                size--;
    
                // Rehash until we encounter null
                Entry e;
                int i;
                for (i = nextIndex(staleSlot, len);
                     (e = tab[i]) != null;
                     i = nextIndex(i, len)) {
                    ThreadLocal<?> k = e.get();
                    if (k == null) {
                        e.value = null;
                        tab[i] = null;
                        size--;
                    } else {
                        int h = k.threadLocalHashCode & (len - 1);
                        if (h != i) {
                            tab[i] = null;
    
                            // Unlike Knuth 6.4 Algorithm R, we must scan until
                            // null because multiple entries could have been stale.
                            while (tab[h] != null)
                                h = nextIndex(h, len);
                            tab[h] = e;
                        }
                    }
                }
                return i;
            }

以get为例，计算hash值，当拿到的entry中的key为空的时候进入 getEntryAfterMiss()中的expungeStaleEntry()方法中，通过源码可以看出，首先是将value置空，然后把整个entry置空，最后重新hash。

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzNTQzNjM0_size_16_color_FFFFFF_t_70]: /images/1611636790110.png