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美团面试,问了ThreadLocal原理,这个回答让我通过了

上周我侥幸通过美团一面( 点击查看一面过程 ),岗位是java后端开发工程师。美团面试官给我进行了二面。面试过程中他问了ThreadLocal原理(上次问线程池,这次问ThreadLocal,美团爸爸这么喜欢线程安全机制么),今天详细讲一讲 ThreadLocal原理

ThreadLocal

ThreadLocal是线程的内部存储类,可以在指定线程内存储数据。只有指定线程可以得到存储数据。

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * {@code get} or {@code set} method) has its own, independently initialized
 * copy of the variable.  {@code ThreadLocal} instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 */
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每个线程都有一个ThreadLocalMap的实例对象,并且通过ThreadLocal管理ThreadLocalMap。

/* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;
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每个新线程都会实例化为一个ThreadLocalMap并且赋值给成员变量ThreadLocals,使用时若已经存在threadLocals则直接使用已经存在的对象。

应用场景

当某些数据是以线程为作用域并且不同线程有不同数据副本时,考虑ThreadLocal。

无状态,副本变量独立后不影响业务逻辑的高并发场景。

如果如果业务逻辑强依赖于副本变量,则不适合用ThreadLocal解决。

get()与set()

set()是调用ThreadLocalMap的set()实现的

public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
//getMap方法
ThreadLocalMap getMap(Thread t) {
      //thred中维护了一个ThreadLocalMap
      return t.threadLocals;
 }
 
//createMap
void createMap(Thread t, T firstValue) {
      //实例化一个新的ThreadLocalMap,并赋值给线程的成员变量threadLocals
      t.threadLocals = new ThreadLocalMap(this, firstValue);
}
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ThreadLocalMap

ThreadLocalMap为每个Thread都维护了一个数组table,ThreadLocal确定了一个数组下标,而这个下标是value存储的对应位置。

美团面试,问了ThreadLocal原理,这个回答让我通过了

ThreadLocalMaps是延迟构造的,因此只有在至少要放置一个条目时才创建。

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);
        }
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ThreadLocalMap初始化时创建了默认长度是16的Entry数组。通过hashCode与length位运算确定索引值i。

每个Thread都有一个ThreadLocalMap类型。相当于每个线程Thread都有一个Entry型的数组table。而一切读取过程都是通过操作这个数组table进行的。

set() 方法

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;
            //通过&运算计算索引
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal<?> k = e.get();
                //如果存在key则覆盖
                if (k == key) {
                    e.value = value;
                    return;
                }
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
            //新建结点插入
            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }
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将threadLocalHashCode与长度进行 位运算 得到索引。

threadLocalHashCode的代码如下:

private final int threadLocalHashCode = nextHashCode();

    /**
     * The next hash code to be given out. Updated atomically. Starts at
     * zero.
     */
    private static AtomicInteger nextHashCode =
        new AtomicInteger();

    /**
     * The difference between successively generated hash codes - turns
     * implicit sequential thread-local IDs into near-optimally spread
     * multiplicative hash values for power-of-two-sized tables.
     */
    private static final int HASH_INCREMENT = 0x61c88647;

    /**
     * Returns the next hash code.
     */
    private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }
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由于是static变量,threadLocalHashCode在每次加载threadLocal类时会重新初始化,同时会自增一次,增加HASH_INCREMENT( 斐波那契散列乘数 ,通过该数散列出来的结果会比较均匀)。

static变量也称作静态变量,静态变量和非静态变量的区别是:静态变量被所有的对象所共享,在内存中只有一个副本,它当且仅当在类初次加载时会被初始化。

而非静态变量是对象所拥有的,在创建对象的时候被初始化,存在多个副本,各个对象拥有的副本互不影响。static成员变量的初始化顺序按照定义的顺序进行初始化。

对于一个ThreadLocal来讲,他的索引值i是确定的。对于不同线程,同一个threadlocal对应的是不同table的同一下标,即是table[i],不同线程之间的table是相互独立的。

get() 方法

计算索引,直接取出

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();
    }
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remove() 方法

/**
  * Remove the entry for key.
  */
   private void remove(ThreadLocal<?> key) {
       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)]) {
           if (e.get() == key) {
               e.clear();
               expungeStaleEntry(i);
               return;
           }
       }
   }
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线程隔离特性

线程隔离特性,只有在线程内才能获取到对应的值,线程外不能访问。

  • Synchronized是通过线程等待,牺牲时间来解决访问冲突

  • ThreadLocal是通过每个线程单独一份存储空间,牺牲空间来解决冲突

内存泄露问题

存在内存泄露问题, 每次使用完ThreadLocal,都调用它的remove()方法,清除数据。

Demo程序

import java.util.concurrent.atomic.AtomicInteger;

/**
 * <h3>Exper1</h3>
 * <p>ThreadLocalId</p>
 *
 * @author : cxc
 * @date : 2020-04-01 23:48
 **/
  public class ThreadLocalId {
      // Atomic integer containing the next thread ID to be assigned
          private static final AtomicInteger nextId = new AtomicInteger(0);

          // Thread local variable containing each thread's ID
          private static final ThreadLocal <Integer> threadId =
              new ThreadLocal<Integer>()
          {
              @Override
              protected Integer initialValue() {
                  return nextId.getAndIncrement();
              }
          };

          // Returns the current thread's unique ID, assigning it if necessary
          public static int get() {
            return threadId.get();
          }
          public static void remove() {
            threadId.remove();
          }
  }

/**
 * <h3>Exper1</h3>
 * <p></p>
 *
 * @author : cxc
 * @date : 2020-04-02 00:07
 **/
public class ThreadLocalMain {
  private static void incrementSameThreadId(){
    try{
      for(int i=0;i<5;i++){
        System.out.println(Thread.currentThread()
        +"_"+i+",threadId:"+
            ThreadLocalId.get());
      }
    }finally {
      ThreadLocalId.remove();
    }
  }

  public static void main(String[] args) {
    incrementSameThreadId();
    new Thread(new Runnable() {
      @Override
      public void run() {
        incrementSameThreadId();
      }
    }).start();
    new Thread(new Runnable() {
      @Override
      public void run() {
        incrementSameThreadId();
      }
    }).start();
  }
}
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总结

咱们玩归玩,闹归闹,别拿面试开玩笑。

ThreadLocal的原理在面试中几乎被问烂了。 Thread的私有数据是存储在ThreadLocalMap,通过ThreadLoacl进行管理。 要了解ThreadLocal的原理,最好多阅读几遍源码,尤其是 ThreadLocalMap的源码部分 。大家面试前要把知识点记牢。

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原文  https://juejin.im/post/5efac08ce51d4534b55a2ed0
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