美团面试问ThreadLocal原理，这个回答我通过了面试！

前言

这是身边朋友的经历，希望可以帮到你们。

ThreadLocal

	/**
 * 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).
 */

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

应用场景

get()与set()

	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);
}

ThreadLocalMap

	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);
        }

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();
        }

	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);
    }

get() 方法

	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();
    }

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;
           }
       }
   }

线程隔离特性

内存泄露问题

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();
  }
}

总结