本文实例讲述了Java ThreadLocal用法。分享给大家供大家参考，具体如下：

目录

• ThreadLocal的基本使用
• ThreadLocal实现原理
• 源码分析(基于openjdk11)
  • get方法：
    • setInitialValue方法
    • getEntry方法
  • set方法
    • ThreadLocalMap的set方法
      • replaceStaleEntry方法
      • cleanSomeSlots方法
      • rehash方法
      • expungeStaleEntries方法
      • resize方法

ThreadLocal实现了Java中线程局部变量。所谓线程局部变量就是保存在每个线程中独有的一些数据，我们知道一个进程中的所有线程是共享该进程的资源的，线程对进程中的资源进行修改会反应到该进程中的其他线程上，如果我们希望一个线程对资源的修改不会影响到其他线程，那么就需要将该资源设为线程局部变量的形式。

ThreadLocal的基本使用

如下示例所示，定义两个ThreadLocal变量，然后分别在主线程和子线程中对线程局部变量进行修改，然后分别获取线程局部变量的值：

public class ThreadLocalTest {

  private static ThreadLocal<String> threadLocal1 = ThreadLocal.withInitial(() -> "threadLocal1 first value");
  private static ThreadLocal<String> threadLocal2 = ThreadLocal.withInitial(() -> "threadLocal2 first value");

  public static void main(String[] args) throws Exception{

    Thread thread = new Thread(() -> {

      System.out.println("================" + Thread.currentThread().getName() + " enter=================");

      // 子线程中打印出初始值
      printThreadLocalInfo();

      // 子线程中设置新值
      threadLocal1.set("new thread threadLocal1 value");
      threadLocal2.set("new thread threadLocal2 value");

      // 子线程打印出新值
      printThreadLocalInfo();

      System.out.println("================" + Thread.currentThread().getName() + " exit=================");
    });

    thread.start();
    // 等待新线程执行
    thread.join();

    // 在main线程打印threadLocal1和threadLocal2，验证子线程对这两个变量的修改是否会影响到main线程中的这两个值
    printThreadLocalInfo();
    // 在main线程中给threadLocal1和threadLocal2设置新值
    threadLocal1.set("main threadLocal1 value");
    threadLocal2.set("main threadLocal2 value");
    // 验证main线程中这两个变量是否为新值
    printThreadLocalInfo();
  }

  private static void printThreadLocalInfo() {
    System.out.println(Thread.currentThread().getName() + ": " + threadLocal1.get());
    System.out.println(Thread.currentThread().getName() + ": " + threadLocal2.get());
  }
}

运行结果如下：

================Thread-0 enter=================
Thread-0: threadLocal1 first value
Thread-0: threadLocal2 first value
Thread-0: new thread threadLocal1 value
Thread-0: new thread threadLocal2 value
================Thread-0 exit=================
main: threadLocal1 first value
main: threadLocal2 first value
main: main threadLocal1 value
main: main threadLocal2 value

如果子线程对threadLocal1和threadLocal2的修改会影响到main线程中的threadLocal1和threadLocal2，那么在main线程第一次printThreadLocalInfo();打印出的应该是修改后的新值，即为new thread threadLocal1 value和new thread threadLocal2 value和，但实际打印结果并不是这样，说明在新线程中对threadLocal1和threadLocal2的修改并不会影响到main线程中的这两个变量，似乎main线程中的threadLocal1和threadLocal2作用域仅局限于main线程，新线程中的threadLocal1和threadLocal2作用域仅局限于新线程，这就是线程局部变量的由来。

ThreadLocal实现原理

如下图所示

每个线程对象里会持有一个java.lang.ThreadLocal.ThreadLocalMap类型的threadLocals成员变量，而ThreadLocalMap里有一个java.lang.ThreadLocal.ThreadLocalMap.Entry[]类型的table成员，这是一个数组，数组元素是Entry类型，Entry中相当于有一个key和value，key指向所有线程共享的java.lang.ThreadLocal对象，value指向各线程私有的变量，这样保证了线程局部变量的隔离性，每个线程只是读取和修改自己所持有的那个value对象，相互之间没有影响。

源码分析(基于openjdk11)

源码包括ThreadLocal和ThreadLocalMap，ThreadLocalMap是ThreadLocal内定义的一个静态内部类，用于存储实际的数据。当调用ThreadLocal的get或者set方法时都有可能创建当前线程的threadLocals成员(ThreadLocalMap类型)。

get方法：

ThreadLocal的get方法定义如下

  /**
   * 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() {
  	// 获取当前线程
    Thread t = Thread.currentThread();
    // 获取当前线程的threadLocals成员变量，这是一个ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    // threadLocals不为null则直接从threadLocals中取出ThreadLocal
    // 对象对应的值
    if (map != null) {
    	// 从map中获取当前ThreadLocal对象对应Entry对象
      ThreadLocalMap.Entry e = map.getEntry(this);
      if (e != null) {
      	// 获取ThreadLocal对象对应的value值
        @SuppressWarnings("unchecked")
        T result = (T)e.value;
        return result;
      }
    }
    // threadLocals为null，则需要创建ThreadLocalMap对象并赋给
    // threadLocals，将当前ThreadLocal对象作为key，调用initialValue
    // 获得的初始值作为value，放置到threadLocals的entry中;
    // 或者threadLocals不为null，但在threadLocals中未
    // 找到当前ThreadLocal对象对应的entry，则需要向threadLocals添加新的
    // entry，该entry以当前的ThreadLocal对象作为key，调用initialValue
    // 获得的值作为value
    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;
  }

当Thread的threadLocals为null，或者在Thread的threadLocals中未找到当前ThreadLocal对象对应的entry，则进入到setInitialValue方法；否则进入到ThreadLocalMap的getEntry方法。

setInitialValue方法

定义如下：

  private T setInitialValue() {
  	// 获取初始值,如果我们在定义ThreadLocal对象时实现了ThreadLocal
  	// 的initialValue方法，就会调用我们自定义的方法来获取初始值，否则
  	// 使用initialValue的默认实现返回null值
    T value = initialValue();
    Thread t = Thread.currentThread();
    // 获取当前线程的threadLocals成员
    ThreadLocalMap map = getMap(t);
    if (map != null) {
    	// 若threadLocals存在则将ThreadLocal对象对应的value设置为初始值
      map.set(this, value);
    } else {
    	// 否则创建threadLocals对象并设置初始值
      createMap(t, value);
    }
    if (this instanceof TerminatingThreadLocal) {
      TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
    }
    return value;
  }

createMap方法实现

  /**
   * 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) {
  	// 创建一个ThreadLocalMap对象，用当前ThreadLocal对象和初始值value来
  	// 构造ThreadLocalMap中table的第一个entry。ThreadLocalMap对象赋
  	// 给线程的threadLocals成员
    t.threadLocals = new ThreadLocalMap(this, firstValue);
  }

ThreadLocalMap的构造方法定义如下：

    /**
     * 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) {
    	// 构造table数组，数组大小为INITIAL_CAPACITY
      table = new Entry[INITIAL_CAPACITY];
      // 计算key(ThreadLocal对象)在table中的索引
      int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
      // 用ThreadLocal对象和value来构造entry对象，并放到table的第i个位置
      table[i] = new Entry(firstKey, firstValue);
      size = 1;
      // 设置table的阈值，当table中元素个数超过该阈值时需要对table
      // 进行resize，通常在调用ThreadLocalMap的set方法时会发生resize
      setThreshold(INITIAL_CAPACITY);
    }
    /**
     * Set the resize threshold to maintain at worst a 2/3 load factor.
     */
    private void setThreshold(int len) {
      threshold = len * 2 / 3;
    }

这里firstKey.threadLocalHashCode是ThreadLocal中定义的一个hashcode，使用该hashcode进行hash运算从而找到该ThreadLocal对象对应的entry在table中的索引。

getEntry方法

定义如下：

    /**
     * Get the entry associated with key. This method
     * itself handles only the fast path: a direct hit of existing
     * key. It otherwise relays to getEntryAfterMiss. This is
     * designed to maximize performance for direct hits, in part
     * by making this method readily inlinable.
     *
     * @param key the thread local object
     * @return the entry associated with key, or null if no such
     */
    private Entry getEntry(ThreadLocal<?> key) {
    	// 根据ThreadLocal的hashcode计算该ThreadLocal对象在table中的位置
      int i = key.threadLocalHashCode & (table.length - 1);
      Entry e = table[i];
      // e为null则table不存在key对应的entry;
      // e.get() != key 可能是由于hash冲突导致key对应的entry在table
      // 的另外一个位置，需要继续查找
      if (e != null && e.get() == key)
        return e;
      else
      	// e==null或者e.get() != key 继续查找key对应的entry
        return getEntryAfterMiss(key, i, e);
    }

getEntryAfterMiss方法定义如下：

    /**
     * Version of getEntry method for use when key is not found in
     * its direct hash slot.
     *
     * @param key the thread local object
     * @param i the table index for key's hash code
     * @param e the entry at table[i]
     * @return the entry associated with key, or null if no such
     */
    private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e){
      Entry[] tab = table;
      int len = tab.length;

			// 从table的第i个位置一直往后找，直到找到键为key的entry为止
      while (e != null) {
        ThreadLocal<?> k = e.get();
        // 若k==key，则找到了entry
        if (k == key)
          return e;
        // k == null 需要删除该entry
        if (k == null)
          expungeStaleEntry(i);
        // k != key && k != null 继续往后寻找，nextIndex就是取(i+1)
        // 即table中第(i+1)个位置的entry
        else
          i = nextIndex(i, len);
        e = tab[i];
      }
      return null;
    }

expungeStaleEntry方法删除key为null的entry，删除后对staleSlot位置的entry和其后第一个为null的entry之间的entry进行一个rehash操作，rehash的目的是降低table发生碰撞的概率:

    /**
     * Expunge a stale entry by rehashing any possibly colliding entries
     * lying between staleSlot and the next null slot. This also expunges
     * any other stale entries encountered before the trailing null. See
     * Knuth, Section 6.4
     *
     * @param staleSlot index of slot known to have null key
     * @return the index of the next null slot after staleSlot
     * (all between staleSlot and this slot will have been checked
     * for expunging).
     */
    private int expungeStaleEntry(int staleSlot) {
      Entry[] tab = table;
      int len = tab.length;

      // expunge entry at staleSlot
      // 删除staleSlot位置的entry
      tab[staleSlot].value = null;
      tab[staleSlot] = null;
      // table中元素个数减一
      size--;

      // Rehash until we encounter null
      // 将table中staleSlot处entry和下一个为null的entry之间的
      // entry重新进行hash放置到新的位置
      // 遇到的entry的key为null则删除该entry
      Entry e;
      int i;
      for (i = nextIndex(staleSlot, len);
         (e = tab[i]) != null;
         i = nextIndex(i, len)) {
        // e是下一个entry
        ThreadLocal<?> k = e.get();
        if (k == null) {
        	// 若entry的key为null，则删除
          e.value = null;
          tab[i] = null;
          size--;
        } else {
        	// entry的key不为null，需要将entry放到新的位置
          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.
            // tab[h]不为null则发生冲突，继续寻找下一个位置
            while (tab[h] != null)
              h = nextIndex(h, len);
            tab[h] = e;
          }
        }
      }
      return i;
    }

set方法

ThreadLocal的set方法定义如下：

  /**
   * Sets the current thread's copy of this thread-local variable
   * to the specified value. Most subclasses will have no need to
   * override this method, relying solely on the {@link #initialValue}
   * method to set the values of thread-locals.
   *
   * @param value the value to be stored in the current thread's copy of
   *    this thread-local.
   */
  public void set(T value) {
    Thread t = Thread.currentThread();
    // 获取当前线程的threadLocals
    ThreadLocalMap map = getMap(t);
    // threadLocals不为null直接设置新值
    if (map != null) {
      map.set(this, value);
    } else {
    	// threadLocals为null则需要创建ThreadLocalMap对象并赋给
    	// Thread的threadLocals成员
      createMap(t, value);
    }
  }

createMap前面已经分析过，接下来分析ThreadLocalMap的set方法

ThreadLocalMap的set方法

ThreadLocalMap的set方法定义如下，将当前的ThreadLocal对象作为key，传入的value为值，用key和value创建entry，放到table中适当的位置：

    /**
     * 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;
      // 用key计算entry在table中的位置
      int i = key.threadLocalHashCode & (len-1);

			// tab[i]不为null的话，则第i个位置已经存在有效的entry，需要继续
			// 往后寻找新的位置
      for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        ThreadLocal<?> k = e.get();

				// 找到与key相同的entry，直接更新value的值
        if (k == key) {
          e.value = value;
          return;
        }

				// 遇到key为null的entry，删除该entry
        if (k == null) {
          replaceStaleEntry(key, value, i);
          return;
        }
      }

			// 此时第i个位置entry为null，将新entry放置到这个位置
      tab[i] = new Entry(key, value);
      int sz = ++size;
      // 试图清除无效的entry，若清除失败并且table中有效entry个数
      // 大于threshold，这进行rehash操作
      if (!cleanSomeSlots(i, sz) && sz >= threshold)
        rehash();
    }

replaceStaleEntry方法

replaceStaleEntry的作用是用set方法传过来的key和value构造entry，将这个entry放到staleSlot后面的某个位置：

    /**
     * Replace a stale entry encountered during a set operation
     * with an entry for the specified key. The value passed in
     * the value parameter is stored in the entry, whether or not
     * an entry already exists for the specified key.
     *
     * As a side effect, this method expunges all stale entries in the
     * "run" containing the stale entry. (A run is a sequence of entries
     * between two null slots.)
     *
     * @param key the key
     * @param value the value to be associated with key
     * @param staleSlot index of the first stale entry encountered while
     *     searching for key.
     */
    private void replaceStaleEntry(ThreadLocal<?> key, Object value,
                    int staleSlot) {
      Entry[] tab = table;
      int len = tab.length;
      Entry e;

      // Back up to check for prior stale entry in current run.
      // We clean out whole runs at a time to avoid continual
      // incremental rehashing due to garbage collector freeing
      // up refs in bunches (i.e., whenever the collector runs).
      // 从staleSlot往前找到第一个key为null的entry的位置
      int slotToExpunge = staleSlot;
      for (int i = prevIndex(staleSlot, len);
         (e = tab[i]) != null;
         i = prevIndex(i, len))
        if (e.get() == null)
          slotToExpunge = i;

      // Find either the key or trailing null slot of run, whichever
      // occurs first
      // 从staleSlot位置往后寻找
      for (int i = nextIndex(staleSlot, len);
         (e = tab[i]) != null;
         i = nextIndex(i, len)) {
        ThreadLocal<?> k = e.get();

        // If we find key, then we need to swap it
        // with the stale entry to maintain hash table order.
        // The newly stale slot, or any other stale slot
        // encountered above it, can then be sent to expungeStaleEntry
        // to remove or rehash all of the other entries in run.
        // 若k与key相同，则直接更新value
        if (k == key) {
          e.value = value;
					// 将原来staleSlot位置的entry放置到第i个位置，此时tab[i]处的entry的key为null
          tab[i] = tab[staleSlot];
          tab[staleSlot] = e;

          // Start expunge at preceding stale entry if it exists
          // 从staleSlot处往前未找到key为null的entry
          if (slotToExpunge == staleSlot)
          	// tab[i]处entry的key为null，也即tab[slotToExpunge]处entry的key为null
            slotToExpunge = i;
          // 清除slotToExpunge位置的entry并进行rehash操作.....
          cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
          return;
        }

        // If we didn't find stale entry on backward scan, the
        // first stale entry seen while scanning for key is the
        // first still present in the run.
        if (k == null && slotToExpunge == staleSlot)
          slotToExpunge = i;
      }

      // If key not found, put new entry in stale slot
      tab[staleSlot].value = null;
      tab[staleSlot] = new Entry(key, value);

      // If there are any other stale entries in run, expunge them
      if (slotToExpunge != staleSlot)
        cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
    }

以下源码只可意会，不可言传…不再做说明

cleanSomeSlots方法

cleanSomeSlots方法：

    /**
     * Heuristically scan some cells looking for stale entries.
     * This is invoked when either a new element is added, or
     * another stale one has been expunged. It performs a
     * logarithmic number of scans, as a balance between no
     * scanning (fast but retains garbage) and a number of scans
     * proportional to number of elements, that would find all
     * garbage but would cause some insertions to take O(n) time.
     *
     * @param i a position known NOT to hold a stale entry. The
     * scan starts at the element after i.
     *
     * @param n scan control: {@code log2(n)} cells are scanned,
     * unless a stale entry is found, in which case
     * {@code log2(table.length)-1} additional cells are scanned.
     * When called from insertions, this parameter is the number
     * of elements, but when from replaceStaleEntry, it is the
     * table length. (Note: all this could be changed to be either
     * more or less aggressive by weighting n instead of just
     * using straight log n. But this version is simple, fast, and
     * seems to work well.)
     *
     * @return true if any stale entries have been removed.
     */
    private boolean cleanSomeSlots(int i, int n) {
      boolean removed = false;
      Entry[] tab = table;
      int len = tab.length;
      do {
        i = nextIndex(i, len);
        Entry e = tab[i];
        if (e != null && e.get() == null) {
          n = len;
          removed = true;
          i = expungeStaleEntry(i);
        }
      } while ( (n >>>= 1) != 0);
      return removed;
    }

rehash方法

rehash方法：

    /**
     * Re-pack and/or re-size the table. First scan the entire
     * table removing stale entries. If this doesn't sufficiently
     * shrink the size of the table, double the table size.
     */
    private void rehash() {
      expungeStaleEntries();

      // Use lower threshold for doubling to avoid hysteresis
      if (size >= threshold - threshold / 4)
        resize();
    }

expungeStaleEntries方法

expungeStaleEntries方法：

    /**
     * Expunge all stale entries in the table.
     */
    private void expungeStaleEntries() {
      Entry[] tab = table;
      int len = tab.length;
      for (int j = 0; j < len; j++) {
        Entry e = tab[j];
        if (e != null && e.get() == null)
          expungeStaleEntry(j);
      }
    }

resize方法

resize方法：

    /**
     * Double the capacity of the table.
     */
    private void resize() {
      Entry[] oldTab = table;
      int oldLen = oldTab.length;
      int newLen = oldLen * 2;
      Entry[] newTab = new Entry[newLen];
      int count = 0;

      for (Entry e : oldTab) {
        if (e != null) {
          ThreadLocal<?> k = e.get();
          if (k == null) {
            e.value = null; // Help the GC
          } else {
            int h = k.threadLocalHashCode & (newLen - 1);
            while (newTab[h] != null)
              h = nextIndex(h, newLen);
            newTab[h] = e;
            count++;
          }
        }
      }

      setThreshold(newLen);
      size = count;
      table = newTab;
    }

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