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());  }}

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

  /**   * 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;  }

  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;  }

  /**   * 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);  }

    /**     * 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;    }

    /**     * 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);    }

    /**     * 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;    }

    /**     * 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;    }

  /**   * 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);    }  }

    /**     * 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();    }

    /**     * 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);    }

    /**     * 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;    }

    /**     * 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();    }

    /**     * 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);      }    }

    /**     * 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;    }