Java中ThreadLocal的一些理解

private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

public static synchronized String formatDate(Date date){
  return simpleDateFormat.format(date);
}

private static final ThreadLocal<SimpleDateFormat> simpleDateFormatThreadLocal = ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));

public static String formatDate(Date date){
  return simpleDateFormatThreadLocal.get().format(date);
}

ThreadLocal<Integer> threadLocal99 = new ThreadLocal<Integer>();
threadLocal99.set(3);
int num = threadLocal99.get();
System.out.println("数字:"+num);
threadLocal99.remove();
System.out.println("数字Empty:"+threadLocal99.get());

public void set(T value) {
  // 获取当前线程
  Thread t = Thread.currentThread();
  // 获取ThreadLocalMap
  ThreadLocal.ThreadLocalMap map = getMap(t);
  // ThreadLocalMap 对象是否为空，不为空则直接将数据放入到ThreadLocalMap中
  if (map != null)
    map.set(this, value);
  else
    createMap(t, value); // ThreadLocalMap对象为空，则先创建对象，再赋值。
}

ThreadLocalMap getMap(Thread t) {
  return t.threadLocals;
}

public class Thread implements Runnable {
	... ...
	/* ThreadLocal values pertaining to this thread. This map is maintained
   * by the ThreadLocal class. */
  ThreadLocal.ThreadLocalMap threadLocals = null;
  ... ...
}

/**
 * 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;
  // 定位在数组中的位置
  int i = key.threadLocalHashCode & (len-1);

  for (Entry e = tab[i];
     e != null;
     e = tab[i = nextIndex(i, len)]) {
    ThreadLocal<?> k = e.get();
    // 如果当前位置不为空，并且当前位置的key和传过来的key相等，那么就会覆盖当前位置的数据
    if (k == key) {
      e.value = value;
      return;
    }
    // 如果当前位置为空，则初始化一个Entry对象，放到当前位置。
    if (k == null) {
      replaceStaleEntry(key, value, i);
      return;
    }
  }
  // 如果当前位置不为空，并且当前位置的key也不等于要赋值的key ，那么将去找下一个空位置，直接将数据放到下一个空位置处。
  tab[i] = new Entry(key, value);
  int sz = ++size;
  if (!cleanSomeSlots(i, sz) && sz >= threshold)
    rehash();
}

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);
}

/**
 * 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;

  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;
}

 ThreadLocal threadLocalMain = new InheritableThreadLocal();
 threadLocalMain.set("主线程变量");
 Thread t = new Thread() {
   @Override
   public void run() {
     super.run();
     System.out.println( "现在获取的变量是 =" + threadLocalMain.get());
   }
 };
 t.start();

if (inheritThreadLocals && parent.inheritableThreadLocals != null)
      this.inheritableThreadLocals =ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);