ThreadLocal.get()返回null，即使在前面初始化它时也是如此

Question

我觉得我可能误解了这件事的运作方式。

我有这样的代码：

public Timestamp startTransaction() {
    cleanupTransactionContext();
    Timestamp timestamp = getLatestTimestamp();
    initThreadLocalVariables(timestamp);
    return getTransactionContext().toString();
}

private Timestamp getTransactionContext() {
    if (transactionContext == null) {
        throw new BadTransactionStateException();
    }
    return transactionContext.get();
}

private void initThreadLocalVariables(Timestamp timestamp) {
    setTransactionContext(timestamp);
}

private void setTransactionContext(Timestamp ts) {
    if (this.transactionContext == null) {
        this.transactionContext = new ThreadLocal<>();
    }
    this.transactionContext.set(ts);
}

我的理解是，ThreadLocal.get()永远不应该返回null (从JDK)：

/**
 * 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() {
    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();
}

因为我之前已经在setTransactionContext中显式地设置了它，它依次调用ThreadLocal.set，它应该创建映射：

   /**
 * 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();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

但是，有时我会在：return getTransactionContext().toString();中获得空指针异常。其他时候，它工作得很好，所以我怀疑某种比赛条件，我只是不知道它会是什么样子。

PS:时间戳类如下所示：

public final class Timestamp {

private final long timeInMilliseconds;
private final long sequenceNumber;

}

但是请注意，这是一个简化的代码版本，它不包括几个检查，以确保这不是null。getLatestTimeStamp值本身是正确的，不会设置为null。

Answer 1

正如@shmosel所指出的，问题在于这段代码不是原子的：

private void setTransactionContext(Timestamp ts) {
  if (this.transactionContext == null) {
    this.transactionContext = new ThreadLocal<>();
  }
  this.transactionContext.set(ts);
}

因此，两个线程可能正在创建ThreadLocal并相互干扰。将线程的创建移动到变量声明中可以解决这个问题，因为ThreadLocal上的后续操作默认是线程安全的。