java - ThreadLocal.get() 返回 null，即使我之前初始化它

Question

我觉得我可能误解了它应该如何工作。

我有这个代码:

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);
}

据我了解，ThreadLocal.get() 永远不应该返回 null(来自 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 中明确设置了它，而它又调用 ThreadLocal.set，所以应该创建 map :

   /**
 * 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，有时我会在以下位置遇到空指针异常:return getTransactionContext().toString();。其他时候它工作得很好，所以我怀疑存在某种竞争条件，我只是看不出它可能是什么。

PS:Timestamp 类如下所示:

public final class Timestamp {

private final long timeInMilliseconds;
private final long sequenceNumber;

}

但请注意，这是代码的简化版本，不包含多项检查以确保其不为空。 getLatestTimeStamp 值本身是正确的，不会被设置为 null。

Answer 1

正如 @shmosel 所指出的 - 问题是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 并互相干扰。将线程的创建移至变量声明本地可以解决该问题，因为 ThreadLocal 上的后续操作默认情况下是线程安全的。