java - 当我尝试在线程内访问静态方法时，它会抛出 NullPointerException

Question

这里我有一个线程

@Component("sessionStateListenerThread")
public class SessionStateListenerThread extends Thread {

    private static final String DEFAULT_RESULT = "expired";
    private List<DeferredResult<String>> subscribed = Collections
            .synchronizedList(new ArrayList<DeferredResult<String>>());

    /**
     * Main thread loop for checking session state.
     */
    @Override
    public void run() {
        while (true) {
            synchronized (subscribed) {
                try {
                    if (null == SecurityHelper.getUserLogin()) {
                        Iterator<DeferredResult<String>> it = subscribed.iterator();
                        while (it.hasNext()) {
                            DeferredResult<String> result = it.next();
                            result.setResult(DEFAULT_RESULT);
                            it.remove();
                        }
                    }
                } catch (NullPointerException e) {
                }
            }
        }
    }

    /**
     * Adds client to listen session state.
     *
     * @param client
     */
    public void addClient(DeferredResult<String> client) {
        synchronized (subscribed) {
            subscribed.add(client);
        }
    };
}

该类用于包装该线程

@Component
@DependsOn("sessionStateListenerThread")
public class SessionStateListener {

    @Autowired
    private SessionStateListenerThread thread;

    @PostConstruct
    public void configThread() {
        thread.setDaemon(true);
        thread.setName("SessionStateChecker");
    }

    /**
     * Starts listener.
     */
    public void startListener() {
        thread.start();
    }

    /**
     * Adds client to write result.
     *
     * @param client object to write result
     */
    public void addSessionStateListener(DeferredResult<String> client) {
        thread.addClient(client);
    }
}

我将方法 startListener 放在 Controller 方法中。同样在 Controller 中，我访问方法 getUserLogin。

String login = SecurityHelper.getUserLogin(); // successfully access it

listener.startListener();

线程中的

方法run也想访问此方法getUserLogin。但是，尽管 login 值不为 null - 调用 SecurityHelper.getUserLogin() 会在线程内抛出 NullPointerException。

有什么问题吗？ getUserLogin 看起来像这样

return SecurityContextHolder.getContext().getAuthentication().getName();

Stacktrace 有点小 (e.printstacktrace())

java.lang.NullPointerException
        at com.ui.web.security.SecurityHelper.getUserLogin(SecurityHelper.java:45)
        at com.ui.web.controller.rest.poll.SessionStateListenerThread.run(SessionState
ListenerThread.java:41)

Answer 1

这是因为您无法将当前 session 数据(上下文)获取到当前请求线程之外的单独线程中。

您将能够将任何 session 数据获取到任何 Controller 方法中，因为任何 Controller 方法都将在请求线程内执行。