Question

作为我之前发布的this question的后续内容，我想知道我遇到问题的原因。

问题是我在使用大量HTML文本更新JLabel时遇到此错误。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at javax.swing.text.html.StyleSheet$ListPainter.paint(Unknown Source)
    at javax.swing.text.html.ListView.paintChild(Unknown Source)
    at javax.swing.text.BoxView.paint(Unknown Source)
    at javax.swing.text.html.BlockView.paint(Unknown Source)
    at javax.swing.text.html.ListView.paint(Unknown Source)
    at javax.swing.text.BoxView.paintChild(Unknown Source)
    at javax.swing.text.BoxView.paint(Unknown Source)
    at javax.swing.text.html.BlockView.paint(Unknown Source)
    at javax.swing.text.BoxView.paintChild(Unknown Source)
    at javax.swing.text.BoxView.paint(Unknown Source)
    at javax.swing.text.html.BlockView.paint(Unknown Source)
    at javax.swing.text.BoxView.paintChild(Unknown Source)
    at javax.swing.text.BoxView.paint(Unknown Source)
    at javax.swing.text.html.BlockView.paint(Unknown Source)
    at javax.swing.plaf.basic.BasicHTML$Renderer.paint(Unknown Source)
    at javax.swing.plaf.basic.BasicLabelUI.paint(Unknown Source)
    at javax.swing.plaf.ComponentUI.update(Unknown Source)
    at javax.swing.JComponent.paintComponent(Unknown Source)
    at javax.swing.JComponent.paint(Unknown Source)
    at javax.swing.JComponent.paintToOffscreen(Unknown Source)
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source)
    at javax.swing.RepaintManager$PaintManager.paint(Unknown Source)
    at javax.swing.RepaintManager.paint(Unknown Source)
    at javax.swing.JComponent._paintImmediately(Unknown Source)
    at javax.swing.JComponent.paintImmediately(Unknown Source)
    at javax.swing.RepaintManager$4.run(Unknown Source)
    at javax.swing.RepaintManager$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.access$1200(Unknown Source)
    at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$500(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

正在设置的HTML由StringBuilder和转换后的.toString生成。这样的事情，但更广泛：

public static String entityOverviewHTML() {
    StringBuilder sb = new StringBuilder();
    List<Entity> entities = Entity.getEntities();

    sb.append("<html><div style='padding: 12px;'>");

    sb.append("<h1>People: alive</h1>");

    // A lot of appending: also loop through getEntities (dynamic, can change)
    // and get contents from each entity in #getEntityInfo below
    if (entities.size() > 0) {
        sb.append("<ul>");
        for (Entity e: entities) {
            getEntityInfo(e);
        }
        sb.append("</ul>");
    }

    sb.append("</div></html>");
    return sb.toString();
}

private static StringBuilder getEntityInfo(Entity e) {
    StringBuilder sbInfo = new StringBuilder();
    // A lot of appending: append info of Entity e such as e.getFirstName()
    sbInfo.append("<li>");
    sbInfo.append(e.getFirstName())
    sbInfo.append("</li>");

    return sbInfo;
}

在某些事件发生后，HTML会发生变化，之后我会调用自定义刷新方法：

public static void bringToFront() {
    getInstance().setVisible(true);
    getInstance().setExtendedState(JFrame.ICONIFIED);
    getInstance().setExtendedState(JFrame.NORMAL);
}

public static void refresh() {
    // text is a JLabel
    text.setText(entityOverviewHTML());
    bringToFront();
}

然后就是这篇帖子顶部的错误发生了，但并不总是！我还没弄清楚为什么会发生这种情况，但我确实发现在将文本重置为空字符串然后调用entityOverviewHTML时会解决问题。

public static void refresh() {
    text.setText(""); // Here we go
    text.setText(entityOverviewHTML());
    bringToFront();
}

文本被定义为类变量：

private static JLabel text = new JLabel();

我想知道的是：为什么？这一系列看似过时的代码如何解决这个问题呢？究竟问题是什么？

Answer 1

问题是你的refresh()方法没有在Swing EDT上调用。

Swing不是线程安全的（https://docs.oracle.com/javase/tutorial/uiswing/concurrency/dispatch.html），这意味着调用text.setText(entityOverviewHTML());会破坏JLabel实例用来显示文本的内部数据结构。

你的刷新方法需要像这样重写：

public static void refresh() {
    try {
        SwingUtilities.invokeAndWait(new Runnable() {
            public void run() {
                text.setText(entityOverviewHTML());
                bringToFront();
            }
        });
    } catch (InvocationTargetException | InterruptedException e) {
        e.printStackTrace();
    }
}

Answer 2

通常在实例化JLabel时，它没有宽度，如果用空文本宽度为零实例化它。当JLabel文本更新时，它将不会显示，因为它的大小设置不正确。

使用首选尺寸：text.setPreferredSize(new Dimension(x, y));然后text.setText(html)

为什么我需要在JLabel中重置setText（）以防止错误？

2 个答案: