作为我先前发布的this question的后续活动,我想知道引起该问题的原因。
问题是,使用大量HTML文本更新JLabel时出现此错误。
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)
设置的HTML由StringBuilder和转换后的
.toString
生成。这样的东西,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;
}
在发生一些事件之后,HTML将会更改,之后我将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();
}
然后是本文顶部的错误发生了,但是并不总是这样!我还没有弄清楚为什么会发生这种情况,但是我确实发现,将文本重置为空字符串然后调用entityOverviewHTML可以解决此问题。
public static void refresh() {
text.setText(""); // Here we go
text.setText(entityOverviewHTML());
bringToFront();
}
文本被定义为Class变量:
private static JLabel text = new JLabel();
我想知道的是:为什么?这行看似过时的代码如何解决该问题?到底是什么问题?
最佳答案
问题是您的refresh()
方法未在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());
破坏了JLabel实例用来显示文本的内部数据结构。
您的刷新方法需要这样重写:
public static void refresh() {
try {
SwingUtilities.invokeAndWait(new Runnable() {
public void run() {
text.setText(entityOverviewHTML());
bringToFront();
}
});
} catch (InvocationTargetException | InterruptedException e) {
e.printStackTrace();
}
}