Question

我使用JEditorPane作为＆＃34;橡皮图章＆＃34;将HTML文本呈现为PDF。我需要文本以特定的宽度换行，并且需要应用白色＆＃34;突出显示＆＃34;在文本背后。因此，我在PDF渲染线程中创建一个JEditorPane，设置文本和样式表，然后将其绘制为PDF图形。但是，我在HTML编辑器工具包的内部深处发现了间歇性的NullPointerException。这可以通过此SSCCE重现：

import javax.swing.*;
import javax.swing.text.View;
import javax.swing.text.html.HTMLDocument;
import java.awt.*;
import java.awt.image.BufferedImage;

/**
 * @author sbarnum
 */
public class TextMarkerUtilsTest {
    public static void main(String[] args) throws Exception {
        Rectangle bounds = new Rectangle(255, 255);
        BufferedImage image = new BufferedImage(bounds.width, bounds.height, BufferedImage.TYPE_INT_RGB);
        Graphics2D d = image.createGraphics();
        d.setClip(bounds);
        for (int i=0; i<1000; i++) {
            JEditorPane renderHelper = new JEditorPane("text/html", "<html><body>This is my text.</body></html>");
            HTMLDocument document = (HTMLDocument) renderHelper.getDocument();
            document.getStyleSheet().addRule("foo{color:black;}");
            View rootView = renderHelper.getUI().getRootView(renderHelper);
            rootView.paint(d, bounds);
        }
    }

}

运行上面的命令会抛出以下异常，通常只需循环几次：

java.lang.NullPointerException
    at sun.font.FontDesignMetrics$MetricsKey.init(FontDesignMetrics.java:199)
    at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:267)
    at sun.swing.SwingUtilities2.getFontMetrics(SwingUtilities2.java:949)
    at javax.swing.JComponent.getFontMetrics(JComponent.java:1599)
    at javax.swing.text.LabelView.getFontMetrics(LabelView.java:154)
    at javax.swing.text.html.InlineView.calculateLongestWordSpanUseWhitespace(InlineView.java:246)
    at javax.swing.text.html.InlineView.calculateLongestWordSpan(InlineView.java:191)
    at javax.swing.text.html.InlineView.getLongestWordSpan(InlineView.java:177)
    at javax.swing.text.html.ParagraphView.calculateMinorAxisRequirements(ParagraphView.java:140)
    at javax.swing.text.BoxView.checkRequests(BoxView.java:918)
    at javax.swing.text.BoxView.getMinimumSpan(BoxView.java:551)
    at javax.swing.text.html.ParagraphView.getMinimumSpan(ParagraphView.java:261)
    at javax.swing.text.BoxView.calculateMinorAxisRequirements(BoxView.java:886)
    at javax.swing.text.html.BlockView.calculateMinorAxisRequirements(BlockView.java:129)
    at javax.swing.text.BoxView.checkRequests(BoxView.java:918)
    at javax.swing.text.BoxView.getMinimumSpan(BoxView.java:551)
    at javax.swing.text.html.BlockView.getMinimumSpan(BlockView.java:361)
    at javax.swing.text.BoxView.calculateMinorAxisRequirements(BoxView.java:886)
    at javax.swing.text.html.BlockView.calculateMinorAxisRequirements(BlockView.java:129)
    at javax.swing.text.BoxView.checkRequests(BoxView.java:918)
    at javax.swing.text.BoxView.setSpanOnAxis(BoxView.java:326)
    at javax.swing.text.BoxView.layout(BoxView.java:691)
    at javax.swing.text.BoxView.setSize(BoxView.java:380)
    at javax.swing.plaf.basic.BasicTextUI$RootView.setSize(BasicTextUI.java:1703)
    at javax.swing.plaf.basic.BasicTextUI$RootView.paint(BasicTextUI.java:1422)
    at com.prosc.msi.model.editor.TextMarkerUtilsTest$1.run(TextMarkerUtilsTest.java:40)
    at java.lang.Thread.run(Thread.java:680)

一些有趣的发现：

如果以上内容在Event Dispatch Thread中运行，则可以正常运行
如果我取消对addRule("foo{color:black;}")的调用，它就可以了（我需要指定规则，但它似乎并不重要，如果添加了任何规则，它就会失败）

问题出在javax.swing.text.GlyphPainter1.sync()，其中javax.swing.text.GlyphView.getFont()返回null。通过设置条件断点，我发现GlyphView在这种情况下是javax.swing.text.html.InlineView。在断点停止后调用getFont()会返回一个非空字体，因此没有及时初始化某些内容。

我意识到swing组件不是线程安全的，但是我不能在后台线程中实例化JEditorPane并在后台线程中安全地操作它，只要只有一个线程正在制作调用组件？

Answer 1

由于您只使用轻量级组件，headless mode可能是一种选择。您可以使用ProcessBuilder使用here来完成GUI的EDT工作。

public static void main(String[] args) throws Exception {
    System.setProperty("java.awt.headless", "true"); 
    EventQueue.invokeLater(new Runnable() {

        @Override
        public void run() {
            Rectangle bounds = new Rectangle(255, 255);
            BufferedImage image = new BufferedImage(
                bounds.width, bounds.height, BufferedImage.TYPE_INT_RGB);
            Graphics2D d = image.createGraphics();
            d.setClip(bounds);
            for (int i = 0; i < 1000; i++) {
                JEditorPane renderHelper = new JEditorPane(
                    "text/html", "<html><body>This is my text.</body></html>");
                HTMLDocument document = (HTMLDocument) renderHelper.getDocument();
                document.getStyleSheet().addRule("foo{color:black;}");
                View rootView = renderHelper.getUI().getRootView(renderHelper);
                rootView.paint(d, bounds);
            }
        }
    });
}

Answer 2

感谢Marko建议查找Event Dispatch Thread的回调，我最终在HTMLDocument.styleChanged()找到了一个。我的子类：

public class ThreadFriendlyHTMLDocument extends HTMLDocument {
    @Override
    protected void styleChanged(final Style style) {
        // to fix GlyphPainter1.sync NullPointerException, we call this in the current thread, instead of the EDT
        DefaultDocumentEvent dde = new DefaultDocumentEvent(0,
                          this.getLength(),
                          DocumentEvent.EventType.CHANGE);
        dde.end();
        fireChangedUpdate(dde);
    }
}

从非EventDispatchThread使用JEditorPane

2 个答案: