document对象中哪个方法可以让输出字段自带下划线

在Java中生成PDF并让输出字段自带下划线，可以使用Apache PDFBox这个开源的Java库。具体实现方法如下：

1. 导入PDFBox库，添加依赖：

<dependency>
    <groupId>org.apache.pdfbox</groupId>
    <artifactId>pdfbox</artifactId>
    <version>2.0.23</version>
</dependency>

2. 创建一个PDF文档对象：

PDDocument doc = new PDDocument();

3. 创建一个页面对象：

PDPage page = new PDPage();
doc.addPage(page);

4. 创建一个表单对象：

PDAcroForm form = new PDAcroForm(doc);
doc.getDocumentCatalog().setAcroForm(form);

5. 设置字体和字号：

PDFont font = PDType1Font.HELVETICA;
int fontSize = 12;

6. 设置下划线样式：

PDAppearanceContentStream cs = new PDAppearanceContentStream(form, field.getWidgets().get(0).getNormalAppearance());
cs.setStrokingColor(Color.BLACK);
cs.setLineWidth(0.5f);

7. 创建一个文本字段对象：

PDTextField field = new PDTextField(form);
field.setPartialName("myText");
form.getFields().add(field);

8. 创建一个字段属性对象：

PDTextFieldAdditionalActions actions = new PDTextFieldAdditionalActions();
field.setActions(actions);

9. 将字段属性对象和下划线样式添加到字段属性对象中：

PDAppearanceDictionary appearanceDictionary = new PDAppearanceDictionary();
PDAppearanceStream appearanceStream = new PDAppearanceStream(doc);
appearanceStream.setBBox(field.getWidgets().get(0).getRectangle());
appearanceDictionary.setNormalAppearance(appearanceStream);
actions.setD(appearanceDictionary);

10. 输出带下划线的字符串：

String text = "带下划线的字符串";
field.setValue(text);
field.setDefaultAppearance(String.format("/%s %d Tf 0 g", font.getName(), fontSize));
cs.beginText();
cs.setFont(font, fontSize);
cs.newLineAtOffset(0, 0);
cs.showText(text);
cs.drawLine(0, -fontSize / 4, font.getStringWidth(text) / 1000 * fontSize, -fontSize / 4);
cs.endText();
cs.close();

11. 将字段添加到页面中：

PDAnnotationWidget widget = field.getWidgets().get(0);
widget.setPage(page);
page.getAnnotations().add(widget);

12. 关闭文档对象：

doc.close();

完整的代码示例：

import java.awt.Color;
import java.io.IOException;

import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.pdmodel.PDPage;
import org.apache.pdfbox.pdmodel.PDPageContentStream;
import org.apache.pdfbox.pdmodel.font.PDFont;
import org.apache.pdfbox.pdmodel.font.PDType1Font;
import org.apache.pdfbox.pdmodel.interactive.annotation.PDAnnotationWidget;
import org.apache.pdfbox.pdmodel.interactive.annotation.PDAppearanceDictionary;
import org.apache.pdfbox.pdmodel.interactive.annotation.PDAppearanceStream;
import org.apache.pdfbox.pdmodel.interactive.annotation.PDTextField;
import org.apache.pdfbox.pdmodel.interactive.annotation.PDTextFieldAdditionalActions;
import org.apache.pdfbox.pdmodel.interactive.form.PDAcroForm;
import org.apache.pdfbox.pdmodel.interactive.form.PDAppearanceContentStream;

public class PDFWithUnderline {
    public static void main(String[] args) throws IOException {
        PDDocument doc = new PDDocument();
        PDPage page = new PDPage();
        doc.addPage(page);
        PDAcroForm form = new PDAcroForm(doc);
        doc.getDocumentCatalog().setAcroForm(form);
        PDFont font = PDType1Font.HELVETICA;
        int fontSize = 12;
        PDTextField field = new PDTextField(form);
        field.setPartialName("myText");
        form.getFields().add(field);
        PDAppearanceDictionary appearanceDictionary = new PDAppearanceDictionary();
        PDAppearanceStream appearanceStream = new PDAppearanceStream(doc);
        appearanceStream.setBBox(field.getWidgets().get(0).getRectangle());
        appearanceDictionary.setNormalAppearance(appearanceStream);
        PDTextFieldAdditionalActions actions = new PDTextFieldAdditionalActions();
        actions.setD(appearanceDictionary);
        field.setActions(actions);
        PDAnnotationWidget widget = field.getWidgets().get(0);
        widget.setPage(page);
        page.getAnnotations().add(widget);
        PDAppearanceContentStream cs = new PDAppearanceContentStream(form, field.getWidgets().get(0).getNormalAppearance());
        cs.setStrokingColor(Color.BLACK);
        cs.setLineWidth(0.5f);
        String text = "带下划线的字符串";
        field.setValue(text);
        field.setDefaultAppearance(String.format("/%s %d Tf 0 g", font.getName(), fontSize));
        cs.beginText();
        cs.setFont(font, fontSize);
        cs.newLineAtOffset(0, 0);
        cs.showText(text);
        cs.drawLine(0, -fontSize / 4, font.getStringWidth(text) / 1000 * fontSize, -fontSize / 4);
        cs.endText();
        cs.close();
        doc.save("pdf_with_underline.pdf");
        doc.close();
    }
}

运行代码后，会生成一个名为pdf_with_underline.pdf的PDF文件，其中包含了一个带下划线的文本字段。