How do I extract CSS styles for specific elements in an HTML document using a Java CSS parser?

CSS Parser for Java

Requirement

The objective is to obtain CSS styles for specific elements within an HTML document using a Java CSS parser.

Solution

CSSParser

A recommended option is CSSParser, a robust parser that offers error feedback. Below is an example of its usage:

<code class="java">import com.steadystate.css.parser.CSSOMParser;
import org.w3c.css.sac.InputSource;
import org.w3c.dom.css.CSSStyleSheet;
import org.w3c.dom.css.CSSRuleList;
import org.w3c.dom.css.CSSRule;
import org.w3c.dom.css.CSSStyleRule;
import org.w3c.dom.css.CSSStyleDeclaration;
import java.io.*;

public class CSSParserTest {

    public static void main(String[] args) {

        CSSParserTest oParser = new CSSParserTest();

        if (oParser.Parse("design.css")) {

            System.out.println("Parsing completed OK");

        } else {

            System.out.println("Unable to parse CSS");

        }
    }

    public boolean Parse(String cssfile) {

        FileOutputStream out = null;
        PrintStream ps = null;
        boolean rtn = false;

        try {

            InputStream stream = oParser.getClass().getResourceAsStream(cssfile);

            out = new FileOutputStream("log.txt");

            if (out != null) {

                ps = new PrintStream(out);
                System.setErr(ps); //redirects stderr to the log file as well

            } else {

                return rtn;

            }

            InputSource source = new InputSource(new InputStreamReader(stream));
            CSSOMParser parser = new CSSOMParser();
            CSSStyleSheet stylesheet = parser.parseStyleSheet(source, null, null);

            CSSRuleList ruleList = stylesheet.getCssRules();

            ps.println("Number of rules: " + ruleList.getLength());

            for (int i = 0; i < ruleList.getLength(); i++) {
                CSSRule rule = ruleList.item(i);
                if (rule instanceof CSSStyleRule) {
                    CSSStyleRule styleRule = (CSSStyleRule) rule;
                    ps.println("selector:" + i + ": " + styleRule.getSelectorText());
                    CSSStyleDeclaration styleDeclaration = styleRule.getStyle();

                    for (int j = 0; j < styleDeclaration.getLength(); j++) {
                        String property = styleDeclaration.item(j);
                        ps.println("property: " + property);
                        ps.println("value: " + styleDeclaration.getPropertyCSSValue(property).getCssText());
                        ps.println("priority: " + styleDeclaration.getPropertyPriority(property));
                    }
                }
            }

            if (out != null) out.close();
            if (stream != null) stream.close();
            rtn = true;
        } catch (IOException ioe) {
            System.err.println("IO Error: " + ioe);
        } catch (Exception e) {
            System.err.println("Error: " + e);
        } finally {
            if (ps != null) ps.close();
        }

        return rtn;

    }

}</code>

This sample demonstrates how to parse a CSS file and extract style information for each selector and property, including their values and priorities.

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