Dynamic Template Injection & In-Memory AST Mutation (JasperDesign API)

package com.enterprise.report.dynamic;

import net.sf.jasperreports.engine.*;
import net.sf.jasperreports.engine.design.*;
import net.sf.jasperreports.engine.type.HorizontalTextAlignEnum;
import net.sf.jasperreports.engine.type.VerticalTextAlignEnum;
import org.springframework.stereotype.Service;

import java.io.InputStream;
import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;

@Service
public class DynamicReportTemplateService {

    // Thread-safe Cache ป้องกันการคอมไพล์ซ้ำซ้อนและป้องกัน Metaspace OOM
    private final Map<String, JasperReport> reportCache = new ConcurrentHashMap<>();

    public record ColumnMetadata(String fieldName, String headerTitle, Class<?> dataType, int width, String pattern) {}

    public JasperReport getOrCreateDynamicReport(List<ColumnMetadata> selectedColumns) throws Exception {
        // 1. สร้าง Fingerprint Hash จากโครงสร้างคอลัมน์ที่ถูกเลือก
        String schemaFingerprint = generateFingerprint(selectedColumns);

        // 2. ดึงจาก Cache ทันทีหากเคยคอมไพล์โครงสร้างนี้ไว้แล้ว (O(1) Access)
        return reportCache.computeIfAbsent(schemaFingerprint, key -> {
            try {
                return buildAndCompileDynamicAST(selectedColumns);
            } catch (Exception e) {
                throw new RuntimeException("Failed to construct dynamic JasperDesign AST", e);
            }
        });
    }

    private JasperReport buildAndCompileDynamicAST(List<ColumnMetadata> columns) throws Exception {
        // 3. โหลด Base Template AST
        InputStream baseStream = getClass().getResourceAsStream("/reports/base_template.jrxml");
        JasperDesign jasperDesign = net.sf.jasperreports.engine.xml.JRXmlLoader.load(baseStream);

        JRDesignBand columnHeaderBand = (JRDesignBand) jasperDesign.getColumnHeader();
        JRDesignBand detailBand = (JRDesignBand) jasperDesign.getDetailSection().getBands()[0];

        int currentX = 0;
        int defaultHeight = 20;

        for (ColumnMetadata col : columns) {
            // 4. ฉีด Field เข้าสู่ Data Model AST
            JRDesignField field = new JRDesignField();
            field.setName(col.fieldName());
            field.setValueClass(col.dataType());
            jasperDesign.addField(field);

            // 5. ฉีด Static Header Text ลงใน ColumnHeader Band
            JRDesignStaticText headerText = new JRDesignStaticText();
            headerText.setX(currentX);
            headerText.setY(0);
            headerText.setWidth(col.width());
            headerText.setHeight(defaultHeight);
            headerText.setText(col.headerTitle());
            headerText.setVerticalTextAlign(VerticalTextAlignEnum.MIDDLE);
            headerText.getFont().setBold(true);
            headerText.getFont().setFontName("Sarabun");
            columnHeaderBand.addElement(headerText);

            // 6. ฉีด Dynamic TextField ลงใน Detail Band
            JRDesignTextField detailField = new JRDesignTextField();
            detailField.setX(currentX);
            detailField.setY(0);
            detailField.setWidth(col.width());
            detailField.setHeight(defaultHeight);
            detailField.setVerticalTextAlign(VerticalTextAlignEnum.MIDDLE);
            detailField.getFont().setFontName("Sarabun");
            
            if (col.pattern() != null) {
                detailField.setPattern(col.pattern());
                detailField.setHorizontalTextAlign(HorizontalTextAlignEnum.RIGHT);
            }

            JRDesignExpression expression = new JRDesignExpression();
            expression.setText("$F{" + col.fieldName() + "}");
            detailField.setExpression(expression);
            detailBand.addElement(detailField);

            // คำนวณพิกัด Snap แนบสนิท ขจัด Merged Cell 100%
            currentX += col.width();
        }

        // 7. คอมไพล์ AST เพียงครั้งเดียวเพื่อส่งเก็บใน Cache
        return JasperCompileManager.compileReport(jasperDesign);
    }

    private String generateFingerprint(List<ColumnMetadata> columns) throws Exception {
        StringBuilder sb = new StringBuilder();
        for (ColumnMetadata col : columns) {
            sb.append(col.fieldName()).append(":").append(col.dataType().getName()).append(":").append(col.width()).append(";");
        }
        MessageDigest digest = MessageDigest.getInstance("SHA-256");
        byte[] hash = digest.digest(sb.toString().getBytes(StandardCharsets.UTF_8));
        return HexFormat.of().formatHex(hash);
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<jasperReport xmlns="http://jasperreports.sourceforge.net/jasperreports"
              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports http://jasperreports.sourceforge.net/xsd/jasperreport.xsd"
              name="BaseDynamicScaffold"
              pageWidth="595" pageHeight="842"
              columnWidth="555" leftMargin="20" rightMargin="20" topMargin="20" bottomMargin="20"
              uuid="a1b2c3d4-e5f6-7890-abcd-ef0123456789">

    <parameter name="REPORT_TITLE" class="java.lang.String"/>

    <title height="50">
        <element kind="staticText" x="0" y="10" width="555" height="30">
            <textElement textAlignment="Center" verticalAlignment="Middle">
                <font fontName="Sarabun" size="16" isBold="true"/>
            </textElement>
            <text><![CDATA[รายงานจัดสรรสินทรัพย์แบบกำหนดโครงสร้างคอลัมน์อัตโนมัติ]]></text>
        </element>
    </title>

    <!-- Bands will be dynamically populated by JasperDesign API at runtime -->
    <columnHeader height="20"/>

    <detail>
        <band height="20"/>
    </detail>

    <pageFooter height="20">
        <element kind="textField" x="0" y="0" width="555" height="20">
            <textElement textAlignment="Center"><font size="8"/></textElement>
            <expression><![CDATA["หน้า " + $V{PAGE_NUMBER} + " (โครงสร้าง Dynamic AST Active)"]]></expression>
        </element>
    </pageFooter>
</jasperReport>