For reference: https://blog.csdn.net/qq263229365/article/details/9089975
I refer to the author's, maybe because I want to do it with springboot + thmmeleaf, so some codes are different, so I wrote this text, hoping to be useful to you.

Step 1: Guide Package

In springboot, you only need to add the

<dependency>
			<groupId>org.jfree</groupId>
			<artifactId>jfreechart</artifactId>
			<version>1.0.19</version>
</dependency>
<dependency>
			<groupId>org.jfree</groupId>
			<artifactId>jfreechart</artifactId>
			<version>1.0.19</version>
</dependency>

Import thymeleaf and freechart's package

Step 2: add a method to the class with main method in the project

//DisplayChart image generation object and set the URL to be accessed
	@Bean
	public ServletRegistrationBean MyServlet() {
		return new ServletRegistrationBean<>(new DisplayChart(),"/chart");
	}
	@RequestMapping("/")
	public String index() {
		return "index";
	}

Note that @ Controller annotation should be added before the class.

Step 3: create the controller class

This is a class of line graphs

import java.awt.Font;
import java.io.IOException;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;

import org.jfree.chart.ChartFactory;
import org.jfree.chart.JFreeChart;
import org.jfree.chart.axis.CategoryAxis;
import org.jfree.chart.axis.NumberAxis;
import org.jfree.chart.plot.CategoryPlot;
import org.jfree.chart.plot.PlotOrientation;
import org.jfree.chart.servlet.ServletUtilities;
import org.jfree.chart.title.LegendTitle;
import org.jfree.data.category.DefaultCategoryDataset;

@Controller
public class makeLineAndShapeChartController {
	
	@RequestMapping("makeLineAndShapeChart")
	public String makeLineAndShapeChart(HttpServletRequest request, HttpServletResponse response, Model model) throws IOException {
         // Define chart object data, data
		DefaultCategoryDataset  dataset  =createDataset();
		JFreeChart  chart = ChartFactory.createLineChart(
			"Broken line diagram", // chart title
             "time", // domain axis label
             "Sales volume(Million)", // range axis label
             dataset, // data
			PlotOrientation.VERTICAL, // orientation
			true, // include legend
			true, // tooltips
			false // urls
				);
		
		CategoryPlot plot = chart.getCategoryPlot();
		// Set icon font 
		chart.getTitle().setFont(new Font("Song style", Font.BOLD, 22)); 
		//Set font for horizontal axis
		CategoryAxis categoryAxis = plot.getDomainAxis();
		 categoryAxis.setLabelFont(new Font("Song style", Font.BOLD, 22));//x-axis title font 
		 categoryAxis.setTickLabelFont(new Font("Song style", Font.BOLD, 18));//x-axis scale font

		 //The following two lines set the font for the legend
		LegendTitle legend = chart.getLegend(0);
		 legend.setItemFont(new Font("Song style", Font.BOLD, 14));
		 //Set font for vertical axis
			NumberAxis rangeAxis = (NumberAxis) plot.getRangeAxis();
		 rangeAxis.setLabelFont(new Font("Song style" , Font.BOLD , 22)); //Set font for axis
		 rangeAxis.setTickLabelFont(new Font("Song style" , Font.BOLD , 22));
		
			rangeAxis.setStandardTickUnits(NumberAxis.createIntegerTickUnits());//Remove the vertical axis font display incomplete
		 rangeAxis.setAutoRangeIncludesZero(true);
		 rangeAxis.setUpperMargin(0.20);
		 rangeAxis.setLabelAngle(Math.PI / 2.0);
		 
		// 6. Convert graphics to pictures and send them to the front desk
			String fileName = ServletUtilities.saveChartAsJPEG(chart, 700, 400, null, request.getSession());
			String chartURL = request.getContextPath() + "/chart?filename=" + fileName;
			model.addAttribute("makeLineAndShapeChart", chartURL);
			
		 return "makeLineAndShapeChart";
		}

		// Generate data
		public static DefaultCategoryDataset createDataset() {
		  DefaultCategoryDataset linedataset = new DefaultCategoryDataset();

		 // Name of each curve
		  String [] series= {"Refrigerator","Color TV","Washing machine"};
		 // Horizontal axis name (column name)
		  String [] month = {"1 month","2 month","3 month","4 month"};
		 //Specific data
		 double [] num = {4,5,6,10,10,15,20,16,10,18,25,19};,
		 
		 int l1 = num.length/series.length; //4
		 int l2 = month.length;
		 int j=0;
		 for(int i=0;i<num.length;i++) {
			 linedataset.addValue(num[i], series[i/l1], month[j]);	
			 j++;
			 if(j==month.length)
				 j=0;
		 }			 
		 return linedataset;
		}		
}

The following cycle is fixed, but I need to get data from the database. There are three parameter arrays, so I write a cycle, but the output is the same. If a fairy wants to use the database, she can transfer the data to the array.

Output on HTML page

This is the index home page

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
	<div style="text-align: center">
        <p>
		   jfreechart Generate line chart<br/>                            
           <a href="makeLineAndShapeChart">Broken line diagram</a>
        </p>       
	</div>
</body>
</html>

Line chart HTML page

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
<div style="text-align: center">
		jfreechart Broken line diagram <br> <br> <img th:src="${makeLineAndShapeChart}"
			width=600 height=400 border=0 color=gray>
	</div>
</body>
</html>

Because I mainly use line chart, so I don't write anything else.
As it is, I sorted out this project that I wrote, and added common histogram, 3D histogram, multi group histogram, pie chart and line chart. I can also go to this link to download all complete codes.
Link address: https://download.csdn.net/download/yjl23332/11931374