增加图片阻塞队列缓冲，当图片减少被使用时候，生产者自动生产保证10个图片缓存。

package com.example.demo.ctrl;

import com.example.demo.util.VerificationImageGenerator;
import org.apache.commons.io.IOUtils;
import org.apache.log4j.Logger;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;

import javax.annotation.PostConstruct;
import javax.imageio.ImageIO;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.*;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

@RequestMapping("/verify")
@Controller
public class VerifyCodeCtrl {

    private static Logger logger = Logger.getLogger(VerifyCodeCtrl.class);

    private static ArrayBlockingQueue<CustomImage> bufferedImages = new ArrayBlockingQueue<>(10);
    private ExecutorService executorService= Executors.newSingleThreadExecutor();
    @PostConstruct
    private void initBufferedImages() {
        executorService.submit(new Producer());
    }

    private static class Producer implements Runnable{
        @Override
        public void run() {
            product();
        }
        private void product(){
            while (true){
                try {
                    bufferedImages.put(generateBufferedImage());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    @RequestMapping(value = "/code",method = RequestMethod.GET)
    public void verifycode(@RequestParam("uid") String uid, HttpServletResponse response, HttpSession session) {
        BufferedImage image = generateRandCode(uid);
        writeToResponse(response, image);
    }

    private void writeToResponse(HttpServletResponse response, BufferedImage image) {
        response.setContentType("image/jpeg");
        response.setHeader("Pragma", "No-cache");
        response.setHeader("Cache-Control", "no-cache");
        response.setDateHeader("Expires", 240);
        ServletOutputStream out = null;
        try {
            out = response.getOutputStream();
            ImageIO.write(image, "JPEG", out);
        }catch (IOException e) {
            logger.error("[VerifyCodeController.verifycode]: " + e);
        }finally {
            IOUtils.closeQuietly(out);
        }
    }

    private BufferedImage generateRandCode(String uid) {
        CustomImage customImage = bufferedImages.poll();
        if (customImage == null) {
            customImage = generateBufferedImage();
        }
        //save uid and code relation.
        return customImage.bufferedImage;
    }

    private static CustomImage generateBufferedImage() {
        CustomImage customImage = new CustomImage();
        String identifying= randomNumberStr(6);
        customImage.verifyCode = identifying;
        customImage.bufferedImage = new VerificationImageGenerator(new Random()).bufferedImage(identifying);
        return customImage;
    }

    public static String randomNumberStr(int length) {
        java.util.List<Integer> list = new ArrayList<>();
        for (int i = 0; i < length; i++) {
            list.add(new Random().nextInt(10));
        }
        Collections.shuffle(list);
        StringBuilder sb = new StringBuilder();
        list.forEach(sb::append);
        return sb.toString();
    }

    private static class CustomImage {
        public String verifyCode;
        public BufferedImage bufferedImage;
    }
}

图片生成类

import java.awt.*;
import java.awt.image.BufferedImage;
import java.util.Random;

public class VerificationImageGenerator {
    private static final int WIDTH = 70;
    private static final int HEIGHT = 35;
    private static final Color IMG_BACK_1 = new Color(165, 42, 42);
    private static final Color IMG_BACK_2 = new Color(238, 18, 137);
    private static final Color IMG_BACK_3 = new Color(100, 149, 237);
    private static final Color IMG_BACK_4 = new Color(238, 238, 0);
    private static final int FONT_SIZE = 25;

    private final Random random;

    public VerificationImageGenerator(Random random) {
        this.random = random;
    }

    public BufferedImage bufferedImage(String identifying) {
        BufferedImage image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics g = image.getGraphics();
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, WIDTH, HEIGHT);
        g.setFont(new Font("Times New Roman", Font.BOLD, FONT_SIZE));
        for(int i = 0; i < identifying.length(); i++){
            if(i+1==1){
                g.setColor(IMG_BACK_1);
            }else if(i+1==2){
                g.setColor(IMG_BACK_2);
            }else if(i+1==3){
                g.setColor(IMG_BACK_3);
            }else if(i+1==4){
                g.setColor(IMG_BACK_4);
            }
            g.drawString(String.valueOf(identifying.charAt(i)), 13 * i + 6, 25);
        }
        //g.drawOval(0, 12, 60, 11);
        for (int i = 0; i < 100; i++) {
            int x = random.nextInt(WIDTH);
            int y = random.nextInt(HEIGHT);
            g.drawLine(x, y, x, y);
        }
        g.dispose();
        return image;
    }
}

测试环境：
64位，win10系统笔记本
CPU：i5-4210U @1.7GHz 2.39GHz
可用RAM：6G
测试结果：

Time Total:0.996687 sec
Time Fastest:0.048034 sec
Time Slowest:0.915648 sec
Success Num:300
Fail Num:0
Response Size:490446 bytes
Requests per second:300.997083 [#/sec]
temporal distribution:[percent %] [time min sec]~[time max sec]
10% 0.048034~0.338240
20% 0.338240~0.416295
30% 0.416295~0.433307
40% 0.433307~0.463329
50% 0.464330~0.741526
60% 0.741526~0.817580
70% 0.820582~0.880625
80% 0.881625~0.891632
90% 0.891633~0.901639
100% 0.901639~0.915648