一文搞定 Java 后端给前端返回图片流的方法

2023-06-19 00:37:59

Java 后端向前端传输图片流的全面指南

简介

在后端开发中，经常需要向前端发送图片，以便在网页或移动应用程序中显示或处理。Java 提供了多种方式来实现此目的，本文将深入探讨这些方法，并提供示例代码和最佳实践。

1. 直接输出图片流

ServletOutputStream

ServletOutputStream 是一个 Java Servlet API 类，允许直接向 HTTP 响应中写入原始字节。我们可以使用它来将图片字节数组直接输出到客户端浏览器。

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

import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;

public class ImageServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String imagePath = req.getParameter("imagePath");

        byte[] imageBytes = getImageBytes(imagePath);

        resp.setContentType("image/jpeg");
        resp.setContentLength(imageBytes.length);

        OutputStream os = resp.getOutputStream();
        os.write(imageBytes);
        os.flush();
        os.close();
    }

    private byte[] getImageBytes(String imagePath) throws IOException {
        FileInputStream fis = new FileInputStream(imagePath);
        byte[] imageBytes = fis.readAllBytes();
        fis.close();
        return imageBytes;
    }
}

ResponseWriter

另一个输出图片流的方法是使用 ResponseWriter。它允许向 HTTP 响应中写入文本或二进制数据。

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

import java.io.FileInputStream;
import java.io.IOException;
import java.io.PrintWriter;

public class ImageServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String imagePath = req.getParameter("imagePath");

        byte[] imageBytes = getImageBytes(imagePath);

        resp.setContentType("image/jpeg");
        resp.setContentLength(imageBytes.length);

        PrintWriter writer = resp.getWriter();
        writer.write(imageBytes);
        writer.flush();
        writer.close();
    }

    private byte[] getImageBytes(String imagePath) throws IOException {
        FileInputStream fis = new FileInputStream(imagePath);
        byte[] imageBytes = fis.readAllBytes();
        fis.close();
        return imageBytes;
    }
}

2. 使用第三方库

Apache Commons IO

Apache Commons IO 是一个流行的 Java 库，提供了 IOUtils 类，可以简化输入和输出操作。它包含一个 toByteArray() 方法，可以将文件转换为字节数组。

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

import org.apache.commons.io.IOUtils;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;

public class ImageServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String imagePath = req.getParameter("imagePath");

        FileInputStream fis = new FileInputStream(imagePath);
        byte[] imageBytes = IOUtils.toByteArray(fis);
        fis.close();

        resp.setContentType("image/jpeg");
        resp.setContentLength(imageBytes.length);

        OutputStream os = resp.getOutputStream();
        os.write(imageBytes);
        os.flush();
        os.close();
    }
}

3. 使用 Spring MVC 框架

Spring MVC 是一个 Java Web 框架，提供了便捷的方法来处理 HTTP 请求和响应。它可以通过 @ResponseBody 注解将方法返回的值直接写入 HTTP 响应。

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

import javax.imageio.ImageIO;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

@Controller
@RequestMapping("/image")
public class ImageController {

    @RequestMapping("/get")
    @ResponseBody
    public byte[] getImage() throws IOException {
        BufferedImage image = ImageIO.read(new File("image.png"));
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ImageIO.write(image, "png", baos);
        byte[] imageBytes = baos.toByteArray();
        return imageBytes;
    }
}