如何从 Jersey REST 服务方法返回 PNG 图像到浏览器

Question

我有一个运行 Jersey REST 资源的 Web 服务器，我想知道如何获取浏览器 img 标签的图像/png 参考；在提交表单或获得 Ajax 响应之后。添加图形的图像处理代码正在工作，只需要以某种方式返回即可。

代码：

@POST
@Path("{fullsize}")
@Consumes(MediaType.MULTIPART_FORM_DATA)
@Produces("image/png")
// Would need to replace void
public void getFullImage(@FormDataParam("photo") InputStream imageIS,
                         @FormDataParam("submit") String extra) {

      BufferedImage image = ImageIO.read(imageIS);

      // .... image processing
      //.... image processing

      return ImageIO.  ..  ?

}

干杯

Answer 1

我不认为在 REST 服务中返回图像数据是个好主意。它占用了应用服务器的内存和 IO 带宽。最好将该任务委托给针对这种传输进行了优化的适当 Web 服务器。您可以通过向图像资源发送重定向来完成此操作（作为带有图像 URI 的 HTTP 302 响应）。这当然假设您的图像被安排为网络内容。

话虽如此，如果您决定确实需要从 Web 服务传输图像数据，您可以使用以下（伪）代码来执行此操作：

@Path("/whatever")
@Produces("image/png")
public Response getFullImage(...) {

    BufferedImage image = ...;

    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ImageIO.write(image, "png", baos);
    byte[] imageData = baos.toByteArray();

    // uncomment line below to send non-streamed
    // return Response.ok(imageData).build();

    // uncomment line below to send streamed
    // return Response.ok(new ByteArrayInputStream(imageData)).build();
}

添加异常处理等

Answer 2

我为此构建了一个具有以下功能的通用方法：

• 如果文件没有在本地修改，则返回“未修改”，Status.NOT_MODIFIED 被发送给调用者。使用Apache Commons Lang
• 使用文件流对象而不是读取文件本身

代码如下：

import org.apache.commons.lang3.time.DateUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

private static final Logger logger = LoggerFactory.getLogger(Utils.class);

@GET
@Path("16x16")
@Produces("image/png")
public Response get16x16PNG(@HeaderParam("If-Modified-Since") String modified) {
    File repositoryFile = new File("c:/temp/myfile.png");
    return returnFile(repositoryFile, modified);
}

/**
 * 
 * Sends the file if modified and "not modified" if not modified
 * future work may put each file with a unique id in a separate folder in tomcat
 *   * use that static URL for each file
 *   * if file is modified, URL of file changes
 *   * -> client always fetches correct file 
 * 
 *     method header for calling method public Response getXY(@HeaderParam("If-Modified-Since") String modified) {
 * 
 * @param file to send
 * @param modified - HeaderField "If-Modified-Since" - may be "null"
 * @return Response to be sent to the client
 */
public static Response returnFile(File file, String modified) {
    if (!file.exists()) {
        return Response.status(Status.NOT_FOUND).build();
    }

    // do we really need to send the file or can send "not modified"?
    if (modified != null) {
        Date modifiedDate = null;

        // we have to switch the locale to ENGLISH as parseDate parses in the default locale
        Locale old = Locale.getDefault();
        Locale.setDefault(Locale.ENGLISH);
        try {
            modifiedDate = DateUtils.parseDate(modified, org.apache.http.impl.cookie.DateUtils.DEFAULT_PATTERNS);
        } catch (ParseException e) {
            logger.error(e.getMessage(), e);
        }
        Locale.setDefault(old);

        if (modifiedDate != null) {
            // modifiedDate does not carry milliseconds, but fileDate does
            // therefore we have to do a range-based comparison
            // 1000 milliseconds = 1 second
            if (file.lastModified()-modifiedDate.getTime() < DateUtils.MILLIS_PER_SECOND) {
                return Response.status(Status.NOT_MODIFIED).build();
            }
        }
    }        
    // we really need to send the file

    try {
        Date fileDate = new Date(file.lastModified());
        return Response.ok(new FileInputStream(file)).lastModified(fileDate).build();
    } catch (FileNotFoundException e) {
        return Response.status(Status.NOT_FOUND).build();
    }
}

/*** copied from org.apache.http.impl.cookie.DateUtils, Apache 2.0 License ***/

/**
 * Date format pattern used to parse HTTP date headers in RFC 1123 format.
 */
public static final String PATTERN_RFC1123 = "EEE, dd MMM yyyy HH:mm:ss zzz";

/**
 * Date format pattern used to parse HTTP date headers in RFC 1036 format.
 */
public static final String PATTERN_RFC1036 = "EEEE, dd-MMM-yy HH:mm:ss zzz";

/**
 * Date format pattern used to parse HTTP date headers in ANSI C
 * <code>asctime()</code> format.
 */
public static final String PATTERN_ASCTIME = "EEE MMM d HH:mm:ss yyyy";

public static final String[] DEFAULT_PATTERNS = new String[] {
    PATTERN_RFC1036,
    PATTERN_RFC1123,
    PATTERN_ASCTIME
};

请注意，区域设置切换似乎不是线程安全的。我认为，最好在全球范围内切换语言环境。不过我不确定副作用...

Answer 3

关于@Perception 的回答，在使用字节数组时确实非常消耗内存，但您也可以简单地写回输出流

@Path("/picture")
public class ProfilePicture {
  @GET
  @Path("/thumbnail")
  @Produces("image/png")
  public StreamingOutput getThumbNail() {
    return new StreamingOutput() {
      @Override
      public void write(OutputStream os) throws IOException, WebApplicationException {
        //... read your stream and write into os
      }
    };
  }
}

Answer 4

如果您有多个图像资源方法，那么值得创建一个MessageBodyWriter 来输出BufferedImage：

@Produces({ "image/png", "image/jpg" })
@Provider
public class BufferedImageBodyWriter implements MessageBodyWriter<BufferedImage>  {
  @Override
  public boolean isWriteable(Class<?> type, Type type1, Annotation[] antns, MediaType mt) {
    return type == BufferedImage.class;
  }

  @Override
  public long getSize(BufferedImage t, Class<?> type, Type type1, Annotation[] antns, MediaType mt) {
    return -1; // not used in JAX-RS 2
  }

  @Override
  public void writeTo(BufferedImage image, Class<?> type, Type type1, Annotation[] antns, MediaType mt, MultivaluedMap<String, Object> mm, OutputStream out) throws IOException, WebApplicationException {
    ImageIO.write(image, mt.getSubtype(), out);
  } 
}

如果为 Jersey 启用了自动发现，则此 MessageBodyWriter 将自动使用，否则需要由自定义应用程序子类返回。请参阅JAX-RS Entity Providers 了解更多信息。

设置完成后，只需从资源方法返回一个BufferedImage，它将作为图像文件数据输出：

@Path("/whatever")
@Produces({"image/png", "image/jpg"})
public Response getFullImage(...) {
  BufferedImage image = ...;
  return Response.ok(image).build();
}

这种方法的几个优点：

• 它写入响应 OutputSteam 而不是中介 BufferedOutputStream
• 它同时支持png和jpg输出（取决于资源方法允许的媒体类型）