servlets - Java Servlet 和 ImageIO 错误

Question

我正在尝试编写一个查看某些 DICOM 文件的 WebApplication。我的想法是通过调用我的 servlet 将 DICOM 文件即时转换为 JPEG。
我使用 dcm4che 2.0.27 来转换文件。

servlet 被称为

<img src="pathToServlet/PathToDICOMFile">.

问题:当我有很多标签(当然处理不同的 DICOM 文件)时，有时我会收到 java.util.ConcurrentModificationException 类型的异常

这是方法，我的 servlet 正在调用:

void convertFile(String path, OutputStream to) throws IOException {
    File myDicomFile = new File(path);

    BufferedImage myJpegImage = null;
    ImageIO.scanForPlugins();
    Iterator<ImageReader> iter = ImageIO.getImageReadersByFormatName("DICOM");

    ImageReader reader = (ImageReader) iter.next();

    DicomImageReadParam param = (DicomImageReadParam) reader.getDefaultReadParam();

    ImageInputStream iis = ImageIO.createImageInputStream(myDicomFile);
    reader.setInput(iis, false);
    myJpegImage = reader.read(0, param);
    iis.close();
    ImageIO.write(myJpegImage, "JPEG", to);

    to.close();

}

“path”是 dicom 文件的绝对路径，“out”只是 response.getOutputStream()。

可以在 3 个地方抛出异常:

ImageIO.getImageReadersByFormatName("DICOM");

myJpegImage = reader.read(0, param);

ImageIO.write(myJpegImage, "JPEG", to);

这是调用 ImageIO.getImageReadersByFormatName("DICOM") 时抛出的堆栈跟踪:

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:806)
at java.util.HashMap$ValueIterator.next(HashMap.java:835)
at javax.imageio.spi.PartialOrderIterator.<init>(PartiallyOrderedSet.java:177)
at javax.imageio.spi.PartiallyOrderedSet.iterator(PartiallyOrderedSet.java:85)
at javax.imageio.spi.SubRegistry.getServiceProviders(ServiceRegistry.java:759)
at javax.imageio.spi.ServiceRegistry.getServiceProviders(ServiceRegistry.java:451)
at javax.imageio.spi.ServiceRegistry.getServiceProviders(ServiceRegistry.java:507)
at javax.imageio.ImageIO.getImageReadersByFormatName(ImageIO.java:708)
at example.project.dicomtest.myDicomConverter.ConvertHelper.convertFile(ConvertHelper.java:32)
at example.project.dicomtest.myDicomConverter.GetImage.doGet(GetImage.java:40)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:185)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:151)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
at org.apache.catalina.core.StandardEnginateValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:269)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)

是不是因为 ImageIO 不是线程安全的？我能做些什么来解决这个问题？
非常感谢您的帮助和最好的问候!

Answer 1

这两行:

ImageIO.scanForPlugins();
Iterator<ImageReader> iter = ImageIO.getImageReadersByFormatName("DICOM");

只需在您的应用程序加载或 servlet 初始化时执行一次。特别是第一行 ImageIO.scanForPlugins()确实在内部改变 ImageIO 类使用的共享数据。这可能是您并发修改异常的原因。考虑将这两行移至 servlet 过滤器，或移至 init(ServletConfig) servlet 的方法。