Java Servlet 和 ImageIO 错误

Question

我正在尝试编写一个 WebApplication 来查看一些 DICOM 文件。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 的调用方式类似于

<img src="pathToServlet/PathToDICOMFile">.

问题：当我有很多标签（当然是针对不同的 DICOM 文件）时，有时我会遇到 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”就是response.getOutputStream()。

异常可以在3个地方抛出：

1. ImageIO.getImageReadersByFormatName("DICOM");
2. myJpegImage = reader.read(0, param);
3. 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您的 servlet 初始化时执行一次。特别是，第一行 ImageIO.scanForPlugins() 确实改变了 ImageIO 类内部使用的共享数据。这可能是您的并发修改异常的原因。考虑将这两行移至 servlet 过滤器或 servlet 的 init(ServletConfig) 方法。

Answer 2

使用同步方法来防止并发修改会有所帮助。阅读这个问题

stackoverflow.com/questions/9884148/use-of-synchronized-method-in-affable-bean-shopping-cart