Java Servlets 和 ImageIO 错误

Question

我正在尝试编写一个Web应用程序，用于查看一些DICOM文件。我的想法是通过调用我的servlet动态将DICOM文件转换为JPEG格式。我使用dcm4che 2.0.27来进行文件转换。

servlet的调用方式如下：

<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，有时会出现java.util.ConcurrentModificationException类型的异常。

这是我的servlet调用的方法：

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，而"out"只是response.getOutputStream()。

异常可能会在以下3个地方抛出：

1. ImageIO.getImageReadersByFormatName("DICOM");
2. myJpegImage = reader.read(0, param);
3. ImageIO.write(myJpegImage, "JPEG", to);

以下是堆栈跟踪，在调用ImageIO.getImageReadersByFormatName("DICOM")时抛出异常：

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)

是因为ImageIO不支持多线程吗？我应该怎么解决这个问题？非常感谢任何帮助，祝好！

- mesx

Answer 1

以下两行代码：

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

只需要在应用程序加载或servlet初始化时执行一次。特别地，第一行ImageIO.scanForPlugins()会改变ImageIO类内部使用的共享数据。这很可能是您遇到并发修改异常的原因。考虑将这两行代码移到servlet过滤器中，或者移到您的servlet的init(ServletConfig)方法中。

- Perception

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非常感谢！那就是解决方案！ - mesx

Answer 2

使用同步方法可以防止并发修改，有所帮助。阅读此问题。

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

- qualebs