java jai_imageio注册机制分析

首先获取ImageIO里的注册机

private static final IIORegistry registry;
 
  static
  {
    IIORegistry temp = null;
    try
    {
      Field field = ImageIO.class.getDeclaredField("theRegistry");
      field.setAccessible(true);
      temp = (IIORegistry)field.get(null);
    }
    catch (Exception e)
    {
      e.printStackTrace();
    }
    finally
    {
      if (temp == null) {
        temp = IIORegistry.getDefaultInstance();
      }
      registry = temp;
    }
  }

然后网注册机里注册读写类

public static void registerServiceProvider(Class<?> clazz)
  {
    Class<?> spiClass = null;
    try
    {
      spiClass = Class.forName(clazz.getName(), true, ClassLoader.getSystemClassLoader());
    }
    catch (ClassNotFoundException localClassNotFoundException) {}
    if (spiClass != null)
    {
      Object spi = registry.getServiceProviderByClass(spiClass);
      if (spi != null) {
        registry.deregisterServiceProvider(spi);
      }
    }
    try
    {
      registry.registerServiceProvider(clazz.newInstance());
    }
    catch (Exception e)
    {
      e.printStackTrace();
    }
  }

这样ImageIO就保留了格式和读写类的对应关系

读写时通过以下代码皆可以调用到读写类

Iterator<ImageReader> readers = ImageIO.getImageReadersByFormatName(param.formatName);

        while (readers.hasNext()) {
            ImageReader reader = readers.next();

            if (param.className == null || param.className.equals(reader.getClass().getName())) {
                LOG.debug("Using Image Reader {}", reader.getClass());
                return reader;
            }
        }