您如何控制Java中的资源缓存？

Question

I am building a Java applet that involves downloading images among other resources from a URL. I have discovered that the images are being cached, and can view them in the Java Control Panel under Temporary Internet Files / View... / Resources. Unfortunately I need to be able to update the images and have these updates appear between executions of the applet but the cache is causing problems.

I can't find any information about what controls the caching of these kinds of resources. What process is caching the resources and how do I control it? In particular how do I set the expiry time for images, or perhaps even specific images?

In case it is relevant, I am downloading the images using code like this: (mt is a MediaTracker object).

public BufferedImage getImageFromUrl(String url)
{
    Image img = null;
    try {
        URL u = new URL(url);
        img = java.awt.Toolkit.getDefaultToolkit().createImage(u);
        mt.addImage(img, numImages++);
        mt.waitForAll();
        ...

Thanks for any help.

Answer 1

Use this to avoid cached images from the server:

URL u = new URL(url);
URLConnection con = u.openConnection();
con.setUseCaches(false);
img = Toolkit.getDefaultToolkit().createImage(new URLImageSource(u, con));

If you want control over the expiry time you can specifically set the Cache-Control or Expires headers by adding lines like:

con.addRequestProperty("Cache-Control", "no-cache, max-age=3600");
con.addRequestProperty("Expires", "Thu, 17 Mar 2011 01:34:00 GMT");

Answer 2

Use a URLConnection to download the image into a byte array. Pass this byte array to createImage() . You may also want to turn off caching by calling setUseCaches(false) on the URLConnection object.