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Dotify Library
There is a newer version: 1.0.7
package org.daisy.dotify.common.xml;

import org.daisy.dotify.common.io.FileIO;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URISyntaxException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import java.util.logging.Logger;

class URLCache {
    private static final Logger LOGGER = Logger.getLogger(URLCache.class.getCanonicalName());
    private final File root;

    URLCache() {
        this(new File(System.getProperty("java.io.tmpdir"), "url-cache"));
    }

    URLCache(File root) {
        this.root = root;
    }

    InputStream openStream(URL url) throws IOException {
        File f;
        if (url.getProtocol().equals("file")) {
            try {
                f = new File(url.toURI());
            } catch (URISyntaxException e) {
                throw new IOException(e);
            }
        } else {
            f = getCacheFile(url);
            if (f == null) {
                return null;
            }
        }
        return new FileInputStream(f);
    }

    /**
     * Updates the cache file.
     *
     * @param url the url to update
     * @return the cache file
     * @throws IOException if download failed
     */
    File updateCacheFile(URL url) throws IOException {
        return getCacheFile(url, true);
    }

    /**
     * Gets the cache file for the specified url.
     *
     * @param url the url to get the file for
     * @return returns the file, or null if the file isn't in the cache
     * @throws IOException if download failed
     */
    File getCacheFile(URL url) throws IOException {
        return getCacheFile(url, false);
    }

    /**
     * Sets the cache file to the contents of the specified input stream.
     *
     * @param url the url to set the cache contents for
     * @param is  the content input stream
     * @return returns true if the operation was successful, false otherwise
     * @throws IOException if the stream couldn't be copied
     */
    boolean setCacheFile(URL url, InputStream is) throws IOException {
        File f = toPath(url);
        if (f != null) {
            copyStream(is, f);
            return true;
        } else {
            return false;
        }
    }

    /**
     * Returns true if the entry is in the cache.
     *
     * @param url
     * @return returns true if the entry is in the cache
     */
    boolean hasEntry(URL url) {
        File f = toPath(url);
        return (f != null && f.exists());
    }

    /**
     * Removes the specified url from the cache.
     *
     * @param url the url to remove from the cache
     * @return returns true if the entry was successfully removed
     */
    boolean removeEntry(URL url) {
        File f = toPath(url);
        if (f != null) {
            return f.delete();
        } else {
            return false;
        }
    }

    private File getCacheFile(URL url, boolean overwrite) throws IOException {
        File f = toPath(url);
        if (f == null) {
            return null;
        }
        if (overwrite || !f.exists()) {
            LOGGER.info("Fetching " + url);
            //Download a copy
            copyStream(url.openStream(), f);
        }
        return f;
    }

    private void copyStream(InputStream source, File target) throws IOException {
        target.getParentFile().mkdirs();
        FileOutputStream os = new FileOutputStream(target);
        FileIO.copy(source, os);
    }

    private File toPath(URL url) {
        if (url.getProtocol() == null || url.getHost() == null || url.getPath() == null ||
                url.getProtocol().equals("") || url.getHost().equals("") || url.getPath().equals("")) {
            return null;
        }
        List list = new ArrayList<>();
        list.add(url.getProtocol());
        list.add(url.getHost());
        list.add(url.getPath());
        StringBuilder sb = new StringBuilder();
        boolean first = true;
        for (String s : list) {
            if (first) {
                first = false;
            } else {
                sb.append("/");
            }
            sb.append(s);
        }
        return new File(root, sb.toString());
    }
}