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SciJava Common is a shared library for SciJava software. It provides a plugin framework, with an extensible mechanism for service discovery, backed by its own annotation processor, so that plugins can be loaded dynamically. It is used by downstream projects in the SciJava ecosystem, such as ImageJ and SCIFIO.
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package org.scijava.download;
import java.io.IOException;
import java.util.Date;
import org.scijava.io.handle.DataHandle;
import org.scijava.io.handle.DataHandleService;
import org.scijava.io.handle.DataHandles;
import org.scijava.io.location.Location;
import org.scijava.plugin.Parameter;
import org.scijava.plugin.Plugin;
import org.scijava.service.AbstractService;
import org.scijava.service.Service;
import org.scijava.task.Task;
import org.scijava.task.TaskService;
/**
* Default implementation of {@link DownloadService}.
*
* @author Curtis Rueden
*/
@Plugin(type = Service.class)
public class DefaultDownloadService extends AbstractService implements
DownloadService
{
@Parameter
private DataHandleService dataHandleService;
@Parameter
private TaskService taskService;
@Override
public Download download(final Location source, final Location destination) {
final Task task = taskService.createTask("Download");
return new DefaultDownload(source, destination, task, () -> {
try (final DataHandle in = dataHandleService.create(source);
final DataHandle out = dataHandleService.create(
destination))
{
task.setStatusMessage("Downloading " + source.getURI());
DataHandles.copy(in, out, task);
}
catch (final IOException exc) {
// TODO: Improve error handling:
// 1. Consider a better exception handling design here.
// 2. Retry at least a few times if something goes wrong.
throw new RuntimeException(exc);
}
});
}
@Override
public Download download(final Location source, final Location destination,
final LocationCache cache)
{
if (cache == null || !cache.canCache(source)) {
// Caching this location is not supported.
return download(source, destination);
}
final Task task = taskService.createTask("Download");
return new DefaultDownload(source, destination, task, () -> {
final Location cached = cache.cachedLocation(source);
try (
final DataHandle sourceHandle = dataHandleService.create(source);
final DataHandle cachedHandle = dataHandleService.create(cached);
final DataHandle destHandle = dataHandleService.create(destination)
)
{
if (isCachedHandleValid(source, cache, sourceHandle, cachedHandle)) {
// The data is cached; download from the cached source instead.
task.setStatusMessage("Retrieving " + source.getURI());
DataHandles.copy(cachedHandle, destHandle, task);
}
else {
// Data is not yet cached; write to the destination _and_ the cache.
task.setStatusMessage("Downloading + caching " + source.getURI());
DataHandles.copy(sourceHandle, //
new MultiWriteHandle(cachedHandle, destHandle), task);
}
}
catch (final IOException exc) {
// TODO: Improve error handling:
// 1. Consider a better exception handling design here.
// 2. Retry at least a few times if something goes wrong.
throw new RuntimeException(exc);
}
});
}
// -- Helper methods --
private boolean isCachedHandleValid(final Location source,
final LocationCache cache, final DataHandle sourceHandle,
final DataHandle cachedHandle) throws IOException
{
if (!cachedHandle.exists()) return false; // No cached data is present.
// Compare data lengths.
final long sourceLen = sourceHandle.length();
final long cachedLen = cachedHandle.length();
if (sourceLen >= 0 && cachedLen >= 0 && sourceLen != cachedLen) {
// Original and cached sources report different lengths; cache is invalid.
return false;
}
// Compare last modified timestamps.
final Date sourceDate = sourceHandle.lastModified();
final Date cachedDate = cachedHandle.lastModified();
if (sourceDate != null && cachedDate != null && //
sourceDate.after(cachedDate))
{
// Source was changed after cache was written; cache is invalid.
return false;
}
// Compare checksums.
final String sourceChecksum = sourceHandle.checksum();
final String cachedChecksum = cache.loadChecksum(source);
if (sourceChecksum != null && cachedChecksum != null && //
!sourceChecksum.equals(cachedChecksum))
{
// Checksums do not match; cache is invalid.
return false;
}
// Everything matched; we're all good.
return true;
}
// -- Helper classes --
private class DefaultDownload implements Download {
private Location source;
private Location destination;
private Task task;
private DefaultDownload(final Location source, final Location destination,
final Task task, final Runnable r)
{
this.source = source;
this.destination = destination;
this.task = task;
task.run(r);
}
@Override
public Location source() {
return source;
}
@Override
public Location destination() {
return destination;
}
@Override
public Task task() {
return task;
}
}
}