WarningInputStream is an InputStream that you can wrap around another InputStream. It's purpose is

Driver.java

private void closeLingeringResources() { //make sure to run this at the end of whatever you're running. It should be part of a cleanup phase.
		final List<InputStreamDescriptor> streams = new ArrayList<>();
		WarningInputStream.retrieveAndClearInputs().forEach(value -> {
			log.error("A resource was not closed\n" + value.describe());
			streams.add(value);
		});
		streams.forEach(value -> {
			getClosingService().submit(() -> {
				try {
					value.getStream().close();
				} catch(final IOException e) {
					log.error("Could not close resource:\n"+value.describe(), e);
				}
			});
		});
	}

sample output.log

Feb 26 14:45:02 pebi.northpool.pvt java[19735]: ERROR 2019-02-26 14:45:02,500 [pool-1-thread-1] nl.northpool.scraper.GenericScraper:lambda$closeLingeringResources$1(165): A resource was not closed
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: regelleistung-mrl
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: at Thread.java:1559 getStackTrace
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: at WarningInputStream.java:27 register
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: at WarningInputStream.java:22 <init>
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: at GenericScraper.java:187 readVersion
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: at GenericScraper.java:81 <init>
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: at GenericHTMLScraper.java:35 <init>
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: at RegelleistungScraper.java:48 <init>
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: at RegelleistungScraper.java:44 <init>
Feb 26 14:45:02 pebi.northpool.pvt java[19735]: at ServletRegelleistungMRL.java:22 initScraper

Usage.java

try(WarningInputStream in = new WarningInputStream(conn.getInputStream())) { //should this stream not close for some reason, you would figure that out when calling closeLingeringResources()
	String xml = IOUtils.toString(saveInput(in, name), Charset.defaultCharset());
	return XmlUtil.parse(xml);
}

WarningInputstream.java

package nl.northpool.scraper;

import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.UUID;

public class WarningInputStream extends InputStream implements AutoCloseable {

	private static Map<UUID, InputStreamDescriptor> inputs = new HashMap<>();

	final InputStream stream;
	final UUID id;

	public WarningInputStream(InputStream stream) {
		this.stream = stream;
		this.id = UUID.randomUUID();
		register();
	}
	
	private synchronized void register() {
		final Thread t = Thread.currentThread();
		inputs.put(id, new InputStreamDescriptor(t.getName(), t.getStackTrace(), this));	
	}
	
	public static synchronized List<InputStreamDescriptor> retrieveAndClearInputs() {
		List<InputStreamDescriptor> values = new ArrayList<>(inputs.values());
		inputs.values().clear();
		return values;
	}

	@Override
	public int read() throws IOException {
		return stream.read();
	}

	@Override
	public int available() throws IOException {
		return stream.available();
	}

	@Override
	public void close() throws IOException {
		inputs.remove(id);
		stream.close();
	}

	public static class InputStreamDescriptor {

		private final String threadName;
		private final StackTraceElement[] elements;
		private final WarningInputStream stream;

		public InputStreamDescriptor(String threadName, StackTraceElement[] elements, WarningInputStream stream) {
			this.threadName = threadName;
			this.elements = elements;
			this.stream = stream;
		}

		public String describe() {
			final StringBuffer buffer = new StringBuffer(threadName + "\n");
			for(final StackTraceElement trace : elements) {
				buffer.append("\tat " + trace.getFileName() + ":" + trace.getLineNumber() + " " + trace.getMethodName() + "\n");
			}
			return buffer.toString();
		}

		@Override
		public String toString() {
			return "InputStreamDescriptor [threadName=" + threadName + ", elements=" + Arrays.toString(elements) + ", stream=" + stream + "]";
		}

		public String getThreadName() {
			return threadName;
		}

		public StackTraceElement[] getElements() {
			return elements;
		}

		public WarningInputStream getStream() {
			return stream;
		}

	}

}