How to Retry Operation n Number of Times in Java?

We have a number of situations where we need to retry a task n-times if it fails (sometimes with some form of back-off-before-retry logic). Generally, if an exception is thrown, the task should be retried up to the max-retry count.

Below is a simple & complete Java example which explains retry logic and callURL logic.

package com.crunchify.tutorials;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;
import java.nio.charset.Charset;

/**
 * @author Crunchify.com
 */

public class CrunchifyRetryNTimes {

	public static void main(String[] args) {

		// Execute this Program as it is once to get correct response.

		// Change URL to
		// https://crunchify.com/wp-content/uploads/code/json.sample2.txt to
		// see retry logic in action..
		String myURL = "https://crunchify.com/wp-content/uploads/code/json.sample.txt";
		CrunchifyRetryOnExceptionStrategy retry = new CrunchifyRetryOnExceptionStrategy();
		while (retry.shouldRetry()) {
			try {
				System.out.println("Requested URL:" + myURL);
				StringBuilder sb = new StringBuilder();
				URLConnection urlConn = null;
				InputStreamReader in = null;
				URL url = new URL(myURL);
				urlConn = url.openConnection();
				if (urlConn != null)
					urlConn.setReadTimeout(60 * 1000);
				if (urlConn != null && urlConn.getInputStream() != null) {
					in = new InputStreamReader(urlConn.getInputStream(),
							Charset.defaultCharset());
					BufferedReader bufferedReader = new BufferedReader(in);
					if (bufferedReader != null) {
						int cp;
						while ((cp = bufferedReader.read()) != -1) {
							sb.append((char) cp);
						}
						bufferedReader.close();
					}
				}
				System.out.println(sb.toString());
				in.close();
				break;

			} catch (Exception e) {
				try {
					System.out.println("in catch.....");
					retry.errorOccured();
				} catch (RuntimeException e1) {
					throw new RuntimeException("Exception while calling URL:"
							+ myURL, e);
				} catch (Exception e1) {
					throw new RuntimeException(e1);
				}
			}
		}
	}

	static class CrunchifyRetryOnExceptionStrategy {
		public static final int DEFAULT_RETRIES = 3;
		public static final long DEFAULT_WAIT_TIME_IN_MILLI = 2000;

		private int numberOfRetries;
		private int numberOfTriesLeft;
		private long timeToWait;

		public CrunchifyRetryOnExceptionStrategy() {
			this(DEFAULT_RETRIES, DEFAULT_WAIT_TIME_IN_MILLI);
		}

		public CrunchifyRetryOnExceptionStrategy(int numberOfRetries,
				long timeToWait) {
			this.numberOfRetries = numberOfRetries;
			numberOfTriesLeft = numberOfRetries;
			this.timeToWait = timeToWait;
		}

		/**
		 * @return true if there are tries left
		 */
		public boolean shouldRetry() {
			return numberOfTriesLeft > 0;
		}

		public void errorOccured() throws Exception {
			if (!shouldRetry()) {
				throw new Exception("Retry Failed: Total " + numberOfRetries
						+ " attempts made at interval " + getTimeToWait()
						+ "ms");
			}
			numberOfTriesLeft--;
			waitUntilNextTry();
		}

		public long getTimeToWait() {
			return timeToWait;
		}

		private void waitUntilNextTry() {
			try {
				Thread.sleep(getTimeToWait());
			} catch (InterruptedException ignored) {
			}
		}
	}
}

Output1:

With URL:  https://crunchify.com/wp-content/uploads/code/json.sample.txt

Requested URL:https://crunchify.com/wp-content/uploads/code/json.sample.txt
{"menu": {
  "id": "file",
  "value": "File",
  "popup": {
    "menuitem": [
      {"value": "New", "onclick": "CreateNewDoc()"},
      {"value": "Open", "onclick": "OpenDoc()"},
      {"value": "Close", "onclick": "CloseDoc()"}
    ]
  }
}}

Output2:

With URL: https://crunchify.com/wp-content/uploads/code/json.sample2.txt

Requested URL:https://crunchify.com/wp-content/uploads/code/json.sample2.txt
in catch.....
Requested URL:https://crunchify.com/wp-content/uploads/code/json.sample2.txt
in catch.....
Requested URL:https://crunchify.com/wp-content/uploads/code/json.sample2.txt
in catch.....
Exception in thread "main" java.lang.RuntimeException: java.lang.Exception: <strong>Retry Failed: Total 3 attempts made at interval 2000ms</strong>
	at com.crunchify.tutorials.CrunchifyRetryNTimes.main(CrunchifyRetryNTimes.java:58)
Caused by: java.lang.Exception: Retry Failed: Total 3 attempts made at interval 2000ms
	at com.crunchify.tutorials.CrunchifyRetryNTimes$CrunchifyRetryOnExceptionStrategy.errorOccured(CrunchifyRetryNTimes.java:93)
	at com.crunchify.tutorials.CrunchifyRetryNTimes.main(CrunchifyRetryNTimes.java:53)

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