Question

Java does not have a retry keyword like Ruby. How can we implement the same sort of functionality?

Answer 1

I wanted to be able to write this retry logic and error handling code in one place and use it for a number of different method calls. There are several ways to do this, but previously in Java 7 I would have just written an abstract class with a single abstract method such as:

public abstract class RetryCommand<T> {
    private int maxRetries;

    public RetryCommand(int maxRetries) {
        this.maxRetries = maxRetries;
    }

    // This abstract command is the method that will be implemented 
    public abstract T command();

    public final T run() throws RuntimeException {
        try {
            return command();
        } catch (Exception e) {
            return retry();
        }
    }

    private final T retry() throws RuntimeException {
        System.out.println("FAILED - Command failed, will be retried " + maxRetries + " times.");
        int retryCounter = 0;
        while (retryCounter < maxRetries) {
            try {
                return command();
            } catch (Exception e) {
                retryCounter++;
                System.out.println("FAILED - Command failed on retry " + retryCounter + " of " + maxRetries + " error: " + ex );
                if (retryCounter >= maxRetries) {
                    System.out.println("Max retries exceeded.");
                    break;
                }
            }
        }
        throw new RuntimeException("Command failed on all of " + maxRetries + " retries");
    }
}

Then in my Gateway code, for each method that I want to wrap with my retry logic I would just do the following:

public class MyGateway {
    private RetryCommand<String> retryCommand;
    public MyGateway(int maxRetries) {
        retryCommand = new RetryCommand<>(maxRetries);
    }

    // Inline create an instance of  the abstract class RetryCommand
    // Define the body of the "command" method
    // Execute the "run" method and return the result
    public String getThing(final String id) {
        return new RetryCommand<String>() {
            public String command() {
                return client.getThatThing(id);
            }
        }.run();
    }
}

The reason for this layout was I could not pass a function as a parameter to a method, like I have done in Scala, Python, and C#. However, now that we have Java 8, we can finally pass functions as parameters using the handy features in java.util.function package!

I hope you got your answer.