java-design-patterns/balking/README.md

---
layout: pattern
title: Balking
folder: balking
permalink: /patterns/balking/
categories: Concurrency
tags:
 - Decoupling
---

## Intent

Balking Pattern is used to prevent an object from executing a certain code if it is in an incomplete 
or inappropriate state.

## Explanation

Real world example

> There's a start-button in a washing machine to initiate the laundry washing. When the washing 
> machine is inactive the button works as expected, but if it's already washing the button does
> nothing.

In plain words

> Using the balking pattern, a certain code executes only if the object is in particular state.

Wikipedia says

> The balking pattern is a software design pattern that only executes an action on an object when 
> the object is in a particular state. For example, if an object reads ZIP files and a calling 
> method invokes a get method on the object when the ZIP file is not open, the object would "balk" 
> at the request.

**Programmatic Example**

In this example implementation, `WashingMachine` is an object that has two states in which it can 
be: ENABLED and WASHING. If the machine is ENABLED, the state changes to WASHING using a thread-safe
method. On the other hand, if it already has been washing and any other thread executes `wash()` 
it won't do that and returns without doing anything.

Here are the relevant parts of the `WashingMachine` class.

```java
@Slf4j
public class WashingMachine {

  private final DelayProvider delayProvider;
  private WashingMachineState washingMachineState;

  public WashingMachine(DelayProvider delayProvider) {
    this.delayProvider = delayProvider;
    this.washingMachineState = WashingMachineState.ENABLED;
  }

  public WashingMachineState getWashingMachineState() {
    return washingMachineState;
  }

  public void wash() {
    synchronized (this) {
      var machineState = getWashingMachineState();
      LOGGER.info("{}: Actual machine state: {}", Thread.currentThread().getName(), machineState);
      if (this.washingMachineState == WashingMachineState.WASHING) {
        LOGGER.error("Cannot wash if the machine has been already washing!");
        return;
      }
      this.washingMachineState = WashingMachineState.WASHING;
    }
    LOGGER.info("{}: Doing the washing", Thread.currentThread().getName());
    this.delayProvider.executeAfterDelay(50, TimeUnit.MILLISECONDS, this::endOfWashing);
  }

  public synchronized void endOfWashing() {
    washingMachineState = WashingMachineState.ENABLED;
    LOGGER.info("{}: Washing completed.", Thread.currentThread().getId());
  }
}
```

Here's the simple `DelayProvider` interface used by the `WashingMachine`.

```java
public interface DelayProvider {
  void executeAfterDelay(long interval, TimeUnit timeUnit, Runnable task);
}
```

Now we introduce the application using the `WashingMachine`.

```java
  public static void main(String... args) {
    final var washingMachine = new WashingMachine();
    var executorService = Executors.newFixedThreadPool(3);
    for (int i = 0; i < 3; i++) {
      executorService.execute(washingMachine::wash);
    }
    executorService.shutdown();
    try {
      executorService.awaitTermination(10, TimeUnit.SECONDS);
    } catch (InterruptedException ie) {
      LOGGER.error("ERROR: Waiting on executor service shutdown!");
      Thread.currentThread().interrupt();
    }
  }
```

Here is the console output of the program.

```
14:02:52.268 [pool-1-thread-2] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-2: Actual machine state: ENABLED
14:02:52.272 [pool-1-thread-2] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-2: Doing the washing
14:02:52.272 [pool-1-thread-3] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-3: Actual machine state: WASHING
14:02:52.273 [pool-1-thread-3] ERROR com.iluwatar.balking.WashingMachine - Cannot wash if the machine has been already washing!
14:02:52.273 [pool-1-thread-1] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-1: Actual machine state: WASHING
14:02:52.273 [pool-1-thread-1] ERROR com.iluwatar.balking.WashingMachine - Cannot wash if the machine has been already washing!
14:02:52.324 [pool-1-thread-2] INFO com.iluwatar.balking.WashingMachine - 14: Washing completed.
```

## Class diagram

![alt text](./etc/balking.png "Balking")

## Applicability

Use the Balking pattern when

* You want to invoke an action on an object only when it is in a particular state
* Objects are generally only in a state that is prone to balking temporarily but for an unknown 
  amount of time

## Related patterns

* [Guarded Suspension Pattern](https://java-design-patterns.com/patterns/guarded-suspension/)
* [Double Checked Locking Pattern](https://java-design-patterns.com/patterns/double-checked-locking/)

## Credits

* [Patterns in Java: A Catalog of Reusable Design Patterns Illustrated with UML, 2nd Edition, Volume 1](https://www.amazon.com/gp/product/0471227293/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=0471227293&linkId=0e39a59ffaab93fb476036fecb637b99)