layout: pattern

title: Balking

folder: balking

permalink: /patterns/balking/

categories: Concurrency

tags:

- Java

- Difficulty-Beginner

Balking Pattern is used to prevent an object from executing certain code if it is an

incomplete or inappropriate state


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

<?xml version="1.0" encoding="UTF-8"?>

<class-diagram version="1.1.13" icons="true" automaticImage="PNG" always-add-relationships="false"

generalizations="true" realizations="true" associations="true" dependencies="false" nesting-relationships="true"

router="FAN">

<class id="1" language="java" name="com.iluwatar.balking.App" project="balking"

file="/balking/src/main/java/com/iluwatar/balking/App.java" binary="false" corner="BOTTOM_RIGHT">

<position height="113" width="114" x="135" y="103"/>

<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"

sort-features="false" accessors="true" visibility="true">

<attributes public="true" package="true" protected="true" private="true" static="true"/>

<operations public="true" package="true" protected="true" private="true" static="true"/>

</display>

</class>

<class id="2" language="java" name="com.iluwatar.balking.WashingMachine" project="balking"

file="/balking/src/main/java/com/iluwatar/balking/WashingMachine.java" binary="false" corner="BOTTOM_RIGHT">

<position height="149" width="268" x="289" y="103"/>

<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"

sort-features="false" accessors="true" visibility="true">

<attributes public="true" package="true" protected="true" private="true" static="true"/>

<operations public="true" package="true" protected="true" private="true" static="true"/>

</display>

</class>

<enumeration id="3" language="java" name="com.iluwatar.balking.WashingMachineState" project="balking"

file="/balking/src/main/java/com/iluwatar/balking/WashingMachineState.java" binary="false" corner="BOTTOM_RIGHT">

<position height="113" width="192" x="289" y="292"/>

<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"

sort-features="false" accessors="true" visibility="true">

<attributes public="true" package="true" protected="true" private="true" static="true"/>

<operations public="true" package="true" protected="true" private="true" static="true"/>

</display>

</enumeration>

<association id="4">

<end type="SOURCE" refId="2" navigable="false">

<attribute id="5" name="washingMachineState"/>

<multiplicity id="6" minimum="0" maximum="1"/>

</end>

<end type="TARGET" refId="3" navigable="true"/>

<display labels="true" multiplicity="true"/>

</association>

<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"

sort-features="false" accessors="true" visibility="true">

<attributes public="true" package="true" protected="true" private="true" static="true"/>

<operations public="true" package="true" protected="true" private="true" static="true"/>

</classifier-display>

<association-display labels="true" multiplicity="true"/>

</class-diagram>

package com.iluwatar.balking {

class App {

- LOGGER : Logger {static}

+ App()

+ main(args : String[]) {static}

}

class WashingMachine {

- LOGGER : Logger {static}

- washingMachineState : WashingMachineState

+ WashingMachine()

+ endOfWashing()

+ getWashingMachineState() : WashingMachineState

+ wash()

}

enum WashingMachineState {

+ ENABLED {static}

+ WASHING {static}

+ valueOf(name : String) : WashingMachineState {static}

+ values() : WashingMachineState[] {static}

}

}

WashingMachine --> "-washingMachineState" WashingMachineState

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

<parent>

<artifactId>java-design-patterns</artifactId>

<groupId>com.iluwatar</groupId>

<version>1.14.0-SNAPSHOT</version>

</parent>

<modelVersion>4.0.0</modelVersion>

<artifactId>balking</artifactId>

<dependencies>

<dependency>

<groupId>junit</groupId>

<artifactId>junit</artifactId>

<scope>test</scope>

</dependency>

</dependencies>

</project>

package com.iluwatar.balking;

import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

public class App {

private static final Logger LOGGER = LoggerFactory.getLogger(App.class);

/**

public static void main(String... args) {

final WashingMachine washingMachine = new WashingMachine();

ExecutorService 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!");

}

}

}

package com.iluwatar.balking;

import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

public class WashingMachine {

private static final Logger LOGGER = LoggerFactory.getLogger(WashingMachine.class);

private WashingMachineState washingMachineState;

public WashingMachine() {

washingMachineState = WashingMachineState.ENABLED;

}

public WashingMachineState getWashingMachineState() {

return washingMachineState;

}

/**

public void wash() {

synchronized (this) {

LOGGER.info("{}: Actual machine state: {}", Thread.currentThread().getName(), getWashingMachineState());

if (washingMachineState == WashingMachineState.WASHING) {

LOGGER.error("ERROR: Cannot wash if the machine has been already washing!");

return;

}

washingMachineState = WashingMachineState.WASHING;

}

LOGGER.info("{}: Doing the washing", Thread.currentThread().getName());

try {

Thread.sleep(50);

} catch (InterruptedException ie) {

ie.printStackTrace();

}

endOfWashing();

}

/**

public synchronized void endOfWashing() {

washingMachineState = WashingMachineState.ENABLED;

LOGGER.info("{}: Washing completed.", Thread.currentThread().getId());

}

}

package com.iluwatar.balking;

public enum WashingMachineState {ENABLED, WASHING}

package com.iluwatar.balking;

public class AppTest {

@org.junit.Test

public void main() throws Exception {

String[] args = {};

App.main(args);

}

}

package com.iluwatar.balking;

import org.junit.Test;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

import static org.junit.Assert.*;

public class WashingMachineTest {

private volatile WashingMachineState machineStateGlobal;

@Test

public void wash() throws Exception {

WashingMachine washingMachine = new WashingMachine();

ExecutorService executorService = Executors.newFixedThreadPool(2);

executorService.execute(washingMachine::wash);

executorService.execute(() -> {

washingMachine.wash();

machineStateGlobal = washingMachine.getWashingMachineState();

});

executorService.shutdown();

try {

executorService.awaitTermination(10, TimeUnit.SECONDS);

} catch (InterruptedException ie) {

ie.printStackTrace();

}

assertEquals(WashingMachineState.WASHING, machineStateGlobal);

}

@Test

public void endOfWashing() throws Exception {

WashingMachine washingMachine = new WashingMachine();

washingMachine.wash();

assertEquals(WashingMachineState.ENABLED, washingMachine.getWashingMachineState());

}

}