Singleton Pattern

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Code Block

class Calculator {
    private total = 0
    def add(a, b) { total++; a + b }
    def getTotalCalulationsgetTotalCalculations() { 'Total Calculations: ' + total }
    String toString() { 'Calc: ' + hashCode()}
}

class Client {
    def calc = new Calculator()
    def executeCalc(a, b) { calc.add(a, b) }
    String toString() { 'Client: ' + hashCode()}
}

...

Code Block

def client = new Client()
assert 3 == client.executeCalc(1, 2)
println "$client, $client.calc, $client.calc.totalCalulationstotalCalculations"

client = new Client()
assert 4 == client.executeCalc(2, 2)
println "$client, $client.calc, $client.calc.totalCalulationstotalCalculations"

Here is the result of running this script (your hashcode values may vary):

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Code Block

// require(groupId:'aopalliance', artifactId:'aopalliance', version:'1.0')
// require(groupId:'com.google.code.guice', artifactId:'guice', version:'1.0')

import com.google.inject.*

interface Calculator {
    def add(a, b)
}

class CalculatorImpl implements Calculator {
    private total = 0
    def add(a, b) { total++; a + b }
    def getTotalCalulationsgetTotalCalculations() { 'Total Calculations: ' + total }
    String toString() { 'Calc: ' + hashCode()}
}

class Client {
    @Inject Calculator calc
    def executeCalc(a, b) { calc.add(a, b) }
    String toString() { 'Client: ' + hashCode()}
}

def injector = Guice.createInjector (
    [configure: { binding ->
        binding.bind(Calculator)
               .to(CalculatorImpl)
               .asEagerSingleton() } ] as Module
)

client = injector.getInstance(Client)    
assert 3 == client.executeCalc(1, 2)
println "$client, $client.calc, $client.calc.totalCalulationstotalCalculations"

client = injector.getInstance(Client)    
assert 4 == client.executeCalc(2, 2)
println "$client, $client.calc, $client.calc.totalCalulationstotalCalculations"

Note the @Inject annotation in the Client class. We can always tell right in the source code which fields will be injected.

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Code Block

import com.google.inject.*

@ImplementedBy(CalculatorImpl)
interface Calculator {
    // as before ...
}

@Singleton
class CalculatorImpl implements Calculator {
    // as before ...
}

class Client {
    // as before ...
}

def injector = Guice.createInjector({} as Module[])

// ...

Note the @Singleton annotation on the CalculatorImpl class and the @ImplementedBy annotation in the Calculator interface.

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Code Block

// require(groupId:'org.springframework', artifactId:'spring-core', version:'2.1m1')
// require(groupId:'org.springframework', artifactId:'spring-beans', version:'2.1m1')

import org.springframework.beans.factory.support.*

interface Calculator {
    def add(a, b)
}

class CalculatorImpl implements Calculator {
    private total = 0
    def add(a, b) { total++; a + b }
    def getTotalCalulationsgetTotalCalculations() { 'Total Calculations: ' + total }
    String toString() { 'Calc: ' + hashCode()}
}

class Client {
    Client(Calculator calc) { this.calc = calc }
    def calc
    def executeCalc(a, b) { calc.add(a, b) }
    String toString() { 'Client: ' + hashCode()}
}

// Here we 'wire' up our dependencies through the API. Alternatively,
// we could use XML-based configuration or the Grails Bean Builder DSL.
def factory = new DefaultListableBeanFactory()
factory.registerBeanDefinition('calc', new RootBeanDefinition(CalculatorImpl))
def beanDef = new RootBeanDefinition(Client, false)
beanDef.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_AUTODETECT)
factory.registerBeanDefinition('client', beanDef)

client = factory.getBean('client')    
assert 3 == client.executeCalc(1, 2)
println "$client, $client.calc, $client.calc.totalCalulationstotalCalculations"

client = factory.getBean('client')    
assert 4 == client.executeCalc(2, 2)
println "$client, $client.calc, $client.calc.totalCalulationstotalCalculations"

And here is the result (your hashcode values will vary):

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Singleton Pattern

Versions Compared

Old Version 13

New Version 14

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