Make System.out a Mock with Spock Test Framework

Introduction

For this year's training for newcomers, we modified all the Java code + wrote tests and made changes so that the tests would guarantee it. At that time, I also used parameters, and when I made it with Spock, it was a memo. (Junit5 made it easier, but it's still a hassle, so I often use spock)

environment

Java8 Groovy 2.5 spock-core 1.2-groovy-2.5 Gradle4.10

This phenomenon

I was creating a test to check what was displayed by converting Java System.out to Mock with spock. At that time, I was hit by a phenomenon that an error occurred at startup if I wrote it smoothly and moved it.

Code in question

Check in Hello World to reproduce the phenomenon. The code of the main body and the test code are as follows.

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

class HelloWorldTest extends Specification {
    def mainTest() {
        setup:
        PrintStream printStream = Mock()
        System.out = printStream

        when:
        HelloWorld.main(null)

        then:
        1 * printStream.println("Hello World")
    }
}

solution

It's OK if you add cglib-nodep and objenesis as Dependency (the error also says so). So, add it and move it, and you're done. The final working .build.gradle is as follows.

plugins {
    id 'java'
    id 'groovy'
}

group 'com.tasogarei'
version '1.0-SNAPSHOT'

sourceCompatibility = 1.8

repositories {
    mavenCentral()
}

dependencies {
    testCompile "org.codehaus.groovy:groovy:2.5.6"
    testCompile "org.spockframework:spock-core:1.2-groovy-2.5"
    testCompile "cglib:cglib-nodep:3.2.10"
    testCompile "org.objenesis:objenesis:3.0.1"
}