System.outをassertするためのRule

SystemOutRule.java

import java.io.ByteArrayOutputStream;
import java.io.PrintStream;

import org.junit.rules.MethodRule;
import org.junit.runners.model.FrameworkMethod;
import org.junit.runners.model.Statement;

public class SystemOutRule implements MethodRule {

	private ByteArrayOutputStream baos;

	@Override
	public Statement apply(Statement base, FrameworkMethod method, Object target) {
		baos = new ByteArrayOutputStream();
		PrintStream out = new PrintStream(baos);
		System.setOut(out);

		return base;
	}

	public String getString() {
		return baos.toString();
	}
}

TestHelloWorld.java

import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;

import org.junit.Rule;
import org.junit.Test;

import sample.SystemOutRule;

public class TestHelloWorld {

	@Rule
	public SystemOutRule rule = new SystemOutRule();

	@Test
	public void test() {

		new HelloWorld().method();

		assertThat(rule.getString(), is("Hello, world!"));
	}
}

class HelloWorld {
	public void method() {
		System.out.print("Hello, world!");
	}

}