import java.io.ByteArrayOutputStream;
import java.io.PrintStream;
import java.util.ArrayList;
import java.util.List;
public class StdOutCaptor extends ByteArrayOutputStream {
private List<String> lines = new ArrayList<>();
private PrintStream originalOut;
@Override
public synchronized void write(byte[] b, int off, int len) {
super.write(b, off, len);
lines.add(new String(b));
}
public void startCapture() {
this.lines = new ArrayList<>();
this.originalOut = System.out;
System.setOut(new PrintStream(this));
}
public void endCapture() {
System.setOut(originalOut);
}
public void assertLineCaptured(String expectedText) {
if (lines.stream().noneMatch(s -> s.contains(expectedText))) {
throw new AssertionError("Expected captured lines to " +
"contain '" + expectedText + "', but was actually: " + lines);
}
}
}public class ThingThatPrintsToStdOut {
public void doThing() {
System.out.println("Doing a thing");
}
}import org.junit.jupiter.api.Test;
class ThingThatPrintsToStdOutTest {
private final StdOutCaptor stdOutCaptor = new StdOutCaptor();
@Test
void printMessageToStdOut() {
ThingThatPrintsToStdOut thing = new ThingThatPrintsToStdOut();
stdOutCaptor.startCapture();
thing.doThing();
stdOutCaptor.endCapture();
stdOutCaptor.assertLineCaptured("Doing a thing");
}
}