Clear Properties

The ClearSystemProperty rule deletes the property before the test and restores the original value of the property when the test is finished.

public void MyTest {
	@Rule
	public final ClearSystemProperty myPropertyIsCleared
	 = new ClearSystemProperty("MyProperty");

	@Test
	public void overrideProperty() {
		assertNull(System.getProperty("MyProperty"));
	}
}

Provide Properties

The ProvideSystemProperty rule provides an arbitrary value for a system property to a test. After the test the original value is restored.

public void MyTest {
	@Rule
	public final ProvideSystemProperty myPropertyHasMyValue
	 = new ProvideSystemProperty("MyProperty", "MyValue");

	@Rule
	public final ProvideSystemProperty otherPropertyIsMissing
	 = new ProvideSystemProperty("OtherProperty", null);

	@Test
	public void overrideProperty() {
		assertEquals("MyValue", System.getProperty("MyProperty"));
		assertNull(System.getProperty("OtherProperty"));
	}
}

You could also use a single instance of the rule to achieve the same effect:

public void MyTest {
	@Rule
	public final ProvideSystemProperty properties
	 = new ProvideSystemProperty("MyProperty", "MyValue").and("OtherProperty", null);

	@Test
	public void overrideProperty() {
		assertEquals("MyValue", System.getProperty("MyProperty"));
		assertNull(System.getProperty("OtherProperty"));
	}
}

You can use a properties file to supply properties for the ProvideSystemProperty rule. The file can be from the file system or the class path. In the first case use

@Rule
public final ProvideSystemProperty properties
 = ProvideSystemProperty.fromFile("/home/myself/example.properties");

and in the second case use

@Rule
public final ProvideSystemProperty properties
 = ProvideSystemProperty.fromResource("example.properties");

Restore Properties

The RestoreSystemProperties rule restores the original value of the property when the test is finished.

public void MyTest {
	@Rule
	public final RestoreSystemProperties restoreSystemProperties
	 = new RestoreSystemProperties("MyProperty");

	@Test
	public void overrideProperty() {
		//after the test the original value of "MyProperty" will be restored.
		System.setProperty("MyProperty", "other value");
		...
	}
}

System.err and System.out

You want to verify the text, which your code writes to System.err or System.out. The StandardErrorStreamLog and StandardOutputStreamLog rule are logging everything written to System.err or System.out. The text is available by calling getLog().

public void MyTest {
	@Rule
	public final StandardErrorStreamLog log = new StandardErrorStreamLog();

	@Test
	public void overrideProperty() {
		System.err.print("hello world");
		assertEquals("hello world", log.getLog());
	}
}

public void MyTest {
	@Rule
	public final StandardOutputStreamLog log = new StandardOutputStreamLog();

	@Test
	public void overrideProperty() {
		System.out.print("hello world");
		assertEquals("hello world", log.getLog());
	}
}

System.in

The TextFromStandardInputStream rule helps you to create tests for classes which read from System.in. You specify the text provided by System.in, by calling provideText(String). The example's class under test reads two numbers from System.in and calculates the sum of these numbers.

Class Under Test

import java.util.Scanner;

public class Summarize {
  public static int sumOfNumbersFromSystemIn() {
    Scanner scanner = new Scanner(System.in);
    int firstSummand = scanner.nextInt();
    int secondSummand = scanner.nextInt();
    return firstSummand + secondSummand;
  }
}

Test

import static org.junit.Assert.*;
import static org.junit.contrib.java.lang.system.TextFromStandardInputStream.*;

import org.junit.Rule;
import org.junit.Test;
import org.junit.contrib.java.lang.system.TextFromStandardInputStream;

public class SummarizeTest {
  @Rule
  public final TextFromStandardInputStream systemInMock
    = emptyStandardInputStream();

  @Test
  public void summarizesTwoNumbers() {
    systemInMock.provideText("1\n2\n");
    assertEquals(3, Summarize.sumOfNumbersFromSystemIn());
  }
}

System.exit()

If your code calls System.exit(), then your test stops and doesn't finish. The ExpectedSystemExit rule allows in-test specification of expected System.exit() calls. Furthermore you cannot use JUnit's assert methods because of the abnormal termination of your code. As a substitute you can provide an Assertion object to the ExpectedSystemExit rule.

Some care must be taken if your system under test creates a new thread and this thread calls System.exit(). In this case you have to ensure that the test does not finish before System.exit() is called.

Class Under Test

public class AppWithExit {
  public static String message;

  public static void doSomethingAndExit() {
    message = "exit ...";
    System.exit(1);		
  }

  public static void doNothing() {
  }
}

Test

public void AppWithExitTest {
  @Rule
  public final ExpectedSystemExit exit = ExpectedSystemExit.none();

  @Test
  public void exits() {
    exit.expectSystemExit();
    AppWithExit.doSomethingAndExit();
  }

  @Test
  public void exitsWithStatusCode1() {
    exit.expectSystemExitWithStatus(1);
    AppWithExit.doSomethingAndExit();
  }

  @Test
  public void writesMessage() {
    exit.expectSystemExitWithStatus(1);
    exit.checkAssertionAfterwards(new Assertion() {
      public void checkAssertion() {
        assertEquals("exit ...", AppWithExit.message);
      }
    });
    AppWithExit.doSomethingAndExit();
  }

  @Test
  public void systemExitWithStatusCode1() {
    exit.expectSystemExitWithStatus(1);
    AppWithExit.doSomethingAndExit();
  }

  @Test
  public void noSystemExit() {
    AppWithExit.doNothing();
    //passes
  }
}

Security Manager

If you need a special security manager to test your code, you may provide it by using the ProvideSecurityManager rule. This rule replaces the system's security manager with yours throughout the test

public void MyTest {
	private final MySecurityManager securityManager
	 = new MySecurityManager();

	@Rule
	public final ProvideSecurityManager provideSecurityManager
	 = new ProvideSecurityManager(securityManager);

	@Test
	public void overrideProperty() {
		assertEquals(securityManager, System.getSecurityManager());
	}
}