Merge pull request dsaff#2 from marcphilipp/ClearSystemProperty

Introduced ClearSystemProperty rule.

system-rules/src/main/java/org/junit/contrib/java/lang/system/ClearSystemProperty.java

@@ -0,0 +1,58 @@
+package org.junit.contrib.java.lang.system;
+
+import static java.lang.System.clearProperty;
+
+import org.junit.rules.ExternalResource;
+
+/**
+ * The {@code ClearSystemProperty} rule clears a system property to a test.
+ * After the test the original value is restored.
+ * 
+ * Let's assume the system property {@code MyProperty} has the value
+ * {@code MyValue}. Now run the test
+ * 
+ * <pre>
+ *   public void MyTest {
+ *     &#064;Rule
+ *     public final ClearSystemProperty myPropertyIsCleared
+ *       = new ClearSystemProperty("MyProperty");
+ * 
+ *     &#064;Test
+ *     public void overrideProperty() {
+ *       assertNull(System.getProperty("MyProperty"));
+ *     }
+ *   }
+ * </pre>
+ * 
+ * The test succeeds and after the test, the system property {@code MyProperty}
+ * has the value {@code MyValue}.
+ */
+public class ClearSystemProperty extends ExternalResource {
+
+	private final RestoreSystemProperties restoreSystemProperty;
+	private final String name;
+
+	public ClearSystemProperty(String name) {
+		this.name = name;
+		this.restoreSystemProperty = new RestoreSystemProperties(name);
+	}
+
+	@Override
+	protected void before() throws Throwable {
+		backupOriginalValue();
+		clearProperty(name);
+	}
+
+	@Override
+	protected void after() {
+		restoreOriginalValue();
+	}
+
+	private void backupOriginalValue() throws Throwable {
+		restoreSystemProperty.before();
+	}
+
+	private void restoreOriginalValue() {
+		restoreSystemProperty.after();
+	}
+}

system-rules/src/test/java/org/junit/contrib/java/lang/system/ClearSystemPropertyTest.java

@@ -0,0 +1,51 @@
+package org.junit.contrib.java.lang.system;
+
+import static java.lang.System.clearProperty;
+import static java.lang.System.getProperty;
+import static java.lang.System.setProperty;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.nullValue;
+import static org.junit.Assert.assertThat;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runners.model.Statement;
+
+public class ClearSystemPropertyTest {
+
+	private static final String ARBITRARY_NAME = "arbitrary property";
+	private static final String ARBITRARY_VALUE = "arbitrary value";
+
+	@Rule
+	public final RestoreSystemProperties restore = new RestoreSystemProperties(
+			ARBITRARY_NAME);
+
+	@Test
+	public void restoresOriginalValue() throws Throwable {
+		setProperty(ARBITRARY_NAME, ARBITRARY_VALUE);
+
+		ClearSystemProperty rule = new ClearSystemProperty(ARBITRARY_NAME);
+		rule.apply(new ClearedValue(), null).evaluate();
+
+		assertThat(getProperty(ARBITRARY_NAME), is(equalTo(ARBITRARY_VALUE)));
+	}
+
+	@Test
+	public void originallyUnsetPropertyRemainsUnset() throws Throwable {
+		clearProperty(ARBITRARY_NAME);
+
+		ClearSystemProperty rule = new ClearSystemProperty(ARBITRARY_NAME);
+		rule.apply(new ClearedValue(), null).evaluate();
+
+		assertThat(getProperty(ARBITRARY_NAME), is(nullValue(String.class)));
+	}
+
+	private class ClearedValue extends Statement {
+
+		@Override
+		public void evaluate() throws Throwable {
+			assertThat(getProperty(ARBITRARY_NAME), is(nullValue(String.class)));
+		}
+	}
+}