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A collection of JUnit rules for testing code which uses java.lang.System.
There is a newer version: 1.19.0
package org.junit.contrib.java.lang.system;

import org.junit.contrib.java.lang.system.internal.RestoreSpecificSystemProperties;
import org.junit.rules.ExternalResource;

/**
 * The {@code ClearSystemProperties} rule clears a set of system
 * properties when the test starts and restores their original values
 * when the test finishes (whether it passes or fails).
 * Supposing that the system property {@code YourProperty} has the
 * value {@code YourValue}. Now run the test
 * 
 * public void YourTest {
 *   @Rule
 *   public final TestRule clearSystemProperties
 *     = new ClearSystemProperties("YourProperty");
 *
 *   @Test
 *   public void verifyProperty() {
 *     assertNull(System.getProperty("YourProperty"));
 *   }
 * }
 * 
 * The test succeeds and afterwards the system property
 * {@code YourProperty} has the value {@code YourValue} again.
 * The {@code ClearSystemProperties} rule accepts a list of
 * properties in case you need to clear multiple properties:
 * 
 * @Rule
 * public final TestRule clearSystemProperties
 *   = new ClearSystemProperties("first", "second", "third");
 * 
 * Clear property for a single test
 * If you want to clear a property for a single test then you can
 * use
 * {@link org.junit.contrib.java.lang.system.RestoreSystemProperties}
 * along with {@link System#clearProperty(String)}.
 * 
 * @Rule
 * public final TestRule restoreSystemProperties
 *   = new RestoreSystemProperties();
 *
 * @Test
 * public void test() {
 *   System.clearProperty("YourProperty");
 *   ...
 * }
 */
public class ClearSystemProperties extends ExternalResource {
	private final RestoreSpecificSystemProperties restoreSystemProperty = new RestoreSpecificSystemProperties();
	private final String[] properties;

	/**
	 * Creates a {@code ClearSystemProperties} rule that clears the specified
	 * properties and restores their original values when the test finishes
	 * (whether it passes or fails).
	 *
	 * @param properties the properties' names.
	 */
	public ClearSystemProperties(String... properties) {
		this.properties = properties;
	}

	/**
	 * Clears the property and restores the value of the property at the point
	 * of clearing it.
	 * This method is deprecated. If you're still using it, please replace your current code
	 * 
	 * @Rule
	 * public final ClearSystemProperties clearSystemProperties = new ClearSystemProperties();
	 *
	 * @Test
	 * public void test() {
	 *   clearSystemProperties.clearProperty("YourProperty");
	 *   ...
	 * }
	 * with this code:
	 * 	 * @Rule
	 * public final TestRule restoreSystemProperties = new RestoreSystemProperties();
	 *
	 * @Test
	 * public void test() {
	 *   System.clearProperty("YourProperty");
	 *   ...
	 * }
	 *
	 * @param property the name of the property.
	 * @since 1.6.0
	 * @deprecated Please use {@link org.junit.contrib.java.lang.system.RestoreSystemProperties}
	 * along with {@link System#clearProperty(String)}.
	 */
	@Deprecated
	public void clearProperty(String property) {
		restoreSystemProperty.add(property);
		System.clearProperty(property);
	}

	@Override
	protected void before() throws Throwable {
		clearProperties();
	}

	@Override
	protected void after() {
		restoreOriginalValue();
	}

	private void clearProperties() {
		for (String property : properties)
			clearProperty(property);
	}

	private void restoreOriginalValue() {
		restoreSystemProperty.restore();
	}
}