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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 static java.lang.System.clearProperty;
import static java.lang.System.setProperty;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Properties;

import org.junit.rules.ExternalResource;

/**
 * The {@code ProvideSystemProperty} rule provides an arbitrary value for a
 * system property to a test. After the test the original value is restored. You
 * can ensure that a property is not set by providing {@code null}.
 * 
 * Let's assume the system property {@code MyProperty} is not set and the system
 * property {@code OtherProperty} has the value {@code OtherValue}. Now run the
 * test
 * 
 * 
 *   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"));
 *     }
 *   }
 * 
 * 
 * The test succeeds and after the test, the system property {@code MyProperty}
 * is not set and the system property {@code OtherProperty} has the value
 * {@code OtherValue}.
 * 
 * You can 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");
 * 
 */
public class ProvideSystemProperty extends ExternalResource {
	private final Map properties = new LinkedHashMap();
	private RestoreSystemProperties restoreSystemProperty;

	public static ProvideSystemProperty fromFile(String name)
			throws IOException {
		FileInputStream fis = new FileInputStream(name);
		return fromInputStream(fis);
	}

	public static ProvideSystemProperty fromResource(String name)
			throws IOException {
		InputStream is = ProvideSystemProperty.class.getResourceAsStream(name);
		return fromInputStream(is);
	}

	private static ProvideSystemProperty fromInputStream(InputStream is)
			throws IOException {
		Properties p = new Properties();
		p.load(is);
		ProvideSystemProperty rule = new ProvideSystemProperty();
		for (Map.Entry property : p.entrySet())
			rule.addProperty((String) property.getKey(),
					(String) property.getValue());
		return rule;
	}

	private ProvideSystemProperty() {
	}

	public ProvideSystemProperty(String name, String value) {
		addProperty(name, value);
	}

	public ProvideSystemProperty and(String name, String value) {
		addProperty(name, value);
		return this;
	}

	private void addProperty(String name, String value) {
		properties.put(name, value);
	}

	@Override
	protected void before() throws Throwable {
		restoreSystemProperty = new RestoreSystemProperties(
				collectPropertyNames());
		restoreSystemProperty.before();
		updateProperties();
	}

	private String[] collectPropertyNames() {
		return properties.keySet().toArray(new String[properties.size()]);
	}

	private void updateProperties() {
		for (Entry property : properties.entrySet()) {
			String name = property.getKey();
			String value = property.getValue();
			updateProperty(name, value);
		}
	}

	private void updateProperty(String name, String value) {
		if (value == null)
			clearProperty(name);
		else
			setProperty(name, value);
	}

	@Override
	protected void after() {
		restoreSystemProperty.after();
	}
}