RestoreSystemProperties sets correct properties.

RestoreSystemProperties now replaces the system properties with a copy
of the original properties. This ensures that the properties behave like
the original properties. That was not the case before. E.g. the
`contains` method returned `false` for a property that has been
originally set but was not modified in the test.

Fixes #43.

Author: stefanbirkner

README.md

@@ -14,7 +14,7 @@ System Rules is available from
     <dependency>
       <groupId>com.github.stefanbirkner</groupId>
       <artifactId>system-rules</artifactId>
-      <version>1.16.0</version>
+      <version>1.16.1</version>
     </dependency>
 
 Please don't forget to add the scope `test` if you're using System

pom.xml

@@ -9,7 +9,7 @@
 	</parent>
 
 	<artifactId>system-rules</artifactId>
-	<version>1.16.0</version>
+	<version>1.16.1</version>
 	<packaging>jar</packaging>
 
 	<name>System Rules</name>

src/main/java/org/junit/contrib/java/lang/system/RestoreSystemProperties.java

@@ -67,7 +67,13 @@ public void add(String property) {
 	@Override
 	protected void before() throws Throwable {
 		originalProperties = getProperties();
-		setProperties(new Properties(originalProperties));
+		setProperties(copyOf(originalProperties));
+	}
+
+	private Properties copyOf(Properties source) {
+		Properties copy = new Properties();
+		copy.putAll(source);
+		return copy;
 	}
 
 	@Override

src/test/java/org/junit/contrib/java/lang/system/RestoreSystemPropertiesTest.java

@@ -1,13 +1,17 @@
 package org.junit.contrib.java.lang.system;
 
 import static java.lang.System.clearProperty;
+import static java.lang.System.getProperties;
 import static java.lang.System.getProperty;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.contrib.java.lang.system.Executor.executeTestWithRule;
 
 import org.junit.After;
 import org.junit.Test;
 import org.junit.rules.TestRule;
+import org.junit.runners.model.Statement;
+
+import java.util.Properties;
 
 public class RestoreSystemPropertiesTest {
 	//ensure that every test uses the same property, because this one is restored after the test
@@ -43,11 +47,22 @@ public void property_that_does_not_exist_before_the_test_does_not_exist_after_th
 	}
 
 	@Test
-	public void property_value_is_unchanged_at_start_of_test() {
+	public void at_start_of_a_test_properties_are_equal_to_the_original_properties() {
 		System.setProperty(PROPERTY_KEY, "dummy value");
-		TestThatCapturesProperties test = new TestThatCapturesProperties();
-		executeTestWithRule(test, rule);
-		assertThat(test.propertiesAtStart)
-			.containsEntry(PROPERTY_KEY, "dummy value");
+		Properties originalProperties = getProperties();
+		executeTestWithRule(
+			assertPropertiesAreEqualTo(originalProperties),
+			rule
+		);
+	}
+
+	private Statement assertPropertiesAreEqualTo(
+			final Properties expectedProperties) {
+		return new Statement() {
+			@Override
+			public void evaluate() throws Throwable {
+				assertThat(getProperties()).isEqualTo(expectedProperties);
+			}
+		};
 	}
 }