replace legacy JUnit 4 rule for system mock by custom helper

pom.xml

@@ -137,12 +137,6 @@
             <version>2.1</version>
             <scope>test</scope>
         </dependency>
-        <dependency>
-            <groupId>com.github.stefanbirkner</groupId>
-            <artifactId>system-rules</artifactId>
-            <version>1.17.2</version>
-            <scope>test</scope>
-        </dependency>
         <dependency>
             <groupId>org.mockito</groupId>
             <artifactId>mockito-core</artifactId>

src/test/java/de/stklcode/jvault/connector/builder/HTTPVaultConnectorBuilderTest.java

@@ -19,11 +19,9 @@ package de.stklcode.jvault.connector.builder;
 import de.stklcode.jvault.connector.HTTPVaultConnector;
 import de.stklcode.jvault.connector.exception.TlsException;
 import de.stklcode.jvault.connector.exception.VaultConnectorException;
-import org.junit.Rule;
+import de.stklcode.jvault.connector.test.EnvironmentMock;
-import org.junit.contrib.java.lang.system.EnvironmentVariables;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
-import org.junit.jupiter.migrationsupport.rules.EnableRuleMigrationSupport;
 
 import java.io.File;
 import java.io.IOException;
@@ -40,7 +38,6 @@ import static org.junit.jupiter.api.Assertions.fail;
  * @author Stefan Kalscheuer
  * @since 0.8.0
  */
-@EnableRuleMigrationSupport
 public class HTTPVaultConnectorBuilderTest {
     private static final String VAULT_ADDR = "https://localhost:8201";
     private static final Integer VAULT_MAX_RETRIES = 13;
@@ -49,9 +46,6 @@ public class HTTPVaultConnectorBuilderTest {
     @TempDir
     File tempDir;
 
-    @Rule
-    public final EnvironmentVariables environment = new EnvironmentVariables();
-
     /**
      * Test building from environment variables
      */
@@ -112,10 +106,10 @@ public class HTTPVaultConnectorBuilderTest {
     }
 
     private void setenv(String vault_addr, String vault_cacert, String vault_max_retries, String vault_token) {
-        environment.set("VAULT_ADDR", vault_addr);
+        EnvironmentMock.setenv("VAULT_ADDR", vault_addr);
-        environment.set("VAULT_CACERT", vault_cacert);
+        EnvironmentMock.setenv("VAULT_CACERT", vault_cacert);
-        environment.set("VAULT_MAX_RETRIES", vault_max_retries);
+        EnvironmentMock.setenv("VAULT_MAX_RETRIES", vault_max_retries);
-        environment.set("VAULT_TOKEN", vault_token);
+        EnvironmentMock.setenv("VAULT_TOKEN", vault_token);
     }
 
     private Object getRequestHelperPrivate(HTTPVaultConnector connector, String fieldName) throws NoSuchFieldException, IllegalAccessException {

src/test/java/de/stklcode/jvault/connector/factory/HTTPVaultConnectorFactoryTest.java

@@ -19,11 +19,9 @@ package de.stklcode.jvault.connector.factory;
 import de.stklcode.jvault.connector.HTTPVaultConnector;
 import de.stklcode.jvault.connector.exception.TlsException;
 import de.stklcode.jvault.connector.exception.VaultConnectorException;
-import org.junit.Rule;
+import de.stklcode.jvault.connector.test.EnvironmentMock;
-import org.junit.contrib.java.lang.system.EnvironmentVariables;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
-import org.junit.jupiter.migrationsupport.rules.EnableRuleMigrationSupport;
 
 import java.io.File;
 import java.io.IOException;
@@ -40,7 +38,6 @@ import static org.junit.jupiter.api.Assertions.fail;
  * @author Stefan Kalscheuer
  * @since 0.6.0
  */
-@EnableRuleMigrationSupport
 public class HTTPVaultConnectorFactoryTest {
     private static String VAULT_ADDR = "https://localhost:8201";
     private static Integer VAULT_MAX_RETRIES = 13;
@@ -49,9 +46,6 @@ public class HTTPVaultConnectorFactoryTest {
     @TempDir
     File tempDir;
 
-    @Rule
-    public final EnvironmentVariables environment = new EnvironmentVariables();
-
     /**
      * Test building from environment variables
      */
@@ -112,10 +106,10 @@ public class HTTPVaultConnectorFactoryTest {
     }
 
     private void setenv(String vault_addr, String vault_cacert, String vault_max_retries, String vault_token) {
-        environment.set("VAULT_ADDR", vault_addr);
+        EnvironmentMock.setenv("VAULT_ADDR", vault_addr);
-        environment.set("VAULT_CACERT", vault_cacert);
+        EnvironmentMock.setenv("VAULT_CACERT", vault_cacert);
-        environment.set("VAULT_MAX_RETRIES", vault_max_retries);
+        EnvironmentMock.setenv("VAULT_MAX_RETRIES", vault_max_retries);
-        environment.set("VAULT_TOKEN", vault_token);
+        EnvironmentMock.setenv("VAULT_TOKEN", vault_token);
     }
 
     private Object getRequestHelperPrivate(HTTPVaultConnector connector, String fieldName) throws NoSuchFieldException, IllegalAccessException {

src/test/java/de/stklcode/jvault/connector/model/response/SecretListResponseTest.java

@@ -26,7 +26,7 @@ import java.util.Map;
 
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.*;
-import static org.junit.Assert.fail;
+import static org.junit.jupiter.api.Assertions.fail;
 
 /**
  * JUnit Test for {@link SecretListResponse} model.
@@ -50,7 +50,6 @@ public class SecretListResponseTest {
      * @throws InvalidResponseException Should not occur
      */
     @Test
-    @SuppressWarnings("unchecked")
     public void getKeysTest() throws InvalidResponseException {
         // Create empty Object.
         SecretListResponse res = new SecretListResponse();

src/test/java/de/stklcode/jvault/connector/test/EnvironmentMock.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2016-2019 Stefan Kalscheuer
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package de.stklcode.jvault.connector.test;
+
+import java.lang.reflect.Field;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.fail;
+
+/**
+ * Test helper to modify system environment.
+ *
+ * @author Stefan Kalscheuer
+ */
+@SuppressWarnings("unchecked")
+public class EnvironmentMock {
+    private static Map<String, String> environment;
+
+    static {
+        try {
+            Map<String, String> originalEnv = System.getenv();
+            Field mapField = originalEnv.getClass().getDeclaredField("m");
+            mapField.setAccessible(true);
+            environment = (Map<String, String>) mapField.get(originalEnv);
+        } catch (NoSuchFieldException | IllegalAccessException | ClassCastException e) {
+            fail("Failed to intercept unmodifiable system environment");
+        }
+    }
+
+    public static void setenv(String key, String value) {
+        if (value != null) {
+            environment.put(key, value);
+        } else {
+            environment.remove(key);
+        }
+    }
+}