Reliability: Add feature flag (#159)

* Add feature flag

* Add URL

Author: mincong-h

reliability/src/test/java/io/mincong/reliability/featureflag/MyJob.java

@@ -0,0 +1,25 @@
+package io.mincong.reliability.featureflag;
+
+import java.util.Set;
+
+/**
+ * @author Mincong Huang
+ * @blog https://mincong.io/2020/11/11/feature-flag/
+ */
+public class MyJob {
+
+  private final Set<String> values;
+
+  public MyJob(Set<String> values) {
+    this.values = values;
+  }
+
+  public void run() {
+    var isNewFeatureEnabled = Boolean.getBoolean("NEW_FEATURE_ENABLED");
+    if (isNewFeatureEnabled) {
+      values.add("new");
+    } else {
+      values.add("old");
+    }
+  }
+}

reliability/src/test/java/io/mincong/reliability/featureflag/MyJobTest.java

@@ -0,0 +1,60 @@
+package io.mincong.reliability.featureflag;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.HashSet;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+/**
+ * @author Mincong Huang
+ * @blog https://mincong.io/2020/11/11/feature-flag/
+ */
+class MyJobTest {
+
+  @AfterEach
+  void tearDown() {
+    System.clearProperty("NEW_FEATURE_ENABLED");
+  }
+
+  @Test
+  void itShouldEnableNewFeature() {
+    // Given
+    System.setProperty("NEW_FEATURE_ENABLED", "True");
+    var set = new HashSet<String>();
+    var job = new MyJob(set);
+
+    // When
+    job.run();
+
+    // Then
+    assertThat(set).containsExactly("new");
+  }
+
+  @Test
+  void itShouldDisableNewFeature() {
+    // Given
+    System.setProperty("NEW_FEATURE_ENABLED", "False");
+    var set = new HashSet<String>();
+    var job = new MyJob(set);
+
+    // When
+    job.run();
+
+    // Then
+    assertThat(set).containsExactly("old");
+  }
+
+  @Test
+  void itShouldFallback() {
+    // Given
+    var set = new HashSet<String>();
+    var job = new MyJob(set);
+
+    // When
+    job.run();
+
+    // Then
+    assertThat(set).containsExactly("old");
+  }
+}