RuleChain#around should not allow null args (#1313)

By throwing IllegalArgEx from around(), we allow for better feedback to users, as the stacktrace will point to the exact line where the null rule is declared.

Author: alb-i986

src/main/java/org/junit/rules/RuleChain.java

@@ -90,10 +90,14 @@ private RuleChain(List<TestRule> rules) {
      * Create a new {@code RuleChain}, which encloses the given {@link TestRule} with
      * the rules of the current {@code RuleChain}.
      *
-     * @param enclosedRule the rule to enclose.
+     * @param enclosedRule the rule to enclose; must not be {@code null}.
      * @return a new {@code RuleChain}.
+     * @throws NullPointerException if the argument {@code enclosedRule} is {@code null}
      */
     public RuleChain around(TestRule enclosedRule) {
+        if (enclosedRule == null) {
+            throw new NullPointerException("The enclosed rule must not be null");
+        }
         List<TestRule> rulesOfNewChain = new ArrayList<TestRule>();
         rulesOfNewChain.add(enclosedRule);
         rulesOfNewChain.addAll(rulesStartingWithInnerMost);

src/test/java/org/junit/rules/RuleChainTest.java

@@ -1,17 +1,26 @@
 package org.junit.rules;
 
 import static java.util.Arrays.asList;
+import static org.hamcrest.CoreMatchers.containsString;
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
 import static org.junit.experimental.results.PrintableResult.testResult;
 import static org.junit.rules.RuleChain.outerRule;
 
+import java.io.PrintWriter;
+import java.io.StringWriter;
 import java.util.ArrayList;
 import java.util.List;
 
 import org.junit.Rule;
 import org.junit.Test;
+import org.junit.internal.Throwables;
 import org.junit.runner.Description;
+import org.junit.runner.JUnitCore;
+import org.junit.runner.Result;
 
 public class RuleChainTest {
     private static final List<String> LOG = new ArrayList<String>();
@@ -55,4 +64,33 @@ public void executeRulesInCorrectOrder() throws Exception {
                 "finished outer rule");
         assertEquals(expectedLog, LOG);
     }
+
+    @Test
+    public void aroundShouldNotAllowNullRules() {
+        RuleChain chain = RuleChain.emptyRuleChain();
+        try {
+            chain.around(null);
+            fail("around() should not allow null rules");
+        } catch (NullPointerException e) {
+            assertThat(e.getMessage(), equalTo("The enclosed rule must not be null"));
+        }
+    }
+
+    public static class RuleChainWithNullRules {
+        @Rule
+        public final RuleChain chain = outerRule(new LoggingRule("outer rule"))
+                .around(null);
+
+        @Test
+        public void example() {}
+    }
+
+    @Test
+    public void whenRuleChainHasNullRuleTheStacktraceShouldPointToIt() {
+        Result result = JUnitCore.runClasses(RuleChainWithNullRules.class);
+
+        assertThat(result.getFailures().size(), equalTo(1));
+        String stacktrace = Throwables.getStacktrace(result.getFailures().get(0).getException());
+        assertThat(stacktrace, containsString("\tat org.junit.rules.RuleChainTest$RuleChainWithNullRules.<init>(RuleChainTest.java:"));
+    }
 }