Improvement: Add error-prone check that forbids extending java.lang.Error

README.md

@@ -185,6 +185,7 @@ Safe Logging can be found at [github.com/palantir/safe-logging](https://github.c
 - `RawTypes`: Avoid raw types; add appropriate type parameters if possible.
 - `VisibleForTestingPackagePrivate`: `@VisibleForTesting` members should be package-private.
 - `OptionalFlatMapOfNullable`: Optional.map functions may return null to safely produce an empty result.
+- `ExtendsErrorOrThrowable`: Avoid extending Error (or subclasses of it) or Throwable directly.
 
 ### Programmatic Application
 

baseline-error-prone/src/main/java/com/palantir/baseline/errorprone/ExtendsErrorOrThrowable.java

@@ -0,0 +1,73 @@
+/*
+ * (c) Copyright 2020 Palantir Technologies Inc. All rights reserved.
+ *
+ * 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 com.palantir.baseline.errorprone;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.fixes.SuggestedFixes;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.google.errorprone.matchers.Matchers;
+import com.google.errorprone.suppliers.Suppliers;
+import com.sun.source.tree.ClassTree;
+import com.sun.source.tree.Tree;
+import com.sun.tools.javac.code.Type;
+import java.util.Optional;
+
+@AutoService(BugChecker.class)
+@BugPattern(
+        name = "ExtendsErrorOrThrowable",
+        link = "https://github.com/palantir/gradle-baseline#baseline-error-prone-checks",
+        linkType = BugPattern.LinkType.CUSTOM,
+        providesFix = BugPattern.ProvidesFix.REQUIRES_HUMAN_ATTENTION,
+        severity = BugPattern.SeverityLevel.ERROR,
+        summary = "Class should not extend Error or Throwable directly. While allowed by java "
+                + "it can lead to surprising behaviour if users end up catching Error or Throwable.")
+public final class ExtendsErrorOrThrowable extends BugChecker implements BugChecker.ClassTreeMatcher {
+    private static final Matcher<ClassTree> IS_ERROR_SUBCLASS = Matchers.isSubtypeOf(Error.class);
+    private static final Matcher<Tree> IS_THROWABLE = Matchers.isSameType(Throwable.class);
+    private static final Matcher<Tree> IS_ERROR_OR_THROWABLE =
+            Matchers.anyOf(Matchers.isSameType(Error.class), Matchers.isSameType(Throwable.class));
+
+    @Override
+    public Description matchClass(ClassTree tree, VisitorState state) {
+        if (tree.getKind() != Tree.Kind.CLASS) {
+            // Don't apply to out interfaces and enums
+            return Description.NO_MATCH;
+        }
+
+        // Match only cases where we extend from any Error type or directly from Throwable
+        if (!(IS_ERROR_SUBCLASS.matches(tree, state) || IS_THROWABLE.matches(tree.getExtendsClause(), state))) {
+            return Description.NO_MATCH;
+        }
+
+        return buildDescription(tree).addFix(buildFix(tree, state)).build();
+    }
+
+    private static Optional<SuggestedFix> buildFix(ClassTree tree, VisitorState state) {
+        if (IS_ERROR_OR_THROWABLE.matches(tree.getExtendsClause(), state)) {
+            Type exceptionType = Suppliers.typeFromClass(RuntimeException.class).get(state);
+            String prettyExceptionType = SuggestedFixes.prettyType(exceptionType, state);
+            return Optional.of(SuggestedFix.replace(tree.getExtendsClause(), prettyExceptionType));
+        } else {
+            return Optional.empty();
+        }
+    }
+}

baseline-error-prone/src/test/java/com/palantir/baseline/errorprone/ExtendsErrorOrThrowableTest.java

@@ -0,0 +1,99 @@
+/*
+ * (c) Copyright 2020 Palantir Technologies Inc. All rights reserved.
+ *
+ * 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 com.palantir.baseline.errorprone;
+
+import com.google.errorprone.BugCheckerRefactoringTestHelper;
+import com.google.errorprone.CompilationTestHelper;
+import org.junit.jupiter.api.Test;
+
+public final class ExtendsErrorOrThrowableTest {
+    @Test
+    void testSimple() {
+        helper().addSourceLines(
+                        "Test.java",
+                        "// BUG: Diagnostic contains: Class should not extend Error or Throwable directly",
+                        "public class Test extends Error {",
+                        "  public Test() {}",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    void testNonJavaLangError() {
+        helper().addSourceLines("Error.java", "public class Error {", "  public Error() {}", "}")
+                .addSourceLines("Test.java", "public class Test extends Error {", "  public Test() {}", "}")
+                .doTest();
+    }
+
+    @Test
+    void testSimpleException() {
+        helper().addSourceLines("Test.java", "public class Test extends RuntimeException {", "  public Test() {}", "}")
+                .expectNoDiagnostics()
+                .doTest();
+    }
+
+    @Test
+    void testSpecificJavaError() {
+        helper().addSourceLines(
+                        "Test.java",
+                        "// BUG: Diagnostic contains: Class should not extend Error or Throwable directly",
+                        "public class Test extends OutOfMemoryError {",
+                        "  public Test() {}",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    void testSpecificJavaErrorNoFix() {
+        fix().addInputLines("Test.java", "public class Test extends OutOfMemoryError {", "  public Test() {}", "}")
+                .expectUnchanged()
+                .doTestExpectingFailure(BugCheckerRefactoringTestHelper.TestMode.TEXT_MATCH);
+    }
+
+    @Test
+    void testFixError() {
+        fix().addInputLines("Test.java", "public class Test extends Error {", "  public Test() {}", "}")
+                .addOutputLines("Test.java", "public class Test extends RuntimeException {", "  public Test() {}", "}")
+                .doTest();
+    }
+
+    @Test
+    void testThrowableDiagnostic() {
+        helper().addSourceLines(
+                        "Test.java",
+                        "// BUG: Diagnostic contains: Class should not extend Error or Throwable directly",
+                        "public class Test extends Throwable {",
+                        "  public Test() {}",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    void testFixThrowable() {
+        fix().addInputLines("Test.java", "public class Test extends Throwable {", "  public Test() {}", "}")
+                .addOutputLines("Test.java", "public class Test extends RuntimeException {", "  public Test() {}", "}")
+                .doTest();
+    }
+
+    private CompilationTestHelper helper() {
+        return CompilationTestHelper.newInstance(ExtendsErrorOrThrowable.class, getClass());
+    }
+
+    private RefactoringValidator fix() {
+        return RefactoringValidator.of(new ExtendsErrorOrThrowable(), getClass());
+    }
+}

changelog/@unreleased/pr-1379.v2.yml

@@ -0,0 +1,7 @@
+type: improvement
+improvement:
+  description: Add error-prone check that forbids extending java.lang.Error (reports
+    ERROR). Provides a fix to replace `extends Error` with `extends RuntimeException`
+    for clearer contract to the consumers.
+  links:
+  - https://github.com/palantir/gradle-baseline/pull/1379