Encourage when/thenReturn over doReturn/when.

core/src/main/java/com/google/errorprone/bugpatterns/MockitoDoSetup.java

@@ -0,0 +1,198 @@
+/*
+ * Copyright 2024 The Error Prone Authors.
+ *
+ * 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.google.errorprone.bugpatterns;
+
+import static com.google.common.collect.Iterables.getLast;
+import static com.google.errorprone.BugPattern.SeverityLevel.WARNING;
+import static com.google.errorprone.matchers.Description.NO_MATCH;
+import static com.google.errorprone.matchers.Matchers.instanceMethod;
+import static com.google.errorprone.matchers.Matchers.staticMethod;
+import static com.google.errorprone.util.ASTHelpers.getReceiver;
+import static com.google.errorprone.util.ASTHelpers.getStartPosition;
+import static com.google.errorprone.util.ASTHelpers.getSymbol;
+import static com.google.errorprone.util.ASTHelpers.hasAnnotation;
+import static com.google.errorprone.util.ASTHelpers.isSameType;
+import static java.lang.String.format;
+
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker.CompilationUnitTreeMatcher;
+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.sun.source.tree.AssignmentTree;
+import com.sun.source.tree.CompilationUnitTree;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MethodInvocationTree;
+import com.sun.source.tree.Tree;
+import com.sun.source.tree.VariableTree;
+import com.sun.source.util.TreePath;
+import com.sun.source.util.TreePathScanner;
+import com.sun.tools.javac.code.Symbol.VarSymbol;
+
+/** A BugPattern; see the summary. */
+@BugPattern(
+    severity = WARNING,
+    summary = "Prefer using when/thenReturn over doReturn/when for additional type safety.")
+public final class MockitoDoSetup extends BugChecker implements CompilationUnitTreeMatcher {
+  @Override
+  public Description matchCompilationUnit(CompilationUnitTree tree, VisitorState state) {
+    ImmutableSet<VarSymbol> spies = findSpies(state);
+    new SuppressibleTreePathScanner<Void, Void>(state) {
+
+      @Override
+      public Void visitMethodInvocation(MethodInvocationTree tree, Void unused) {
+        handle(tree);
+        return super.visitMethodInvocation(tree, null);
+      }
+
+      private void handle(MethodInvocationTree tree) {
+        if (!DO_STUBBER.matches(tree, state)) {
+          return;
+        }
+        TreePath whenPath = getCurrentPath().getParentPath().getParentPath();
+        Tree whenCall = whenPath.getLeaf();
+        if (!(whenCall instanceof MethodInvocationTree)
+            || !INSTANCE_WHEN.matches((MethodInvocationTree) whenCall, state)) {
+          return;
+        }
+        if (isSpy(((MethodInvocationTree) whenCall).getArguments().get(0))) {
+          return;
+        }
+        Tree mockedMethod = whenPath.getParentPath().getParentPath().getLeaf();
+
+        if (!(mockedMethod instanceof MethodInvocationTree)) {
+          return;
+        }
+        if (isSameType(
+            getSymbol((MethodInvocationTree) mockedMethod).getReturnType(),
+            state.getSymtab().voidType,
+            state)) {
+          return;
+        }
+
+        SuggestedFix.Builder fix = SuggestedFix.builder();
+        var when = SuggestedFixes.qualifyStaticImport("org.mockito.Mockito.when", fix, state);
+        fix.replace(((MethodInvocationTree) whenCall).getMethodSelect(), when)
+            .replace(state.getEndPosition(whenCall) - 1, state.getEndPosition(whenCall), "")
+            .postfixWith(
+                mockedMethod,
+                format(
+                    ").%s(%s)",
+                    NAME_MAPPINGS.get(getSymbol(tree).getSimpleName().toString()),
+                    getParameterSource(tree, state)));
+
+        state.reportMatch(describeMatch(tree, fix.build()));
+      }
+
+      private boolean isSpy(ExpressionTree tree) {
+        var symbol = getSymbol(tree);
+        return symbol != null
+            && (spies.contains(symbol) || hasAnnotation(symbol, "org.mockito.Spy", state));
+      }
+    }.scan(state.getPath(), null);
+    return NO_MATCH;
+  }
+
+  private static String getParameterSource(MethodInvocationTree tree, VisitorState state) {
+    return state
+        .getSourceCode()
+        .subSequence(
+            getStartPosition(tree.getArguments().get(0)),
+            state.getEndPosition(getLast(tree.getArguments())))
+        .toString();
+  }
+
+  private static ImmutableSet<VarSymbol> findSpies(VisitorState state) {
+    // NOTES: This is extremely conservative in at least two ways.
+    // 1) We ignore an entire mock if _any_ method is mocked to throw, not just the relevant method.
+    // 2) We could still refactor if the thenThrow comes _after_, or if the _only_ call is
+    // thenThrow.
+    ImmutableSet.Builder<VarSymbol> spiesOrThrows = ImmutableSet.builder();
+    new TreePathScanner<Void, Void>() {
+      @Override
+      public Void visitVariable(VariableTree tree, Void unused) {
+        if (tree.getInitializer() != null && SPY.matches(tree.getInitializer(), state)) {
+          spiesOrThrows.add(getSymbol(tree));
+        }
+        return super.visitVariable(tree, null);
+      }
+
+      @Override
+      public Void visitMethodInvocation(MethodInvocationTree tree, Void unused) {
+        if (DO_THROW.matches(tree, state)) {
+          var whenCall = getCurrentPath().getParentPath().getParentPath().getLeaf();
+          if ((whenCall instanceof MethodInvocationTree)
+              && INSTANCE_WHEN.matches((MethodInvocationTree) whenCall, state)) {
+            var whenTarget = getSymbol(((MethodInvocationTree) whenCall).getArguments().get(0));
+            if (whenTarget instanceof VarSymbol) {
+              spiesOrThrows.add((VarSymbol) whenTarget);
+            }
+          }
+        }
+        if (THEN_THROW.matches(tree, state)) {
+          var receiver = getReceiver(tree);
+          if (STATIC_WHEN.matches(receiver, state)) {
+            var mock = getReceiver(((MethodInvocationTree) receiver).getArguments().get(0));
+            var mockSymbol = getSymbol(mock);
+            if (mockSymbol instanceof VarSymbol) {
+              spiesOrThrows.add((VarSymbol) mockSymbol);
+            }
+          }
+        }
+        return super.visitMethodInvocation(tree, null);
+      }
+
+      @Override
+      public Void visitAssignment(AssignmentTree tree, Void unused) {
+        if (SPY.matches(tree.getExpression(), state)) {
+          var symbol = getSymbol(tree.getVariable());
+          if (symbol instanceof VarSymbol) {
+            spiesOrThrows.add((VarSymbol) symbol);
+          }
+        }
+        return super.visitAssignment(tree, null);
+      }
+    }.scan(state.getPath().getCompilationUnit(), null);
+    return spiesOrThrows.build();
+  }
+
+  private static final ImmutableMap<String, String> NAME_MAPPINGS =
+      ImmutableMap.of(
+          "doAnswer", "thenAnswer",
+          "doReturn", "thenReturn",
+          "doThrow", "thenThrow");
+  private static final Matcher<ExpressionTree> DO_STUBBER =
+      staticMethod().onClass("org.mockito.Mockito").namedAnyOf(NAME_MAPPINGS.keySet());
+
+  private static final Matcher<ExpressionTree> INSTANCE_WHEN =
+      instanceMethod().onDescendantOf("org.mockito.stubbing.Stubber").named("when");
+
+  private static final Matcher<ExpressionTree> SPY =
+      staticMethod().onClass("org.mockito.Mockito").named("spy");
+
+  private static final Matcher<ExpressionTree> DO_THROW =
+      staticMethod().onClass("org.mockito.Mockito").named("doThrow");
+
+  private static final Matcher<ExpressionTree> STATIC_WHEN =
+      staticMethod().onClass("org.mockito.Mockito").named("when");
+
+  private static final Matcher<ExpressionTree> THEN_THROW =
+      instanceMethod().onDescendantOf("org.mockito.stubbing.OngoingStubbing").named("thenThrow");
+}

core/src/main/java/com/google/errorprone/scanner/BuiltInCheckerSuppliers.java

@@ -244,6 +244,7 @@
 import com.google.errorprone.bugpatterns.MixedDescriptors;
 import com.google.errorprone.bugpatterns.MixedMutabilityReturnType;
 import com.google.errorprone.bugpatterns.MockNotUsedInProduction;
+import com.google.errorprone.bugpatterns.MockitoDoSetup;
 import com.google.errorprone.bugpatterns.MockitoUsage;
 import com.google.errorprone.bugpatterns.ModifiedButNotUsed;
 import com.google.errorprone.bugpatterns.ModifyCollectionInEnhancedForLoop;
@@ -1169,6 +1170,7 @@ public static ScannerSupplier warningChecks() {
           MissingBraces.class,
           MissingDefault.class,
           MixedArrayDimensions.class,
+          MockitoDoSetup.class,
           MoreThanOneQualifier.class,
           MultiVariableDeclaration.class,
           MultipleTopLevelClasses.class,

core/src/test/java/com/google/errorprone/bugpatterns/MockitoDoSetupTest.java

@@ -0,0 +1,120 @@
+/*
+ * Copyright 2024 The Error Prone Authors.
+ *
+ * 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.google.errorprone.bugpatterns;
+
+import com.google.errorprone.BugCheckerRefactoringTestHelper;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public final class MockitoDoSetupTest {
+  private final BugCheckerRefactoringTestHelper helper =
+      BugCheckerRefactoringTestHelper.newInstance(MockitoDoSetup.class, getClass());
+
+  @Test
+  public void happy() {
+    helper
+        .addInputLines(
+            "Test.java",
+            "import org.mockito.Mockito;",
+            "public class Test {",
+            "  public int test(Test test) {",
+            "    Mockito.doReturn(1).when(test).test(null);",
+            "    return 1;",
+            "  }",
+            "}")
+        .addOutputLines(
+            "Test.java",
+            "import static org.mockito.Mockito.when;",
+            "import org.mockito.Mockito;",
+            "public class Test {",
+            "  public int test(Test test) {",
+            "    when(test.test(null)).thenReturn(1);",
+            "    return 1;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void ignoresSpiesCreatedByAnnotation() {
+    helper
+        .addInputLines(
+            "Test.java",
+            "import org.mockito.Mockito;",
+            "public class Test {",
+            "  @org.mockito.Spy Test test;",
+            "  public int test() {",
+            "    Mockito.doReturn(1).when(test).test();",
+            "    return 1;",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
+
+  @Test
+  public void ignoresSpiesCreatedByStaticMethod() {
+    helper
+        .addInputLines(
+            "Test.java",
+            "import org.mockito.Mockito;",
+            "public class Test {",
+            "  Test test = Mockito.spy(Test.class);",
+            "  public int test() {",
+            "    Mockito.doReturn(1).when(test).test();",
+            "    return 1;",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
+
+  @Test
+  public void ignoresMocksConfiguredToThrow_viaThenThrow() {
+    helper
+        .addInputLines(
+            "Test.java",
+            "import org.mockito.Mockito;",
+            "public class Test {",
+            "  public int test(Test test) {",
+            "    Mockito.doReturn(1).when(test).test(null);",
+            "    Mockito.when(test.test(null)).thenThrow(new Exception());",
+            "    return 1;",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
+
+  @Test
+  public void ignoresMocksConfiguredToThrow_viaDoThrow() {
+    helper
+        .addInputLines(
+            "Test.java",
+            "import org.mockito.Mockito;",
+            "public class Test {",
+            "  public int test(Test test) {",
+            "    Mockito.doReturn(1).when(test).test(null);",
+            "    Mockito.doThrow(new Exception()).when(test).test(null);",
+            "    return 1;",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
+}

docs/bugpattern/MockitoDoSetup.md

@@ -0,0 +1,22 @@
+Prefer using the format
+
+```java
+  when(mock.mockedMethod(...)).thenReturn(returnValue);
+```
+
+to initialise mocks, rather than,
+
+```java
+  doReturn(returnValue).when(mock).mockedMethod(...);
+```
+
+Mockito recommends the `when`/`thenReturn` syntax as it is both more readable
+and provides type-safety: the return type of the stubbed method is checked
+against the stubbed value at compile time.
+
+There are certain situations where `doReturn` is required:
+
+*   Overriding previous stubbing where the method will *throw*, as `when` makes
+    an actual method call.
+*   Overriding a `spy` where the method call where calling the spied method
+    brings undesired side-effects.