Add ErrorProne MockitoInternalUsage checker

This checker will warn on any usage of a Mockito internal symbol. Since
we do not guarantee API stability on symbols in our internal packages,
users should not rely on them. This checker can be used to enforce that
they are unused.

subprojects/errorprone/src/main/java/org/mockito/errorprone/bugpatterns/MockitoInternalUsage.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2017 Mockito contributors
+ * This program is made available under the terms of the MIT License.
+ */
+package org.mockito.errorprone.bugpatterns;
+
+import static com.google.errorprone.matchers.Matchers.packageStartsWith;
+
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.BugPattern.ProvidesFix;
+import com.google.errorprone.BugPattern.SeverityLevel;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.bugpatterns.BugChecker.MemberSelectTreeMatcher;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.google.errorprone.util.ASTHelpers;
+import com.sun.source.tree.MemberSelectTree;
+import com.sun.source.tree.Tree;
+import com.sun.tools.javac.code.Symbol;
+
+/**
+ * {@link com.google.errorprone.bugpatterns.BugChecker} that detects usages of
+ * org.mockito.internal.*
+ */
+@BugPattern(
+    name = "MockitoInternalUsage",
+    summary = "org.mockito.internal.* is a private API and should not be used by clients",
+    explanation =
+        "Classes under `org.mockito.internal.*` are internal implementation details and are"
+            + " not part of Mockito's public API. Mockito team does not support them, and they"
+            + " may change at any time. Depending on them may break your code when you upgrade"
+            + " to new versions of Mockito."
+            + "This checker ensures that your code will not break with future Mockito upgrades."
+            + "Mockito's public API is documented at"
+            + " https://www.javadoc.io/doc/org.mockito/mockito-core/. If you believe that there"
+            + " is no replacement available in the public API for your use-case, contact the"
+            + " Mockito team at https://github.com/mockito/mockito/issues.",
+    severity = SeverityLevel.WARNING,
+    providesFix = ProvidesFix.REQUIRES_HUMAN_ATTENTION)
+public class MockitoInternalUsage extends BugChecker implements MemberSelectTreeMatcher {
+
+  private static final Matcher<Tree> INSIDE_MOCKITO = packageStartsWith("org.mockito");
+
+  @Override
+  public Description matchMemberSelect(MemberSelectTree tree, VisitorState state) {
+    if (INSIDE_MOCKITO.matches(tree, state)) {
+      return Description.NO_MATCH;
+    }
+    Symbol symbol = ASTHelpers.getSymbol(tree);
+    if (symbol != null && symbol.getQualifiedName().toString().startsWith("org.mockito.internal")) {
+      return describeMatch(tree);
+    }
+    return Description.NO_MATCH;
+  }
+}

subprojects/errorprone/src/test/java/org/mockito/errorprone/bugpatterns/MockitoInternalUsageTest.java

@@ -0,0 +1,117 @@
+/*
+ * Copyright 2018 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 org.mockito.errorprone.bugpatterns;
+
+import com.google.errorprone.CompilationTestHelper;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/**
+ * Verify that there are no usages of Mockito internal implementations.
+ *
+ * @author tvanderlippe@google.com (Tim van der Lippe)
+ */
+@RunWith(JUnit4.class)
+public class MockitoInternalUsageTest {
+
+  private CompilationTestHelper compilationHelper;
+
+  @Before
+  public void setUp() {
+    compilationHelper = CompilationTestHelper.newInstance(MockitoInternalUsage.class, getClass());
+  }
+
+  @Test
+  public void testPositiveCases() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "import java.io.Serializable;",
+            "class MockitoInternalUsagePositiveCases {",
+            "  public void newObject() {",
+            "    // BUG: Diagnostic contains:",
+            "    new org.mockito.internal.MockitoCore();",
+            "    // BUG: Diagnostic contains:",
+            "    new InternalConsumer(new org.mockito.internal.MockitoCore());",
+            "  }",
+            "  public void staticMethodInvocation() {",
+            "    // BUG: Diagnostic contains:",
+            "    org.mockito.internal.configuration.GlobalConfiguration.validate();",
+            "  }",
+            "  public void variableTypeDeclaration() {",
+            "    // BUG: Diagnostic contains:",
+            "    org.mockito.internal.stubbing.InvocationContainerImpl container = null;",
+            "  }",
+            "  // BUG: Diagnostic contains:",
+            "  class ExtendsClause extends org.mockito.internal.MockitoCore {}",
+            "  // BUG: Diagnostic contains:",
+            "  abstract class ImplementsClause implements"
+                + " org.mockito.internal.junit.MockitoTestListener {}",
+            "  abstract class SecondImplementsClause",
+            "      // BUG: Diagnostic contains:",
+            "      implements Serializable, org.mockito.internal.junit.MockitoTestListener {}",
+            "  // BUG: Diagnostic contains:",
+            "  class ExtendsGeneric<T extends org.mockito.internal.stubbing.InvocationContainerImpl>"
+                + " {}",
+            "  // BUG: Diagnostic contains:",
+            "  class SecondExtendsGeneric<R, T extends"
+                + " org.mockito.internal.stubbing.InvocationContainerImpl> {}",
+            "  class FieldClause {",
+            "    // BUG: Diagnostic contains:",
+            "    org.mockito.internal.MockitoCore core;",
+            "  }",
+            "  class MethodArgumentClause {",
+            "    // BUG: Diagnostic contains:",
+            "    public void methodArgument(org.mockito.internal.MockitoCore core) {}",
+            "  }",
+            "  static class InternalConsumer {",
+            "    // BUG: Diagnostic contains:",
+            "    InternalConsumer(org.mockito.internal.MockitoCore core) {}",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void testPositiveImportsCases() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "package foo;",
+            "// BUG: Diagnostic contains:",
+            "import org.mockito.internal.MockitoCore;",
+            "// BUG: Diagnostic contains:",
+            "import org.mockito.internal.stubbing.InvocationContainerImpl;",
+            "// BUG: Diagnostic contains:",
+            "import org.mockito.internal.*;",
+            "class Test {}")
+        .doTest();
+  }
+
+  @Test
+  public void testNegativeCases() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "package org.mockito;",
+            "import org.mockito.internal.MockitoCore;",
+            "class Mockito {}")
+        .doTest();
+  }
+}