8071961: Add javac lint warning when a default constructor is created

Reviewed-by: jjg, jlahoda, abuckley, erikj, mcimadamore

make/CompileJavaModules.gmk

@@ -76,6 +76,7 @@ java.datatransfer_COPY += flavormap.properties
 
 ################################################################################
 
+java.desktop_DISABLED_WARNINGS += missing-explicit-ctor
 java.desktop_DOCLINT += -Xdoclint:all/protected,-reference \
     '-Xdoclint/package:java.*,javax.*'
 java.desktop_COPY += .gif .png .wav .txt .xml .css .pf
@@ -298,6 +299,10 @@ java.xml.crypto_CLEAN += .properties
 
 ################################################################################
 
+jdk.accessibility_DISABLED_WARNINGS += missing-explicit-ctor
+
+################################################################################
+
 jdk.charsets_COPY += .dat
 
 ################################################################################
@@ -347,10 +352,19 @@ jdk.javadoc_COPY += .xml .css .js .png
 
 ################################################################################
 
+jdk.jartool_DISABLED_WARNINGS += missing-explicit-ctor
 jdk.jartool_JAVAC_FLAGS += -XDstringConcat=inline
 
 ################################################################################
 
+jdk.httpserver_DISABLED_WARNINGS += missing-explicit-ctor
+
+################################################################################
+
+jdk.unsupported.desktop_DISABLED_WARNINGS += missing-explicit-ctor
+
+################################################################################
+
 # No SCTP implementation on Mac OS X or AIX. These classes should be excluded.
 SCTP_IMPL_CLASSES = \
     $(TOPDIR)/src/jdk.sctp/unix/classes/sun/nio/ch/sctp/AssociationChange.java \

src/jdk.compiler/share/classes/com/sun/tools/javac/code/Lint.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -210,6 +210,11 @@ public class Lint
          */
         FINALLY("finally"),
 
+        /**
+          * Warn about compiler generation of a default constructor.
+          */
+        MISSING_EXPLICIT_CTOR("missing-explicit-ctor"),
+
         /**
          * Warn about module system related issues.
          */

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java

@@ -29,6 +29,7 @@ import java.util.*;
 import java.util.function.Supplier;
 
 import javax.lang.model.element.ElementKind;
+import javax.lang.model.element.NestingKind;
 import javax.tools.JavaFileManager;
 
 import com.sun.tools.javac.code.*;
@@ -3827,6 +3828,59 @@ public class Check {
         }
     }
 
+    /**
+     * Check for a default constructor in an exported package.
+     */
+    void checkDefaultConstructor(ClassSymbol c, DiagnosticPosition pos) {
+        if (lint.isEnabled(LintCategory.MISSING_EXPLICIT_CTOR) &&
+            ((c.flags() & (ENUM | RECORD)) == 0) &&
+            !c.isAnonymous() &&
+            ((c.flags() & PUBLIC) != 0) &&
+            Feature.MODULES.allowedInSource(source)) {
+            NestingKind nestingKind = c.getNestingKind();
+            switch (nestingKind) {
+                case ANONYMOUS,
+                     LOCAL -> {return;}
+                case TOP_LEVEL -> {;} // No additional checks needed
+                case MEMBER -> {
+                    // For nested member classes, all the enclosing
+                    // classes must be public.
+                    Symbol owner = c.owner;
+                    while (owner != null && owner.kind == TYP) {
+                        if ((owner.flags() & PUBLIC) == 0)
+                            return;
+                        owner = owner.owner;
+                    }
+                }
+            }
+
+            // Only check classes in named packages exported by its module
+            PackageSymbol pkg = c.packge();
+            if (!pkg.isUnnamed()) {
+                ModuleSymbol modle = pkg.modle;
+                for (ExportsDirective exportDir : modle.exports) {
+                    // Report warning only if the containing
+                    // package is unconditionally exported
+                    if (exportDir.packge.equals(pkg)) {
+                        if (exportDir.modules == null || exportDir.modules.isEmpty()) {
+                            // Warning may be suppressed by
+                            // annotations; check again for being
+                            // enabled in the deferred context.
+                            deferredLintHandler.report(() -> {
+                                if (lint.isEnabled(LintCategory.MISSING_EXPLICIT_CTOR))
+                                   log.warning(LintCategory.MISSING_EXPLICIT_CTOR,
+                                               pos, Warnings.MissingExplicitCtor(c, pkg, modle));
+                                                       });
+                        } else {
+                            return;
+                        }
+                    }
+                }
+            }
+        }
+        return;
+    }
+
     private class ConversionWarner extends Warner {
         final String uncheckedKey;
         final Type found;

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/TypeEnter.java

@@ -1000,6 +1000,7 @@ public class TypeEnter implements Completer {
             // Add default constructor if needed.
             DefaultConstructorHelper helper = getDefaultConstructorHelper(env);
             if (helper != null) {
+                chk.checkDefaultConstructor(sym, tree.pos());
                 defaultConstructor = defaultConstructor(make.at(tree.pos), helper);
                 tree.defs = tree.defs.prepend(defaultConstructor);
             }

src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties

@@ -1760,6 +1760,10 @@ compiler.warn.dir.path.element.not.found=\
 compiler.warn.dir.path.element.not.directory=\
     bad path element "{0}": not a directory
 
+# 0: symbol, 1: symbol, 2: symbol
+compiler.warn.missing-explicit-ctor=\
+    class {0} in exported package {1} declares no explicit constructors, thereby exposing a default constructor to clients of module {2}
+
 compiler.warn.finally.cannot.complete=\
     finally clause cannot complete normally
 

src/jdk.compiler/share/classes/com/sun/tools/javac/resources/javac.properties

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 1999, 2019, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 1999, 2020, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # This code is free software; you can redistribute it and/or modify it
@@ -182,6 +182,9 @@ javac.opt.Xlint.desc.cast=\
 javac.opt.Xlint.desc.classfile=\
     Warn about issues related to classfile contents.
 
+javac.opt.Xlint.desc.missing-explicit-ctor=\
+    Warn about missing explicit constructors in public classes in exported packages.
+
 javac.opt.Xlint.desc.deprecation=\
     Warn about use of deprecated items.
 

test/langtools/tools/javac/diags/examples/WarnDefaultCtor/DefaultCtor.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+// key: compiler.warn.missing-explicit-ctor
+// options: -Xlint:missing-explicit-ctor

test/langtools/tools/javac/diags/examples/WarnDefaultCtor/modulesourcepath/defaultctor/module-info.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+module defaultctor {
+    exports pkg;
+}

test/langtools/tools/javac/diags/examples/WarnDefaultCtor/modulesourcepath/defaultctor/pkg/Foo.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package pkg;
+
+public class Foo {
+    // No explicit constructor.
+}

test/langtools/tools/javac/warnings/DefaultCtor/DefaultCtorWarningToolBox.java

@@ -0,0 +1,369 @@
+/*
+ * Copyright (c) 2019, 2020, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8071961
+ * @summary Verify expected default constructor warnings are producted or not produced
+ * @library /tools/lib
+ * @modules jdk.compiler/com.sun.tools.javac.api
+ *          jdk.compiler/com.sun.tools.javac.main
+ * @build toolbox.ToolBox toolbox.JavacTask toolbox.TestRunner
+ * @run main DefaultCtorWarningToolBox
+ */
+
+import java.io.IOException;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Arrays;
+import java.util.List;
+
+import java.io.InputStream;
+import java.nio.file.Files;
+import java.util.EnumSet;
+import javax.tools.JavaFileManager;
+import javax.tools.JavaFileObject;
+import javax.tools.StandardLocation;
+import javax.tools.ToolProvider;
+import toolbox.JavacTask;
+import toolbox.Task;
+import toolbox.Task.Expect;
+import toolbox.TestRunner;
+import toolbox.ToolBox;
+
+public class DefaultCtorWarningToolBox extends TestRunner {
+
+    private final ToolBox tb = new ToolBox();
+    private final String fileSep = System.getProperty("file.separator");
+
+    public DefaultCtorWarningToolBox() {
+        super(System.err);
+    }
+
+    public static void main(String... args) throws Exception {
+        new DefaultCtorWarningToolBox().runTests();
+    }
+
+    @Test
+    public void testWithAndWithOutLint(Path base) throws IOException {
+        Path src = base.resolve("src");
+
+        tb.writeJavaFiles(src,
+                          MOD_INFO_SRC,
+                          PKG1_BAR_SRC, PKG1_FOO_SRC,
+                          PKG2_BAZ_SRC, PKG2_QUUX_SRC,
+                          PKG3_CORGE_SRC, PKG3_GRAULT_SRC
+                          );
+        Path classes = base.resolve("classes");
+        tb.createDirectories(classes);
+
+        List<String> log;
+        List<String> expected = List.of("");
+
+        // Warning disabled, no messages expected
+        log = new JavacTask(tb)
+                .options("-Xlint:-missing-explicit-ctor", "-Werror")
+                .outdir(classes)
+                .files(tb.findJavaFiles(src))
+                .run(Expect.SUCCESS)
+                .writeAll()
+                .getOutputLines(Task.OutputKind.DIRECT);
+
+        if (!expected.equals(log)) {
+            throw new AssertionError("Unexpected output: " + log);
+        }
+
+        expected =
+            List.of("Foo.java:4:8: compiler.warn.missing-explicit-ctor: pkg1.Foo, pkg1, mod",
+                    "Foo.java:12:12: compiler.warn.missing-explicit-ctor: pkg1.Foo.FooNest, pkg1, mod",
+                    "Foo.java:16:19: compiler.warn.missing-explicit-ctor: pkg1.Foo.StaticFooNest, pkg1, mod",
+                    "3 warnings");
+
+        // Warning enable,
+        log = new JavacTask(tb)
+            .options("-Xlint:missing-explicit-ctor", "-XDrawDiagnostics")
+                .outdir(classes)
+                .files(tb.findJavaFiles(src))
+                .run(Expect.SUCCESS)
+                .writeAll()
+                .getOutputLines(Task.OutputKind.DIRECT);
+
+        if (!expected.equals(log)) {
+            throw new AssertionError("Unexpected output: " + log);
+        }
+    }
+
+    protected void runTests() throws Exception {
+        runTests(m -> new Object[] { Paths.get(m.getName()).toAbsolutePath() });
+    }
+
+    private static final String MOD_INFO_SRC =
+        """
+        module mod {
+            exports pkg1;
+            // Do *not* export pkg2.
+             exports pkg3 to java.base;
+        }
+        """;
+
+    private static final String PKG1_BAR_SRC =
+        """
+        package pkg1;
+
+        // Neither the top-level class nor the nested classes should generate
+        // a warning since Bar is not public.
+
+        // No explicit constructor; use a default.
+        class Bar {
+
+            // No explicit constructor; use a default.
+            public class FooNest {
+            }
+
+            // No explicit constructor; use a default.
+            public static class StaticFooNest {
+            }
+
+            // Package-access classes
+
+            // No explicit constructor; use a default.
+            /*package*/ class PkgFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            /*package*/ static class PkgStaticFooNest {
+            }
+            // Private classes
+
+            // No explicit constructor; use a default.
+            private class PrvFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            private static class PrvStaticFooNest {
+            }
+        }
+        """;
+
+    private  static final String PKG1_FOO_SRC =
+        """
+        package pkg1;
+
+        // No explicit constructor; use a default.
+        public class Foo {
+
+            /*
+             * Of the nexted classes, only FooNest and StaticFooNest should
+             * generate warnings.
+             */
+
+            // No explicit constructor; use a default.
+            public class FooNest {
+            }
+
+            // No explicit constructor; use a default.
+            public static class StaticFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            @SuppressWarnings("missing-explicit-ctor")
+            public static class SuppressedStaticFooNest {
+            }
+
+            // Package-access classes
+
+            // No explicit constructor; use a default.
+            /*package*/ class PkgFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            /*package*/ static class PkgStaticFooNest {
+            }
+            // Private classes
+
+            // No explicit constructor; use a default.
+            private class PrvFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            private static class PrvStaticFooNest {
+            }
+        }
+        """;
+
+    private static final String PKG2_BAZ_SRC =
+        """
+        package pkg2;
+
+        // None of these classes should generate warnings since pkg2 is not
+        // exported unconditionally.
+
+        // No explicit constructor; use a default.
+        public class Baz {
+
+            // No explicit constructor; use a default.
+            public class FooNest {
+            }
+
+            // No explicit constructor; use a default.
+            public static class StaticFooNest {
+            }
+
+            // Package-access classes
+
+            // No explicit constructor; use a default.
+            /*package*/ class PkgFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            /*package*/ static class PkgStaticFooNest {
+            }
+            // Private classes
+
+            // No explicit constructor; use a default.
+            private class PrvFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            private static class PrvStaticFooNest {
+            }
+        }
+        """;
+
+    private static final String PKG2_QUUX_SRC =
+        """
+        package pkg2;
+
+        // Neither the top-level class nor the nested classes should generate
+        // a warning since Bar is not public.
+
+        // No explicit constructor; use a default.
+        class Quux {
+
+            // No explicit constructor; use a default.
+            public class FooNest {
+            }
+
+            // No explicit constructor; use a default.
+            public static class StaticFooNest {
+            }
+
+            // Package-access classes
+
+            // No explicit constructor; use a default.
+            /*package*/ class PkgFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            /*package*/ static class PkgStaticFooNest {
+            }
+            // Private classes
+
+            // No explicit constructor; use a default.
+            private class PrvFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            private static class PrvStaticFooNest {
+            }
+        }
+        """;
+
+    private static final String PKG3_CORGE_SRC =
+        """
+        package pkg3;
+
+        // None of these classes should generate warnings since pkg3 is not
+        // exported unconditionally.
+
+        // No explicit constructor; use a default.
+        public class Corge {
+
+            // No explicit constructor; use a default.
+            public class FooNest {
+            }
+
+            // No explicit constructor; use a default.
+            public static class StaticFooNest {
+            }
+
+            // Package-access classes
+
+            // No explicit constructor; use a default.
+            /*package*/ class PkgFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            /*package*/ static class PkgStaticFooNest {
+            }
+            // Private classes
+
+            // No explicit constructor; use a default.
+            private class PrvFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            private static class PrvStaticFooNest {
+            }
+        }
+        """;
+
+    private static final String PKG3_GRAULT_SRC =
+        """
+        package pkg3;
+
+        // None of these classes should generate warnings since pkg3 is not
+        // exported unconditionally.
+
+        // No explicit constructor; use a default.
+        class Grault {
+
+            // No explicit constructor; use a default.
+            public class FooNest {
+            }
+
+            // No explicit constructor; use a default.
+            public static class StaticFooNest {
+            }
+
+            // Package-access classes
+
+            // No explicit constructor; use a default.
+            /*package*/ class PkgFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            /*package*/ static class PkgStaticFooNest {
+            }
+            // Private classes
+
+            // No explicit constructor; use a default.
+            private class PrvFooNest {
+            }
+
+            // No explicit constructor; use a default.
+            private static class PrvStaticFooNest {
+            }
+        }
+        """;
+}

test/langtools/tools/javac/warnings/DefaultCtor/NoWarningCases.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8071961
+ * @compile -Xlint:missing-explicit-ctor -Werror --release 8 NoWarningCases.java
+ * @compile -Xlint:missing-explicit-ctor -Werror             NoWarningCases.java
+ */
+
+public class NoWarningCases {
+    // No explicit constructor; use a default.
+
+    public enum NestedEnum {
+        FOO,
+        BAR;
+        // No explicit constructor; use implicit one.
+    }
+}

test/langtools/tools/javac/warnings/DefaultCtor/NoWarningRecord.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8071961
+ * @compile -Xlint:missing-explicit-ctor -Werror
+ *          --enable-preview -source ${jdk.version} NoWarningRecord.java
+ */
+
+public record NoWarningRecord(/* no components */) {
+    // No explicit constructor; use canonical one.
+}