[Kotlin] Attach JvmStatic annotation to each method in companion object (#6052)

* Attach JvmStatic annotation to each method of companion object

Kotlin does not have static accessor so companion object used instead of static.
It's so natural. But when use kotlin companion object methods on java it is very inconvenient.
```java
GeneratedClassByFlatBuffer.Companion.someMethod()
```

If use @JvmStatic annotation it can be shorten like below.
```java
GeneratedClassByFlatBuffer.someMethod()
```

* Formatting by Idea Google C++ style

* Add comments - Commit for missing cla

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Add comments - Commit for missing cla

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Reset code formatting except modified lines

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Pass missing flag to validateVersion method

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Add annotations to missing method in companion object

* addVector
* createVector
* endVector
* tableCreator

And also I tried add compiler option for generate annotation who don't like this operation.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Modify unmatched option name in compiler usage

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Add missing operation for finishSizePrefixed and finishStructBuffer method.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Add compiled example with --kotlin-gen-jvmstatic option.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Fix Compiler.md

Change option name from --gen-jvm-static-annotation to --kotlin-gen-jvmstatic

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Use IDLOptions reference instead of bool parameter.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Change option name - kotlin_gen_jvmstatic to gen_jvmstatic

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Use IDLOptions reference instead of bool parameter and missing process @JvmStatic as suffix.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Add code generation for --gen-jvmstatic option

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Miss typo directory for including.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Miss typo variable suffix for including.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Fix camel case to snake case.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Rollback generate code for gen_jvmstatic option.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Delete generated test files.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* add missing new line at end of file.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

* Remove generated test file by command line.

Signed-off-by: Yoon KyongSik <sam1287@gmail.com>

Co-authored-by: sam <sam@jennifersoft.com>

4 files changed, 75 insertions, 44 deletions

diff --git a/docs/source/Compiler.md b/docs/source/Compiler.md
index 87b4275c..e5d065c4 100644
--- a/docs/source/Compiler.md
+++ b/docs/source/Compiler.md
@@ -116,6 +116,9 @@ Additional options:
 
 -   `--gen-generated` : Add @Generated annotation for Java.
 
+-   `--gen-jvmstatic` : Add @JvmStatic annotation for Kotlin methods
+    in companion object for interop from Java to Kotlin.
+
 -   `--gen-all` : Generate not just code for the current schema files, but
     for all files it includes as well. If the language uses a single file for
     output (by default the case for C++ and JS), all code will end up in
diff --git a/include/flatbuffers/idl.h b/include/flatbuffers/idl.h
index 70a2b787..847c976a 100644
--- a/include/flatbuffers/idl.h
+++ b/include/flatbuffers/idl.h
@@ -509,6 +509,7 @@ struct ServiceDef : public Definition {
 
 // Container of options that may apply to any of the source/text generators.
 struct IDLOptions {
+  bool gen_jvmstatic;
   // Use flexbuffers instead for binary and text generation
   bool use_flexbuffers;
   bool strict_json;
@@ -604,7 +605,8 @@ struct IDLOptions {
   bool set_empty_vectors_to_null;
 
   IDLOptions()
-      : use_flexbuffers(false),
+      : gen_jvmstatic(false),
+        use_flexbuffers(false),
         strict_json(false),
         skip_js_exports(false),
         use_goog_js_export_format(false),
diff --git a/src/flatc.cpp b/src/flatc.cpp
index 947e78d7..4a77cce5 100644
--- a/src/flatc.cpp
+++ b/src/flatc.cpp
@@ -106,6 +106,8 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     "  --gen-nullable         Add Clang _Nullable for C++ pointer. or @Nullable for Java\n"
     "  --java-checkerframe    work Add @Pure for Java.\n"
     "  --gen-generated        Add @Generated annotation for Java\n"
+    "  --gen-jvmstatic        Add @JvmStatic annotation for Kotlin methods\n"
+    "                         in companion object for interop from Java to Kotlin.\n"
     "  --gen-all              Generate not just code for the current schema files,\n"
     "                         but for all files it includes as well.\n"
     "                         If the language uses a single file for output (by default\n"
@@ -363,6 +365,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.cs_gen_json_serializer = true;
       } else if (arg == "--flexbuffers") {
         opts.use_flexbuffers = true;
+      } else if (arg == "--gen-jvmstatic") {
+        opts.gen_jvmstatic = true;
       } else if (arg == "--cpp-std") {
         if (++argi >= argc)
           Error("missing C++ standard specification" + arg, true);
diff --git a/src/idl_gen_kotlin.cpp b/src/idl_gen_kotlin.cpp
index df032f77..2c0c8d43 100644
--- a/src/idl_gen_kotlin.cpp
+++ b/src/idl_gen_kotlin.cpp
@@ -88,7 +88,7 @@ class KotlinGenerator : public BaseGenerator {
       auto &struct_def = **it;
       if (!parser_.opts.one_file)
         cur_name_space_ = struct_def.defined_namespace;
-      GenStruct(struct_def, structWriter);
+      GenStruct(struct_def, structWriter, parser_.opts);
       if (parser_.opts.one_file) {
         one_file_code += structWriter.ToString();
       } else {
@@ -276,7 +276,7 @@ class KotlinGenerator : public BaseGenerator {
           if (enum_def.MinValue()->IsNonZero())
             writer += " - " + enum_def.MinValue()->name + ".toInt()\\";
           writer += "]";
-        });
+        }, parser_.opts.gen_jvmstatic);
       }
     });
     writer.DecrementIdentLevel();
@@ -417,7 +417,7 @@ class KotlinGenerator : public BaseGenerator {
     return key_offset;
   }
 
-  void GenStruct(StructDef &struct_def, CodeWriter &writer) const {
+  void GenStruct(StructDef &struct_def, CodeWriter &writer, IDLOptions options) const {
     if (struct_def.generated) return;
 
     GenerateComment(struct_def.doc_comment, writer, &comment_config);
@@ -458,13 +458,13 @@ class KotlinGenerator : public BaseGenerator {
           // runtime.
           GenerateFunOneLine(writer, "validateVersion", "", "", [&]() {
             writer += "Constants.FLATBUFFERS_1_12_0()";
-          });
+          }, options.gen_jvmstatic);
 
-          GenerateGetRootAsAccessors(Esc(struct_def.name), writer);
-          GenerateBufferHasIdentifier(struct_def, writer);
-          GenerateTableCreator(struct_def, writer);
+          GenerateGetRootAsAccessors(Esc(struct_def.name), writer, options);
+          GenerateBufferHasIdentifier(struct_def, writer, options);
+          GenerateTableCreator(struct_def, writer, options);
 
-          GenerateStartStructMethod(struct_def, writer);
+          GenerateStartStructMethod(struct_def, writer, options);
 
           // Static Add for fields
           auto fields = struct_def.fields.vec;
@@ -474,29 +474,29 @@ class KotlinGenerator : public BaseGenerator {
             field_pos++;
             if (field.deprecated) continue;
             if (field.key) key_field = &field;
-            GenerateAddField(NumToString(field_pos), field, writer);
+            GenerateAddField(NumToString(field_pos), field, writer, options);
 
             if (field.value.type.base_type == BASE_TYPE_VECTOR) {
               auto vector_type = field.value.type.VectorType();
               if (!IsStruct(vector_type)) {
-                GenerateCreateVectorField(field, writer);
+                GenerateCreateVectorField(field, writer, options);
               }
-              GenerateStartVectorField(field, writer);
+              GenerateStartVectorField(field, writer, options);
             }
           }
 
-          GenerateEndStructMethod(struct_def, writer);
+          GenerateEndStructMethod(struct_def, writer, options);
           auto file_identifier = parser_.file_identifier_;
           if (parser_.root_struct_def_ == &struct_def) {
-            GenerateFinishStructBuffer(struct_def, file_identifier, writer);
-            GenerateFinishSizePrefixed(struct_def, file_identifier, writer);
+            GenerateFinishStructBuffer(struct_def, file_identifier, writer, options);
+            GenerateFinishSizePrefixed(struct_def, file_identifier, writer, options);
           }
 
           if (struct_def.has_key) {
-            GenerateLookupByKey(key_field, struct_def, writer);
+            GenerateLookupByKey(key_field, struct_def, writer, options);
           }
         } else {
-          GenerateStaticConstructor(struct_def, writer);
+          GenerateStaticConstructor(struct_def, writer, options);
         }
       });
     }
@@ -508,7 +508,7 @@ class KotlinGenerator : public BaseGenerator {
 
   // TODO: move key_field to reference instead of pointer
   void GenerateLookupByKey(FieldDef *key_field, StructDef &struct_def,
-                           CodeWriter &writer) const {
+                           CodeWriter &writer, const IDLOptions options) const {
     std::stringstream params;
     params << "obj: " << Esc(struct_def.name) << "?"
            << ", ";
@@ -564,31 +564,34 @@ class KotlinGenerator : public BaseGenerator {
       writer += "return null";
     };
     GenerateFun(writer, "__lookup_by_key", params.str(),
-                Esc(struct_def.name) + "?", statements);
+                Esc(struct_def.name) + "?", statements, options.gen_jvmstatic);
   }
 
   void GenerateFinishSizePrefixed(StructDef &struct_def,
                                   const std::string &identifier,
-                                  CodeWriter &writer) const {
+                                  CodeWriter &writer,
+                                  const IDLOptions options) const {
     auto id = identifier.length() > 0 ? ", \"" + identifier + "\"" : "";
     auto params = "builder: FlatBufferBuilder, offset: Int";
     auto method_name = "finishSizePrefixed" + Esc(struct_def.name) + "Buffer";
     GenerateFunOneLine(writer, method_name, params, "", [&]() {
       writer += "builder.finishSizePrefixed(offset" + id + ")";
-    });
+    }, options.gen_jvmstatic);
   }
   void GenerateFinishStructBuffer(StructDef &struct_def,
                                   const std::string &identifier,
-                                  CodeWriter &writer) const {
+                                  CodeWriter &writer,
+                                  const IDLOptions options) const {
     auto id = identifier.length() > 0 ? ", \"" + identifier + "\"" : "";
     auto params = "builder: FlatBufferBuilder, offset: Int";
     auto method_name = "finish" + Esc(struct_def.name) + "Buffer";
     GenerateFunOneLine(writer, method_name, params, "",
-                       [&]() { writer += "builder.finish(offset" + id + ")"; });
+                       [&]() { writer += "builder.finish(offset" + id + ")"; },
+                       options.gen_jvmstatic);
   }
 
-  void GenerateEndStructMethod(StructDef &struct_def,
-                               CodeWriter &writer) const {
+  void GenerateEndStructMethod(StructDef &struct_def, CodeWriter &writer,
+                               const IDLOptions options) const {
     // Generate end{{TableName}}(builder: FlatBufferBuilder) method
     auto name = "end" + Esc(struct_def.name);
     auto params = "builder: FlatBufferBuilder";
@@ -606,11 +609,12 @@ class KotlinGenerator : public BaseGenerator {
       }
       writer.DecrementIdentLevel();
       writer += "return o";
-    });
+    }, options.gen_jvmstatic);
   }
 
   // Generate a method to create a vector from a Kotlin array.
-  void GenerateCreateVectorField(FieldDef &field, CodeWriter &writer) const {
+  void GenerateCreateVectorField(FieldDef &field, CodeWriter &writer,
+                                 const IDLOptions options) const {
     auto vector_type = field.value.type.VectorType();
     auto method_name = "create" + MakeCamel(Esc(field.name)) + "Vector";
     auto params = "builder: FlatBufferBuilder, data: " +
@@ -628,10 +632,11 @@ class KotlinGenerator : public BaseGenerator {
       writer.DecrementIdentLevel();
       writer += "}";
       writer += "return builder.endVector()";
-    });
+    }, options.gen_jvmstatic);
   }
 
-  void GenerateStartVectorField(FieldDef &field, CodeWriter &writer) const {
+  void GenerateStartVectorField(FieldDef &field, CodeWriter &writer,
+                                const IDLOptions options) const {
     // Generate a method to start a vector, data to be added manually
     // after.
     auto vector_type = field.value.type.VectorType();
@@ -643,11 +648,11 @@ class KotlinGenerator : public BaseGenerator {
         writer, "start" + MakeCamel(Esc(field.name) + "Vector", true), params,
         "", [&]() {
           writer += "builder.startVector({{size}}, numElems, {{align}})";
-        });
+        }, options.gen_jvmstatic);
   }
 
   void GenerateAddField(std::string field_pos, FieldDef &field,
-                        CodeWriter &writer) const {
+                        CodeWriter &writer, const IDLOptions options) const {
     auto field_type = GenTypeBasic(field.value.type.base_type);
     auto secondArg = MakeCamel(Esc(field.name), false) + ": " + field_type;
     GenerateFunOneLine(writer, "add" + MakeCamel(Esc(field.name), true),
@@ -662,7 +667,7 @@ class KotlinGenerator : public BaseGenerator {
 
                          writer += "builder.add{{method_name}}({{pos}}, \\";
                          writer += "{{field_name}}{{cast}}, {{default}})";
-                       });
+                       }, options.gen_jvmstatic);
   }
 
   static std::string ToSignedType(const Type &type) {
@@ -703,17 +708,18 @@ class KotlinGenerator : public BaseGenerator {
   }
 
   // fun startMonster(builder: FlatBufferBuilder) = builder.startTable(11)
-  void GenerateStartStructMethod(StructDef &struct_def,
-                                 CodeWriter &code) const {
+  void GenerateStartStructMethod(StructDef &struct_def, CodeWriter &code,
+                                 const IDLOptions options) const {
     GenerateFunOneLine(code, "start" + Esc(struct_def.name),
                        "builder: FlatBufferBuilder", "", [&]() {
                          code += "builder.startTable(" +
                                  NumToString(struct_def.fields.vec.size()) +
                                  ")";
-                       });
+                       }, options.gen_jvmstatic);
   }
 
-  void GenerateTableCreator(StructDef &struct_def, CodeWriter &writer) const {
+  void GenerateTableCreator(StructDef &struct_def, CodeWriter &writer,
+                            const IDLOptions options) const {
     // Generate a method that creates a table in one go. This is only possible
     // when the table has no struct fields, since those have to be created
     // inline, and there's no way to do so in Java.
@@ -776,11 +782,11 @@ class KotlinGenerator : public BaseGenerator {
           }
         }
         writer += "return end{{struct_name}}(builder)";
-      });
+      }, options.gen_jvmstatic);
     }
   }
   void GenerateBufferHasIdentifier(StructDef &struct_def,
-                                   CodeWriter &writer) const {
+                                   CodeWriter &writer, IDLOptions options) const {
     auto file_identifier = parser_.file_identifier_;
     // Check if a buffer has the identifier.
     if (parser_.root_struct_def_ != &struct_def || !file_identifier.length())
@@ -790,7 +796,7 @@ class KotlinGenerator : public BaseGenerator {
                        "Boolean", [&]() {
                          writer += "__has_identifier(_bb, \"" +
                                    file_identifier + "\")";
-                       });
+                       }, options.gen_jvmstatic);
   }
 
   void GenerateStructGetters(StructDef &struct_def, CodeWriter &writer) const {
@@ -1218,18 +1224,21 @@ class KotlinGenerator : public BaseGenerator {
   }
 
   static void GenerateGetRootAsAccessors(const std::string &struct_name,
-                                         CodeWriter &writer) {
+                                         CodeWriter &writer,
+                                         IDLOptions options) {
     // Generate a special accessor for the table that when used as the root
     // ex: fun getRootAsMonster(_bb: ByteBuffer): Monster {...}
     writer.SetValue("gr_name", struct_name);
     writer.SetValue("gr_method", "getRootAs" + struct_name);
 
     // create convenience method that doesn't require an existing object
+    GenerateJvmStaticAnnotation(writer, options.gen_jvmstatic);
     writer += "fun {{gr_method}}(_bb: ByteBuffer): {{gr_name}} = \\";
     writer += "{{gr_method}}(_bb, {{gr_name}}())";
 
     // create method that allows object reuse
     // ex: fun Monster getRootAsMonster(_bb: ByteBuffer, obj: Monster) {...}
+    GenerateJvmStaticAnnotation(writer, options.gen_jvmstatic);
     writer +=
         "fun {{gr_method}}"
         "(_bb: ByteBuffer, obj: {{gr_name}}): {{gr_name}} {";
@@ -1243,13 +1252,14 @@ class KotlinGenerator : public BaseGenerator {
   }
 
   static void GenerateStaticConstructor(const StructDef &struct_def,
-                                        CodeWriter &code) {
+                                        CodeWriter &code,
+                                        const IDLOptions options) {
     // create a struct constructor function
     auto params = StructConstructorParams(struct_def);
     GenerateFun(code, "create" + Esc(struct_def.name), params, "Int", [&]() {
       GenStructBody(struct_def, code, "");
       code += "return builder.offset()";
-    });
+    }, options.gen_jvmstatic);
   }
 
   static std::string StructConstructorParams(const StructDef &struct_def,
@@ -1323,7 +1333,8 @@ class KotlinGenerator : public BaseGenerator {
   static void GenerateFun(CodeWriter &writer, const std::string &name,
                           const std::string &params,
                           const std::string &returnType,
-                          const std::function<void()> &body) {
+                          const std::function<void()> &body,
+                          bool gen_jvmstatic = false) {
     // Generates Kotlin function
     // e.g.:
     // fun path(j: Int): Vec3 {
@@ -1333,6 +1344,7 @@ class KotlinGenerator : public BaseGenerator {
     writer.SetValue("name", name);
     writer.SetValue("params", params);
     writer.SetValue("return_type", noreturn ? "" : ": " + returnType);
+    GenerateJvmStaticAnnotation(writer, gen_jvmstatic);
     writer += "fun {{name}}({{params}}) {{return_type}} {";
     writer.IncrementIdentLevel();
     body();
@@ -1343,7 +1355,8 @@ class KotlinGenerator : public BaseGenerator {
   static void GenerateFunOneLine(CodeWriter &writer, const std::string &name,
                                  const std::string &params,
                                  const std::string &returnType,
-                                 const std::function<void()> &body) {
+                                 const std::function<void()> &body,
+                                 bool gen_jvmstatic = false) {
     // Generates Kotlin function
     // e.g.:
     // fun path(j: Int): Vec3 = return path(Vec3(), j)
@@ -1351,6 +1364,7 @@ class KotlinGenerator : public BaseGenerator {
     writer.SetValue("params", params);
     writer.SetValue("return_type_p",
                     returnType.empty() ? "" : " : " + returnType);
+    GenerateJvmStaticAnnotation(writer, gen_jvmstatic);
     writer += "fun {{name}}({{params}}){{return_type_p}} = \\";
     body();
   }
@@ -1428,6 +1442,14 @@ class KotlinGenerator : public BaseGenerator {
     }
   }
 
+  // Prepend @JvmStatic to methods in companion object.
+  static void GenerateJvmStaticAnnotation(CodeWriter &code,
+                                          bool gen_jvmstatic) {
+    if (gen_jvmstatic) {
+      code += "@JvmStatic";
+    }
+  }
+
   // This tracks the current namespace used to determine if a type need to be
   // prefixed by its namespace
   const Namespace *cur_name_space_;