Support to auto-generate Java files from flatbuffer (#3749)

* auto gen flatbuffers for Java

* Add auto_gen_tool.py

* Refine

* Add a comment

* address comments.

* Address comments.

* Addressed

* Refine

* Address comments

* Fix typo

* Add exception

* Address comments.

* Refine

* Fix lint

* Fix

* Fix lint and address comment.

* Fix lint error

java/modify_generated_java_flatbuffers_files.py

@@ -0,0 +1,55 @@
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import os
+import sys
+
+"""
+This script is used for modifying the generated java flatbuffer
+files for the reason: The package declaration in Java is different
+from python and C++, and there is no option in the flatc command
+to specify package(namepsace) for Java specially.
+
+USAGE:
+    python modify_generated_java_flatbuffers_file.py RAY_HOME
+
+RAY_HOME: The root directory of Ray project.
+"""
+
+# constants declarations
+PACKAGE_DECLARATION = "package org.ray.runtime.generated;"
+
+
+def add_new_line(file, line_num, text):
+    with open(file, "r") as file_handler:
+        lines = file_handler.readlines()
+        if (line_num <= 0) or (line_num > len(lines) + 1):
+            return False
+
+    lines.insert(line_num - 1, text + os.linesep)
+    with open(file, "w") as file_handler:
+        for line in lines:
+            file_handler.write(line)
+
+    return True
+
+
+def add_package_declarations(generated_root_path):
+    file_names = os.listdir(generated_root_path)
+    for file_name in file_names:
+        if not file_name.endswith(".java"):
+            continue
+        full_name = os.path.join(generated_root_path, file_name)
+        success = add_new_line(full_name, 2, PACKAGE_DECLARATION)
+        if not success:
+            raise RuntimeError("Failed to add package declarations, "
+                               "file name is %s" % full_name)
+
+
+if __name__ == "__main__":
+    ray_home = sys.argv[1]
+    root_path = os.path.join(
+        ray_home,
+        "java/runtime/src/main/java/org/ray/runtime/generated")
+    add_package_declarations(root_path)