Cleanup CMake makefiles for nlohmann_json.

CHANGELOG.md

@@ -26,6 +26,8 @@ Increment the:
   [1871](https://github.com/open-telemetry/opentelemetry-cpp/pull/1871)
 * [BUILD] Migrate from @bazel_tools//platforms to [Bazel Platforms](https://github.com/bazelbuild/platforms)
   to enable Bazel 6.0.0 compatibility [#1873](https://github.com/open-telemetry/opentelemetry-cpp/pull/1873)
+* [BUILD] Cleanup CMake makefiles for nlohmann_json
+  [#1912](https://github.com/open-telemetry/opentelemetry-cpp/pull/1912)
 
 ## [1.8.1] 2022-12-04
 

CMakeLists.txt

@@ -340,32 +340,9 @@ if(WITH_ELASTICSEARCH
 else()
   set(USE_NLOHMANN_JSON OFF)
 endif()
+
 if((NOT WITH_API_ONLY) AND USE_NLOHMANN_JSON)
-  # nlohmann_json package is required for most SDK build configurations
-  find_package(nlohmann_json QUIET)
-  set(nlohmann_json_clone FALSE)
-  if(nlohmann_json_FOUND)
-    message("Using external nlohmann::json")
-  elseif(EXISTS ${PROJECT_SOURCE_DIR}/.git
-         AND EXISTS
-             ${PROJECT_SOURCE_DIR}/third_party/nlohmann-json/CMakeLists.txt)
-    message("Trying to use local nlohmann::json from submodule")
-    set(JSON_BuildTests
-        OFF
-        CACHE INTERNAL "")
-    set(JSON_Install
-        ON
-        CACHE INTERNAL "")
-    # This option allows to link nlohmann_json::nlohmann_json target
-    add_subdirectory(${PROJECT_SOURCE_DIR}/third_party/nlohmann-json)
-    # This option allows to add header to include directories
-    include_directories(
-      ${PROJECT_SOURCE_DIR}/third_party/nlohmann-json/single_include)
-  else()
-    set(nlohmann_json_clone TRUE)
-    include(cmake/nlohmann-json.cmake)
-    message("\nnlohmann_json package was not found. Cloning from github")
-  endif()
+  include(cmake/nlohmann-json.cmake)
 endif()
 
 if(OTELCPP_MAINTAINER_MODE)

cmake/nlohmann-json.cmake

@@ -1,42 +1,98 @@
-if("${nlohmann-json}" STREQUAL "")
-    set(nlohmann-json "develop")
-endif()
-include(ExternalProject)
-ExternalProject_Add(nlohmann_json_download
-    PREFIX third_party
-    GIT_REPOSITORY https://github.com/nlohmann/json.git
-    GIT_TAG
-        "${nlohmann-json}"
-    UPDATE_COMMAND ""
-    CMAKE_ARGS
-      -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
-      -DJSON_BuildTests=OFF
-      -DJSON_Install=ON
-      -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
-    TEST_AFTER_INSTALL
-        0
-    DOWNLOAD_NO_PROGRESS
-        1
-    LOG_CONFIGURE
-        1
-    LOG_BUILD
-        1
-    LOG_INSTALL
-        1
-)
+# Copyright The OpenTelemetry Authors
+# SPDX-License-Identifier: Apache-2.0
+
+#
+# The dependency on nlohmann_json can be provided different ways.
+# By order of decreasing priority, options are:
+#
+# 1 - Search for a nlohmann_json package
+#
+# Packages installed on the local machine are used if found.
+#
+# The nlohmann_json dependency is not installed,
+# as it already is.
+#
+# 2 - Search for a nlohmann_json git submodule
+#
+# When git submodule is used,
+# the nlohmann_json code is located in:
+# third_party/nlohmann-json
+#
+# The nlohmann_json dependency is installed,
+# by building the sub directory with JSON_Install=ON
+#
+# 3 - Download nlohmann_json from github
+#
+# Code from the development branch is used,
+# unless a specific release tag is provided
+# in variable ${nlohmann-json}
+#
+# The nlohmann_json dependency is installed,
+# by building the downloaded code with JSON_Install=ON
+#
+
 
-ExternalProject_Get_Property(nlohmann_json_download INSTALL_DIR)
-SET(NLOHMANN_JSON_INCLUDE_DIR ${INSTALL_DIR}/src/nlohmann_json_download/single_include)
-add_library(nlohmann_json_ INTERFACE)
-target_include_directories(nlohmann_json_ INTERFACE
-    "$<BUILD_INTERFACE:${NLOHMANN_JSON_INCLUDE_DIR}>"
-    "$<INSTALL_INTERFACE:include>")
-add_dependencies(nlohmann_json_ nlohmann_json_download)
-add_library(nlohmann_json::nlohmann_json ALIAS nlohmann_json_)
+# nlohmann_json package is required for most SDK build configurations
+find_package(nlohmann_json QUIET)
+if(nlohmann_json_FOUND)
+  message(STATUS "nlohmann::json dependency satisfied by: package")
+elseif(EXISTS ${PROJECT_SOURCE_DIR}/.git
+       AND EXISTS
+           ${PROJECT_SOURCE_DIR}/third_party/nlohmann-json/CMakeLists.txt)
+  message(STATUS "nlohmann::json dependency satisfied by: git submodule")
+  set(JSON_BuildTests
+      OFF
+      CACHE INTERNAL "")
+  set(JSON_Install
+      ON
+      CACHE INTERNAL "")
+  # This option allows to link nlohmann_json::nlohmann_json target
+  add_subdirectory(${PROJECT_SOURCE_DIR}/third_party/nlohmann-json)
+  # This option allows to add header to include directories
+  include_directories(
+    ${PROJECT_SOURCE_DIR}/third_party/nlohmann-json/single_include)
+else()
+  if("${nlohmann-json}" STREQUAL "")
+      set(nlohmann-json "develop")
+  endif()
+  message(STATUS "nlohmann::json dependency satisfied by: github download")
+  include(ExternalProject)
+  ExternalProject_Add(nlohmann_json_download
+      PREFIX third_party
+      GIT_REPOSITORY https://github.com/nlohmann/json.git
+      GIT_TAG
+          "${nlohmann-json}"
+      UPDATE_COMMAND ""
+      CMAKE_ARGS
+        -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
+        -DJSON_BuildTests=OFF
+        -DJSON_Install=ON
+        -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
+      TEST_AFTER_INSTALL
+          0
+      DOWNLOAD_NO_PROGRESS
+          1
+      LOG_CONFIGURE
+          1
+      LOG_BUILD
+          1
+      LOG_INSTALL
+          1
+  )
 
-install(
-  TARGETS nlohmann_json_
-  EXPORT "${PROJECT_NAME}-target"
-  RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
-  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
-  ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
+  ExternalProject_Get_Property(nlohmann_json_download INSTALL_DIR)
+  SET(NLOHMANN_JSON_INCLUDE_DIR ${INSTALL_DIR}/src/nlohmann_json_download/single_include)
+  add_library(nlohmann_json_ INTERFACE)
+  target_include_directories(nlohmann_json_ INTERFACE
+      "$<BUILD_INTERFACE:${NLOHMANN_JSON_INCLUDE_DIR}>"
+      "$<INSTALL_INTERFACE:include>")
+  add_dependencies(nlohmann_json_ nlohmann_json_download)
+  add_library(nlohmann_json::nlohmann_json ALIAS nlohmann_json_)
+
+  install(
+    TARGETS nlohmann_json_
+    EXPORT "${PROJECT_NAME}-target"
+    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
+endif()