aosp12/system/extras/libjsonpb

libjsonpbparse

This library provides functions to parse a JSON file to a structured Protobuf message.

At this time of writing, libprotobuf-cpp-full is at version 3.0.0-beta, and unknown fields in a JSON file cannot be ignored. Do NOT use this library in vendor / recovery until libprotobuf-cpp-full is updated.

Using libjsoncpp in parser code

Since libjsonpbparse cannot be used in vendor / recovery processes yet, libjsoncpp is used instead. However, there are notable differences in the logic of libjsoncpp and libprotobuf when parsing JSON files.

• There are no implicit string to integer conversion in libjsoncpp. Hence:
  • If the Protobuf schema uses 64-bit integers ((s|fixed|u|)int64):
    • The JSON file must use strings (to pass tests in libjsonpbverify)
    • Parser code (that uses libjsoncpp) must explicitly convert strings to integers. Example:

      strtoull(value.asString(), 0, 10)
      

  • If the Protobuf schema uses special floating point values:
    • The JSON file must use strings (e.g. "NaN", "Infinity", "-Infinity")
    • Parser code must explicitly handle these cases. Example:

      double d;
      if (value.isNumeric()) {
        d = value.asDouble();
      } else {
        auto&& s = value.asString();
        if (s == "NaN") d = std::numeric_limits<double>::quiet_NaN();
        else if (s == "Infinity") d = std::numeric_limits<double>::infinity();
        else if (s == "-Infinity") d = -std::numeric_limits<double>::infinity();
      }
      

• libprotobuf accepts either lowerCamelCase (or json_name option if it is defined) or the original field name as keys in the input JSON file. The test in libjsonpbverify explicitly check this case to avoid ambiguity; only the original field name (or json_name option if it is defined) can be used.

Once libprotobuf in the source tree is updated to a higher version and libjsonpbparse is updated to ignore unknown fields in JSON files, all parsing code must be converted to use libjsonpbparse for consistency.

libjsonpbverify

This library provides functions and tests to examine a JSON file and validate it against a Protobuf message definition.

In addition to a validity check that libprotobuf can convert the JSON file to a Protobuf message (using libjsonpbparse), it also checks the following:

• Whether there are fields unknown to the schema. All fields in the JSON file must be well defined in the schema.
• Whether the Protobuf file defines JSON keys clearly. The JSON keys must be the json_name option of a Protobuf field, or name of a Protobuf field if json_name is not defined. lowerCamelCase supported by libprotobuf is explicitly disallowed (unless explicitly used in json_name). For example, in the following Protobuf file, only keys foo_bar and barBaz are allowed in the JSON file:

  message Foo {
      string foo_bar = 1;
      string bar_baz = 2 [json_name = "barBaz"];
  }
  

• Whether json == convert_to_json(convert_to_pb(json)), using libprotobuf. This imposes additional restrictions including:
  • Enum values must be present as names (not integer values) in the JSON file.
  • 64-bit integers and special floating point values (infinity, NaN) must always be strings.

Defining a JSON schema using Protobuf

Check JSON Mapping before defining a Protobuf object as a JSON schema. In general:

• Use proto3. libjsonverify does not support proto2.
• JSON booleans should be bool.
• JSON numbers should be (s|fixed|u|)int32, float, or double in the schema
• JSON strings are generally strings, but if you want to impose more restrictions on the string, you can also use Timestamp, bytes, float or double (if NaN and infinity are valid values), enumerations, etc.
  • If a custom enumeration is used, parser code should NOT error when the enumeration value name is unknown, as enumeration definitions may be extended in the future.
• JSON arrays should be repeated fields.
• JSON objects should be a well-defined message, unless you have a good reason to use map<string, T>.
• Don't use Any; it defeats the purpose of having the schema.

Validating a JSON file against a Protobuf definition

Example:

#include <jsonpb/verify.h>
using namespace ::android::jsonpb;
std::unique_ptr<JsonSchemaTestConfig> CreateCgroupsParam() {

}
INSTANTIATE_TEST_SUITE_P(LibProcessgroupProto, JsonSchemaTest,
                         ::testing::Values(MakeTestParam<Cgroups>("cgroups.json")));