用Google的Gson严格分析JSON？ [英] Strict JSON parsing with Google's Gson?

问题描述

假设我使用Google的Gson库将JSON解析为Java数据结构。

如果有一个Java字段有一个简单的方法来抛出异常没有相应的JSON？也就是说，我希望要求JSON具有Java结构中的所有字段。

解决方案

Gson没有JSON模式验证功能用于指定特定元素必须存在，并且它没有办法指定必须填充Java成员。可能有这样的功能可用，比如使用 @Required 注释。请前往 Gson问题列表，并提出增强请求。

使用Gson，您可以执行指定的JSON元素与自定义的反序列化器一起存在。

  //输出：
 // [MyObject：element1 = value1，element2 = value2，element3 = value3] 
 // [MyObject：element1 = value1，element2 = value2，element3 = null] 
 //线程main中的异常com.google.gson.JsonParseException：必填字段未找到：element2 
 
 import java.lang.reflect.Type; 
 import java.util.ArrayList; 
 import java.util.List; 
 
 import com.google.gson.Gson; 
 import com.google.gson.GsonBuilder; 
 import com.google.gson.JsonDeserializationContext; 
 import com.google.gson.JsonDeserializer; 
 import com.google.gson.JsonElement; 
 import com.google.gson.JsonObject; 
 import com.google.gson.JsonParseException; 
 
 public class Foo 
 {
 static String jsonInput1 ={\element1 \：\value1 \，\element2 \： \ value2\，\ element3\：\ value3\}; 
 static String jsonInput2 ={\element1 \：\value1 \，\element2 \：\value2 \}; 
 static String jsonInput3 ={\element1 \：\value1 \，\element3 \：\value3 \}; 
 
 public static void main（String [] args）
 {
 GsonBuilder gsonBuilder = new GsonBuilder（）; 
 MyDeserializer deserializer = new MyDeserializer（）; 
 deserializer.registerRequiredField（element2）; 
 gsonBuilder.registerTypeAdapter（MyObject.class，deserializer）; 
 Gson gson = gsonBuilder.create（）; 
 MyObject object1 = gson.fromJson（jsonInput1，MyObject.class）; 
 System.out.println（object1）; 
 MyObject object2 = gson.fromJson（jsonInput2，MyObject.class）; 
 System.out.println（object2）; 
 MyObject object3 = gson.fromJson（jsonInput3，MyObject.class）; 
 System.out.println（object3）; 
} 
} 
 
 class MyObject 
 {
 String element1; 
 String element2; 
 String element3; 
 
 @Override 
 public String toString（）
 {
 return String.format（
[MyObject：element1 =％s，element2 =％s ，element3 =％s]，
 element1，element2，element3）; 
} 
} 
 
类MyDeserializer实现了JsonDeserializer< MyObject> 
 {
列表< String> requiredFields = new ArrayList< String>（）; 
 
 void registerRequiredField（String fieldName）
 {
 requiredFields.add（fieldName）; 
 
 $ b @Override 
 public MyObject反序列化（JsonElement json，类型typeOfT，JsonDeserializationContext上下文）
抛出JsonParseException 
 {
 JsonObject jsonObject = （JsonObject）json; 
 for（String fieldName：requiredFields）
 {
 if（jsonObject.get（fieldName）== null）
 {
 throw new JsonParseException（Required Field Not Found ：+ fieldName）; 
} 
} 
返回新的Gson（）。fromJson（json，MyObject.class）; 
} 
} 
  

更好的方法可能是使用API JSON模式验证。 杰克逊至少有一个基本的实施可用。 JSON工具看起来比较成熟。

这是杰克逊的一个例子。

  //输出：
 //验证jsonInput1 ... 
 / /验证jsonInput2 ... 
 //验证jsonInput3 ... 
 // $ .element2：缺失并且不是可选
 // [MyObject：element1 = value1，element2 = value2 ，element3 = value3] 
 // [MyObject：element1 = value1，element2 = value2，element3 = null] 
 // [MyObject：element1 = value1，element2 = null，element3 = value3] 
 
 import java.util.List; 
 
导入org.codehaus.jackson.map.ObjectMapper; 
 
导入eu.vahlas.json.schema.JSONSchema; 
 import eu.vahlas.json.schema.JSONSchemaProvider; 
 import eu.vahlas.json.schema.impl.JacksonSchemaProvider; 
 
 public class Foo 
 {
 static String jsonSchema = 
{+ 
\description \：\Serialized MyObject规格\，+ 
\type\：[\object \]，+ 
\properties \：+ 
{+ 
\element1 \：{\type \：\string \}，+ 
\element2 \ ：{\type \：\string \，\optional \：false}，+ 
\element3 \：{\type \\ \\：\string \，\optional \：true}+ 
}+ 
};; 
 
 static String jsonInput1 ={\element1 \：\value1 \，\element2 \：\value2 \，\element3 \ ：\ value3\ }; 
 static String jsonInput2 ={\element1 \：\value1 \，\element2 \：\value2 \}; 
 static String jsonInput3 ={\element1 \：\value1 \，\element3 \：\value3 \}; 
 
 public static void main（String [] args）抛出异常
 {
 ObjectMapper mapper = new ObjectMapper（）; 
 JSONSchemaProvider schemaProvider = new JacksonSchemaProvider（mapper）; 
 JSONSchema schema = schemaProvider.getSchema（jsonSchema）; 
 
 System.out.println（Validating jsonInput1 ...）; 
 validateAndLogErrors（jsonInput1，schema）; 
 System.out.println（Validating jsonInput2 ...）; 
 validateAndLogErrors（jsonInput2，schema）; 
 System.out.println（Validating jsonInput3 ...）; 
 validateAndLogErrors（jsonInput3，schema）; 
 
 MyObject object1 = mapper.readValue（jsonInput1，MyObject.class）; 
 System.out.println（object1）; 
 MyObject object2 = mapper.readValue（jsonInput2，MyObject.class）; 
 System.out.println（object2）; 
 MyObject object3 = mapper.readValue（jsonInput3，MyObject.class）; 
 System.out.println（object3）; 
 
 
 static void validateAndLogErrors（String jsonInput，JSONSchema schema）
 {
 List< String> errors = schema.validate（jsonInput）; 
 for（String error：errors）
 {
 System.out.println（error）; 
} 
} 
} 
 
 class MyObject 
 {
 String element1; 
 String element2; 
 String element3; 
 
 void setElement1（String element1）
 {
 this.element1 = element1; 
} 
 
 void setElement2（String element2）
 {
 this.element2 = element2; 
} 
 
 void setElement3（String element3）
 {
 this.element3 = element3; 
 
 $ b @Override 
 public String toString（）
 {
 return String.format（
[MyObject：element1 =％s ，element2 =％s，element3 =％s]，
 element1，element2，element3）; 
} 
} 
  

Suppose I am using Google's Gson library to parse JSON into Java data structures.

Is there an easy way to throw an exception if there is a Java field that has no corresponding JSON? That is, I wish to require the JSON to have all the fields in the Java structure.

解决方案

Gson doesn't have a JSON schema validation feature to specify that a particular element must be present, and it doesn't have a way to specify that a Java member must be populated. It might be nice to have such a feature available, such as with an @Required annotation. Head on over to the Gson Issues List and put in an enhancement request.

With Gson, you could enforce that specified JSON elements are present with a custom deserializer.

// output: 
//   [MyObject: element1=value1, element2=value2, element3=value3]
//   [MyObject: element1=value1, element2=value2, element3=null]
//   Exception in thread "main" com.google.gson.JsonParseException: Required Field Not Found: element2

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.List;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;

public class Foo
{
  static String jsonInput1 = "{\"element1\":\"value1\",\"element2\":\"value2\",\"element3\":\"value3\"}";
  static String jsonInput2 = "{\"element1\":\"value1\",\"element2\":\"value2\"}";
  static String jsonInput3 = "{\"element1\":\"value1\",\"element3\":\"value3\"}";

  public static void main(String[] args)
  {
    GsonBuilder gsonBuilder = new GsonBuilder();
    MyDeserializer deserializer = new MyDeserializer();
    deserializer.registerRequiredField("element2");
    gsonBuilder.registerTypeAdapter(MyObject.class, deserializer);
    Gson gson = gsonBuilder.create();
    MyObject object1 = gson.fromJson(jsonInput1, MyObject.class);
    System.out.println(object1);
    MyObject object2 = gson.fromJson(jsonInput2, MyObject.class);
    System.out.println(object2);
    MyObject object3 = gson.fromJson(jsonInput3, MyObject.class);
    System.out.println(object3);
  }
}

class MyObject
{
  String element1;
  String element2;
  String element3;

  @Override
  public String toString()
  {
    return String.format(
        "[MyObject: element1=%s, element2=%s, element3=%s]",
        element1, element2, element3);
  }
}

class MyDeserializer implements JsonDeserializer<MyObject>
{
  List<String> requiredFields = new ArrayList<String>();

  void registerRequiredField(String fieldName)
  {
    requiredFields.add(fieldName);
  }

  @Override
  public MyObject deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
      throws JsonParseException
  {
    JsonObject jsonObject = (JsonObject) json;
    for (String fieldName : requiredFields)
    {
      if (jsonObject.get(fieldName) == null)
      {
        throw new JsonParseException("Required Field Not Found: " + fieldName);
      }
    }
    return new Gson().fromJson(json, MyObject.class);
  }
}

A preferable approach might be to use an API that provides JSON Schema validation. Jackson has at least a rudimentary implementation available. JSON Tools looks to have a more mature one.

Here's an example with Jackson.

// output: 
// Validating jsonInput1...
// Validating jsonInput2...
// Validating jsonInput3...
// $.element2: is missing and it is not optional
// [MyObject: element1=value1, element2=value2, element3=value3]
// [MyObject: element1=value1, element2=value2, element3=null]
// [MyObject: element1=value1, element2=null, element3=value3]

import java.util.List;

import org.codehaus.jackson.map.ObjectMapper;

import eu.vahlas.json.schema.JSONSchema;
import eu.vahlas.json.schema.JSONSchemaProvider;
import eu.vahlas.json.schema.impl.JacksonSchemaProvider;

public class Foo
{
  static String jsonSchema = 
    "{" + 
        "\"description\":\"Serialized MyObject Specification\"," + 
        "\"type\":[\"object\"]," + 
        "\"properties\":" + 
        "{" + 
            "\"element1\":{\"type\":\"string\"}," + 
            "\"element2\":{\"type\":\"string\",\"optional\":false}," + 
            "\"element3\":{\"type\":\"string\",\"optional\":true}" + 
        "}" + 
    "}";;

  static String jsonInput1 = "{\"element1\":\"value1\",\"element2\":\"value2\",\"element3\":\"value3\"}";
  static String jsonInput2 = "{\"element1\":\"value1\",\"element2\":\"value2\"}";
  static String jsonInput3 = "{\"element1\":\"value1\",\"element3\":\"value3\"}";

  public static void main(String[] args) throws Exception
  {
    ObjectMapper mapper = new ObjectMapper();
    JSONSchemaProvider schemaProvider = new JacksonSchemaProvider(mapper);
    JSONSchema schema = schemaProvider.getSchema(jsonSchema);

    System.out.println("Validating jsonInput1...");
    validateAndLogErrors(jsonInput1, schema);
    System.out.println("Validating jsonInput2...");
    validateAndLogErrors(jsonInput2, schema);
    System.out.println("Validating jsonInput3...");
    validateAndLogErrors(jsonInput3, schema);

    MyObject object1 = mapper.readValue(jsonInput1, MyObject.class);
    System.out.println(object1);
    MyObject object2 = mapper.readValue(jsonInput2, MyObject.class);
    System.out.println(object2);
    MyObject object3 = mapper.readValue(jsonInput3, MyObject.class);
    System.out.println(object3);
  }

  static void validateAndLogErrors(String jsonInput, JSONSchema schema)
  {
    List<String> errors = schema.validate(jsonInput);
    for (String error : errors)
    {
      System.out.println(error);
    }
  }
}

class MyObject
{
  String element1;
  String element2;
  String element3;

  void setElement1(String element1)
  {
    this.element1 = element1;
  }

  void setElement2(String element2)
  {
    this.element2 = element2;
  }

  void setElement3(String element3)
  {
    this.element3 = element3;
  }

  @Override
  public String toString()
  {
    return String.format(
        "[MyObject: element1=%s, element2=%s, element3=%s]",
        element1, element2, element3);
  }
}

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