探究一下Gson中的高级主题

最新推荐文章于 2024-10-17 14:19:48 发布

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本文介绍 Gson 库在 Java 中处理复杂 JSON 数据的高级技巧，包括对象嵌套序列化与反序列化、自定义序列化器和反序列化器、解决对象引用重复问题等。

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经过Gson最简单入门，Gson常用用法之后，再探讨一下Gson中的高级主题.这里主要讨论两个，对象的嵌套&&对象的引用.

对象嵌套

定义要解析的POJO——Book&&Author

package com.example;

public class Author {

    private int id;
    private String name;
    //getter && setter
    //为了便于观察效果，这里重载一下toString
     @Override
    public String toString() {
        return "id: "+id+"\tname: "+name;
    }

package com.example;

import java.util.Arrays;

public class Book {

    private Author[] authors;
    private String title;
      //getter && setter
    //为了便于观察效果，这里重载一下toString
     @Override
    public String toString() {
        String infoMsg = "Book POJO msg as follow:\n";
        System.out.println(infoMsg);
        return "title:  " + title + "\t\t\tauthors: " + Arrays.toString(authors);
    }

使用Annotation
- 调用&&测试
这里Author&&Book中的成员变量和JSON中的key一致

package com.example;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import java.io.IOException;

public class Main {

    public static void main(final String[] args) throws IOException {
        // Configure GSON
        final GsonBuilder gsonBuilder = new GsonBuilder();
        //在生产环境中最好屏蔽，由于JSON格式化，可能产生大的JSON对象
        gsonBuilder.setPrettyPrinting();
        final Gson gson = gsonBuilder.create();

        final Book javaPuzzlers = new Book();
        javaPuzzlers.setTitle("English ABC");

        Author authorA=new Author();
        authorA.setId(1235);
        authorA.setName("gu");

        Author authorB=new Author();
        authorB .setId(1432);
        authorB.setName("hang");

        Author[] authors={authorA,authorB};

        javaPuzzlers.setAuthors(authors);

        // Format to JSON
        final String json = gson.toJson(javaPuzzlers);
        System.out.println(json);

    }
}

打印结果：

{
  "authors": [
    {
      "id": 1235,
      "name": "gu"
    },
    {
      "id": 1432,
      "name": "hang"
    }
  ],
  "title": "English ABC"
}

当然了，如果成员变量名称不一致，采用SerializedName Annotation也可。不熟悉的同学请看这里.

使用自定义Serializer&&Deserializer

完成Book序列化方法
这个与之前的大致相同，不同点就是对于内嵌对象的处理上，

package com.example;

import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;

import java.lang.reflect.Type;

public class BookSerializer implements JsonSerializer<Book> {

    @Override
    public JsonElement serialize(final Book book, final Type typeOfSrc, final JsonSerializationContext context) {
        final JsonObject jsonObject = new JsonObject();
        jsonObject.addProperty("title", book.getTitle());
        //*******************************************************
        JsonElement authorElement = context.serialize(book.getAuthors(), Author[].class);
        //*******************************************************
        jsonObject.add("authors", authorElement);

        return jsonObject;
    }
}

仅仅调用这一句，就可以完成内嵌对象的解析.
这个context就去调用内嵌对象Author的serialize()，去完成对象的序列化，自然内嵌对象的序列化就和之前讲的Gson的常用操作没有任何区别.

注意：

  public JsonElement serialize(Object src, Type typeOfSrc);

java中Book数组代表的Type就为Book[].class;
如果是一个单独的Book对象，自然就写成Book.class;
如果是一个List<Book>

import com.google.gson.reflect.TypeToken;
import java.lang.reflect.Type;
import java.util.List;
Type type=new TypeToken<List<Book>>(){}.getType();

调用&&测试

package com.example;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import java.io.IOException;

public class Main {

    public static void main(final String[] args) throws IOException {
        final GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setPrettyPrinting();
        //注册自定义序列化接口
        gsonBuilder.registerTypeAdapter(Book.class,new BookSerializer());
        //如果Author也使用了自定义Serializer，也需要在这里进行注册
        final Gson gson = gsonBuilder.create();

        final Book javaPuzzlers = new Book();
        javaPuzzlers.setTitle("English ABC");

        Author authorA=new Author();
        authorA.setId(1235);
        authorA.setName("gu");

        Author authorB=new Author();
        authorB .setId(1432);
        authorB.setName("hang");

        Author[] authors={authorA,authorB};

        javaPuzzlers.setAuthors(authors);

        // Format to JSON
        final String json = gson.toJson(javaPuzzlers);
        System.out.println(json);

    }
}

测试结果：

{
  "authors": [
    {
      "id": 1235,
      "name": "gu"
    },
    {
      "id": 1432,
      "name": "hang"
    }
  ],
  "title": "English ABC"
}

反序列话非常类似，在这里罗列一下代码.

package com.example;

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;

import java.lang.reflect.Type;

public class BookDeserializer implements JsonDeserializer<Book> {

    @Override
    public Book deserialize(JsonElement jsonElement, Type type, JsonDeserializationContext jsonDeserializationContext) throws JsonParseException {
        JsonObject jsonObject = jsonElement.getAsJsonObject();

        JsonElement titleElement = jsonObject.get("title");
        String title = "";
        //注意：如果要反序列化的JSON中没有title这个key，不进行null判断，会崩溃
        if (titleElement != null) {
            title = titleElement.getAsString();
        }
        //*******************************反序列化内嵌对象***********
        Author[] authors = jsonDeserializationContext.deserialize(jsonObject.get("authors"), Author[].class);
        //*********************************反序列化内嵌对象***********

        Book book = new Book();
        book.setTitle(title);
        book.setAuthors(authors);
        return book;
    }
}

调用&&测试

package com.example;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import java.io.IOException;

public class Main {
public static final String TEST="{\n" +
        "  \"authors\": [\n" +
        "    {\n" +
        "      \"id\": 1235,\n" +
        "      \"name\": \"gu\"\n" +
        "    },\n" +
        "    {\n" +
        "      \"id\": 1432,\n" +
        "      \"name\": \"hang\"\n" +
        "    }\n" +
        "  ],\n" +
        "  \"title\": \"English ABC\"\n" +
        "}";
    public static void main(final String[] args) throws IOException {
        final GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setPrettyPrinting();
        //注册自定义反序列化接口
        gsonBuilder.registerTypeAdapter(Book.class,new BookSerializer());
        //如果Author也使用了自定义Serializer，也需要在这里进行注册
        final Gson gson = gsonBuilder.create();
        Book book=gson.fromJson(TEST,Book.class);
        System.out.println(book);

    }
}

测试结果：

Book POJO msg as follow:

title:  English ABC         authors: [id: 1235  name: gu, id: 1432  name: hang]

对象引用

{
  'authors': [
    {
      'id': 1,
      'name': 'Joshua Bloch'
    },
    {
      'id': 2,
      'name': 'Neal Gafter'
    }
  ],
  'books': [
    {
      'title': 'Java Puzzlers: Traps, Pitfalls, and Corner Cases',
      'authors':[1, 2]
    },
    {
      'title': 'Effective Java (2nd Edition)',
      'authors':[1]
    }
  ]
}

客户端接收到这样的数据，如何填充Book中的Author的name字段呢？下面就探究其中的一种实现.
1.将上述Gson格式字段保存到项目根路径下的Input.json文件中.
2.定义新的类型Data用于接收全部数据

package com.example;

import java.util.Arrays;

public class Data {

    private Author[] authors;
    private Book[] books;
    //getter && setter
    //重载toString
     @Override
    public String toString() {
        return "authors: "+ Arrays.toString(authors)+"\tbooks:"+Arrays.toString(books);
    }

3.定义AuthorDeserializer完成反序列化

package com.example;

import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

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;
import com.google.gson.JsonPrimitive;

public class AuthorDeserializer implements JsonDeserializer<Author> {

    private final ThreadLocal<Map<Integer, Author>> cache = new ThreadLocal<Map<Integer, Author>>() {
        @Override
        protected Map<Integer, Author> initialValue() {
            return new HashMap<>();
        }
    };

    @Override
    public Author deserialize(final JsonElement json, final Type typeOfT, final JsonDeserializationContext context)
            throws JsonParseException {

        // Only the ID is available
        if (json.isJsonPrimitive()) {
            final JsonPrimitive primitive = json.getAsJsonPrimitive();
            return getOrCreate(primitive.getAsInt());
        }

        // The whole object is available
        if (json.isJsonObject()) {
            final JsonObject jsonObject = json.getAsJsonObject();

            final Author author = getOrCreate(jsonObject.get("id").getAsInt());
            author.setName(jsonObject.get("name").getAsString());
            return author;
        }

        throw new JsonParseException("Unexpected JSON type: " + json.getClass().getSimpleName());
    }

    private Author getOrCreate(final int id) {
        Author author = cache.get().get(id);
        if (author == null) {
            author = new Author();
            author.setId(id);
            cache.get().put(id, author);
        }
        return author;
    }
}

这里ThreadLocal<Map<Integer, Author>> cache持有了所有Author对象的引用，并且将相同id的Author指向同一个Author，这与指针挺像，修改id对应的Author将会修改所有持有该id的Author.

package com.example;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.Reader;

public class Main {

    public static void main(final String[] args) throws IOException {
        final GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setPrettyPrinting();

        //注册自定义反序列化接口
        gsonBuilder.registerTypeAdapter(Author.class, new AuthorDeserializer());
        try (Reader reader = new InputStreamReader(new FileInputStream("Input.json"))) {

            final Gson gson = gsonBuilder.create();
            Data data = gson.fromJson(reader, Data.class);
            System.out.println(data);
        }


    }
}

测试结果：

authors: [id: 1 name: Joshua Bloch, id: 2   name: Neal Gafter]  books:[title:  Java Puzzlers: Traps, Pitfalls, and Corner Cases         authors: [id: 1 name: Joshua Bloch, id: 2   name: Neal Gafter], title:  Effective Java (2nd Edition)            authors: [id: 1 name: Joshua Bloch]]

参考地址：http://www.javacreed.com/gson-deserialiser-example/

引入

package com.example;

import java.io.IOException;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class Example1 {

    public static void main(final String[] args) throws IOException {
        final GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.setPrettyPrinting();
        final Gson gson = gsonBuilder.create();

        final Author joshuaBloch = new Author();
        joshuaBloch.setId(1);
        joshuaBloch.setName("Joshua Bloch");

        final Author nealGafter = new Author();
        nealGafter.setId(2);
        nealGafter.setName("Neal Gafter");

        final Book javaPuzzlers = new Book();
        javaPuzzlers.setTitle("Java Puzzlers: Traps, Pitfalls, and Corner Cases");
        javaPuzzlers.setAuthors(new Author[] { joshuaBloch, nealGafter });

        final Book effectiveJava = new Book();
        effectiveJava.setTitle("Effective Java (2nd Edition)");
        effectiveJava.setAuthors(new Author[] { joshuaBloch });

        final Book[] books = new Book[] { javaPuzzlers, effectiveJava };

        final String json = gson.toJson(books);
        System.out.println(json);
    }
}

打印结果：

[
  {
    "authors": [
      {
        "id": 1,
        "name": "Joshua Bloch"
      },
      {
        "id": 2,
        "name": "Neal Gafter"
      }
    ],
    "title": "Java Puzzlers: Traps, Pitfalls, and Corner Cases"
  },
  {
    "authors": [
      {
        "id": 1,
        "name": "Joshua Bloch"
      }
    ],
    "title": "Effective Java (2nd Edition)"
  }
]

很显然这里id=1的author出现了两次，可以缩减为1？

在Book中添加一个返回id数组的方法

  public int[] getAuthorsIds() {
    final int[] ids = new int[authors.length];
    for (int i = 0; i < ids.length; i++) {
      ids[i] = authors[i].getId();
    }
    return ids;
  }

修改BookSerializer方法，Author字段仅仅填充ids

package com.example;

import java.lang.reflect.Type;

import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;

public class BookSerializer implements JsonSerializer<Book> {

    @Override
    public JsonElement serialize(final Book book, final Type typeOfSrc, final JsonSerializationContext context) {
        final JsonObject jsonObject = new JsonObject();
        jsonObject.addProperty("title", book.getTitle());
        final JsonElement jsonAuthors = context.serialize(book.getAuthorsIds());
//   old:    JsonElement authorElement = context.serialize(book.getAuthors(), Author[].class);
        jsonObject.add("authors", jsonAuthors);

        return jsonObject;
    }
}

调用&&测试

package com.example;

import java.io.IOException;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class Example1 {

    public static void main(final String[] args) throws IOException {
        Book[] books = null;
        //制造数据
        {
            final Author joshuaBloch = new Author();
            joshuaBloch.setId(1);
            joshuaBloch.setName("Joshua Bloch");

            final Author nealGafter = new Author();
            nealGafter.setId(2);
            nealGafter.setName("Neal Gafter");

            final Book javaPuzzlers = new Book();
            javaPuzzlers.setTitle("Java Puzzlers: Traps, Pitfalls, and Corner Cases");
            javaPuzzlers.setAuthors(new Author[]{joshuaBloch, nealGafter});

            final Book effectiveJava = new Book();
            effectiveJava.setTitle("Effective Java (2nd Edition)");
            effectiveJava.setAuthors(new Author[]{joshuaBloch});

            books = new Book[]{javaPuzzlers, effectiveJava};
        }
        //精简冗余数据
        {
            GsonBuilder gsonBuilder = new GsonBuilder();
            gsonBuilder.registerTypeAdapter(Book.class, new BookSerializer());
            Gson gson = gsonBuilder.create();
            String bookArrString = gson.toJson(books);
            System.out.println(bookArrString);
        }


    }
}

打印结果：

[
  {
    "title": "Java Puzzlers: Traps, Pitfalls, and Corner Cases",
    "authors": [
      1,
      2
    ]
  },
  {
    "title": "Effective Java (2nd Edition)",
    "authors": [
      1
    ]
  }
]