原文链接：http://www.dubby.cn/detail.html?id=9069

我们在这里使用jackson-core提供的JsonParser和JsonGenerator来实现基本的序列化和反序列化。

1.数据和实体类

我们先定义出JSON字符串:

{
  "id":123456789,
  "text":"我是杨正，我在http://www.dubby.cn",
  "fromUserId":123456, 
  "toUserId":789,
  "languageCode":"zh"
}

给出我们等会解析出来后承载数据的POJO:

public class TwitterEntry {
    long _id;
    String _text;
    int _fromUserId, _toUserId;
    String _languageCode;

    public TwitterEntry() {
    }

    public void setId(long id) {
        _id = id;
    }

    public void setText(String text) {
        _text = text;
    }

    public void setFromUserId(int id) {
        _fromUserId = id;
    }

    public void setToUserId(int id) {
        _toUserId = id;
    }

    public void setLanguageCode(String languageCode) {
        _languageCode = languageCode;
    }

    public long getId() {
        return _id;
    }

    public String getText() {
        return _text;
    }

    public int getFromUserId() {
        return _fromUserId;
    }

    public int getToUserId() {
        return _toUserId;
    }

    public String getLanguageCode() {
        return _languageCode;
    }

    public String toString() {
        return "[Tweet, id: " + _id + ", text: " + _text + "', from: " + _fromUserId + ", to: " + _toUserId + ", lang: " + _languageCode + "]";
    }
}

2.反序列化Unmarshalling(JSON->POJO)

private static TwitterEntry read(JsonParser jp) throws IOException {
        // 检查是否是JSON
        if (jp.nextToken() != JsonToken.START_OBJECT) {
            throw new IOException("Expected data to start with an Object");
        }
        TwitterEntry result = new TwitterEntry();
        // 遍历属性，并一个一个的赋值
        while (jp.nextToken() != JsonToken.END_OBJECT) {
            String fieldName = jp.getCurrentName();
            jp.nextToken();
            switch (fieldName) {
                case "id":
                    result.setId(jp.getLongValue());
                    break;
                case "text":
                    result.setText(jp.getText());
                    break;
                case "fromUserId":
                    result.setFromUserId(jp.getIntValue());
                    break;
                case "toUserId":
                    result.setToUserId(jp.getIntValue());
                    break;
                case "languageCode":
                    result.setLanguageCode(jp.getText());
                    break;
                default:
                    throw new IOException("Unrecognized field '" + fieldName + "'");
            }
        }
        //关闭 parser
        jp.close();
        return result;
    }

调用地方如下:

JsonFactory jsonF = new JsonFactory();

        String jsonStr = "{\n" +
                "  \"id\":123456789,\n" + 
                "  \"text\":\"我是杨正，我在http://www.dubby.cn\",\n" +
                "  \"fromUserId\":123456, \n" +
                "  \"toUserId\":789,\n" +
                "  \"languageCode\":\"zh\"\n" +
                "}";
        System.out.println(jsonStr);

        JsonParser jp = jsonF.createParser(jsonStr);
        TwitterEntry entry = read(jp);
        System.out.println(entry.toString());

3.序列化Marshalling(POJO -> JSON)

private static void write(JsonGenerator jg, TwitterEntry entry) throws IOException {
        jg.writeStartObject();
        jg.writeNumberField("id", entry.getId());
        jg.writeStringField("text", entry.getText());
        jg.writeNumberField("fromUserId", entry.getFromUserId());
        jg.writeNumberField("toUserId", entry.getToUserId());
        jg.writeStringField("langugeCode", entry.getLanguageCode());
        jg.writeEndObject();
        jg.close();
    }

调用如下:

ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        JsonGenerator jg = jsonF.createGenerator(byteArrayOutputStream, JsonEncoding.UTF8);
        jg.useDefaultPrettyPrinter();
        write(jg, entry);
        System.out.println(byteArrayOutputStream.toString());

4.更多的选项

4.1 JsonFactory的选项（命名规范化）

只针对属性名

JsonFactory f = new JsonFactory();
f.disable(JsonFactory.Feature.CANONICALIZE_FIELD_NAMES);

4.2 JsonGenerator的选项

JsonFactory f = new JsonFactory();
f.enable(JsonGenerator.Feature.ESCAPE_NON_ASCII);
f.disable(JsonGenerator.Feature.AUTO_CLOSE_TARGET);
JsonGenerator g = f.createGenerator(destination);
g.enable(JsonGenerator.Feature.STRICT_DUPLICATE_DETECTION);

4.3 JsonParser的选项

JsonFactory f = new JsonFactory();
f.enable(JsonParser.Feature.ALLOW_COMMENTS);
f.disable(JsonParser.Feature.ALLOW_SINGLE_QUOTES);
JsonParser p = f.createParser(jsonSource);
p.enable(JsonParser.Feature.STRICT_DUPLICATE_DETECTION);

这里给出一个示例:

JsonFactory jsonF = new JsonFactory();

        String jsonStr = "{\n" +
                "  \"id\":123456789,\n" + "//注释1\n" + "/*注释1*/\n" +
                "  \"text\":\"我是杨正，我在http://www.dubby.cn\",\n" +
                "  \"fromUserId\":123456, \n" +
                "  \"toUserId\":789,\n" +
                "  \"languageCode\":\"zh\"\n" +
                "}";
        System.out.println(jsonStr);

        JsonParser jp = jsonF.createParser(jsonStr);
        //特意在上面的JSON字符串种加入一些注释，如果在此处不开启允许注释，会报错
        jp.enable(JsonParser.Feature.ALLOW_COMMENTS);
        TwitterEntry entry = read(jp);
        System.out.println(entry.toString());

结果:

{
  "id":123456789,
//注释1
/*注释1*/
  "text":"我是杨正，我在http://www.dubby.cn",
  "fromUserId":123456, 
  "toUserId":789,
  "languageCode":"zh"
}
[Tweet, id: 123456789, text: 我是杨正，我在http://www.dubby.cn', from: 123456, to: 789, lang: zh]