Java常用开源库: fastjson, gson

fastjson

官方资料
https://mvnrepository.com/artifact/com.alibaba/fastjson
https://github.com/alibaba/fastjson/wiki/Quick-Start-CN
国内开源的一贯通病, 就是不好好写文档

基础用法

    public static class Data{
        public String str = "str";
        public int i32 = 1;
    }

    public static void main(String[] args) throws Throwable{
        Data data = new Data();
        String jsonStr = JSONObject.toJSONString(data);
        Data obj = JSON.parseObject(jsonStr, Data.class);
    }

输出优雅的字符串(pretty)

String jsonStr = JSONObject.toJSONString(data, SerializerFeature.PrettyFormat);

其它的特性, 可以查看SerializerFeature的说明, 可以添加多次特性, 如

String jsonStr = JSONObject.toJSONString(data, PrettyFormat, SkipTransientField);

过滤Filter

3种方式,

1. 通过关键字transient修饰

    public static class Data{
        public String str = "str";
        public transient int i32 = 1; //不被序列化
    }

2. 通过JSONField注解

    public static class Data{
        public String str = "str";
        @JSONField(serialize = false)
        public int i32 = 1; //不被序列化
    }

3. 通过filter

     SimplePropertyPreFilter filter = new SimplePropertyPreFilter(Data.class, "i32");
     String str = JSONObject.toJSONString(data, filter, PrettyFormat);
     System.out.println(str);

修改key名称

    public static class Data{
        public String str = "str";
        @JSONField(name="myI32", serialize = true)
        public int i32 = 1;
        public Date date = new Date();
        public MyType myType = new MyType();
    }

修改Date的序列化格式

public static class Data{
    @JSONField(ordinal = 2)
    public String str = "str";
    @JSONField(ordinal = 3)
    public int i32 = 5;

    @JSONField(name="myDate", format="dd/MM/yyyy", ordinal = 1)
    public Date date = new Date();
    @JSONField(ordinal = 4)
    public MyType myType = new MyType();
}

泛型

     List<Data> list = Arrays.asList(new Data());
     String str = JSONObject.toJSONString(list, PrettyFormat);
     List<Data> out = JSON.parseObject(str, new TypeReference<List<Data>>(){});

自定义序列化/反序列化

对于fastjson已定义的类型, 修改不了.

			//也可以使用全局对象SerializeConfig.getGlobalInstance()
			SerializeConfig serializeConfig = new SerializeConfig();
            serializeConfig.put(MyType.class, new ObjectSerializer(){
                public void write(JSONSerializer serializer, Object object, Object fieldName, Type fieldType, 		int features) throws IOException {
                    System.out.println("my serializer");
                    SerializeWriter out = serializer.out;
                    MyType value = (MyType) object;
                    if (value == null) {
                        return;
                    }
                     //把MyType作为字符串序列化
                    out.write("\"my type\"");
                }
            });
            String str = JSONObject.toJSONString(data, serializeConfig, PrettyFormat);
            System.out.println(str);
            //输出{"date":1549940906729,"myType":"my type","str":"str","i32":5}

			//或者ParserConfig.getGlobalInstance()
            ParserConfig parseConfig = new ParserConfig();
            parseConfig.putDeserializer(MyType.class, new ObjectDeserializer() {
                public <T> T deserialze(DefaultJSONParser parser, Type type, Object fieldName) {
                    System.out.println("my Deserializer, type" + type + ", fieldName: " + fieldName);
                    JSONLexer lexer = parser.lexer;
                    if(lexer.token() == JSONToken.LITERAL_STRING) { //本例以引号"开始
                        System.out.println("value: " + lexer.stringVal()); //输出 value: my type
                        lexer.nextToken(JSONToken.LITERAL_STRING); // 跳到下一个标记, 本例以引号"结束
                    }
                    return (T)new MyType();
                }
                public int getFastMatchToken() {
                    //自定义序列化好像用不到.
                    return 0;
                }
            });
            Data out = JSON.parseObject(str, Data.class, parseConfig);

gson

有了fastjson之后, 几乎没有在用gson了.
https://mvnrepository.com/artifact/com.google.code.gson/gson
https://github.com/google/gson/blob/master/UserGuide.md
官方文档已经很详细了. 例子就不多说了.

fastjson与gson的序列化差异

性能方面就不说了, 应该是fastjson快. (号称比protobuf还快)

类型	fastjson序列化	gson序列化
Date	long类型的毫秒数, 例如1549939829382	字符串形式的GMT时间, 例如Feb 12, 2019 10:39:59 AM
Number	小数与整数可以互转	小数与整数不能互转, 会抛出异常

所以还是fastjson好用.