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Fastjson输出null为空字符串

2024-09-10 16:08:49   215人阅读

 

JSONObject.toJSONString(Object object, SerializerFeature... features)

SerializerFeature有用的一些枚举值

QuoteFieldNames———-输出key时是否使用双引号,默认为true
WriteMapNullValue——–是否输出值为null的字段,默认为false
WriteNullNumberAsZero—-数值字段如果为null,输出为0,而非null
WriteNullListAsEmpty—–List字段如果为null,输出为[],而非null
WriteNullStringAsEmpty—字符类型字段如果为null,输出为”“,而非null
WriteNullBooleanAsFalse–Boolean字段如果为null,输出为false,而非null

上面的SerializerFeature主要是针对Object对象序列化转换时的情况（这个时候才能判断参数的类型），而在Map中，你放进入了null就是null，进行序列化时已经没法判断它原来的类型了，所以并没有起作用。要使用SerializerFeature里相关null的参数，应该传入对象进行序列化。

对规则的理解：

  1. SerializerFeature.WriteMapNullValue 是否输出值为null的字段,默认为false也就是说有null时会输出而不是忽略（默认策略是忽略，所以看不到为null的字段）

  2. WriteNullStringAsEmpty—字符类型字段如果为null,输出为”“,而非null 
    注意是字段是字段是字段，而不是json.put("key",null)，所以用它时，字段为null的可以转换为空字符串。

  3. 如果让输出的json中所有为null的字符串都变成空字符串，最简单的做法就是加一个值过滤器，这样就避免了有的字段为null，有的字段为空字符的现象。

示例代码

import java.util.ArrayList;import java.util.HashMap;import java.util.HashSet;import java.util.List;import java.util.Map;import java.util.Set;import com.alibaba.fastjson.JSON;import com.alibaba.fastjson.JSONObject;import com.alibaba.fastjson.serializer.SerializerFeature;import com.alibaba.fastjson.serializer.ValueFilter;public class Demo1 {    public class Student {        private String name;        private int age;        private boolean isMale;        private Student gf;        public String getName() {            return name;        }        public void setName(String name) {            this.name = name;        }        public int getAge() {            return age;        }        public void setAge(int age) {            this.age = age;        }        public boolean isMale() {            return isMale;        }        public void setMale(boolean isMale) {            this.isMale = isMale;        }        public Student getGf() {            return gf;        }        public void setGf(Student gf) {            this.gf = gf;        }    }    private static ValueFilter filter = new ValueFilter() {        @Override        public Object process(Object obj, String s, Object v) {            if (v == null)                return "";            return v;        }    };    public static void main(String[] args) {        new Demo1().foo();        new Demo1().bar();    }    private void foo() {        System.out.println("foo()---------------------------");        JSONObject j1 = new JSONObject();        j1.put("name", "zhangsan");        j1.put("age", 13);        j1.put("isMale", true);        j1.put("gf", null);        Map<String, Object> fav = new HashMap<String, Object>();        Set<String> books = new HashSet<String>();        books.add("三国");        books.add("史记");        fav.put("history", books);        String[] arts = new String[] {};        fav.put("arts", arts);        String[] musics = new String[] { "北京欢迎你", "画心" };        fav.put("musics", musics);        List<String> sports = new ArrayList<String>();        fav.put("sports", sports);        j1.put("fav", fav);        List<Student> classmates = new ArrayList<Student>();        classmates.add(new Student());        Student lisi = new Student();        lisi.setMale(false);        lisi.setAge(11);        classmates.add(lisi);        Student zhangsan = new Student();        zhangsan.setAge(13);        zhangsan.setName("张三");        zhangsan.setMale(true);        zhangsan.setGf(lisi);        classmates.add(zhangsan);        j1.put("classmates", classmates);        String str = null;        j1.put("str", str);        System.out.println(j1.toString());        System.out            .println(JSON.toJSONString(j1, SerializerFeature.WriteMapNullValue, SerializerFeature.WriteNullStringAsEmpty));        System.out.println(            JSON.toJSONString(j1, filter, SerializerFeature.WriteMapNullValue, SerializerFeature.WriteNullStringAsEmpty));        System.out.println(JSON.toJSONString(j1, SerializerFeature.WriteNullStringAsEmpty));        System.out.println(JSON.toJSONString(j1, filter, SerializerFeature.WriteNullStringAsEmpty));        Map<String, JSONObject> m = new HashMap<String, JSONObject>();        m.put("key", j1);        System.out.println(            JSON.toJSONString(m, SerializerFeature.WriteNonStringKeyAsString, SerializerFeature.WriteNullStringAsEmpty));        System.out.println(JSON.toJSONString(m, filter, SerializerFeature.WriteNonStringKeyAsString,            SerializerFeature.WriteNullStringAsEmpty));    }    private void bar() {        System.out.println("bar()---------------------------");        Student zhangsan = new Student();        zhangsan.setAge(13);        zhangsan.setName("张三");        zhangsan.setMale(true);        Student lisi = new Student();        // lisi.setName("lisi");        lisi.setMale(false);        lisi.setAge(11);        zhangsan.setGf(lisi);        System.out.println(            JSON.toJSONString(zhangsan, SerializerFeature.WriteMapNullValue, SerializerFeature.WriteNullStringAsEmpty));        System.out.println(JSON.toJSONString(zhangsan, SerializerFeature.WriteMapNullValue));        System.out.println(JSON.toJSONString(zhangsan, SerializerFeature.WriteNullStringAsEmpty));        System.out.println(JSON.toJSONString(zhangsan));        System.out.println(JSON.toJSONString(zhangsan, filter));        System.out.println(JSON.toJSONString(zhangsan, filter, SerializerFeature.WriteMapNullValue,            SerializerFeature.WriteNullStringAsEmpty));    }}

 

foo()---------------------------{"isMale":true,"name":"zhangsan","classmates":[{"age":0,"male":false},{"age":11,"male":false},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13}{"str":null,"isMale":true,"name":"zhangsan","classmates":[{"age":0,"gf":null,"male":false,"name":""},{"age":11,"gf":null,"male":false,"name":""},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13,"gf":null}{"str":"","isMale":true,"name":"zhangsan","classmates":[{"age":0,"gf":"","male":false,"name":""},{"age":11,"gf":"","male":false,"name":""},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13,"gf":""}{"isMale":true,"name":"zhangsan","classmates":[{"age":0,"male":false},{"age":11,"male":false},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13}{"str":"","isMale":true,"name":"zhangsan","classmates":[{"age":0,"gf":"","male":false,"name":""},{"age":11,"gf":"","male":false,"name":""},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13,"gf":""}{"key":{"isMale":true,"name":"zhangsan","classmates":[{"age":0,"male":false},{"age":11,"male":false},{"age":13,"gf":{"$ref":"$.key.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13}}{"key":{"str":"","isMale":true,"name":"zhangsan","classmates":[{"age":0,"gf":"","male":false,"name":""},{"age":11,"gf":"","male":false,"name":""},{"age":13,"gf":{"$ref":"$.key.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13,"gf":""}}bar()---------------------------{"age":13,"gf":{"age":11,"gf":null,"male":false,"name":""},"male":true,"name":"张三"}{"age":13,"gf":{"age":11,"gf":null,"male":false,"name":null},"male":true,"name":"张三"}{"age":13,"gf":{"age":11,"male":false},"male":true,"name":"张三"}{"age":13,"gf":{"age":11,"male":false},"male":true,"name":"张三"}{"age":13,"gf":{"age":11,"gf":"","male":false,"name":""},"male":true,"name":"张三"}{"age":13,"gf":{"age":11,"gf":"","male":false,"name":""},"male":true,"name":"张三"}

 

Fastjson输出null为空字符串


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