json数据格式+gson解析json问题总结

一、使用gson解析json数据时，如果出现某一个属性为null的情况（且并无报错信息），首先检查json文件的属性名称和实体类的属性名称是否一致。

举例如下（正常情况，此处读取本地的一个json文件）：

json文件

{"name":"Tim",
"age":"40",
"job":"basketball",
"hobby":"dai"}

解析代码如下

String jsonstring=FileUtil.readAssets(ctx,"tim.json");
		Gson g=new Gson();
		tim=g.fromJson(jsonstring, Tim.class);

实体类如下

package com.example.jsondemoerror;

public class Tim {
	private String name;
	private String age;
	private String job;
	private String hobby;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getAge() {
		return age;
	}
	public void setAge(String age) {
		this.age = age;
	}
	public String getJob() {
		return job;
	}
	public void setJob(String job) {
		this.job = job;
	}
	public String getHobby() {
		return hobby;
	}
	public void setHobby(String hobby) {
		this.hobby = hobby;
	}
	@Override
	public String toString() {
		return "Tim [name=" + name + ", age=" + age + ", job=" + job
				+ ", hobby=" + hobby + "]";
	}
	

	
}

json文件的各属性名称和实体类的名称一致时，解析成功。

若出现名称不一致：

1、json文件的名称写错，实体类未错，比如hobby写为hobb，出现Tim [name=Tim, age=40, job=basketball, hobby=null]。

2、实体类属性名称写错，json未错，比如把hobby写为hobb，出现Tim [name=Tim, age=40, job=basketball, hobb=null]。

二、使用gson可以直接解析出实体类的集合。

json文件

{"name":"Tim",
"age":"40",
"job":"basketball",
"hobby":
[{"name":"sing","time":"1"},
{"name":"dance","time":"1"},
{"name":"swim","time":"1"},
{"name":"dai","time":"1"}]}

实体类如下

package com.example.jsondemoerror;

import java.util.List;

public class Tims {
    private String name;
    private String age;
    private String job;
    private List<Hobby> hobby;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getAge() {
        return age;
    }
    public void setAge(String age) {
        this.age = age;
    }
    public String getJob() {
        return job;
    }
    public void setJob(String job) {
        this.job = job;
    }
    public List<Hobby> getHobby() {
        return hobby;
    }
    public void setHobby(List<Hobby> hobby) {
        this.hobby = hobby;
    }
    @Override
    public String toString() {
        return "Tims [name=" + name + ", age=" + age + ", job=" + job
                + ", hobby=" + hobby + "]";
    }
    
    

    
}

解析代码

String jsonstring=FileUtil.readAssets(ctx,"tims.json");
        Gson g=new Gson();
        tims=g.fromJson(jsonstring, Tims.class);
        Log.d("JsonErrorActivity", tims.toString()+tims.getHobby().get(0).getName());

log内容

Tims [name=Tim, age=40, job=basketball, hobby=[Hobby [name=sing, time=1], Hobby [name=dance, time=1], Hobby [name=swim, time=1], Hobby [name=dai, time=1]]]sing

三、使用gson可以解析出实体类。

json文件

{"name":"Tim",
"age":"40",
"job":"basketball",
"hobby":
{"name":"sing","time":"1"}
}

实体类

package com.example.jsondemoerror;

public class Timsh {
    private String name;
    private String age;
    private String job;
    private Hobby hobby;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getAge() {
        return age;
    }
    public void setAge(String age) {
        this.age = age;
    }
    public String getJob() {
        return job;
    }
    public void setJob(String job) {
        this.job = job;
    }
    public Hobby getHobby() {
        return hobby;
    }
    public void setHobby(Hobby hobby) {
        this.hobby = hobby;
    }
    @Override
    public String toString() {
        return "Timsh [name=" + name + ", age=" + age + ", job=" + job
                + ", hobby=" + hobby + "]";
    }
    
    

    
}

解析代码

String jsonstring=FileUtil.readAssets(ctx,"timsh.json");
        Gson g=new Gson();
        timsh=g.fromJson(jsonstring, Timsh.class);
        Log.d("JsonErrorActivity", timsh.toString()+timsh.getHobby().getName());

log内容

Timsh [name=Tim, age=40, job=basketball, hobby=Hobby [name=sing, time=1]]sing

四、使用gson转对象为json时需注意为空的属性将不会进行转换，切勿理解为会转换为空值。

实体类：

package gson;

public class Tim {

    private String name;
    private String age;
    private String job;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAge() {
        return age;
    }

    public void setAge(String age) {
        this.age = age;
    }

    public String getJob() {
        return job;
    }

    public void setJob(String job) {
        this.job = job;
    }

    @Override
    public String toString() {
        return "Tim [name=" + name + ", age=" + age + ", job=" + job + "]";
    }

}

测试例子：

package com.test.gson;

import com.google.gson.Gson;

public class Demo {

    public static void main(String[] args) {
        Tim tim = new Tim();
        tim.setAge("20");
        tim.setName("tim");
        tim.setJob(null);
        Gson gson = new Gson();
        System.out.println(gson.toJson(tim));

    }

}

console打印：

{"name":"tim","age":"40"}

因为未设置job属性，转换的json格式数据无此属性。

五、gson转换json若想要体现属性为null或者未设置的属性为null，采用如下方式：

package com.test.gson;

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

public class Demo {

    public static void main(String[] args) {
        Tim tim = new Tim();
        tim.setAge("20");
        tim.setName("tim");
        tim.setJob(null);
        // Gson gson = new Gson();
        // System.out.println(gson.toJson(tim));
        Gson gson = new GsonBuilder().serializeNulls().create();
        System.out.println(gson.toJson(tim));
    }
}

结果：

{"name":"tim","age":"20","job":null}

六、json的数据格式：

json的数据格式有：字符串、数字、布尔值、对象（包括空值null）、数组，举例如下：

package com.test.gson;

import java.util.ArrayList;
import java.util.List;

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

public class JsonDemo {

    public static void main(String[] args) {
        NormalJson normalJson = new JsonDemo().new NormalJson();
        normalJson.setPropertyString("hello world");
        normalJson.setPropertyInt(1);
        normalJson.setPropertyBoolean(true);
        normalJson.setPropertyObject(new Object());
        normalJson.setPropertyNull(null);
        List<String> list = new ArrayList<String>();
        list.add("a");
        list.add("b");
        normalJson.setPropertyArray(list);
        Gson gson = new GsonBuilder().serializeNulls().create();
        System.out.println(gson.toJson(normalJson));
    }

    public class NormalJson {
        private String propertyString;
        private int propertyInt;
        private boolean propertyBoolean;
        private Object propertyObject;
        private Object propertyNull;
        private List<String> propertyArray;

        public String getPropertyString() {
            return propertyString;
        }

        public void setPropertyString(String propertyString) {
            this.propertyString = propertyString;
        }

        public int getPropertyInt() {
            return propertyInt;
        }

        public void setPropertyInt(int propertyInt) {
            this.propertyInt = propertyInt;
        }

        public boolean isPropertyBoolean() {
            return propertyBoolean;
        }

        public void setPropertyBoolean(boolean propertyBoolean) {
            this.propertyBoolean = propertyBoolean;
        }

        public Object getPropertyObject() {
            return propertyObject;
        }

        public void setPropertyObject(Object propertyObject) {
            this.propertyObject = propertyObject;
        }

        public List<String> getPropertyArray() {
            return propertyArray;
        }

        public void setPropertyArray(List<String> propertyArray) {
            this.propertyArray = propertyArray;
        }

        public Object getPropertyNull() {
            return propertyNull;
        }

        public void setPropertyNull(Object propertyNull) {
            this.propertyNull = propertyNull;
        }

        @Override
        public String toString() {
            return "NormalJson [propertyString=" + propertyString + ", propertyInt=" + propertyInt
                    + ", propertyBoolean=" + propertyBoolean + ", propertyObject=" + propertyObject + ", propertyNull="
                    + propertyNull + ", propertyArray=" + propertyArray + "]";
        }
    }
}

运行结果：

{"propertyString":"hello world","propertyInt":1,"propertyBoolean":true,"propertyObject":{},"propertyNull":null,"propertyArray":["a","b"]}

七、gson库解析json为map时对于数字的处理：

解析为map时：

　　若用object接收value值时，int、long、float、double都会转为double类型；

　　若用string接收value值时，int、long、float、double都会按原有类型转换；

　　若用Integer接收value值时，int、long、float、double都会转为intege类型；

　　若用Double接收value值时，int、long、float、double都会转为double类型。

示例如下：

package com.test.gson;

import java.util.Map;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class GsonMapDemo {

    public static void main(String[] args) {
        JsonMap jsonMap = new GsonMapDemo().new JsonMap();
        jsonMap.setA(1);
        jsonMap.setB(2);
        jsonMap.setC(3);
        jsonMap.setD(4);
        Gson gson = new Gson();
        System.out.println(gson.toJson(jsonMap));
        System.out.println(gson.fromJson(gson.toJson(jsonMap), new TypeToken<Map<String, String>>() {
        }.getType()));
        System.out.println(gson.fromJson(gson.toJson(jsonMap), new TypeToken<Map<String, Object>>() {
        }.getType()));
        System.out.println(gson.fromJson(gson.toJson(jsonMap), new TypeToken<Map<String, Integer>>() {
        }.getType()));
        System.out.println(gson.fromJson(gson.toJson(jsonMap), new TypeToken<Map<String, Double>>() {
        }.getType()));

    }

    public class JsonMap {
        private int a;
        private long b;
        private float c;
        private double d;

        public int getA() {
            return a;
        }

        public void setA(int a) {
            this.a = a;
        }

        public long getB() {
            return b;
        }

        public void setB(long b) {
            this.b = b;
        }

        public float getC() {
            return c;
        }

        public void setC(float c) {
            this.c = c;
        }

        public double getD() {
            return d;
        }

        public void setD(double d) {
            this.d = d;
        }

        @Override
        public String toString() {
            return "JsonMap [a=" + a + ", b=" + b + ", c=" + c + ", d=" + d + "]";
        }

    }

}

运行结果如下：

{"a":1,"b":2,"c":3.0,"d":4.0}
{a=1, b=2, c=3.0, d=4.0}
{a=1.0, b=2.0, c=3.0, d=4.0}
{a=1, b=2, c=3, d=4}
{a=1.0, b=2.0, c=3.0, d=4.0}