Gson解析JSON的方法分享
下文笔者讲述Gson解析json的方法及示例分享,如下所示
Gson对外提供两个方法 toJson()和fromJson() 用于对象和JSON之间互相转换例:
toJson()
对象转换为json文件
Gson gson = new Gson();
// 1 java对象转json文件
gson.toJson(obj, new FileWriter("D:\\test.json"));
// 2. java对象转json字符串
String json = gson.toJson(obj);
fromJson()JSON转为Java对象
Gson gson = new Gson();
// 1.json文件转java对象
Object object = gson.fromJson(new FileReader("D:\\test.json"), Object.class);
// 2.json字符串转java对象
String json = "{'name' : 'java265'}";
Object object = gson.fromJson(json, Staff.class);
Gson解析JSON的示例
1.下载Gson pom.xml <dependency> <groupId>com.google.code.gson</groupId> <artifactId>gson</artifactId> <version>2.8.5</version> </dependency> 2. Java对象 Staff.java package com.java265; import java.math.BigDecimal; import java.util.Arrays; import java.util.list; import java.util.Map; public class Staff { private String name; private int age; private String[] position; // array private List<String> skills; // list private Map<String, BigDecimal> salary; // map //getters and setters } 3. Java对象到JSON package com.java265; import com.google.gson.Gson; import com.google.gson.GsonBuilder; import java.io.FileWriter; import java.io.IOException; import java.math.BigDecimal; import java.util.Arrays; import java.util.HashMap; import java.util.Map; public class GsonExample1 { public static void main(String[] args) { // pretty print Gson gson = new GsonBuilder().setPrettyPrinting().create(); Staff staff = createStaffObject(); // Java objects to String String json = gson.toJson(staff); //System.out.println(json); // Java objects to File try (FileWriter writer = new FileWriter("d:\\test\\staff.json")) { gson.toJson(staff, writer); } catch (IOException e) { e.printStackTrace(); } } private static Staff createStaffObject() { Staff staff = new Staff(); staff.setName("java265"); staff.setAge(18); staff.setPosition(new String[]{"CIO", "CTO", "CEO"}); Map<String, BigDecimal> salary = new HashMap() {{ put("2010", new BigDecimal(20000)); put("2012", new BigDecimal(88000)); put("2018", new BigDecimal(62000)); }}; staff.setSalary(salary); staff.setSkills(Arrays.asList("java", "python", "node", "kotlin")); return staff; } } 4. JSON到Java对象 package com.java265; import com.google.gson.Gson; import java.io.FileReader; import java.io.IOException; import java.io.Reader; public class GsonExample2 { public static void main(String[] args) { Gson gson = new Gson(); try (Reader reader = new FileReader("d:\\test\\staff.json")) { // Convert JSON File to Java Object Staff staff = gson.fromJson(reader, Staff.class); // print staff System.out.println(staff); } catch (IOException e) { e.printStackTrace(); } } } ----运行以上代码,将输出以下信息 Staff{name='java265', age=18, position=[CIO,CTO,CEO], skills=[java, python, node, kotlin], salary={2018=62000, 2012=88000, 2010=20000}}
Gson输出美化后的JSON
package com.java265;
import com.google.gson.Gson;
public class GsonExample3 {
public static void main(String[] args) {
// compact print
Gson gson = new Gson();
String[] lang = {"Java", "Node", "C#", "JavaScript"};
String json = gson.toJson(lang);
System.out.println(json);
}
}
----运行以上代码,将输出以下信息----
["Java","Node","C#","JavaScript"]
//美化后的输出打印
GsonExample4.java
package com.java265;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
public class GsonExample4 {
public static void main(String[] args) {
// pretty print
Gson gson = new GsonBuilder().setPrettyPrinting().create();
String[] lang = {"Java", "Node", "C#", "JavaScript"};
String json = gson.toJson(lang);
System.out.println(json);
}
}
----运行以上代码,将输出以下信息-----
[
"Java",
"Node",
"C#",
"JavaScript"
]
Gson不参与序列化字段
GSON默认情况下，transient和static字段将不参与Gson序列化 此时我们可以通过excludeFieldsWithModifiers方法覆盖其默认设置 如: 只设置static修饰的字段不参与Gson序列化 import java.lang.reflect.Modifier; Gson gson = new GsonBuilder() .excludeFieldsWithModifiers(Modifier.STATIC) .create(); 设置static和transient修饰的字段不参与Gson序列化 Gson gson = new GsonBuilder() .excludeFieldsWithModifiers(Modifier.STATIC, Modifier.TRANSIENT) .create(); static和transient修饰的字段参与Gson序列化 Gson gson = new GsonBuilder() .excludeFieldsWithModifiers() .create();
使用@Expose注解字段
Gson gson = new GsonBuilder()
.excludeFieldsWithoutExposeAnnotation()
.create();
当启用.excludeFieldsWithoutExposeAnnotation()模式
则将排除所有没有@Expose字段
例:
import com.google.gson.annotations.Expose;
public class Staff {
@Expose(serialize = true, deserialize = true)
private String name;
@Expose
private int age;
@Expose(serialize = false, deserialize = true)
private String[] position;
private List<String> skills;
private Map<String, BigDecimal> salary;
如果将上述Java对象转换为JSON，则输出将如下所示
{
"name": "java265",
"age": 18
}
使用ExclusionStrategies接口实现自定义排除字段
例:自定义注释@ExcludeField ExcludeField.java package com.java265; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Retention(RetentionPolicy.Runtime) @Target({ElementType.FIELD}) public @interface ExcludeField { } 一个ExclusionStrategy用于定义应排除或跳过哪些字段。 package com.java265; import com.google.gson.ExclusionStrategy; import com.google.gson.FieldAttributes; public class CustomExclusionStrategy implements ExclusionStrategy { private final Class<?> typeToSkip; public CustomExclusionStrategy(Class<?> typeToSkip) { this.typeToSkip = typeToSkip; } @Override public boolean shouldSkipField(FieldAttributes f) { // if field name 'salary`, skip if ("salary".equals(f.getName())) { return true; } // if found @ExcludeField, skip if (f.getAnnotation(ExcludeField.class) != null) { return true; } return false; } @Override public boolean shouldSkipClass(Class<?> clazz) { return (clazz == typeToSkip); } } 再次检查staff对象。 Staff.java public class Staff { private String name; private int age; @ExcludeField private String[] position; private List<String> skills; private Map<String, BigDecimal> salary; //启用ExclusionStrategy模式 Gson gson = new GsonBuilder() .setExclusionStrategies(new CustomExclusionStrategy(List.class)) // exclude all List fields. .create(); //字段名salary ， @ExcludeField字段和List类型字段将被排除 {"name":"java265","age":18}
空对象支持
null对象字段将被忽略
package com.java265;
import com.google.gson.Gson;
public class GsonExample5 {
public static void main(String[] args) {
Gson gson = new Gson();
Staff staff = createStaffObject();
String json = gson.toJson(staff);
System.out.println(json);
}
private static Staff createStaffObject() {
Staff staff = new Staff();
staff.setName("java265");
staff.setAge(18);
return staff;
}
}
----运行以上代码,将输出以下信息-----
{"name":"java265","age":18}
//显示空值的设置
Gson gson = new GsonBuilder().serializeNulls().create();
{"name":"java265","age":18,"position":null,"skills":null,"salary":null}
JSON字段命名设置
只需使用 @SerializedName
即可以自定义字段名称
例
public class Staff {
@SerializedName("java265_name")
private String name;
----将输出以下信息-----
{"java265_name":"abc"}
版本支持
import com.google.gson.annotations.Since;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
public class Staff {
@Since(1.0)
private String name;
@Since(2.0)
private int age;
@Since(3.0)
private String[] position;
private List<String> skills;
private Map<String, BigDecimal> salary;
在此示例中，将排除字段position （版本3）。
Gson gson = new GsonBuilder()
.serializeNulls()
.setVersion(2.0) // version <= 2.0 will be included.
.create();
-----运行以上代码,将输出以下信息-----
{"name":null,"age":0,"skills":null,"salary":null}
JSON转换为Map
package com.java265;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.util.Map;
public class GsonExample5 {
public static void main(String[] args) {
Gson gson = new Gson();
String json = "{\"name\":\"java265\", \"age\"18}";
Map<String, Object> map = gson.fromJson(json, new TypeToken<Map<String, Object>>() {}.getType());
map.forEach((x, y) -> System.out.println("key : " + x + " , value : " + y));
}
}
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