转载自：https://segmentfault.com/a/1190000005717319

 

Jackson

jackson-databind文档

Jackson可以轻松的将Java对象转换成json对象和xml文档，同样也可以将json、xml转换成Java对象。在项目中如果要引入Jackson，可以直接利用Maven或者Gradle引入：

 

<properties> ... <!-- Use the latest version whenever possible. --> <jackson.version>2.7.0</jackson.version> ... </properties> <dependencies> ... <dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-databind</artifactId> <version>${jackson.version}</version> </dependency> ... </dependencies>

注意，databind项目已经自动依赖了jackson-core与jackson-annotation，不需要额外重复引入。

Convert Java to JSON

首先声明有一个简单的POJO:

 

// Note: can use getters/setters as well; here we just use public fields directly: public class MyValue { public String name; public int age; // NOTE: if using getters/setters, can keep fields `protected` or `private` }

然后创建一个ObjectMapper实例用于进行转化：

 

ObjectMapper mapper = new ObjectMapper(); // create once, reuse

 

MyValue value = mapper.readValue(new File("data.json"), MyValue.class); // or: value = mapper.readValue(new URL("http://some.com/api/entry.json"), MyValue.class); // or: value = mapper.readValue("{\"name\":\"Bob\", \"age\":13}", MyValue.class);

我们可以参考一个实例，将某个Staff的信息转化为JSON然后写入到文件中，首先来定义实体类：

 

package com.mkyong.json; import java.math.BigDecimal; import java.util.List; public class Staff { private String name; private int age; private String position; private BigDecimal salary; private List<String> skills; //getters and setters

然后具体的将Java实体类转化为JSON的语句为：

 

package com.mkyong.json; import java.io.File; import java.io.IOException; import java.math.BigDecimal; import java.util.ArrayList; import java.util.List; import com.fasterxml.jackson.core.JsonGenerationException; import com.fasterxml.jackson.databind.JsonMappingException; import com.fasterxml.jackson.databind.ObjectMapper; public class Jackson2Example { public static void main(String[] args) { Jackson2Example obj = new Jackson2Example(); obj.run(); } private void run() { ObjectMapper mapper = new ObjectMapper(); Staff staff = createDummyObject(); try { // Convert object to JSON string and save into a file directly mapper.writeValue(new File("D:\\staff.json"), staff); // Convert object to JSON string String jsonInString = mapper.writeValueAsString(staff); System.out.println(jsonInString); // Convert object to JSON string and pretty print jsonInString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(staff); System.out.println(jsonInString); } catch (JsonGenerationException e) { e.printStackTrace(); } catch (JsonMappingException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } private Staff createDummyObject() { Staff staff = new Staff(); staff.setName("mkyong"); staff.setAge(33); staff.setPosition("Developer"); staff.setSalary(new BigDecimal("7500")); List<String> skills = new ArrayList<>(); skills.add("java"); skills.add("python"); staff.setSkills(skills); return staff; } }

最终的输出为：

 

//new json file is created in D:\\staff.json" {"name":"mkyong","age":33,"position":"Developer","salary":7500,"skills":["java","python"]} { "name" : "mkyong", "age" : 33, "position" : "Developer", "salary" : 7500, "skills" : [ "java", "python" ] }

Properties:属性处理

Rename:属性重命名

 

public class Name { @JsonProperty("firstName") public String _first_name; }

在将Name实体类转化为JSON的时候，就会变成：

 

{ "firstName" : "Bob" }

Ignore:属性忽略

 

public class Value { public int value; @JsonIgnore public int internalValue; }

最终生成的JSON是如下格式：

 

{ "value" : 42 }

也可以在类的头部统一声明:

 

@JsonIgnoreProperties({ "extra", "uselessValue" }) public class Value { public int value; }

那么如下的JSON字符串也是可以被转化为该实体类的:

 

{ "value" : 42, "extra" : "fluffy", "uselessValue" : -13 }

对于意外地未知属性，也可以统一忽略：

 

@JsonIgnoreProperties(ignoreUnknown=true) public class PojoWithAny { public int value; }

@JsonView:动态控制展示的成员变量

首先定义一个简单的View控制类：

 

package com.mkyong.json; public class Views { public static class Normal{}; public static class Manager extends Normal{}; }

在下面的代码实现中，如果是选择了Normal View，那么salary属性将会被隐藏，而在Manager View状态下，任何属性都会被展示。

 

package com.mkyong.json; import java.math.BigDecimal; import java.util.List; import com.fasterxml.jackson.annotation.JsonView; public class Staff { @JsonView(Views.Normal.class) private String name; @JsonView(Views.Normal.class) private int age; @JsonView(Views.Normal.class) private String position; @JsonView(Views.Manager.class) private BigDecimal salary; @JsonView(Views.Normal.class) private List<String> skills;

在进行Object转化为JSON的过程中，进行视图控制：

 

package com.mkyong.json; import java.io.IOException; import java.math.BigDecimal; import java.util.ArrayList; import java.util.List; import com.fasterxml.jackson.core.JsonGenerationException; import com.fasterxml.jackson.databind.JsonMappingException; import com.fasterxml.jackson.databind.ObjectMapper; public class Jackson2Example { public static void main(String[] args) { Jackson2Example obj = new Jackson2Example(); obj.run(); } private void run() { ObjectMapper mapper = new ObjectMapper(); Staff staff = createDummyObject(); try { // Salary will be hidden System.out.println("Normal View"); String normalView = mapper.writerWithView(Views.Normal.class).writeValueAsString(staff); System.out.println(normalView); String jsonInString = "{\"name\":\"mkyong\",\"age\":33,\"position\":\"Developer\",\"salary\":7500,\"skills\":[\"java\",\"python\"]}"; Staff normalStaff = mapper.readerWithView(Views.Normal.class).forType(Staff.class).readValue(jsonInString); System.out.println(normalStaff); // Display everything System.out.println("\nManager View"); String managerView = mapper.writerWithView(Views.Manager.class).writeValueAsString(staff); System.out.println(managerView); Staff managerStaff = mapper.readerWithView(Views.Manager.class).forType(Staff.class).readValue(jsonInString); System.out.println(managerStaff); } catch (JsonGenerationException e) { e.printStackTrace(); } catch (JsonMappingException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } private Staff createDummyObject() { Staff staff = new Staff(); staff.setName("mkyong"); staff.setAge(33); staff.setPosition("Developer"); staff.setSalary(new BigDecimal("7500")); List<String> skills = new ArrayList<>(); skills.add("java"); skills.add("python"); staff.setSkills(skills); return staff; } }

最终输出的结果为:

 

Normal View {"name":"mkyong","age":33,"position":"Developer","skills":["java","python"]} Staff [name=mkyong, age=33, position=Developer, salary=null, skills=[java, python]] Manager View {"name":"mkyong","age":33,"position":"Developer","salary":7500,"skills":["java","python"]} Staff [name=mkyong, age=33, position=Developer, salary=7500, skills=[java, python]]

Convert JSON to Java

将JSON转化为Java的实体类同样需要用到ObjectMapper对象：

 

mapper.writeValue(new File("result.json"), myResultObject); // or: byte[] jsonBytes = mapper.writeValueAsBytes(myResultObject); // or: String jsonString = mapper.writeValueAsString(myResultObject);

而如果我们要将JSON转化为Java中的List或者Map的话，可以采用如下方式：

 

//将某个JSON转化为List String json = "[{\"name\":\"mkyong\"}, {\"name\":\"laplap\"}]"; List<Staff> list = mapper.readValue(json, new TypeReference<List<Staff>>(){}); //将某个JSON转化为Map String json = "{\"name\":\"mkyong\", \"age\":33}"; Map<String, Object> map = mapper.readValue(json, new TypeReference<Map<String,Object>>(){});

还是来看一个实例，：

 

package com.mkyong.json; import java.io.File; import java.io.IOException; import java.math.BigDecimal; import java.util.ArrayList; import java.util.List; import com.fasterxml.jackson.core.JsonGenerationException; import com.fasterxml.jackson.databind.JsonMappingException; import com.fasterxml.jackson.databind.ObjectMapper; public class Jackson2Example { public static void main(String[] args) { Jackson2Example obj = new Jackson2Example(); obj.run(); } private void run() { ObjectMapper mapper = new ObjectMapper(); try { // Convert JSON string from file to Object Staff staff = mapper.readValue(new File("D:\\staff.json"), Staff.class); System.out.println(staff); // Convert JSON string to Object String jsonInString = "{\"name\":\"mkyong\",\"salary\":7500,\"skills\":[\"java\",\"python\"]}"; Staff staff1 = mapper.readValue(jsonInString, Staff.class); System.out.println(staff1); //Pretty print String prettyStaff1 = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(staff1); System.out.println(prettyStaff1); } catch (JsonGenerationException e) { e.printStackTrace(); } catch (JsonMappingException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } }

最终的输出为：

 

Staff [name=mkyong, age=33, position=Developer, salary=7500, skills=[java, python]] Staff [name=mkyong, age=0, position=null, salary=7500, skills=[java, python]] { "name" : "mkyong", "age" : 0, "position" : null, "salary" : 7500, "skills" : [ "java", "python" ] }

Tree Model:抽象的JSON数据类型，类似于FastJSON中的JSONObject

 

// can be read as generic JsonNode, if it can be Object or Array; or, // if known to be Object, as ObjectNode, if array, ArrayNode etc: ObjectNode root = mapper.readTree("stuff.json"); String name = root.get("name").asText(); int age = root.get("age").asInt(); // can modify as well: this adds child Object as property 'other', set property 'type' root.with("other").put("type", "student"); String json = mapper.writeValueAsString(root); // with above, we end up with something like as 'json' String: // { // "name" : "Bob", "age" : 13, // "other" : { // "type" : "student" // } // }

Constructor:自定义构造器

默认情况下，Jackson使用默认的构造器创建新的对象，不过你也可以使用@JsonCreator与@JsonProperty注解来自定义对象创建函数与值的绑定。

 

public class CtorPOJO { private final int _x, _y; @JsonCreator public CtorPOJO(@JsonProperty("x") int x, @JsonProperty("y") int y) { _x = x; _y = y; } }

 

public class DelegatingPOJO { private final int _x, _y; @JsonCreator public DelegatingPOJO(Map<String,Object> delegate) { _x = (Integer) delegate.get("x"); _y = (Integer) delegate.get("y"); } }