Java - Jackson JSON Java Parser API

文章目录

概述

依赖

Jackson JSON示例

基础数据

JSON转换为Java对象

Jackson JSON –将JSON转换为Map

概述

Jackson JSON Java Parser非常流行，并且也用于Spring框架。

Jackson JSON Parser API提供了将JSON转换为POJO对象的简便方法，并支持从JSON数据轻松转换为Map。

Jackson也支持泛型，并直接将它们从JSON转换为对象。

依赖

要在我们的项目中使用Jackson JSON Java API，我们可以将其添加到项目构建路径中

com.fasterxml.jackson.core jackson-databind 2.2.3

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jackson-databind jar依赖于jackson-core和jackson-annotations库，因此，如果直接将它们添加到构建路径，请确保将所有三个添加在一起，否则会出现运行时错误。

Jackson JSON示例

基础数据

对于从JSON到POJO / Java对象转换的示例，我们将使用一个嵌套对象和数组的复杂示例。、

我们将在Java对象中使用数组，列表和Map进行转换。 json存储在文件employee.txt中，其结构如下

{ "id": 123, "name": "Pankaj", "permanent": true, "address": { "street": "Albany Dr", "city": "San Jose", "zipcode": 95129 }, "phoneNumbers": [ 123456, 987654 ], "role": "Manager", "cities": [ "Los Angeles", "New York" ], "properties": { "age": "29 years", "salary": "1000 USD" } }

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有以下与json数据相对应的java类。

【Address 】

public class Address { private String street; private String city; private int zipcode; public String getStreet() { return street; } public void setStreet(String street) { this.street = street; } public String getCity() { return city; } public void setCity(String city) { this.city = city; } public int getZipcode() { return zipcode; } public void setZipcode(int zipcode) { this.zipcode = zipcode; } @Override public String toString(){ return getStreet() + ", "+getCity()+", "+getZipcode(); }

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【Employee 】

import java.util.Arrays; import java.util.List; import java.util.Map; public class Employee { private int id; private String name; private boolean permanent; private Address address; private long[] phoneNumbers; private String role; private List cities; private Map properties; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public boolean isPermanent() { return permanent; } public void setPermanent(boolean permanent) { this.permanent = permanent; } public Address getAddress() { return address; } public void setAddress(Address address) { this.address = address; } public long[] getPhoneNumbers() { return phoneNumbers; } public void setPhoneNumbers(long[] phoneNumbers) { this.phoneNumbers = phoneNumbers; } public String getRole() { return role; } public void setRole(String role) { this.role = role; } public List getCities() { return cities; } public void setCities(List cities) { this.cities = cities; } public Map getProperties() { return properties; } public void setProperties(Map properties) { this.properties = properties; } @Override public String toString(){ StringBuilder sb = new StringBuilder(); sb.append("***** Employee Details *****\n"); sb.append("ID="+getId()+"\n"); sb.append("Name="+getName()+"\n"); sb.append("Permanent="+isPermanent()+"\n"); sb.append("Role="+getRole()+"\n"); sb.append("Phone Numbers="+Arrays.toString(getPhoneNumbers())+"\n"); sb.append("Address="+getAddress()+"\n"); sb.append("Cities="+Arrays.toString(getCities().toArray())+"\n"); sb.append("Properties="+getProperties()+"\n"); sb.append("*****************************"); return sb.toString(); } }

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Employee是代表根json对象的Java bean。现在，让我们看看如何使用Jackson JSON解析器API将JSON转换为Java对象

JSON转换为Java对象

import java.io.File; import java.io.IOException; import java.io.StringWriter; import java.nio.file.Files; import java.nio.file.Paths; import java.util.ArrayList; import java.util.HashMap; import java.util.Iterator; import java.util.List; import java.util.Map; import com.fasterxml.jackson.core.type.TypeReference; import com.fasterxml.jackson.databind.JsonNode; import com.fasterxml.jackson.databind.ObjectMapper; import com.fasterxml.jackson.databind.SerializationFeature; import com.fasterxml.jackson.databind.node.ObjectNode; import com.journaldev.jackson.model.Address; import com.journaldev.jackson.model.Employee; public class JacksonObjectMapperExample { public static void main(String[] args) throws IOException { //read json file data to String byte[] jsonData = Files.readAllBytes(Paths.get("employee.txt")); //create ObjectMapper instance ObjectMapper objectMapper = new ObjectMapper(); //convert json string to object Employee emp = objectMapper.readValue(jsonData, Employee.class); System.out.println("Employee Object\n"+emp); //convert Object to json string Employee emp1 = createEmployee(); //configure Object mapper for pretty print objectMapper.configure(SerializationFeature.INDENT_OUTPUT, true); //writing to console, can write to any output stream such as file StringWriter stringEmp = new StringWriter(); objectMapper.writeValue(stringEmp, emp1); System.out.println("Employee JSON is\n"+stringEmp); } public static Employee createEmployee() { Employee emp = new Employee(); emp.setId(100); emp.setName("David"); emp.setPermanent(false); emp.setPhoneNumbers(new long[] { 123456, 987654 }); emp.setRole("Manager"); Address add = new Address(); add.setCity("Bangalore"); add.setStreet("BTM 1st Stage"); add.setZipcode(560100); emp.setAddress(add); List cities = new ArrayList(); cities.add("Los Angeles"); cities.add("New York"); emp.setCities(cities); Map props = new HashMap(); props.put("salary", "1000 Rs"); props.put("age", "28 years"); emp.setProperties(props); return emp; } }

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输出

Employee Object ***** Employee Details ***** ID=123 Name=Pankaj Permanent=true Role=Manager Phone Numbers=[123456, 987654] Address=Albany Dr, San Jose, 95129 Cities=[Los Angeles, New York] Properties={age=29 years, salary=1000 USD} ***************************** Employee JSON is //printing same as above json file data

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com.fasterxml.jackson.databind.ObjectMapper是Jackson API中最重要的类，它提供readValue（）和writeValue（）方法以将JSON转换为Java Object以及将Java Object转换为JSON。

ObjectMapper类可以重用，并且可以将其作为Singleton对象初始化一次。有很多重载版本的readValue（）和writeValue（）方法可用于字节数组，File，输入/输出流和Reader / Writer对象。

Jackson JSON –将JSON转换为Map

在data.txt文件中有一个如下所示的JSON对象：

{ "name": "David", "role": "Manager", "city": "Los Angeles" }

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2.2 Jackson JSON –读取特定的JSON密钥

2.3 Jackson JSON –编辑JSON文档

2.4 Jackson JSON流API示例

API Java JSON

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