【转载】对Json进行处理的Google Gson API教程

对Json进行处理的Google Gson API教程

    在上一篇文章中，我们已经接触了针对JSON处理的Java API，你很容易就会发现，它并不容易使用，无论你是否必须将JSON转换为Java对象，或者其他需求，你都需要写上很多与目标JSON结构高度耦合的代码。

    这也是为什么我开始留意其他能做到自行转换格式的API，Gson映入了我的眼帘。Gson是开源的，并已被广泛应用于JSON和Java中，Gson使用Java反射API，提供了诸多易于使用的方式将JSON转换为Java，反之亦然。

    你可以从google的代码站点下载到Gson的jar文件，或者如果你正在使用maven，那么你所需要做的所有事情仅仅是添加以下依赖。

< dependencies >

    < dependency >
      < groupId >com.google.code.gson groupId >
      < artifactId >gson artifactId >
      < version >2.2.4 version >
    dependency >
dependencies >

Gson是非常强大的API，它支持Java泛型，支持现成的JSON与Java对象的转换，只要对象的成员名称与JSON中的一致即可。如果针对Java bean和JSON要使用不同的名称，那么可以使用@SerializedName注解来映射JSON和Java类中的变量。

我们来看一个复杂示例，在JSON中含有嵌套对象以及数组，我们要将其映射到Java bean的属性（List、Map、Array类型等）中。

{
  "empID" : 100,
  "name" : "David" ,
  "permanent" : false ,
  "address" : {
    "street" : "BTM 1st Stage" ,
    "city" : "Bangalore" ,
    "zipcode" : 560100
  },
  "phoneNumbers" : [
    123456,
    987654
  ],
  "role" : "Manager" ,
  "cities" : [
    "Los Angeles" ,
    "New York"
  ],
  "properties" : {
    "age" : "28 years" ,
    "salary" : "1000 Rs"
  }
}

    建立Java bean类，将JSON转换为Java对象。

    Employee.java

package com.journaldev.json.model;

import java.util.Arrays;
import java.util.List;
import java.util.Map;

import com.google.gson.annotations.SerializedName;

public class Employee {

    @SerializedName ( "empID" )
    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;
    }

    @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();
    }
    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;
    }
}

   Address.java

package com.journaldev.json.model;

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();
    }
}

    下面是Java程序，展示了如何将JSON转换为Java对象，反之亦然。

    EmployeeGsonExample.java

package com.journaldev.json.gson;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.journaldev.json.model.Address;
import com.journaldev.json.model.Employee;

public class EmployeeGsonExample {

    public static void main(String[] args) throws IOException {
        Employee emp = createEmployee();

        // Get Gson object
        Gson gson = new GsonBuilder().setPrettyPrinting().create();

        // read JSON file data as String
        String fileData = new String(Files.readAllBytes(Paths
                .get( "employee.txt" )));

        // parse json string to object
        Employee emp1 = gson.fromJson(fileData, Employee. class );

        // print object data
        System.out.println( "nnEmployee Objectnn" + emp1);

        // create JSON String from Object
        String jsonEmp = gson.toJson(emp);
        System.out.print(jsonEmp);

    }

    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;
    }
}

Gson是主类，它暴露出fromJson()和toJson()方法进行转换工作，对于默认实现，可以直接创建对象，也可以使用GsonBuilder类提供的实用选项进行转换，比如整齐打印，字段命名转换，排除字段，日期格式化，等等。

当运行以上程序时，可以看到以下Java对象的输出。

Employee Object

***** Employee Details *****
ID=100
Name=David
Permanent= false
Role=Manager
Phone Numbers=[123456, 987654]
Address=BTM 1st Stage, Bangalore, 560100
Cities=[Los Angeles, New York]
Properties={age=28 years, salary=1000 Rs}
*****************************

    你可以看到，使用Gson是多么的容易，这就是为什么它在JSON处理方面如此风靡。

    以上的JSON处理方式是我们所熟知的对象模型，因为整个JSON被一次性的转换为对象了，在大多数情况下这足够了，然而如果JSON确实非常庞大，我们不想将其全部一次性置入内存，Gson也提供了Streaming API。

    我们来看一个例子，它展示了如何使用Streaming API进行JSON到Java对象的转换。

    EmployeeGsonReader.java

package com.journaldev.json.gson;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.journaldev.json.model.Address;
import com.journaldev.json.model.Employee;

public class EmployeeGsonReader {

    public static void main(String[] args) throws IOException {
        InputStream is = new FileInputStream( "employee.txt" );
        InputStreamReader isr = new InputStreamReader(is);

        //create JsonReader object
        JsonReader reader = new JsonReader(isr);

        //create objects
        Employee emp = new Employee();
        Address add = new Address();
        emp.setAddress(add);
        List phoneNums = new ArrayList();
        emp.setCities( new ArrayList());
        emp.setProperties( new HashMap());
        String key = null ;
        boolean insidePropertiesObj= false ;

        key = parseJSON(reader, emp, phoneNums, key, insidePropertiesObj);

        long [] nums = new long [phoneNums.size()];
        int index = 0 ;
        for (Long l :phoneNums){
            nums[index++] = l;
        }
        emp.setPhoneNumbers(nums);

        reader.close();
        //print employee object
        System.out.println( "Employee Objectnn" +emp);
    }

    private static String parseJSON(JsonReader reader, Employee emp,
            List phoneNums, String key, boolean insidePropertiesObj) throws IOException {

        //loop to read all tokens
                while (reader.hasNext()){
                    //get next token
                    JsonToken token = reader.peek();

                    switch (token){
                    case BEGIN_OBJECT:
                        reader.beginObject();
                        if ( "address" .equals(key) || "properties" .equals(key)){
                            while (reader.hasNext()){
                            parseJSON(reader, emp,phoneNums, key, insidePropertiesObj);
                            }
                            reader.endObject();
                        }
                        break ;
                    case END_OBJECT:
                        reader.endObject();
                        if (insidePropertiesObj) insidePropertiesObj= false ;
                        break ;
                    case BEGIN_ARRAY:
                        reader.beginArray();
                        if ( "phoneNumbers" .equals(key) || "cities" .equals(key)){
                            while (reader.hasNext()){
                                parseJSON(reader, emp,phoneNums, key, insidePropertiesObj);
                                }
                            reader.endArray();
                        }
                        break ;
                    case END_ARRAY:
                        reader.endArray();
                        break ;
                    case NAME:
                        key = reader.nextName();
                        if ( "properties" .equals(key)) insidePropertiesObj= true ;
                        break ;
                    case BOOLEAN:
                        if ( "permanent" .equals(key)) emp.setPermanent(reader.nextBoolean());
                        else {
                            System.out.println( "Unknown item found with key=" +key);
                            //skip value to ignore it
                            reader.skipValue();
                        }
                        break ;
                    case NUMBER:
                        if ( "empID" .equals(key)) emp.setId(reader.nextInt());
                        else if ( "phoneNumbers" .equals(key)) phoneNums.add(reader.nextLong());
                        else if ( "zipcode" .equals(key)) emp.getAddress().setZipcode(reader.nextInt());
                        else {
                            System.out.println( "Unknown item found with key=" +key);
                            //skip value to ignore it
                            reader.skipValue();
                        }
                        break ;