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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,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,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。
1 2 3 4 5 6 7 8 | < dependencies > <!-- Gson dependency --> < 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。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | { "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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 | 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<String> cities; private Map<String, String> 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<String> getCities() { return cities; } public void setCities(List<String> cities) { this .cities = cities; } public Map<String, String> getProperties() { return properties; } public void setProperties(Map<String, String> properties) { this .properties = properties; } } |
Address.java
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 | 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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 | 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<String> cities = new ArrayList<String>(); cities.add( "Los Angeles" ); cities.add( "New York" ); emp.setCities(cities); Map<String, String> props = new HashMap<String, String>(); 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。
1 2 3 4 5 6 7 8 9 10 11 12 | 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。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 | 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<Long> phoneNums = new ArrayList<Long>(); emp.setCities( new ArrayList<String>()); emp.setProperties( new HashMap<String, String>()); 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<Long> 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 ; case STRING: setStringValues(emp, key, reader.nextString(), insidePropertiesObj); break ; case NULL: System.out.println( "Null value for key" +key); reader.nextNull(); break ; case END_DOCUMENT: System.out.println( "End of Document Reached" ); break ; default : System.out.println( "This part will never execute" ); break ; } } return key; } private static void setStringValues(Employee emp, String key, String value, boolean insidePropertiesObj) { if ( "name" .equals(key)) emp.setName(value); else if ( "role" .equals(key)) emp.setRole(value); else if ( "cities" .equals(key)) emp.getCities().add(value); else if ( "street" .equals(key)) emp.getAddress().setStreet(value); else if ( "city" .equals(key)) emp.getAddress().setCity(value); else { //add to emp properties map if (insidePropertiesObj){ emp.getProperties().put(key, value); } else { System.out.println( "Unknown data found with key=" +key+ " value=" +value); } } } } |
由于JSON是一个递归语言(译注:JSON本身并不是“语言”,而是一种表示方法),我们也需要针对数组和嵌套对象递归地调用解析方法。JsonToken是JsonReader中next()方法所返回的Java枚举类型,我们可以用其配合条件逻辑或switch case语句进行转换工作。根据以上代码,你应该能够理解这不是一个简单的实现,如果JSON确实非常复杂,那么代码将会变得极难维护,所以要避免使用这种方式,除非没有其他出路。
我们来看一下如何使用Gson Streaming API写出Employee对象。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 | package com.journaldev.json.gson; import java.io.IOException; import java.io.OutputStreamWriter; import java.util.Set; import com.google.gson.stream.JsonWriter; import com.journaldev.json.model.Employee; public class EmployeeGsonWriter { public static void main(String[] args) throws IOException { Employee emp = EmployeeGsonExample.createEmployee(); //writing on console, we can initialize with FileOutputStream to write to file OutputStreamWriter out = new OutputStreamWriter(System.out); JsonWriter writer = new JsonWriter(out); //set indentation for pretty print writer.setIndent( "t" ); //start writing writer.beginObject(); //{ writer.name( "id" ).value(emp.getId()); // "id": 123 writer.name( "name" ).value(emp.getName()); // "name": "David" writer.name( "permanent" ).value(emp.isPermanent()); // "permanent": false writer.name( "address" ).beginObject(); // "address": { writer.name( "street" ).value(emp.getAddress().getStreet()); // "street": "BTM 1st Stage" writer.name( "city" ).value(emp.getAddress().getCity()); // "city": "Bangalore" writer.name( "zipcode" ).value(emp.getAddress().getZipcode()); // "zipcode": 560100 writer.endObject(); // } writer.name( "phoneNumbers" ).beginArray(); // "phoneNumbers": [ for ( long num : emp.getPhoneNumbers()) writer.value(num); //123456,987654 writer.endArray(); // ] writer.name( "role" ).value(emp.getRole()); // "role": "Manager" writer.name( "cities" ).beginArray(); // "cities": [ for (String c : emp.getCities()) writer.value(c); //"Los Angeles","New York" writer.endArray(); // ] writer.name( "properties" ).beginObject(); //"properties": { Set<String> keySet = emp.getProperties().keySet(); for (String key : keySet) writer.name( "key" ).value(emp.getProperties().get(key)); //"age": "28 years","salary": "1000 Rs" writer.endObject(); // } writer.endObject(); // } writer.flush(); //close writer writer.close(); } } |
从Java对象到JSON的转换,与使用streaming API解析相比,相对容易一些,默认情况下JsonWriter会以一种紧凑的格式写入JSON,但也可以设置缩进进行整齐打印。
这就是Gson API演示教程的所有内容,如果你遇到任何问题,请告诉我。以下链接可以下载项目,你可以玩一玩Gson提供的多种选项。
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