Jackson 和 Gson 都是库/API。 这两个库都使用 JSON 数据。 这些也用于将 JSON 数据序列化和反序列化为 Java 对象。 这两个库都是 Java 的完整库，提供 JSON 数据绑定支持并能够处理复杂的数据类型。 这些库提供对 Java 泛型的支持。
Jackson 和 Gson 都可以反序列化 JSON 数据，而无需对实体类进行任何更改。 为了在代码中使用 Jackson 和 Gson，我们需要在 pom.xml 文件中添加以下依赖项。
Gson依赖
<dependency> <groupId>com.google.code.gson</groupId> <artifactId>gson</artifactId> <version>2.9.0</version> </dependency> Jackson依赖
<dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-core</artifactId> <version>2.13.2</version> </dependency> 下面来看看如何使用 Jackson 和 Gson 库来序列化和反序列化 JSON 数据。
Gson序列化
将 Java 对象序列化为 JSON 有两种方法，即简单序列化(使用 toJson() 方法)和自定义序列化(使用自定义序列化程序)。
在简单序列化中，我们使用 Gson 库的 toJson() 方法来序列化一个 Java 对象。通过以下方式使用 toJson() 方法：
String jsonString = new Gson().toJson(object); 下面举个例子来看看如何使用它来序列化特定的 Java 对象。
文件代码：GsonSerializationExample1.java
//import required classes and package if any import java.util.Scanner; import com.google.gson.Gson; //create class GsonSerializationExample1 public class GsonSerializationExample1 { //main() methods start public static void main(String args[]) { // create Scanner class object Scanner sc= new Scanner(System.in); College colg; String name, university, street, state, city, rank; System.out.println("Enter College Name:"); name = sc.nextLine(); System.out.println("Enter University Name:"); university = sc.nextLine(); System.out.println("Enter Street:"); street = sc.nextLine(); System.out.println("Enter City:"); city = sc.nextLine(); System.out.println("Enter State:"); state = sc.nextLine(); System.out.println("Enter College Rank:"); rank = sc.nextLine(); // set values to College object by using constructor colg = new College(name, university, new Address(street, state, city), rank); // serialize College using Gson String jsonString = new Gson().toJson(colg); System.out.println(jsonString); //close Scanner class object sc.close(); } } //create class College class College { //Creating properties of College class public String name; public String university; public Address address; public String rank; // constructor College(String name, String university, Address address, String rank){ this.name = name; this.university = university; this.address = address; this.rank = rank; } } //create class Address class Address { public String street; public String state; public String city; Address(String street, String state, String city){ this.street = street; this.state = state; this.city = city; } } 但是，也可以使用自定义序列化程序来序列化 Java 对象。 通过使用自定义序列化程序，可以修改标准行为。 可以处理空值、使用带有 html 的输出格式化程序、添加新输出或从输出中排除属性。
创建序列化程序文件：CollegeGsonSerializer.java –
import java.lang.reflect.Type; import java.text.SimpleDateFormat; import java.util.Date; import java.util.List; import java.util.stream.Collectors; import com.google.gson.JsonElement; import com.google.gson.JsonObject; import com.google.gson.JsonSerializationContext; import com.google.gson.JsonSerializer; // create custom serializer for College class public class CollegeGsonSerializer implements JsonSerializer<College> { // create an instance of SimpleDateFormat class private SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy"); @Override public JsonElement serialize(College colg, Type type, JsonSerializationContext context) { // create an instance of JsonObject JsonObject colgJsonObj = new JsonObject(); // use addProperty() method to add properties to JsonObject colgJsonObj.addProperty("<strong>College Name</strong>", colg.getCollegeName()); colgJsonObj.addProperty("<strong>University Name</strong>", colg.getUniversityName()); colgJsonObj.addProperty("<strong>College Est.</strong>", colg.getEst() != null ? sdf.format(colg.getEst()) : null); colgJsonObj.addProperty("<strong>College Address</strong> ", colg.getAddress().size() > 0 ? convertAddress(colg.getAddress()) : null); colgJsonObj.addProperty("College Rank", colg.getRank() != null ? colg.getRank() : null); return colgJsonObj; } private String convertAddress(List<String> address) { return address.stream() .collect(Collectors.joining("-")); } } //create class College class College { //Creating properties of College class public String name; public String university; public List<String> address; public Date est; public String rank; // constructor College(String name, String university, List<String> address, Date est, String rank){ this.name = name; this.university = university; this.address = address; this.est = est; this.rank = rank; } //Getter and Setters public String getCollegeName() { return name; } public String getUniversityName() { return university; } public Date getEst() { return est; } public List<String> getAddress() { return address; } public String getRank() { return rank; } @Override public String toString() { // TODO Auto-generated method stub return ""College [College Name: "+ name + ", College University = " + university + ", College Est. = " + est + ", Address = "+address+", College Rank = "+rank+"]"; } } 现在，使用上面的序列化器来序列化 College 类，示例代码：GsonSerializationExample2.java
import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.ArrayList; import java.util.List; //import required classes and package if any import java.util.Scanner; import com.google.gson.Gson; import com.google.gson.GsonBuilder; //create class GsonSerializationExample2 to serialize POJO using custom serializer public class GsonSerializationExample2 { //main() methods start public static void main(String args[]) { // create Scanner class object Scanner sc= new Scanner(System.in); College colg; String name, university, add1, add2, rank, est; List<String> address = new ArrayList<String>(); System.out.println("Enter College Name:"); name = sc.nextLine(); System.out.println("Enter University Name:"); university = sc.nextLine(); System.out.println("Enter College Address:"); add1 = sc.nextLine(); System.out.println("Enter University Address:"); add2 = sc.nextLine(); System.out.println("Enter College Est. in dd-mm-yyy format: "); est = sc.nextLine(); System.out.println("Enter College Rank:"); rank = sc.nextLine(); address.add(add1); address.add(add2); Gson gson = new GsonBuilder() .setPrettyPrinting() .excludeFieldsWithoutExposeAnnotation() .serializeNulls() .disableHtmlEscaping() .registerTypeAdapter(College.class, new CollegeGsonSerializer()) .create(); // create an instance of SimpleDateFormat SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy"); try { colg = new College(name, university, address, sdf.parse(est), rank); // convert college to JSON String jsonString = gson.toJson(colg); System.out.println("After serializing College using custom serializer: "+jsonString); } catch (ParseException e) { e.printStackTrace(); } //close Scanner class object sc.close(); } } Jackson序列化
就像 Gson 一样，也可以通过两种方式使用 Jackson 将 Java 对象序列化为 JSON，即简单序列化(使用 writeValueAsString() 方法)和自定义序列化(使用自定义序列化程序)。
在简单序列化中，使用 Jackson 库的 writeValueAsString() 方法来序列化 Java 对象。通过以下方式使用 writeValueAsString() 方法：
ObjectMapper mapper = new ObjectMapper(); String jsonString = mapper.writeValueAsString(Object); 下面举个例子来了解如何使用它来序列化特定的 Java 对象。示例代码：JacksonSerializationExample1.java
//import required classes and package if any import java.util.Scanner; import com.fasterxml.jackson.core.JsonProcessingException; import com.fasterxml.jackson.databind.ObjectMapper; //create class JacksonSerializationExample1 public class JacksonSerializationExample1 { //main() methods start public static void main(String args[]) { // create an instance of ObjectMapper class ObjectMapper mapper = new ObjectMapper(); // create Scanner class object Scanner sc= new Scanner(System.in); College colg; String name, university, street, state, city, rank; System.out.println("Enter College Name:"); name = sc.nextLine(); System.out.println("Enter University Name:"); university = sc.nextLine(); System.out.println("Enter Street:"); street = sc.nextLine(); System.out.println("Enter City:"); city = sc.nextLine(); System.out.println("Enter State:"); state = sc.nextLine(); System.out.println("Enter College Rank:"); rank = sc.nextLine(); // set values to College object by using constructor colg = new College(name, university, new Address(street, state, city), rank); String jsonString; try { // serialize College using Jackson jsonString = mapper.writeValueAsString(colg); System.out.println(jsonString); } catch (JsonProcessingException e) { // TODO Auto-generated catch block e.printStackTrace(); } //close Scanner class object sc.close(); } } //create class College class College { //Creating properties of College class public String name; public String university; public Address address; public String rank; // constructor College(String name, String university, Address address, String rank){ this.name = name; this.university = university; this.address = address; this.rank = rank; } } //create class Address class Address { public String street; public String state; public String city; Address(String street, String state, String city){ this.street = street; this.state = state; this.city = city; } } 就像 Gson 自定义序列化一样，也可以通过简单地创建一个序列化器来执行 Jackson 自定义序列化。 College.java 创建一个自定义序列化程序，稍后使用 Jackson 进行序列化。
代码文件：CollegeJacksonSerializer.java
import java.io.IOException; import java.text.SimpleDateFormat; import java.util.Date; import java.util.List; import java.util.stream.Collectors; import com.fasterxml.jackson.core.JsonGenerator; import com.fasterxml.jackson.databind.SerializerProvider; import com.fasterxml.jackson.databind.ser.std.StdSerializer; // create custom serializer for College class public class CollegeJacksonSerializer extends StdSerializer<College> { // create an instance of SimpleDateFormat class private SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy"); public CollegeJacksonSerializer(Class t) { super(t); } @Override public void serialize(College colg, JsonGenerator gen, SerializerProvider provider) throws IOException { gen.writeStartObject(); gen.writeStringField("College Name ", colg.getCollegeName()); gen.writeStringField("College University ", colg.getUniversityName()); gen.writeStringField("Address ", colg.getAddress().stream().collect(Collectors.joining("-"))); gen.writeObjectField("College Est. ", colg.getEst() != null ? sdf.format(colg.getEst()) : null); gen.writeStringField("College Rank: ", colg.getRank()); gen.writeEndObject(); } } //create class College class College { //Creating properties of College class public String name; public String university; public List<String> address; public Date est; public String rank; // constructor College(String name, String university, List<String> address, Date est, String rank){ this.name = name; this.university = university; this.address = address; this.est = est; this.rank = rank; } //Getter and Setters public String getCollegeName() { return name; } public String getUniversityName() { return university; } public Date getEst() { return est; } public List<String> getAddress() { return address; } public String getRank() { return rank; } @Override public String toString() { // TODO Auto-generated method stub return "/"College [College Name: "+ name + ", College University = " + university + ", College Est. = " + est + ", Address = "+address+", College Rank = "+rank+"]"; } } 下面使用上面的序列化器来序列化 College 类。文件：JacksonSerializationExample2.java –
import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.ArrayList; import java.util.List; //import required classes and package if any import java.util.Scanner; import com.fasterxml.jackson.core.JsonProcessingException; import com.fasterxml.jackson.core.util.DefaultPrettyPrinter; import com.fasterxml.jackson.databind.ObjectMapper; import com.fasterxml.jackson.databind.module.SimpleModule; //create class JacksonSerializationExample2 to serialize POJO using the custom serializer public class JacksonSerializationExample2 { //main() methods start public static void main(String args[]) throws ParseException { // create Scanner class object Scanner sc= new Scanner(System.in); College colg; String name, university, add1, add2, rank, est; List<String> address = new ArrayList<String>(); System.out.println("Enter College Name:"); name = sc.nextLine(); System.out.println("Enter University Name:"); university = sc.nextLine(); System.out.println("Enter College Address:"); add1 = sc.nextLine(); System.out.println("Enter University Address:"); add2 = sc.nextLine(); System.out.println("Enter College Est. in dd-mm-yyy format: "); est = sc.nextLine(); System.out.println("Enter College Rank:"); rank = sc.nextLine(); address.add(add1); address.add(add2); // create an instance of SimpleDateFormat SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy"); colg = new College(name, university, address, sdf.parse(est), rank); SimpleModule module = new SimpleModule(); module.addSerializer(new CollegeJacksonSerializer(College.class)); ObjectMapper mapper = new ObjectMapper(); try { String jsonResult = mapper .registerModule(module) .writer(new DefaultPrettyPrinter()) .writeValueAsString(colg); System.out.println(jsonResult); } catch (JsonProcessingException e) { // TODO Auto-generated catch block e.printStackTrace(); } //close Scanner class object sc.close(); } } Gson反序列化
反序列化是将 JSON 字符串转换为 Java 对象的另一个重要概念。在两个实体类中都实现了 toString() 方法来说明输出。还可以通过两种方式将 JSON 字符串反序列化为 Java 对象，即简单反序列化和自定义反序列化。
下面以一个简单反序列化的例子来了解如何将 JSON 字符串反序列化为 Java 对象。
文件：GsonDeserializationExample1.java –
import java.text.ParseException; import java.util.Date; import java.util.List; import com.google.gson.Gson; //create class GsonDeserializationExample1 to deserialize JSON string into POJO public class GsonDeserializationExample1 { //main() methods start public static void main(String args[]) throws ParseException { String jsonString = "{/"name/":/"JavaDeveloper/",/"expReq/":2,/"valid/":/"05-04-2022/"," + "/"ctc/":700001,/"employees/":[{/"empName/":/"Nicholas/",/"empId/":/"12001/"," + "/"empAdd/":/"Noida/",/"empContact/":/"123456/"},{/"empName/":/"Brittany/"," + "/"empId/":/"1002/",/"empAdd/":/"Noida/",/"empContact/":/"123457/"}]}"; JobPosition position = new Gson().fromJson(jsonString, JobPosition.class); System.out.println(position); } } //create class JobPosition class JobPosition { //Creating properties of College class public String name; public int expReq; public String valid; public int ctc; public List<Employee> employees; // constructor JobPosition(String name, int expReq, List<Employee> employees, String valid, int ctc){ this.name = name; this.expReq = expReq; this.valid = valid; this.ctc = ctc; this.employees = employees; } @Override public String toString() { // TODO Auto-generated method stub return "/"Job Position [Job Position: "+ name + ", Experience Required: " + expReq + ", Last Date To Apply: " + valid + ", CTC: "+ctc+", Employees Details "+employees+"]"; } } //create class Employee class Employee { public String empName; public String empId; public String empAdd; public String empContact; Employee(String empName, String empId, String empAdd, String empContact){ this.empName = empName; this.empId = empId; this.empAdd = empAdd; this.empContact = empContact; } @Override public String toString() { // TODO Auto-generated method stub return "/"Employee [Employee Name: "+ empName + ", Employee Id: " + empId + ", Employee Address: " + empAdd + ", Employee Contact: "+empContact+"]"; } } 在自定义反序列化中，使用自定义反序列化器。它允许修改标准的解串器行为。使用反序列化器来反映 Last Date To Apply 的正确时区。
下面再举一个自定义反序列化的例子，将 JSON 字符串反序列化为 Java 对象。
JobPositionGsonDeserializer.java
import java.lang.reflect.Type; import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.ArrayList; import com.google.gson.JsonArray; import com.google.gson.JsonDeserializationContext; import com.google.gson.JsonDeserializer; import com.google.gson.JsonElement; import com.google.gson.JsonObject; import com.google.gson.JsonParseException; public class JobPositionGsonDeserializer implements JsonDeserializer<JobPosition> { // create an instance of the SimpleDateFormat private SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy"); @Override public JobPosition deserialize(JsonElement element, Type typeOfT, JsonDeserializationContext context) throws JsonParseException { // read element as JSON Object JsonObject obj = element.getAsJsonObject(); JsonElement JobPositionName = obj.get("name"); JsonElement expRequired = obj.get("expReq"); JsonElement ctc = obj.get("ctc"); JsonElement dateToApply = obj.get("valid"); JsonArray employeesData = obj.getAsJsonArray("employees"); ArrayList<String> empList = new ArrayList<String>(); if (employeesData != null) { for (int i = 0; i < employeesData.size(); i++) { empList.add(employeesData.get(i).getAsString()); } } JobPosition pos; try { pos = new JobPosition(JobPositionName.getAsString(), expRequired.getAsInt(), empList, sdf.parse(dateToApply.getAsString()), ctc.getAsInt()); return pos; } catch (ParseException e) { // TODO Auto-generated catch block e.printStackTrace(); } return null; } } GsonDeserializationExample2.java import java.text.ParseException; import java.util.Date; import java.util.List; import com.google.gson.Gson; import com.google.gson.GsonBuilder; //create class GsonDeserializationExample2 to deserialize JSON string into POJO using custom deserializer public class GsonDeserializationExample2 { //main() methods start public static void main(String args[]) throws ParseException { String jsonString = "{/"name/":/"JavaDeveloper/",/"expReq/":2," + "/"valid/":/"05-04-2022/",/"ctc/":700001,/"employees/":[/"Nicholas/",/"Brittany/",Sharon]}"; Gson gson = new GsonBuilder() .registerTypeAdapter(JobPosition.class, new JobPositionGsonDeserializer()) .create(); JobPosition position = gson.fromJson(jsonString, JobPosition.class); System.out.println(position); } } //create class JobPosition class JobPosition { //Creating properties of College class public String name; public int expReq; public Date valid; public int ctc; public List<String> employees; // constructor JobPosition(String name, int expReq, List<String> employees, Date valid, int ctc){ this.name = name; this.expReq = expReq; this.valid = valid; this.ctc = ctc; this.employees = employees; } @Override public String toString() { // TODO Auto-generated method stub return "/"Job Position [Job Position: "+ name + ", Experience Required: " + expReq + ", Last Date To Apply: " + valid + ", CTC: "+ctc+", Employees Details "+employees+"]"; } } Jackson反序列化
就像 Gson 一样，可以通过两种方式对 JSON 字符串进行反序列化，即简单反序列化和自定义反序列化。下面举一个例子来了解如何使用 Jackson 反序列化带有/不带有反序列化器的 JSON 字符串。
JacksonDeserializationExample1.java –
import java.text.ParseException; import java.util.List; import com.fasterxml.jackson.core.JsonProcessingException; import com.fasterxml.jackson.databind.JsonMappingException; import com.fasterxml.jackson.databind.ObjectMapper; //create class JacksonDeserializationExample1 to deserialize JSON string into POJO public class JacksonDeserializationExample1 { //main() methods start public static void main(String args[]) throws ParseException { String jsonString = "{/"name/":/"JavaDeveloper/",/"expReq/":2," + "/"valid/":/"05-04-2022/",/"ctc/":700000,/"employees/":[/"Brittany/",/"Nicholas/",/"Sharon/"]}"; // create an instance of ObjectMapper class ObjectMapper mapper = new ObjectMapper(); // deserialize jsonString into POJO try { JobPosition position = mapper.readValue(jsonString, JobPosition.class); System.out.println(position); } catch (JsonMappingException e) { e.printStackTrace(); } catch (JsonProcessingException e) { e.printStackTrace(); } } } //create class JobPosition class JobPosition { //Creating properties of College class public String name; public int expReq; public String valid; public int ctc; public List<String> employees; // default constructor JobPosition(){ this.name = null; this.expReq = 0; this.valid = null; this.ctc = 0; this.employees = null; } // constructor JobPosition(String name, int expReq, List<String> employees, String valid, int ctc){ this.name = name; this.expReq = expReq; this.valid = valid; this.ctc = ctc; this.employees = employees; } @Override public String toString() { // TODO Auto-generated method stub return "/"Job Position [Job Position: "+ name + ", Experience Required: " + expReq + ", Last Date To Apply: " + valid + ", CTC: "+ctc+", Employees Details "+employees+"]"; } } 文件：JacksonDeserializationExample2.java
import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.Date; import java.util.List; import com.fasterxml.jackson.core.JsonProcessingException; import com.fasterxml.jackson.databind.JsonMappingException; import com.fasterxml.jackson.databind.ObjectMapper; //create class JacksonDeserializationExample2 to deserialize JSON string into POJO public class JacksonDeserializationExample2 { //main() methods start public static void main(String args[]) throws ParseException { String jsonString = "{/"name/":/"JavaDeveloper/",/"expReq/":2," + "/"valid/":/"2022-09-21T12:00:00+01:00/",/"ctc/":700000,/"employees/":[/"Brittany/",/"Nicholas/",/"Sharon/"]}"; // create an instance of SimpleDateFormat class SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss"); // create an instance of ObjectMapper class ObjectMapper mapper = new ObjectMapper(); // add date format to ObjectMapper class mapper.setDateFormat(sdf); // deserialize jsonString into POJO try { JobPosition position = mapper.readValue(jsonString, JobPosition.class); System.out.println(position); } catch (JsonMappingException e) { e.printStackTrace(); } catch (JsonProcessingException e) { e.printStackTrace(); } } } //create class JobPosition class JobPosition { //Creating properties of College class public String name; public int expReq; public Date valid; public int ctc; public List<String> employees; // default constructor JobPosition(){ this.name = null; this.expReq = 0; this.valid = null; this.ctc = 0; this.employees = null; } // constructor JobPosition(String name, int expReq, List<String> employees, Date valid, int ctc){ this.name = name; this.expReq = expReq; this.valid = valid; this.ctc = ctc; this.employees = employees; } @Override public String toString() { // TODO Auto-generated method stub return "/"Job Position [Job Position: "+ name + ", Experience Required: " + expReq + ", Last Date To Apply: " + valid + ", CTC: "+ctc+", Employees Details "+employees+"]"; } } 原创文章，作者：ItWorker，如若转载，请注明出处：https://blog.ytso.com/264213.html