Gson 简明教程

Gson - Quick Guide

Gson - Overview

Google Gson 是一个简单的基于 Java 的库，用于序列化 Java 对象到 JSON，反之亦然。它是由 Google 开发的开源库。

Google Gson is a simple Java-based library to serialize Java objects to JSON and vice versa. It is an open-source library developed by Google.

以下各点突出了你为何应该使用该库——

The following points highlight why you should be using this library −

Standardized − Gson is a standardized library that is managed by Google.
Efficient − It is a reliable, fast, and efficient extension to the Java standard library.
Optimized − The library is highly optimized.
Support Generics − It provides extensive support for generics.
Supports complex inner classes − It supports complex objects with deep inheritance hierarchies.

Features of Gson

以下是 Gson 的一些最突出特点的列表——

Here is a list of some of the most prominent features of Gson −

Easy to use − Gson API provides a high-level facade to simplify commonly used use-cases.
No need to create mapping − Gson API provides default mapping for most of the objects to be serialized.
Performance − Gson is quite fast and is of low memory footprint. It is suitable for large object graphs or systems.
Clean JSON − Gson creates a clean and compact JSON result which is easy to read.
No Dependency − Gson library does not require any other library apart from JDK.
Open Source − Gson library is open source; it is freely available.

Three Ways of Processing JSON

Gson 提供三种 JSON 处理备选方案——

Gson provides three alternative ways to process JSON −

Streaming API

它将 JSON 内容读入和写入为离散事件。 JsonReader 和 JsonWriter 以 token 形式读/写数据，称为 JsonToken 。

It reads and writes JSON content as discrete events. JsonReader and JsonWriter read/write the data as token, referred as JsonToken.

在三种处理 JSON 的方法中它是最强大的方法。它开销最低而且读/写操作非常快。它类似于 XML 的 Stax 解析器。

It is the most powerful approach among the three approaches to process JSON. It has the lowest overhead and it is quite fast in read/write operations. It is analogous to Stax parser for XML.

Tree Model

它准备一个 JSON 文档的内存树表示。它构建一个 JsonObject 节点树。它是一种灵活的方法，类似于 XML 的 DOM 解析器。

It prepares an in-memory tree representation of the JSON document. It builds a tree of JsonObject nodes. It is a flexible approach and is analogous to DOM parser for XML.

Data Binding

它使用属性访问器将 JSON 转换为 POJO（普通旧 Java 对象）以及从它们转换 JSON。Gson 使用数据类型适配器读/写 JSON。它类似于 XML 的 JAXB 解析器。

It converts JSON to and from POJO (Plain Old Java Object) using property accessor. Gson reads/writes JSON using data type adapters. It is analogous to JAXB parser for XML.

Gson - Environment Setup

Local Environment Setup

如果你仍然想为 Java 编程语言设置本地环境，那么本部分将指导你如何在计算机上下载和设置 Java。请按照以下给出的步骤设置环境。

If you still want to set up a local environment for Java programming language, then this section will guide you on how to download and set up Java on your machine. Please follow the steps given below, to set up the environment.

Java SE 可以从链接 Download Java 免费获得。因此，你可以根据你的操作系统下载一个版本。

Java SE is freely available from the link Download Java. So you download a version based on your operating system.

按照说明下载 Java 并运行 .exe ，在您的机器上安装 Java。在您的机器上安装 Java 后，您需要设置环境变量以指向它们正确的安装目录。

Follow the instructions to download Java and run the .exe to install Java on your machine. Once you have installed Java on your machine, you would need to set the environment variables to point to their correct installation directories.

Setting up the Path in Windows 2000/XP

假设你已将 Java 安装在 c:\Program Files\java\jdk 目录中 −

Assuming you have installed Java in c:\Program Files\java\jdk directory −

Right-click on 'My Computer' and select 'Properties'.
Click on the 'Environment variables' button under the 'Advanced' tab.
Next, alter the 'Path' variable so that it also contains the path to the Java executable. For example, if the path is currently set to 'C:\WINDOWS\SYSTEM32', then change your path to read 'C:\WINDOWS\SYSTEM32;c:\Program Files\java\jdk\bin'.

Setting up the Path in Windows 95 / 98 / ME

假设你已将 Java 安装在 c:\Program Files\java\jdk 目录中 −

Assuming you have installed Java in c:\Program Files\java\jdk directory −

Edit the 'C:\autoexec.bat' file and add the following line at the end: 'SET PATH=%PATH%;C:\Program Files\java\jdk\bin'

Setting up the Path for Linux, UNIX, Solaris, FreeBSD

环境变量 PATH 应该被设置为指向已安装 Java 二进制文件的位置。如果您这样做遇到问题，请参阅您的 shell 文档。

The environment variable PATH should be set to point to where the Java binaries have been installed. Refer to your shell documentation if you have trouble doing this.

例如，如果您使用 bash 作为您的 shell，那么您将在您的“'bashrc: export PATH=/path/ to/java:$PATH'”末尾添加以下行。

For example, if you use bash as your shell, then you would add the following line to the end of your '.bashrc: export PATH=/path/to/java:$PATH'

Popular Java Editors

要编写您的 Java 程序，您将需要一个文本编辑器。市场上有许多成熟的 IDE 可用。但现在，您可以考虑以下之一 −

To write your Java programs, you will need a text editor. There are quite a few sophisticated IDEs available in the market. But for now, you can consider one of the following −

Notepad − On Windows, you can use any simple text editor like Notepad (Recommended for this tutorial) or TextPad.
Netbeans − It is a Java IDE that is open-source and free which can be downloaded from https://netbeans.org/index.html.
Eclipse − It is also a Java IDE developed by the Eclipse open-source community and can be downloaded from https://www.eclipse.org/.

Download Gson Archive

从 gson-2.3.1.jar 下载 Gson jar 文件的最新版本。在撰写本教程时，我们下载了 gson-2.3.1.jar 并将其复制到了 C:\>gson 文件夹。

Download the latest version of Gson jar file from gson-2.3.1.jar. At the time of writing this tutorial, we downloaded gson-2.3.1.jar and copied it into C:\>gson folder.

Archive name

Windows

gson-2.3.1.jar

Linux

gson-2.3.1.jar

Mac

gson-2.3.1.jar

Set Gson Environment

设置 GSON_HOME 环境变量，使其指向 Gson jar 在您的机器上存储的基本目录位置。

Set the GSON_HOME environment variable to point to the base directory location where Gson jar is stored on your machine.

Output

Windows

Set the environment variable GSON_HOME to C:\gson

Linux

export GSON_HOME=/usr/local/gson

Mac

export GSON_HOME=/Library/gson

Set CLASSPATH variable

设置 CLASSPATH 环境变量以指向 Gson jar 位置。

Set the CLASSPATH environment variable to point to the Gson jar location.

Output

Windows

Set the environment variable CLASSPATH to %CLASSPATH%;%GSON_HOME%\gson-2.3.1.jar;.;

Linux

export CLASSPATH=$CLASSPATH:$GSON_HOME/gson-2.3.1.jar:.

Mac

export CLASSPATH=$CLASSPATH:$GSON_HOME/gson-2.3.1.jar:.

Gson - First Application

在详细了解 Google Gson 库之前，让我们来看看一个示例是如何发挥作用的。在此示例中，我们创建了一个 Student 类。我们将使用学生详细信息创建一个 JSON 字符串，并将其反序列化为 student 对象，然后将其序列化为 JSON 字符串。

Before going into the details of the Google Gson library, let’s see an application in action. In this example, we’ve created a Student class. We’ll create a JSON string with student details and deserialize it to student object and then serialize it to an JSON String.

Example

在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class GsonTester {
   public static void main(String[] args) {
      String jsonString = "{\"name\":\"Mahesh\", \"age\":21}";

      GsonBuilder builder = new GsonBuilder();
      builder.setPrettyPrinting();

      Gson gson = builder.create();
      Student student = gson.fromJson(jsonString, Student.class);
      System.out.println(student);

      jsonString = gson.toJson(student);
      System.out.println(jsonString);
   }
}

class Student {
   private String name;
   private int age;
   public Student(){}

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public int getAge() {
      return age;
   }

   public void setAge(int age) {
      this.age = age;
   }

   public String toString() {
      return "Student [ name: "+name+", age: "+ age+ " ]";
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 来查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

Student [ name: Mahesh, age: 21 ]
{
   "name" : "Mahesh",
   "age" : 21
}

Steps to Remember

以下是需要考虑的重要步骤。

Following are the important steps to be considered here.

Step 1 − Create Gson object using GsonBuilder

创建一个 Gson 对象，这是一个可重用对象。

Create a Gson object. It is a reusable object.

GsonBuilder builder = new GsonBuilder();
builder.setPrettyPrinting();
Gson gson = builder.create();

Step 2 − Deserialize JSON to Object

使用 fromJson() 方法从 JSON 中获取对象，将 Json 字符串/Json 字符串源和对象类型作为参数传入。

Use fromJson() method to get the Object from the JSON. Pass Json string / source of Json string and object type as parameter.

//Object to JSON Conversion
Student student = gson.fromJson(jsonString, Student.class);

Step 3 − Serialize Object to JSON

使用 toJson() 方法获取一个对象的 JSON 字符串表示形式。

Use toJson() method to get the JSON string representation of an object.

//Object to JSON Conversion
jsonString = gson.toJson(student);

Gson - Class

Gson 是 Google Gson 库的主要操作员类。它提供将 Java 对象转换成匹配 JSON 构造的过程，反之亦然。使用 GsonBuilder 首先构建 Gson，然后使用 toJson(Object) 或 fromJson(String, Class) 方法来读/写 JSON 构造。

Gson is the main actor class of Google Gson library. It provides functionalities to convert Java objects to matching JSON constructs and vice versa. Gson is first constructed using GsonBuilder and then, toJson(Object) or fromJson(String, Class) methods are used to read/write JSON constructs.

Class Declaration

以下为 com.google.gson.Gson 类的声明 −

Following is the declaration for com.google.gson.Gson class −

public final class Gson
   extends Object

Constructors

Sr.No

Constructor & Description

Gson() Constructs a Gson object with default configuration.

Class Methods

Sr.No	Method & Description
1	<T> T fromJson(JsonElement json, Class<T> classOfT) This method deserializes the Json read from the specified parse tree into an object of the specified type.
2	<T> T fromJson(JsonElement json, Type typeOfT) This method deserializes the Json read from the specified parse tree into an object of the specified type.
3	<T> T fromJson(JsonReader reader, Type typeOfT) Reads the next JSON value from reader and convert it to an object of type typeOfT.
4	<T> T fromJson(Reader json, Class<T> classOfT) This method deserializes the Json read from the specified reader into an object of the specified class.
5	<T> T fromJson(Reader json, Type typeOfT) This method deserializes the Json read from the specified reader into an object of the specified type.
6	<T> T fromJson(String json, Class<T> classOfT) This method deserializes the specified Json into an object of the specified class.
7	<T> T fromJson(String json, Type typeOfT) This method deserializes the specified Json into an object of the specified type.
8	<T> TypeAdapter<T> getAdapter(Class<T> type) Returns the type adapter for type.
9	<T> TypeAdapter<T> getAdapter(TypeToken<T> type) Returns the type adapter for type.
10	<T> TypeAdapter<T> getDelegateAdapter(TypeAdapterFactory skipPast, TypeToken<T> type) This method is used to get an alternate type adapter for the specified type.
11	String toJson(JsonElement jsonElement) Converts a tree of JsonElements into its equivalent JSON representation.
12	void toJson(JsonElement jsonElement, Appendable writer) Writes out the equivalent JSON for a tree of JsonElements.
13	void toJson(JsonElement jsonElement, JsonWriter writer) Writes the JSON for jsonElement to writer.
14	String toJson(Object src) This method serializes the specified object into its equivalent Json representation.
15	void toJson(Object src, Appendable writer) This method serializes the specified object into its equivalent Json representation.
16	String toJson(Object src, Type typeOfSrc) This method serializes the specified object, including those of generic types, into its equivalent Json representation.
17	void toJson(Object src, Type typeOfSrc, Appendable writer) This method serializes the specified object, including those of generic types, into its equivalent Json representation.
18	void toJson(Object src, Type typeOfSrc, JsonWriter writer) Writes the JSON representation of src of type typeOfSrc to writer.
19	JsonElement toJsonTree(Object src) This method serializes the specified object into its equivalent representation as a tree of JsonElements.
20	JsonElement toJsonTree(Object src, Type typeOfSrc) This method serializes the specified object, including those of generic types, into its equivalent representation as a tree of JsonElements.
21	String toString()

Methods inherited

此类从以下类继承方法 −

This class inherits methods from the following class −

java.lang.Object

Example

在任何您选择的编辑器中创建以下 Java 程序，并将它保存在，比如，C:/> GSON_WORKSPACE

Create the following Java program using any editor of your choice, and save it at, say, C:/> GSON_WORKSPACE

File − GsonTester.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class GsonTester {
   public static void main(String[] args) {
      String jsonString = "{\"name\":\"Mahesh\", \"age\":21}";

      GsonBuilder builder = new GsonBuilder();
      builder.setPrettyPrinting();

      Gson gson = builder.create();
      Student student = gson.fromJson(jsonString, Student.class);
      System.out.println(student);

      jsonString = gson.toJson(student);
      System.out.println(jsonString);
   }
}

class Student {
   private String name;
   private int age;
   public Student(){}

   public String getName() {
      return name;
   }
   public void setName(String name) {
      this.name = name;
   }
   public int getAge() {
      return age;
   }
   public void setAge(int age) {
      this.age = age;
   }
   public String toString() {
      return "Student [ name: "+name+", age: "+ age+ " ]";
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 来查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出

Verify the output

Student [ name: Mahesh, age: 21 ]
{
   "name" : "Mahesh",
   "age" : 21
}

Gson - Object Serialization

让我们将 Java 对象序列化到 Json 文件，然后读取该 Json 文件以返回该对象。在此示例中，我们创建了 Student 类。我们将创建一个 student.json 文件，该文件将有一个 json 的 Student 对象表示形式。

Let’s serialize a Java object to a Json file and then read that Json file to get the object back. In this example, we’ve created a Student class. We’ll create a student.json file which will have a json representation of Student object.

Example

在 C:>GSON_WORKSPACE 中创建名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File - GsonTester.java

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class GsonTester {
   public static void main(String args[]) {

      GsonTester tester = new GsonTester();
      try {
         Student student = new Student();
         student.setAge(10);
         student.setName("Mahesh");
         tester.writeJSON(student);
         Student student1 = tester.readJSON();
         System.out.println(student1);
      }
      catch(FileNotFoundException e) {
         e.printStackTrace();
      }
      catch(IOException e) {
         e.printStackTrace();
      }
   }

   private void writeJSON(Student student) throws IOException {
      GsonBuilder builder = new GsonBuilder();
      Gson gson = builder.create();
      FileWriter writer = new FileWriter("student.json");
      writer.write(gson.toJson(student));
      writer.close();
   }

   private Student readJSON() throws FileNotFoundException {
      GsonBuilder builder = new GsonBuilder();
      Gson gson = builder.create();
      BufferedReader bufferedReader = new BufferedReader(
         new FileReader("student.json"));

      Student student = gson.fromJson(bufferedReader, Student.class);
      return student;
   }
}

class Student {
   private String name;
   private int age;
   public Student(){}

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public int getAge() {
      return age;
   }

   public void setAge(int age) {
      this.age = age;
   }

   public String toString() {
      return "Student [ name: "+name+", age: "+ age+ " ]";
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 来查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出

Verify the Output

Student [ name: Mahesh, age: 10 ]

Gson - Data Binding

数据绑定 API 用于使用属性访问器或注释将 JSON 转换为 POJO（纯旧 Java 对象）并从 POJO 转换回来。它有两种类型。

Data Binding API is used to convert JSON to and from POJO (Plain Old Java Object) using property accessor or using annotations. It is of two types.

Primitives Data Binding − Converts JSON to and from Java Maps, Lists, Strings, Numbers, Booleans, and NULL objects.
Objects Data Binding − Converts JSON to and from any JAVA type.

Gson 读写 JSON，针对两种数据绑定。数据绑定类似于用于 XML 的 JAXB 解析器。

Gson reads/writes JSON for both types of data bindings. Data Binding is analogous to JAXB parser for XML.

Primitives Data Binding

原始数据绑定是指将 JSON 映射到 JAVA 核心数据类型和内置集合。Gson 提供了各种内置适配器，可用于序列化/反序列化原始数据类型。

Primitives data binding refers to mapping of JSON to JAVA Core data types and inbuilt collections. Gson provides various inbuilt adapters which can be used to serialize/deserialize primitive data types.

Example

让我们看看原始数据绑定的实际应用。在这里，我们将直接将 JAVA 基本类型映射到 JSON，反之亦然。

Let’s see primitive data binding in action. Here we’ll map JAVA basic types directly to JSON and vice versa.

在 C:>Gson_WORKSPACE 中创建名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>Gson_WORKSPACE.

File − GsonTester.java

import java.util.Arrays;
import com.google.gson.Gson;

public class GsonTester {
   public static void main(String args[]) {

      Gson gson = new Gson();
      String name = "Mahesh Kumar";
      long rollNo = 1;
      boolean verified = false;
      int[] marks = {100,90,85};

      //Serialization
      System.out.println("{");
      System.out.println("name: " + gson.toJson(name) +",");
      System.out.println("rollNo: " + gson.toJson(rollNo) +",");
      System.out.println("verified: " + gson.toJson(verified) +",");
      System.out.println("marks:" + gson.toJson(marks));
      System.out.println("}");

      //De-serialization
      name = gson.fromJson("\"Mahesh Kumar\"", String.class);
      rollNo = gson.fromJson("1", Long.class);
      verified = gson.fromJson("false", Boolean.class);
      marks = gson.fromJson("[100,90,85]", int[].class);

      System.out.println("name: " + name);
      System.out.println("rollNo: " + rollNo);
      System.out.println("verified: " +verified);
      System.out.println("marks:" + Arrays.toString(marks));
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{
   name: "Mahesh Kumar",
   rollNo: 1,
   verified: false,
   marks:[100,90,85]
}

name: Mahesh Kumar
rollNo: 1
verified: false
marks:[100, 90, 85]

Gson - Object Data Binding

对象数据绑定是指 JSON 与任何 JAVA 对象之间的映射。

Object data binding refers to mapping of JSON to any JAVA Object.

//Create a Gson instance
Gson gson = new Gson();

//map Student object to JSON content
String jsonString = gson.toJson(student);

//map JSON content to Student object
Student student1 = gson.fromJson(jsonString, Student.class);

Example

让我们看看对象数据绑定在实际使用中的情况。在这里，我们将直接将 JAVA 对象映射到 JSON，反之亦然。

Let’s see object data binding in action. Here we’ll map JAVA Object directly to JSON and vice versa.

在 C:\>GSON_WORKSPACE 创建一个名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File - GsonTester.java

import com.google.gson.Gson;

public class GsonTester {
   public static void main(String args[]) {

      Gson gson = new Gson();
      Student student = new Student();
      student.setAge(10);
      student.setName("Mahesh");

      String jsonString = gson.toJson(student);
      System.out.println(jsonString);

      Student student1 = gson.fromJson(jsonString, Student.class);
      System.out.println(student1);
   }
}

class Student {
   private String name;
   private int age;
   public Student(){}

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public int getAge() {
      return age;
   }

   public void setAge(int age) {
      this.age = age;
   }

   public String toString() {
      return "Student [ name: "+name+", age: "+ age+ " ]";
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{"name":"Mahesh","age":10}
Student [ name: Mahesh, age: 10 ]

Gson - Tree Model

树形模型准备了一个 JSON 文档的内存树表示。它构建一个 JsonObject 节点树。它是一种灵活的方法，类似于 XML 的 DOM 解析器。

Tree Model prepares an in-memory tree representation of the JSON document. It builds a tree of JsonObject nodes. It is a flexible approach and is analogous to DOM parser for XML.

Create Tree from JSON

JsonParser 在读取 JSON 后提供一个指向树形根节点的指针。根节点可用于遍历整个树形。考虑以下代码片段，以获取提供的 JSON 字符串的根节点。

JsonParser provides a pointer to the root node of the tree after reading the JSON. Root Node can be used to traverse the complete tree. Consider the following code snippet to get the root node of a provided JSON String.

//Create an JsonParser instance
JsonParser parser = new JsonParser();

String jsonString =
"{\"name\":\"Mahesh Kumar\", \"age\":21,\"verified\":false,\"marks\": [100,90,85]}";

//create tree from JSON
JsonElement rootNode = parser.parse(jsonString);

Traversing Tree Model

在遍历树形时，使用相对根节点的路径获取每个节点，并处理数据。以下代码片段展示了如何遍历树形。

Get each node using relative path to the root node while traversing the tree and process the data. The following code snippet shows how you can traverse a tree.

JsonObject details = rootNode.getAsJsonObject();

JsonElement nameNode = details.get("name");
System.out.println("Name: " +nameNode.getAsString());

JsonElement ageNode = details.get("age");
System.out.println("Age: " + ageNode.getAsInt());

Example

在 C:\>GSON_WORKSPACE 创建一个名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.JsonArray;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParser;
import com.google.gson.JsonPrimitive;

public class GsonTester {
   public static void main(String args[]) {

      String jsonString =
         "{\"name\":\"Mahesh Kumar\", \"age\":21,\"verified\":false,\"marks\": [100,90,85]}";
      JsonParser parser = new JsonParser();
      JsonElement rootNode = parser.parse(jsonString);

      if (rootNode.isJsonObject()) {
         JsonObject details = rootNode.getAsJsonObject();
         JsonElement nameNode = details.get("name");
         System.out.println("Name: " +nameNode.getAsString());

         JsonElement ageNode = details.get("age");
         System.out.println("Age: " + ageNode.getAsInt());

         JsonElement verifiedNode = details.get("verified");
         System.out.println("Verified: " + (verifiedNode.getAsBoolean() ? "Yes":"No"));
         JsonArray marks = details.getAsJsonArray("marks");

         for (int i = 0; i < marks.size(); i++) {
            JsonPrimitive value = marks.get(i).getAsJsonPrimitive();
            System.out.print(value.getAsInt() + " ");
         }
      }
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

Name: Mahesh Kumar
Age: 21
Verified: No
100 90 85

Gson - Streaming

Streaming API 用于逐个读取 JSON token。它将 JSON 内容读入和写入为离散事件。 JsonReader 和 JsonWriter 以 token 形式读/写数据，称为 JsonToken 。

Streaming API is used to read JSON token by token. It reads and writes JSON content as discrete events. JsonReader and JsonWriter read/write the data as token, referred as JsonToken.

在三种处理 JSON 的方法中它是最强大的方法。它开销最低而且读/写操作非常快。它类似于 XML 的 Stax 解析器。

It is the most powerful approach among the three approaches to process JSON. It has the lowest overhead and it is quite fast in read/write operations. It is analogous to Stax parser for XML.

在本章中，我们将展示 GSON 流式 API 以读取 JSON 数据。流式 API 使用 token 的概念，Json 的每个细节都必须小心处理。

In this chapter, we will showcase the usage of GSON streaming APIs to read JSON data. Streaming API works with the concept of token and every details of Json is to be handled carefully.

//create JsonReader object and pass it the json source or json text.
JsonReader reader = new JsonReader(new StringReader(jsonString));

//start reading json
reader.beginObject();

//get the next token
JsonToken token = reader.peek();

//check the type of the token
if (token.equals(JsonToken.NAME)) {
   //get the current token
   fieldname = reader.nextName();
}

Example

让我们看看 JsonReader 如何发挥作用。在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Let’s see JsonReader in action. Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File - GsonTester.java

import java.io.IOException;
import java.io.StringReader;

import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;

public class GsonTester {
   public static void main(String args[]) {

      String jsonString =
         "{\"name\":\"Mahesh Kumar\", \"age\":21,\"verified\":false,\"marks\": [100,90,85]}";
      JsonReader reader = new JsonReader(new StringReader(jsonString));
      try {
         handleJsonObject(reader);
      }
      catch (IOException e) {
         e.printStackTrace();
      }
   }

   private static void handleJsonObject(JsonReader reader) throws IOException {
      reader.beginObject();
      String fieldname = null;

      while (reader.hasNext()) {
         JsonToken token = reader.peek();

         if (token.equals(JsonToken.BEGIN_ARRAY)) {
            System.out.print("Marks [ ");
            handleJsonArray(reader);
            System.out.print("]");
         } else if (token.equals(JsonToken.END_OBJECT)) {
            reader.endObject();
            return;
         } else {
            if (token.equals(JsonToken.NAME)) {
               //get the current token
               fieldname = reader.nextName();
            }

            if ("name".equals(fieldname)) {
               //move to next token
               token = reader.peek();
               System.out.println("Name: "+reader.nextString());
            }

            if("age".equals(fieldname)) {
               //move to next token
               token = reader.peek();
               System.out.println("Age:" + reader.nextInt());
            }

            if("verified".equals(fieldname)) {
               //move to next token
               token = reader.peek();
               System.out.println("Verified:" + reader.nextBoolean());
            }
         }
      }
   }

   private static void handleJsonArray(JsonReader reader) throws IOException {
      reader.beginArray();
      String fieldname = null;

      while (true) {
         JsonToken token = reader.peek();

         if (token.equals(JsonToken.END_ARRAY)) {
            reader.endArray();
            break;
         } else if (token.equals(JsonToken.BEGIN_OBJECT)) {
            handleJsonObject(reader);
         } else if (token.equals(JsonToken.END_OBJECT)) {
            reader.endObject();
         } else {
            System.out.print(reader.nextInt() + " ");
         }
      }
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

Name: Mahesh Kumar
Age:21
Verified:false
Marks [ 100 90 85 ]

Gson - Serialization Examples

在本章中，我们将讨论数组、集合和泛型的序列化/反序列化。

In this chapter, we will discuss the serialization/deserialization of arrays, collections, and generics.

Array Example

int[] marks = {100,90,85};
//Serialization
System.out.println("marks:" + gson.toJson(marks));

//De-serialization
marks = gson.fromJson("[100,90,85]", int[].class);
System.out.println("marks:" + Arrays.toString(marks));

Example

让我们看看 Array 序列化/反序列化如何发挥作用。在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Let’s see Array serialization/de-serialization in action. Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import java.util.Arrays;
import com.google.gson.Gson;

public class GsonTester {
   public static void main(String args[]) {

      Gson gson = new Gson();
      int[] marks = {100,90,85};
      String[] names = {"Ram","Shyam","Mohan"};

      //Serialization
      System.out.print("{");
      System.out.print("marks:" + gson.toJson(marks) + ",");
      System.out.print("names:" + gson.toJson(names));
      System.out.println("}");

      //De-serialization
      marks = gson.fromJson("[100,90,85]", int[].class);
      names = gson.fromJson("[\"Ram\",\"Shyam\",\"Mohan\"]", String[].class);
      System.out.println("marks:" + Arrays.toString(marks));
      System.out.println("names:" + Arrays.toString(names));
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 来查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{marks:[100,90,85],names:["Ram","Shyam","Mohan"]}
marks:[100, 90, 85]
names:[Ram, Shyam, Mohan]

Collections Example

List marks = new ArrayList();
//Serialization
System.out.println("marks:" + gson.toJson(marks));

//De-serialization
//get the type of the collection.
Type listType = new TypeToken<list>(){}.getType();

//pass the type of collection
marks = gson.fromJson("[100,90,85]", listType);
System.out.println("marks:" +marks);</list>

Example

让我们看看 Collection 序列化/反序列化如何发挥作用。在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Let’s see Collection serialization/de-serialization in action. Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Collection;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class GsonTester {
   public static void main(String args[]) {

      Gson gson = new Gson();
      Collection<Integer> marks = new ArrayList<Integer>();
      marks.add(100);
      marks.add(90);
      marks.add(85);

      //Serialization
      System.out.print("{");
      System.out.print("marks:" + gson.toJson(marks));
      System.out.println("}");

      //De-serialization
      Type listType = new TypeToken<Collection<Integer>>(){}.getType();
      marks = gson.fromJson("[100,90,85]", listType);
      System.out.println("marks:" +marks);
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{marks:[100,90,85]}
marks:[100, 90, 85]

Generics Example

Gson 使用 Java 反射 API 来获取 Json 文本要映射到的对象的类型。但是对于泛型，此信息会在序列化期间丢失。为了解决这个问题，Gson 提供了一个名为 com.google.gson.reflect.TypeToken 的类来存储泛型对象的类型。

Gson uses Java reflection API to get the type of the object to which a Json text is to be mapped. But with generics, this information is lost during serialization. To counter this problem, Gson provides a class com.google.gson.reflect.TypeToken to store the type of the generic object.

Example

让我们看看 Generics 序列化/反序列化如何发挥作用。在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Let’s see Generics serialization/de-serialization in action. Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import java.lang.reflect.Type;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class GsonTester {
   public static void main(String args[]) {

      // create a shape class of type circle.
      Shape<Circle> shape = new Shape<Circle>();

      // Create a Circle object
      Circle circle = new Circle(5.0);

      //assign circle to shape
      shape.setShape(circle);
      Gson gson = new Gson();

      // Define a Type shapeType of type circle.
      Type shapeType = new TypeToken<Shape<Circle>>() {}.getType();

      //Serialize the json as ShapeType
      String jsonString = gson.toJson(shape, shapeType);
      System.out.println(jsonString);
      Shape shape1 = gson.fromJson(jsonString, Shape.class);

      System.out.println(shape1.get().getClass());
      System.out.println(shape1.get().toString());
      System.out.println(shape1.getArea());
      Shape shape2 = gson.fromJson(jsonString, shapeType);
      System.out.println(shape2.get().getClass());
      System.out.println(shape2.get().toString());
      System.out.println(shape2.getArea());
   }
}

class Shape <T> {
   public T shape;

   public void setShape(T shape) {
      this.shape = shape;
   }
   public T get() {
      return shape;
   }
   public double getArea() {
      if(shape instanceof Circle) {
         return ((Circle) shape).getArea();
      } else {
         return 0.0;
      }
   }
}

class Circle {
   private double radius;

   public Circle(double radius){
      this.radius = radius;
   }
   public String toString() {
      return "Circle";
   }
   public double getRadius() {
      return radius;
   }
   public void setRadius(double radius) {
      this.radius = radius;
   }
   public double getArea() {
      return (radius*radius*3.14);
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{"shape":{"radius":5.0}}
class com.google.gson.internal.LinkedTreeMap
{radius = 5.0}
0.0
class Circle
Circle
78.5

Gson - Serializing Inner Classes

在本章中，我们将讲解包含内部类的类的序列化/反序列化。

In this chapter, we will explain serialization/deserialization of classes having inner classes.

Nested Inner Class example

Student student = new Student();
student.setRollNo(1);
Student.Name name = student.new Name();
name.firstName = "Mahesh";
name.lastName = "Kumar";

student.setName(name);
//serialize inner class object
String nameString = gson.toJson(name);
System.out.println(nameString);

//deserialize inner class object
name = gson.fromJson(nameString,Student.Name.class);
System.out.println(name.getClass());

Example

让我们看一个序列化/反序列化带有内部类的类的示例。在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Let’s see an example of serialization/de-serialization of class with an inner class in action. Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson;

public class GsonTester {
   public static void main(String args[]) {

      Student student = new Student();
      student.setRollNo(1);
      Student.Name name = student.new Name();

      name.firstName = "Mahesh";
      name.lastName = "Kumar";
      student.setName(name);
      Gson gson = new Gson();

      String jsonString = gson.toJson(student);
      System.out.println(jsonString);
      student = gson.fromJson(jsonString, Student.class);

      System.out.println("Roll No: "+ student.getRollNo());
      System.out.println("First Name: "+ student.getName().firstName);
      System.out.println("Last Name: "+ student.getName().lastName);

      String nameString = gson.toJson(name);
      System.out.println(nameString);

      name = gson.fromJson(nameString,Student.Name.class);
      System.out.println(name.getClass());
      System.out.println("First Name: "+ name.firstName);
      System.out.println("Last Name: "+ name.lastName);
   }
}

class Student {
   private int rollNo;
   private Name name;

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }

   public Name getName() {
      return name;
   }

   public void setName(Name name) {
      this.name = name;
   }

   class Name {
      public String firstName;
      public String lastName;
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 来查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{"rollNo":1,"name":{"firstName":"Mahesh","lastName":"Kumar"}}
Roll No: 1
First Name: Mahesh
Last Name: Kumar

{"firstName":"Mahesh","lastName":"Kumar"}
class Student$Name
First Name: Mahesh
Last Name: Kumar

Nested Static Inner Class Example

Student student = new Student();
student.setRollNo(1);
Student.Name name = new Student.Name();

name.firstName = "Mahesh";
name.lastName = "Kumar";
student.setName(name);

//serialize static inner class object
String nameString = gson.toJson(name);
System.out.println(nameString);

//deserialize static inner class object
name = gson.fromJson(nameString,Student.Name.class);
System.out.println(name.getClass());

Example

让我们看一个序列化/反序列化带有静态内部类的类的示例。在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Let’s see an example of serialization/de-serialization of class with a static inner class in action. Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson;

public class GsonTester {
   public static void main(String args[]) {

      Student student = new Student();
      student.setRollNo(1);
      Student.Name name = new Student.Name();

      name.firstName = "Mahesh";
      name.lastName = "Kumar";
      student.setName(name);

      Gson gson = new Gson();
      String jsonString = gson.toJson(student);
      System.out.println(jsonString);
      student = gson.fromJson(jsonString, Student.class);

      System.out.println("Roll No: "+ student.getRollNo());
      System.out.println("First Name: "+ student.getName().firstName);
      System.out.println("Last Name: "+ student.getName().lastName);
      String nameString = gson.toJson(name);
      System.out.println(nameString);

      name = gson.fromJson(nameString,Student.Name.class);
      System.out.println(name.getClass());
      System.out.println("First Name: "+ name.firstName);
      System.out.println("Last Name: "+ name.lastName);
   }
}

class Student {
   private int rollNo;
   private Name name;

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }

   public Name getName() {
      return name;
   }

   public void setName(Name name) {
      this.name = name;
   }

   static class Name {
      public String firstName;
      public String lastName;
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{"rollNo":1,"name":{"firstName":"Mahesh","lastName":"Kumar"}}
Roll No: 1
First Name: Mahesh
Last Name: Kumar

{"firstName":"Mahesh","lastName":"Kumar"}
class Student$Name
First Name: Mahesh
Last Name: Kumar

Gson - Custom Type Adapters

Gson 使用其内置适配器执行对象的序列化/反序列化，它还支持自定义适配器。让我们讨论如何创建自定义适配器以及如何使用它。

Gson performs the serialization/deserialization of objects using its inbuilt adapters. It also supports custom adapters. Let’s discuss how you can create a custom adapter and how you can use it.

Create a Custom Adapter

通过扩展 TypeAdapter 类并向其传入目标对象类型来创建自定义适配器。覆盖 read 和 write 方法，分别执行自定义反序列化和序列化。

Create a custom adapter by extending the TypeAdapter class and passing it the type of object targeted. Override the read and write methods to do perform custom deserialization and serialization respectively.

class StudentAdapter extends TypeAdapter<Student> {
   @Override
   public Student read(JsonReader reader) throws IOException {
      ...
   }

   @Override
   public void write(JsonWriter writer, Student student) throws IOException {
   }
}

Register the Custom Adapter

使用 GsonBuilder 注册自定义适配器，并使用 GsonBuilder 创建 Gson 实例。

GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(Student.class, new StudentAdapter());
Gson gson = builder.create();

Use the Adapter

Gson 现在将使用自定义适配器将 Json 文本转换为对象，反之亦然。

Gson will now use the custom adapter to convert Json text to object and vice versa.

String jsonString = "{\"name\":\"Mahesh\", \"rollNo\":1}";
Student student = gson.fromJson(jsonString, Student.class);
System.out.println(student);
jsonString = gson.toJson(student);
System.out.println(jsonString);

Example

让我们看一个自定义类型适配器的示例。在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Let’s see an example of custom type adapter in action. Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import java.io.IOException;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.TypeAdapter;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;

public class GsonTester {
   public static void main(String args[]) {

      GsonBuilder builder = new GsonBuilder();
      builder.registerTypeAdapter(Student.class, new StudentAdapter());
      builder.setPrettyPrinting();
      Gson gson = builder.create();

      String jsonString = "{\"name\":\"Mahesh\", \"rollNo\":1}";
      Student student = gson.fromJson(jsonString, Student.class);
      System.out.println(student);

      jsonString = gson.toJson(student);
      System.out.println(jsonString);
   }
}

class StudentAdapter extends TypeAdapter<Student> {
   @Override
   public Student read(JsonReader reader) throws IOException {
      Student student = new Student();
      reader.beginObject();
      String fieldname = null;

      while (reader.hasNext()) {
         JsonToken token = reader.peek();

         if (token.equals(JsonToken.NAME)) {
            //get the current token
            fieldname = reader.nextName();
         }

         if ("name".equals(fieldname)) {
            //move to next token
            token = reader.peek();
            student.setName(reader.nextString());
         }

         if("rollNo".equals(fieldname)) {
            //move to next token
            token = reader.peek();
            student.setRollNo(reader.nextInt());
         }
      }
      reader.endObject();
      return student;
   }

   @Override
   public void write(JsonWriter writer, Student student) throws IOException {
      writer.beginObject();
      writer.name("name");
      writer.value(student.getName());
      writer.name("rollNo");
      writer.value(student.getRollNo());
      writer.endObject();
   }
}

class Student {
   private int rollNo;
   private String name;

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public String toString() {
      return "Student[ name = "+name+", roll no: "+rollNo+ "]";
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

Student[ name = Mahesh, roll no: 1]
{
   "name": "Mahesh",
   "rollNo": 1
}

Gson - Null Object Support

默认情况下，Gson 会忽略 NULL 值，生成经过优化的 Json 内容。但 GsonBuilder 提供了使用 GsonBuilder.serializeNulls() 方法在 Json 输出中显示 NULL 值的标志。

Gson by default generates optimized Json content ignoring the NULL values. But GsonBuilder provides flags to show NULL values in the Json output using the GsonBuilder.serializeNulls() method.

GsonBuilder builder = new GsonBuilder();
builder.serializeNulls();
Gson gson = builder.create();

Example without serializeNulls Call

在 C:\>GSON_WORKSPACE 创建一个名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File - GsonTester.java

import com.google.gson.Gson;

public class GsonTester {
   public static void main(String args[]) {

      Gson gson = new Gson();

      Student student = new Student();
      student.setRollNo(1);
      String jsonString = gson.toJson(student);

      System.out.println(jsonString);
      student = gson.fromJson(jsonString, Student.class);
      System.out.println(student);
   }
}

class Student {
   private int rollNo;
   private String name;

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public String toString() {
      return "Student[ name = "+name+", roll no: "+rollNo+ "]";
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 来查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{"rollNo": 1}
Student[ name = null, roll no: 1]

Example with serializeNulls call

在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File - GsonTester.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class GsonTester {
   public static void main(String args[]) {

      GsonBuilder builder = new GsonBuilder();
      builder.serializeNulls();
      builder.setPrettyPrinting();
      Gson gson = builder.create();

      Student student = new Student();
      student.setRollNo(1);
      String jsonString = gson.toJson(student);

      System.out.println(jsonString);
      student = gson.fromJson(jsonString, Student.class);
      System.out.println(student);
   }
}
class Student {
   private int rollNo;
   private String name;

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public String toString() {
      return "Student[ name = "+name+", roll no: "+rollNo+ "]";
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{
   "rollNo": 1,
   "name": null
}
Student[ name = null, roll no: 1]

Gson - Versioning Support

Gson 提供 @Since 注释来控制基于其不同版本的类的 Json 序列化/反序列化。考虑具有版本支持的以下类。在此类中，我们最初定义了两个变量 rollNo 和 name ，然后我们在后面将 verified 添加为一个新变量。使用 @Since，我们定义了 rollNo 和 name 为版本 1.0，并验证为版本 1.1。

Gson provides @Since annotation to control the Json serialization/deserialization of a class based on its various versions. Consider the following class with versioning support. In this class, we’ve initially defined two variables rollNo and name and later on, we added verified as a new variable. Using @Since, we’ve defined rollNo and name as of version 1.0 and verified to be of version 1.1.

class Student {
   @Since(1.0)
   private int rollNo;

   @Since(1.0)
   private String name;

   @Since(1.1)
   private boolean verified;
}

GsonBuilder 提供 setVersion() 方法来序列化此类版本化类。

GsonBuilder provides the setVersion() method to serialize such versioned class.

GsonBuilder builder = new GsonBuilder();
builder.setVersion(1.0);
Gson gson = builder.create();

Example

让我们看一个版本支持的示例。在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Let’s see an example of versioning support in action. Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File - GsonTester.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.annotations.Since;

public class GsonTester {
   public static void main(String args[]) {

      GsonBuilder builder = new GsonBuilder();
      builder.setVersion(1.0);
      Gson gson = builder.create();

      Student student = new Student();
      student.setRollNo(1);
      student.setName("Mahesh Kumar");
      student.setVerified(true);

      String jsonString = gson.toJson(student);
      System.out.println(jsonString);

      gson = new Gson();
      jsonString = gson.toJson(student);
      System.out.println(jsonString);
   }
}

class Student {
   @Since(1.0)
   private int rollNo;

   @Since(1.0)
   private String name;

   @Since(1.1)
   private boolean verified;

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public void setVerified(boolean verified) {
      this.verified = verified;
   }

   public boolean isVerified() {
      return verified;
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 来查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出。

Verify the output.

{"rollNo":1,"name":"Mahesh Kumar"}
{"rollNo":1,"name":"Mahesh Kumar","verified":true}

Gson - Excluding fields from Serialization

默认情况下，GSON 会将瞬态和静态字段排除在序列化/反序列化过程中。让我们来看一下下面的示例。

By default, GSON excludes transient and static fields from the serialization/deserialization process. Let’s take a look at the following example.

Example

在 C:\>GSON_WORKSPACE 创建一个名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class GsonTester {
   public static void main(String args[]) {

      GsonBuilder builder = new GsonBuilder();
      Gson gson = builder.create();

      Student student = new Student();
      student.setRollNo(1);
      student.setName("Mahesh Kumar");
      student.setVerified(true);
      student.setId(1);
      student.className = "VI";

      String jsonString = gson.toJson(student);
      System.out.println(jsonString);
   }
}

class Student {
   private int rollNo;
   private String name;
   private boolean verified;
   private transient int id;
   public static String className;

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public void setVerified(boolean verified) {
      this.verified = verified;
   }

   public boolean isVerified() {
      return verified;
   }

   public int getId() {
      return id;
   }

   public void setId(int id) {
      this.id = id;
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出

Verify the output

{"rollNo":1,"name":"Mahesh Kumar","verified":true}

Using excludeFieldsWithModifiers

GsonBuilder 提供对使用 excludeFieldsWithModifiers() 方法排除具有特殊修饰符的字段的控制，以排除序列化/反序列化过程。请参阅以下示例。

GsonBuilder provides control over excluding fields with particular modifier using excludeFieldsWithModifiers() method from serialization/deserialization process. See the following example.

Example

在 C:\>GSON_WORKSPACE 中创建一个名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import java.lang.reflect.Modifier;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class GsonTester {
   public static void main(String args[]) {

      GsonBuilder builder = new GsonBuilder();
      builder.excludeFieldsWithModifiers(Modifier.TRANSIENT);
      Gson gson = builder.create();

      Student student = new Student();
      student.setRollNo(1);
      student.setName("Mahesh Kumar");
      student.setVerified(true);
      student.setId(1);
      student.className = "VI";

      String jsonString = gson.toJson(student);
      System.out.println(jsonString);
   }
}

class Student {
   private int rollNo;
   private String name;
   private boolean verified;
   private transient int id;
   public static String className;

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public void setVerified(boolean verified) {
      this.verified = verified;
   }

   public boolean isVerified() {
      return verified;
   }

   public int getId() {
      return id;
   }

   public void setId(int id) {
      this.id = id;
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出

Verify the output

{"rollNo":1,"name":"Mahesh Kumar","verified":true,"className":"VI"}

Using @Expose Annotation

Gson 提供 @Expose 注释来控制基于类的作用域的 Json 序列化/反序列化。考虑具有变量具有 @Expose 支持的以下类。此类中， name 和 rollno 变量公开序列化。然后，我们使用 GsonBuilder.excludeFieldsWithoutExposeAnnotation() 方法来指示仅公开变量将被序列化/反序列化。请参阅以下示例。

Gson provides @Expose annotation to control the Json serialization/deserialization of a class based on its scope. Consider the following class with a variable having @Expose support. In this class, name and rollno variables are to be exposed for serialization. Then we’ve used the GsonBuilder.excludeFieldsWithoutExposeAnnotation() method to indicate that only exposed variables are to be serialized/deserialized. See the following example.

Example

在 C:\>GSON_WORKSPACE 创建一个名为 GsonTester 的 Java 类文件。

Create a Java class file named GsonTester in C:\>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.annotations.Expose;

public class GsonTester {
   public static void main(String args[]) {

      GsonBuilder builder = new GsonBuilder();
      builder.excludeFieldsWithoutExposeAnnotation();
      Gson gson = builder.create();

      Student student = new Student();
      student.setRollNo(1);
      student.setName("Mahesh Kumar");
      student.setVerified(true);
      student.setId(1);
      student.className = "VI";

      String jsonString = gson.toJson(student);
      System.out.println(jsonString);
   }
}
class Student {
   @Expose
   private int rollNo;

   @Expose
   private String name;
   private boolean verified;
   private int id;
   public static String className;

   public int getRollNo() {
      return rollNo;
   }
   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }
   public String getName() {
      return name;
   }
   public void setName(String name) {
      this.name = name;
   }
   public void setVerified(boolean verified) {
      this.verified = verified;
   }
   public boolean isVerified() {
      return verified;
   }
   public int getId() {
      return id;
   }
   public void setId(int id) {
      this.id = id;
   }
}

Verify the result

使用以下 javac 编译器编译类：

Compile the classes using javac compiler as follows −

C:\GSON_WORKSPACE>javac GsonTester.java

现在运行 GsonTester 以查看结果 −

Now run the GsonTester to see the result −

C:\GSON_WORKSPACE>java GsonTester

验证输出

Verify the output

{"rollNo":1,"name":"Mahesh Kumar"}