如何在Java应用程序中设置 Jackson？

JSON（Javascript Object Notation）是 Web 应用程序世界中最流行的数据交换格式。浏览器可以轻松解析 json 请求并将它们转换为 javascript 对象。服务器解析 json 请求，处理它们，并生成新的 json 响应。 JSON 具有自我描述性且易于理解。将Java对象转换为json的过程称为序列化，将json转换为Java对象的过程称为反序列化。

考虑下面的示例图来了解 json 的文件结构，如下所示：

插图：

我们有一个 Student 类，它有 id、name、address、city、hobby 等属性。下面我们来了解一下对应的json文件是怎样的：

{"id":"S1122","name":"Jane","address":"XYZ Street","city":"Mumbai","hobby":"Badminton, Dancing"}

JSON 数据以名称/值对的形式写入，其中名称是属性/属性名称，值是该属性的值。

现在让我们在继续为任何Java应用程序设置 Jackson 之前讨论一下 Jackson JSON 库。

对于Java应用程序，使用 Json字符串是非常困难的。因此，在Java应用程序中，我们需要一个 json 解析器来解析 Json 文件并将它们转换为Java对象。
Jackson 就是这样一种用于解析和生成 Json 文件的Java Json 库。它内置了 Object Mapper 类，用于解析 json 文件并将其反序列化为自定义Java对象。它有助于从Java对象生成 json。
Jackson 还有一个 Jackson Json Parser 和 Jackson Json Generator，它一次解析并生成一个 json 标记。

设置：-

要在我们的应用程序中使用 Jackson 库，我们需要在我们的 maven 项目的 pom.xml 文件中添加以下依赖项。


   com.fasterxml.jackson.core
   jackson-core
   2.9.6



   com.fasterxml.jackson.core
   jackson-annotations
   2.9.6



   com.fasterxml.jackson.core
   jackson-databind
   2.9.6

On adding this dependency in pom.xml, the following jar files get added in Maven dependencies folder in eclipse:

jackson-core-2.9.6.jar
jackson-annotations-2.9.6.jar
jackson-databind-2.9.6.jar

编程需要懂一点英语

Note: If we are not using maven project, then we need to download and add these jar files in our classpath.

编程需要懂一点英语

实现：我们来了解一下Jackson库是如何解析json文件并生成的。

让我们考虑具有名称、id、deptName、salary、 rating 等属性的 Employee 类。我们使用 Jackson 库从 Employee 对象生成一个 json 文件。我们更新了它的属性之一——deptName。我们将员工对象序列化为 json 文件，然后将其反序列化回员工对象，并使用更新的 deptName 属性值。

示例 1

Java

//  Java Program to Illustrate Setting Up of Jackson by
//  parsing Jackson library json files and
//  generating the same
 
// Importing required classes
import java.io.*;
 
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an employee object with it's attributes
        // set
        Employee employee = getEmployee();
        ObjectMapper mapper = new ObjectMapper();
 
        // Try block to check for exceptions
        try {
 
            // Serializes emp object to a file employee.json
            mapper.writeValue(
                new File(
                    "/home/suchitra/Desktop/suchitra/projects/java-concurrency-examples/jackson-parsing/src/main/resources/employee.json"),
                employee);
 
            // Deserializes emp object in json string format
            String empJson
                = mapper.writeValueAsString(employee);
            System.out.println(
                "The employee object in json format:"
                + empJson);
            System.out.println(
                "Updating the dept of emp object");
 
            // Update deptName attribute of emp object
            employee.setDeptName("Devops");
            System.out.println(
                "Deserializing updated emp json ");
 
            // Reading from updated json and deserializes it
            // to emp object
            Employee updatedEmp = mapper.readValue(
                mapper.writeValueAsString(employee),
                Employee.class);
 
            // Print and display the updated employee object
            System.out.println("Updated emp object is "
                               + updatedEmp.toString());
        }
  
        // Catch block to handle exceptions
 
        // Catch block 1
        // Handling JsonGenerationException
        catch (JsonGenerationException e) {
 
            // Display the exception along with line number
            // using printStackTrace() method
            e.printStackTrace();
        }
 
        // Catch block 2
        // Handling JsonmappingException
        catch (JsonMappingException e) {
 
            // Display the exception along with line number
            // using printStackTrace() method
            e.printStackTrace();
        }
 
        // Catch block 3
        // handling generic I/O exceptions
        catch (IOException e) {
 
            // Display the exception along with line number
            // using printStackTrace() method
            e.printStackTrace();
        }
    }
 
    // Method 2
    // To get the employees
    private static Employee getEmployee()
    {
        // Creating an object of Employee class
        Employee emp = new Employee();
        emp.setId("E010890");
        emp.setName("James");
        emp.setDeptName("DBMS");
        emp.setRating(5);
        emp.setSalary(1000000.00);
 
        // Returning the employee
        return emp;
    }
}
 
// Class 2
// Helper class
class Employee {
 
    // Member variables of this class
    private String id;
    private String name;
    private String deptName;
    private double salary;
    private int rating;
 
    // Member methods of this class
    public String getId() { return id; }
    public void setId(String id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public String getDeptName() { return deptName; }
    public void setDeptName(String deptName)
    {
        // This keyword refers to current instance
        this.deptName = deptName;
    }
 
    public double getSalary() { return salary; }
    public void setSalary(double salary)
    {
        this.salary = salary;
    }
    public int getRating() { return rating; }
    public void setRating(int rating)
    {
        this.rating = rating;
    }
 
    @Override public String toString()
    {
        return "Employee [id=" + id + ", name=" + name
            + ", deptName=" + deptName + ", salary="
            + salary + ", rating=" + rating + "]";
    }
}

Java

//  Java Program to Illustrate Setting Up of Jackson by
//  Reading an object from an InputStream
// Using Object Mapper & deserializing to object
 
// Importing required classes
import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.*;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
 
// Class 1
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an object mapper instance
        ObjectMapper mapper = new ObjectMapper();
 
        // Try block to check for exceptions
        try {
 
            // input stream points to a json file in
            // src/main/resources folder
            InputStream inputStream = new FileInputStream(
                "/home/suchitra/Desktop/suchitra/projects/java-concurrency-examples/jackson-parsing/src/main/resources/employee.json");
 
            // Deserializes from json file to employee
            // object
            Employee emp = mapper.readValue(inputStream,
                                            Employee.class);
            System.out.println(emp.toString());
        }
 
        // Catch blocks to handle the exceptions
 
        // Catch block 1
        // Handling FileNotFoundException
        catch (FileNotFoundException e) {
 
            // Displaying the exception along with line
            // number using printStackTrace()
            e.printStackTrace();
        }
 
        // Catch block 2
        catch (JsonParseException e) {
 
            // Displaying the exception along with line
            // number using printStackTrace()
            e.printStackTrace();
        }
 
        // Catch block 3
        catch (JsonMappingException e) {
            e.printStackTrace();
        }
 
        // Catch block 4
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}
}
 
// Class 2
// Helper class
class Employee {
 
    // Member variables of this class
    private String id;
    private String name;
    private String deptName;
    private double salary;
    private int rating;
 
    // Member methods of this class
    public String getId() { return id; }
    public void setId(String id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public String getDeptName() { return deptName; }
    public void setDeptName(String deptName)
    {
        // This keyword refers to current object itself
        this.deptName = deptName;
    }
 
    public double getSalary() { return salary; }
 
    public void setSalary(double salary)
    {
        this.salary = salary;
    }
 
    public int getRating() { return rating; }
 
    public void setRating(int rating)
    {
        this.rating = rating;
    }
 
    @Override public String toString()
    {
        return "Employee [id=" + id + ", name=" + name
            + ", deptName=" + deptName + ", salary="
            + salary + ", rating=" + rating + "]";
    }
}

输出：

The employee object in json format:{"id":"E010890","name":"James","deptName":"DBMS","salary":1000000.0,"rating":5}
Updating the dept of emp object
Deserializing updated emp json 
Updated emp object is Employee [id=E010890, name=James, deptName=Devops, salary=1000000.0, rating=5]

In the src/main/resources folder, employee.json is created.

编程需要懂一点英语

现在让我们进入下一个示例，我们将使用 Jackson 从使用 Object Mapper 的 InputStream 读取对象并将其反序列化为Java对象。

Note: Here we will be having a file named employee.json in src/main/resources folder

编程需要懂一点英语

我们将使用 ObjectMapper 类的readValue() 方法来读取文件。

{"id":"E010890","name":"James","deptName":"DBMS","salary":1000000.0,"rating":5}

例子

Java

{"id":"E010890","name":"James","deptName":"DBMS","salary":1000000.0,"rating":5}

输出：

ObjectMapper mapper = new ObjectMapper();
InputStream inputStream = new FileInputStream("file-path"); 
Employee emp = mapper.readValue(inputStream, Employee.class);