Java的UnrecognizedPropertyException

在 Restful Web 服务应用程序中，我们解析一个 JSON 请求字符串，然后将其映射到一个Java类。使用 Jackson 库将 JSON字符串解组到Java类。如果 JSON字符串包含无法映射到Java类属性的属性，那么我们会遇到 UnrecognizedPropertyException。让我们通过下面给出的插图来理解这一点。

Illustration:

We have an Employee class with attributes like name, id, department and salary. We then create a json string with all these properties.  We will then try to deserialize the JSON string to Employee class.

在eclipse中设置时，我们需要创建一个maven项目，并在'pom.xml文件'中添加如下依赖。

        

            com.fasterxml.jackson.core
            jackson-databind
            2.2.3
        

    

这将在我们的项目中下载以下 jars。

jackson-databind-2.2.3.jar
jackson-annotations-2.2.3.jar
jackson-core-2.2.3.jar

现在继续，让我们使用 Jackson 库进行序列化和反序列化，其中我们使用 Jackson 库将员工对象序列化为在src/main/resources 文件夹中创建的 json 文件。 Jackson 的库也有API s，它几乎打印了下面实现的 json 文件。

例子

Java

// Java Program to Serialize and Deserialize
// Using the Jackson library
 
// Importing input output classes
// Importing Jackson classes
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.*;
import java.io.File;
import java.io.IOException;
 
// Helper class
class Employee {
 
    // Member variables of this class
    private String id;
    private String name;
    private String deptName;
    private double salary;
 
    // Member functions
    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.deptName = deptName;
    }
    public double getSalary() { return salary; }
    public void setSalary(double salary)
    {
        this.salary = salary;
    }
}
 
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // creates an object mapper instance
        ObjectMapper mapper = new ObjectMapper();
 
        // creates an employee instance.
        Employee employee = generateEmployee();
 
        // java object to json
 
        // Try block to check for exceptions
        try {
 
            // Serializes the java object to json and
            // creates a file in src/main/resources
            mapper.writeValue(
                new File(
                    "/home/suchitra/Desktop/suchitra/projects/java-concurrency-examples/json-parsing/src/main/resources/emp.json"),
                employee);
 
            // Creating a json string
            String empJson
                = mapper.writeValueAsString(employee);
 
            // Print and display
            System.out.println(empJson);
 
            // Pretty print
            String empJsonPrettyPrint
                = mapper.writerWithDefaultPrettyPrinter()
                      .writeValueAsString(employee);
 
            System.out.println("Pretty print format");
            System.out.println(empJsonPrettyPrint);
        }
 
        // Catch block 2
        // Catch block to handle exceptions
        // related to json generation
        catch (JsonGenerationException e) {
 
            // Print the exception along with line number
            // using printStackTrace() method
            e.printStackTrace();
        }
 
        // Catch block 2
        // Catch block to handle exceptions
        // related to json mapping
        catch (JsonMappingException e) {
            e.printStackTrace();
        }
 
        // Catch block 3
        // Catch block to handle basic exceptions
        catch (IOException e) {
            e.printStackTrace();
        }
 
        // Convert to a json string
    }
 
    private static Employee generateEmployee()
    {
 
        Employee employee = new Employee();
        employee.setId("e01010");
        employee.setName("Jane");
        employee.setDeptName("Sales");
        employee.setSalary(100000.00);
 
        return employee;
    }
}

输出：

{"id":"e01010","name":"Jane","deptName":"Sales","salary":100000.0}
Pretty print format
{
  "id" : "e01010",
  "name" : "Jane",
  "deptName" : "Sales",
  "salary" : 100000.0
}

文件 emp.json 在 src/main/resources 文件夹中创建。

{"id":"e01010","name":"Jane","deptName":"Sales","salary":100000.0}

现在让我们通过一个例子来理解UnrecognizedPropertyException，我们在解析json文件时是什么时候得到这个异常的。

执行：

• 我们创建了一个 json 文件，其中添加了一个新属性作为 phoneNumber。但是，该属性不存在于 Employee 类中。
• 当 Jackson 库尝试将 json 字段值解组/反序列化为类属性时，它无法识别此属性并抛出 UnrecognizedPropertyException。
• 我们在 src/main/resources 中创建一个文件 emp1.json。文件 emp1.json 有一个新属性 phoneNumber，它不是 Employee 类的一部分。

{"id":"e01010","name":"Jane","deptName":"Sales","salary":100000.0,"phoneNumber": 98218281812}

例子

Java

// Java Program to Illustrate UnrecognizedPropertyException
// While parsing json files
 
// Importing input output classes
import java.io.*;
 
// Class 1
// Helper class
class Employee {
 
    // Member variables of this class
    private String id;
    private String name;
    private String deptName;
    private double salary;
 
    // Member functions 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.deptName = deptName;
    }
 
    public double getSalary() { return salary; }
 
    public void setSalary(double salary)
    {
        this.salary = salary;
    }
 
    @Override public String toString()
    {
        return "Employee [id=" + id + ", name=" + name
            + ", deptName=" + deptName
            + ", salary=" + salary + "]";
    }
}
 
// Class 2
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating an object of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
 
        // Try block to check for exceptions
        try {
 
            // mapper reads from json file and deserializes
            // it to Employee object
            Employee emp = mapper.readValue(
                new File(
                    "/home/suchitra/Desktop/suchitra/projects/java-concurrency-examples/json-parsing/src/main/resources/emp1.json"),
                Employee.class);
            System.out.println(
                "Reading values of employee attributes from a json file");
            System.out.println(emp.toString());
        }
 
        // Catch Block 1
        // Handling json parsing exception
        catch (JsonParseException e) {
 
            // Print and display the exception along with
            // line number using printStackTrace() method
            e.printStackTrace();
        }
 
        // Catch Block 2
        // Handling json mapping exception
        catch (JsonMappingException e) {
            e.printStackTrace();
        }
 
        // Catch Block 3
        // Catch block to handle basc exceptios
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}

输出：

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "phoneNumber" (class com.sample.json.Employee), not marked as ignorable (4 known properties: , "deptName", "salary", "id", "name"])
 at [Source: /home/suchitra/Desktop/suchitra/projects/java-concurrency-examples/json-parsing/src/main/resources/emp1.json; line: 1, column: 93] (through reference chain: com.sample.json.Employee["phoneNumber"])
    at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:79)
    at com.fasterxml.jackson.databind.DeserializationContext.reportUnknownProperty(DeserializationContext.java:555)
    at com.fasterxml.jackson.databind.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:708)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownProperty(BeanDeserializerBase.java:1160)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:315)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:121)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2888)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1988)
    at com.sample.json.TestParsingException.main(TestParsingException.java:19)

Note: The exception states that it is unable to recognize the property phoneNumber which is a part of emp1.json file. This is because it is unable to map this property with the attributes of the Employee class.

现在让我们讨论摆脱这个异常的方法。以下是我们可以合并的两种方式，如下所示：

1. 禁用对象映射器对未知属性进行反序列化。
2. 在类级别@JsonIgnoreProperties 提供注释。

方法 1：禁用对象映射器对未知属性进行反序列化。

为了解决这个问题，我们禁用对象映射器对未知属性进行反序列化。因此，对象映射器将仅解组/反序列化 json 文件中映射到Java类的那些属性。我们有 emp1.json，它类似于前面的例子。

emp1.json
{"id":"e01010","name":"Jane","deptName":"Sales","salary":100000.0,"phoneNumber": 98218281812}

例子

Java

// Java Program to Remove UnrecognizedPropertyException by
// Disabling the object mapper to deserialize on unknown
// properties.
 
package com.sample.json;
// Importing input output classes
import java.io.*;
 
// Class 1
// Helper class
public class Employee {
 
    // Member variables of this class
    private String id;
    private String name;
    private String deptName;
    private double salary;
 
    // 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.deptName = deptName;
    }
    public double getSalary() { return salary; }
    public void setSalary(double salary)
    {
        this.salary = salary;
    }
    @Override public String toString()
    {
        return "Employee [id=" + id + ", name=" + name
            + ", deptName=" + deptName
            + ", salary=" + salary + "]";
    }
}
 
// Class 2
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating an object of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
 
        // Try block to check for exceptions
        try {
 
            // Disabling the mapper object to deserialize
            // properties which cannot be mapped to java
            // class
            mapper.disable(DeserializationFeature
                               .FAIL_ON_UNKNOWN_PROPERTIES);
 
            // mapper reading the json file and
            // deserializes it to employee object
            Employee emp = mapper.readValue(
                new File(
                    "/home/suchitra/Desktop/suchitra/projects/java-concurrency-examples/json-parsing/src/main/resources/emp1.json"),
                Employee.class);
 
            // Display message only
            System.out.println(
                "Reading values of employee attributes from a json file");
 
            // Print and display
            System.out.println(emp.toString());
        }
 
        // Catch block to handle exceptions
 
        // Catch block 1 to deal with json parsing exception
        catch (JsonParseException e) {
            e.printStackTrace();
        }
 
        // Catch block 2 to deal with json mapping exception
        catch (JsonMappingException e) {
            e.printStackTrace();
        }
 
        // Catch block 2 to deal with basic I/O exceptions
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}

输出：

Reading values of employee attributes from a json file
Employee [id=e01010, name=Jane, deptName=Sales, salary=100000.0]

方法二：在类级别提供注解@JsonIgnoreProperties

另一种方法是在类级别提供注释@JsonIgnoreProperties(ignoreUnknown = true)。这个注解告诉 jackson 忽略那些不能映射到Java类的属性。这与上一个示例中的 emp1.json 相同。

emp1.json
{"id":"e01010","name":"Jane","deptName":"Sales","salary":100000.0,"phoneNumber": 98218281812}

例子

Java

// Java Program to Remove UnrecognizedPropertyException by
// Providing an annotation at the class level
// @JsonIgnoreProperties
 
// Importing required classes
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import java.io.*;
 
@JsonIgnoreProperties(ignoreUnknown = true)
 
// Class 1
// Helper class
public class Employee {
 
    // Member variables of this class
    private String id;
    private String name;
    private String deptName;
    private double salary;
 
    // Mamber functions 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.deptName = deptName;
    }
    public double getSalary() { return salary; }
    public void setSalary(double salary)
    {
        this.salary = salary;
    }
    @Override public String toString()
    {
        return "Employee [id=" + id + ", name=" + name
            + ", deptName=" + deptName
            + ", salary=" + salary + "]";
    }
}
 
// Class 2
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an object of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
 
        // Try block to check for exceptions
        try {
 
            // Mapper reads from emp1.json and deserializes
            // it to employee object. It ignores those
            // properties which are not mapped to java class
            Employee emp = mapper.readValue(
                new File(
                    "/home/suchitra/Desktop/suchitra/projects/java-concurrency-examples/json-parsing/src/main/resources/emp1.json"),
                Employee.class);
 
            System.out.println(
                "Reading values of employee attributes from a json file");
 
            System.out.println(emp.toString());
        }
 
        // Catch block to handle the exceptions
 
        // Catch Block 1
        catch (JsonParseException e) {
 
            // Print the exception along wth line number
            // using printStackTrace() method
            e.printStackTrace();
        }
 
        // Catch Block 2
        catch (JsonMappingException e) {
            e.printStackTrace();
        }
 
        // Catch Block 3
        // Handling basic I/O exceptions
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}

输出：

Reading values of employee attributes from a json file
Employee [id=e01010, name=Jane, deptName=Sales, salary=100000.0]