Java 中的 UnrecognizedPropertyException 异常
原文:https://www . geeksforgeeks . org/unrecorded property exception-in-Java/
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。使用杰克逊库将 JSON 字符串解组到 java 类中。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unrecognized property exception。让我们通过下面的插图来理解这一点。
说明:
我们有一个 Employee 类，有姓名、身份证、部门、工资等属性。然后我们用所有这些属性创建一个 json 字符串。然后，我们将尝试将 JSON 字符串反序列化为 Employee 类。
在 eclipse 中设置时，我们需要创建一个 maven 项目，并在“POM . XML 文件”中添加以下依赖项。
<dependencies>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.2.3</version>
</dependency>
</dependencies>
这将在我们的项目中下载以下 jar。
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 employee 对象序列化为 json 文件，该文件是在 src/main/resources 文件夹中创建的。Jackson 的库也有 API s，它可以打印 json 文件，如下图所示。
例
爪哇
// 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，当我们解析 json 文件时，什么时候会得到这个异常。
实施:
- 我们创建了一个 json 文件，其中添加了一个新属性 phoneNumber。但是，该属性不在 Employee 类中。
- 当 Jackson 库试图将 json 字段值解组/反序列化为类属性时，它无法识别该属性，并引发 UnrecognizedPropertyException。
- 我们在 src/main/resources 中创建了一个文件 emp1.json。文件 emp1.json 有一个不属于 Employee 类的新属性 phoneNumber。
{"id":"e01010","name":"Jane","deptName":"Sales","salary":100000.0,"phoneNumber": 98218281812}
例
爪哇
// 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 basic exceptions
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)
注意:异常表示无法识别属于 emp1.json 文件一部分的属性 phoneNumber。这是因为它无法将此属性与 Employee 类的属性进行映射。
现在让我们讨论一个摆脱这种例外的方法。下面是我们可以合并的两种方法，如下所示:
- Disable object mapper to deserialize unknown attributes.
- Provide class-level comments @ JsonIgnoreProperties.
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，对象映射器将只解组/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。
<u>emp1.json</u>
{"id":"e01010","name":"Jane","deptName":"Sales","salary":100000.0,"phoneNumber": 98218281812}
例
爪哇
// 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 2: 在类级别提供注释@ JsonIgnoreProperties
另一种方法是在类级别@ JsonIgnoreProperties 提供注释(ignoreUnknown = true)。这个注释告诉 jackson 忽略那些不能映射到 java 类的属性。这与前面示例中的 emp1.json 相同。
<u>emp1.json</u>
{"id":"e01010","name":"Jane","deptName":"Sales","salary":100000.0,"phoneNumber": 98218281812}
例
爪哇
// 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]
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