How to fix: Java Reflection Error: Unable to access private property

How to solve: Java reflection error: Unable to access private properties

Introduction:
In the Java development process, we often use reflection to dynamically obtain and operational information. However, sometimes we encounter a common error when using the reflection API: private properties cannot be accessed. This article will describe the cause of this problem and provide code examples of the solution.

Problem description:
When we use reflection to obtain the private properties of a class, we sometimes receive an IllegalAccessException exception, indicating that we do not have sufficient permissions to access the property.

Cause analysis:
The reason for this problem is that the attributes or methods of the class are restricted to private access rights. Java's access control permissions are divided into four levels: public, protected, default (that is, without modifiers) and private. The reflection API can only access public-level properties and methods by default, so when we try to access private properties, an IllegalAccessException will be thrown.

Solution:
To solve this problem, we can access private properties in the following two ways:

1. Use the setAccessible() method
   setAccessible() is A method of the AccessibleObject class that allows us to bypass access checks at runtime even if the property is marked as private. Using the setAccessible(true) method allows us to gain access to private properties. The following is a sample code that uses the setAccessible() method to get and set private properties:

import java.lang.reflect.Field;

public class ReflectionExample {
    private String privateField = "私有属性";

    public static void main(String[] args) throws IllegalAccessException, NoSuchFieldException {
        ReflectionExample example = new ReflectionExample();
        Field privateField = ReflectionExample.class.getDeclaredField("privateField");
        privateField.setAccessible(true);
        System.out.println("私有属性的值：" + privateField.get(example));
        privateField.set(example, "修改后的值");
        System.out.println("修改后的私有属性的值：" + privateField.get(example));
    }
}

In the above code, we get the class by calling the getDeclaredField() method Private property privateField. Then, we obtain access to the private property by calling the setAccessible(true) method, and obtain the value of the private property using the get() method. Finally, we set the new value of the private property using the set() method.

2. Use the getDeclaredFields() method
   In addition to obtaining a single attribute through the getDeclaredField() method, we can also use the getDeclaredFields() method to obtain the class All properties (including private properties) in the property, and then access and change the value of the private property based on the property name. Here is a sample code that uses the getDeclaredFields() method to get and set a private property:

import java.lang.reflect.Field;

public class ReflectionExample {
    private String privateField = "私有属性";

    public static void main(String[] args) throws IllegalAccessException {
        ReflectionExample example = new ReflectionExample();
        Field[] fields = ReflectionExample.class.getDeclaredFields();

        for (Field field : fields) {
            field.setAccessible(true);
            if (field.getName().equals("privateField")) {
                System.out.println("私有属性的值：" + field.get(example));
                field.set(example, "修改后的值");
                System.out.println("修改后的私有属性的值：" + field.get(example));
                break;
            }
        }
    }
}

In the above code, we get and set a private property by calling getDeclaredFields()The method obtains all attributes in the class. Then, we traverse the attribute array and obtain and change the value of the private attribute by determining whether the attribute name is consistent with the target attribute name.

Conclusion:
By using the setAccessible() method or getDeclaredFields() method, we can access and operate private properties in reflection. However, it is important to avoid abusing reflection to access and modify private properties to maintain code readability and security. When using reflection, you should clearly understand the design and purpose of the reflected class, and try to follow the object-oriented encapsulation principles.

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