How to use the setAccessible() method of Java reflection

1. What is Java's access check

Access check is to check whether the use of member attributes and member methods complies with access rights (public, protected, default, private).

It’s a bit too theoretical. Simply put, if the access permission of a class member (attribute or method) is private, then the member can only be used in the current class; if a class If the access permission of a member is public, then the member can be used in any class; if the access permission of a member of a class is default, then the member can only be used in classes under the same package; if the access permission of a member of a class is If the permission is protected, then the member can be used in classes under the same package and in subclasses of the class under other packages.

If the access permission of a class member is default, but you use the member in another package, When compiling, the compiler will perform an access check and discover the use of the member It is inconsistent with the given access rights, so an error will be reported.

For example, create a People class under the com.example package with four member variables. Under the com.example.app package (which is a package different from com.example), use the four member variables of the People class.

package com.example;

package com.example;

public class People {

    private int privateVar = 1;
            int defaultVar = 2;
    protected int protectedVar = 3;
    public int publicVar = 4;
}


package com.example.app;

import com.example.People;
public class TestMain {
    public static void main(String[] args) {
        People p = new People();
        System.out.println(p.privateVar);
        System.out.println(p.defaultVar);
        System.out.println(p.protectedVar);
        System.out.println(p.publicVar);
    }
}

After compilation, it is prompted that the use of publicVar complies with the public access rights, so there is no error.

I believe everyone understands what access checking is. So, what is the access checking of reflective objects?

The member attributes, member methods, and constructors of a class are abstracted into Field, Method, and Counstructor classes respectively during reflection.

We can use Field to access the member attributes of the object. The compiler does not know the access permissions of the member attributes and only knows it at runtime. Therefore, the check of access rights to reflective objects (such as Field) can only be left to the virtual machine.

If, when the virtual machine is running, it is found that the use of members is inconsistent with the given access rights, the following code

package com.example.app;

import com.example.People;
import java.lang.reflect.Field;
public class TestMain {
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        People p = new People();
        Class cl = p.getClass();
        
        // 利用反射访问private修饰的成员变量 
        Field f = cl.getDeclaredField("privateVar")
        System.out.println(f.get(p));        
    }
}

When running, an exception will be thrown: java.lang.IllegalAccessException

To sum up, access checking can be performed by the compiler at compile time or the virtual machine at runtime (mainly for reflection)

2. setAccessible() method Introduction

　setAccessible(boolean flag) method is a method in the AccessibleObject class, which is the common parent class of Field, Method, and Constructor. When Field, Method or Constructor (all three are reflection objects) are used to set fields (set(Object obj, Object value)) or get fields (get(Object obj) respectively) ), when calling a method (invoke(Object obj, Object... args)) or creating and initializing a new instance of a class (newInstance(Object... initargs)), the Perform runtime access checks.

Quoted from "Java Core Technology Tenth Edition"

Note: The method name setAccessible is easily misleading. People feel that the accessibility of members is set. For example, if they feel that public-modified members can be accessed by any class, the accessibility flag is true; if they feel that private-modified members can only be accessed by this class, the accessibility flag is false. In fact, no matter what the access rights are, the value of the access flag is false.

The test code is as follows:

package com.example.app;

import com.example.People;
import java.lang.reflect.Field;
public class TestMain {
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        People p = new People();
        Class cl = p.getClass();
        // 打印输出所有成员变量的名字及可访问标志
        for (Field f : cl.getDeclaredFields()) {
            System.out.println(f.getName() + ": " + f.isAccessible());
        }
    }
}

Input result:

The API above makes it very clear that this is accessible The flag indicates whether to block the Java language access check, The default value is false, (tested above)

The default value can be modified by setAccessible(true) , this will block the (runtime) access check of the Java language, so that the private members of the object can be accessed without reporting an error.

package com.example.app;

import com.example.People;

import java.lang.reflect.Field;

public class TestMain {

    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        People p = new People();
        Class cl = p.getClass();

        for (Field f : cl.getDeclaredFields()) {
            //屏蔽对象的访问检查
            f.setAccessible(true);
            // 访问不符合访问权限的成员属性
            System.out.println(f.getName() + " = " + f.get(p));
        }
    }
}

Input result:

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