Java Reflection - EdmondFrank's 时光足迹

Java Reflection - Private Fields and Methods

Note: This only works when running the code as a standalone Java application, like you do with unit tests and regular applications. if you try to do this inside a Java Applet, you will need to fiddle around with SecurityManager.

Accessing Private Fields

To access a private field you will need to call Class.getDeclaredFiled(String name) method

The methods Class.getField(String name) only return public fields.

Example

import java.lang.reflect.Field;

class PrivateObject {
    private String privateString = null;
    public PrivateObject(String privateString) {
        this.privateString = privateString;
    }
}
public class test {
    public static void main(String[] args) throws NoSuchFieldException {
        PrivateObject privateObject = new PrivateObject("The Private Value");
        Field privateStringField = privateObject.getClass().getDeclaredField("privateString");
        privateStringField.setAccessible(true);
        try {
            String oldFieldValue = (String) privateStringField.get(privateObject);
            System.out.println("fieldValue: " + oldFieldValue);
            String newFieldValue = oldFieldValue.replace("The", "That");
            privateStringField.set(privateObject, newFieldValue);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

PS: by calling Field.setAccessible(true) just turn off the access checks for this particular Field instance, for reflection only. Now you can access it even if it is private/protected or package scope. even if the caller is not part of those scopes But you still can’t access the field using normal code which would be disallowed by compiler.

Accessing Private Methods

To access a private method you will need to call the Class.getDeclaredMethod(String name, Class[] parameterTypes)

The methods Class.getMethod(String name, Class[] parameterTypes) only return public methods

Example

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

class PrivateObject {
    private String privateString = null;
    public PrivateObject(String privateString) {
        this.privateString = privateString;
    }

    private String getPrivateString() {
        return this.privateString;
    }
}
public class test {
    public static void main(String[] args) throws NoSuchMethodException {
        PrivateObject privateObject = new PrivateObject("The Private Value");

        try {
            Method privateStringMethod = PrivateObject.class.getDeclaredMethod("getPrivateString", null);
            privateStringMethod.setAccessible(true);
            String returnValue = (String) privateStringMethod.invoke(privateObject, null);

            System.out.println("returnValue = " + returnValue);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }
}