使用反射设置私有字段值

我有两个班: Father和 Child

public class Father implements Serializable, JSONInterface {


private String a_field;


//setter and getter here


}


public class Child extends Father {
//empty class
}

通过反思，我想在 Child类中设置 a_field:

Class<?> clazz = Class.forName("Child");
Object cc = clazz.newInstance();


Field f1 = cc.getClass().getField("a_field");
f1.set(cc, "reflecting on life");
String str1 = (String) f1.get(cc.getClass());
System.out.println("field: " + str1);

但我有个例外:

线程“ main”java.lang.NoSuchFieldException: a _ field 中的异常

但如果我尝试:

Child child = new Child();
child.setA_field("123");

很管用。

使用 setter 方法，我遇到了同样的问题:

method = cc.getClass().getMethod("setA_field");
method.invoke(cc, new Object[] { "aaaaaaaaaaaaaa" });
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As per the Javadoc of Class.getField (emphasis mine):

Returns a Field object that reflects the specified public member field of the class or interface represented by this Class object.

This method only returns public fields. Since a_field is private, it won't be found.

Here's a working code:

public class Main {


public static void main(String[] args) throws Exception {
Class<?> clazz = Class.forName("Child");
Object cc = clazz.newInstance();


Field f1 = cc.getClass().getField("a_field");
f1.set(cc, "reflecting on life");
String str1 = (String) f1.get(cc);
System.out.println("field: " + str1);
}


}


class Father implements Serializable {
public String a_field;
}


class Child extends Father {
//empty class
}

Note that I also changed your line String str1 = (String) f1.get(cc.getClass()); to String str1 = (String) f1.get(cc); because you need to give the object of the field, not the class.

If you want to keep your field private, then you need to retrieve the getter / setter method and invoke those instead. The code you have given does not work because, to get a method, you also need to specify it's arguments, so

cc.getClass().getMethod("setA_field");

must be

cc.getClass().getMethod("setA_field", String.class);

Here's a working code:

public class Main {


public static void main(String[] args) throws Exception {
Class<?> clazz = Class.forName("Child");
Object cc = clazz.newInstance();
cc.getClass().getMethod("setA_field", String.class).invoke(cc, "aaaaaaaaaaaaaa");
String str1 = (String) cc.getClass().getMethod("getA_field").invoke(cc);
System.out.println("field: " + str1);
}


}


class Father implements Serializable {


private String a_field;


public String getA_field() {
return a_field;
}


public void setA_field(String a_field) {
this.a_field = a_field;
}


}


class Child extends Father {
//empty class
}
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最佳答案

To access a private field you need to set Field::setAccessible to true. You can pull the field off the super class. This code works:

Class<?> clazz = Child.class;
Object cc = clazz.newInstance();


Field f1 = cc.getClass().getSuperclass().getDeclaredField("a_field");
f1.setAccessible(true);
f1.set(cc, "reflecting on life");
String str1 = (String) f1.get(cc);
System.out.println("field: " + str1);
小开

This one can access private fields as well without having to do anything

import org.apache.commons.lang3.reflect.FieldUtils;
Object value = FieldUtils.readField(entity, fieldName, true);
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Using FieldUtils from the Apache Commons Lang 3:

FieldUtils.writeField(childInstance, "a_field", "Hello", true);

The true forces it to set, even if the field is private.

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Kotlin verison

Get private variable using below extension functions

fun <T : Any> T.getPrivateProperty(variableName: String): Any? {
return javaClass.getDeclaredField(variableName).let { field ->
field.isAccessible = true
return@let field.get(this)
}
}

Set private variable value get the variable

fun <T : Any> T.setAndReturnPrivateProperty(variableName: String, data: Any): Any? {
return javaClass.getDeclaredField(variableName).let { field ->
field.isAccessible = true
field.set(this, data)
return@let field.get(this)
}
}

Get variable use:

val bool = <your_class_object>.getPrivateProperty("your_variable") as String

Set and get variable use:

val bool = <your_class_object>.setAndReturnPrivateProperty("your_variable", true) as Boolean
val str = <your_class_object>.setAndReturnPrivateProperty("your_variable", "Hello") as String

Java version

public class RefUtil {


public static Field setFieldValue(Object object, String fieldName, Object valueTobeSet) throws NoSuchFieldException, IllegalAccessException {
Field field = getField(object.getClass(), fieldName);
field.setAccessible(true);
field.set(object, valueTobeSet);
return field;
}


public static Object getPrivateFieldValue(Object object, String fieldName) throws NoSuchFieldException, IllegalAccessException {
Field field = getField(object.getClass(), fieldName);
field.setAccessible(true);
return field.get(object);
}


private static Field getField(Class mClass, String fieldName) throws NoSuchFieldException {
try {
return mClass.getDeclaredField(fieldName);
} catch (NoSuchFieldException e) {
Class superClass = mClass.getSuperclass();
if (superClass == null) {
throw e;
} else {
return getField(superClass, fieldName);
}
}
}
}

Set private value use

RefUtil.setFieldValue(<your_class_object>, "your_variableName", newValue);

Get private value use

Object value = RefUtil.getPrivateFieldValue(<your_class_object>, "your_variableName");

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