Question

我正在使用一个不是开源的项目，我需要修改一个或多个类。

在一个课程中是关注集合：

private Map<Integer, TTP> ttp = new HashMap<>();

我需要做的就是使用反射并在这里使用concurrenthashmap。我试过跟随代码，但它不起作用。

Field f = ..getClass().getDeclaredField("ttp");
f.setAccessible(true);
f.set(null, new ConcurrentHashMap<>());

Answer 1

希望这是你想要做的事情：

import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class Test {

    private Map ttp = new HashMap(); 

    public  void test() {
        Field declaredField =  null;
        try {

            declaredField = Test.class.getDeclaredField("ttp");
            boolean accessible = declaredField.isAccessible();

            declaredField.setAccessible(true);

            ConcurrentHashMap<Object, Object> concHashMap = new ConcurrentHashMap<Object, Object>();
            concHashMap.put("key1", "value1");
            declaredField.set(this, concHashMap);
            Object value = ttp.get("key1");

            System.out.println(value);

            declaredField.setAccessible(accessible);

        } catch (NoSuchFieldException 
                | SecurityException
                | IllegalArgumentException 
                | IllegalAccessException e) {
            e.printStackTrace();
        }

    }

    public static void main(String... args) {
        Test test = new Test();
        test.test(); 
    }
}

打印：

value1

Answer 2

值得一读Oracle Java Tutorial - Getting and Setting Field Values

Field#set(Object object, Object value)将指定对象参数上此Field对象所表示的字段设置为指定的新值。

应该是这样的

f.set(objectOfTheClass, new ConcurrentHashMap<>());

您无法在null对象中设置任何值如果尝试过，则会产生NullPointerException

注意：通过反射设置字段值会产生一定量的性能开销，因为必须进行各种操作，例如验证访问权限。从运行时的角度来看，效果是相同的，并且操作就像在类代码中直接更改值一样原子化。

Answer 3

即使字段位于超类

中，下面的方法也会在对象上设置字段

/**
 * Sets a field value on a given object
 *
 * @param targetObject the object to set the field value on
 * @param fieldName    exact name of the field
 * @param fieldValue   value to set on the field
 * @return true if the value was successfully set, false otherwise
 */
public static boolean setField(Object targetObject, String fieldName, Object fieldValue) {
    Field field;
    try {
        field = targetObject.getClass().getDeclaredField(fieldName);
    } catch (NoSuchFieldException e) {
        field = null;
    }
    Class superClass = targetObject.getClass().getSuperclass();
    while (field == null && superClass != null) {
        try {
            field = superClass.getDeclaredField(fieldName);
        } catch (NoSuchFieldException e) {
            superClass = superClass.getSuperclass();
        }
    }
    if (field == null) {
        return false;
    }
    field.setAccessible(true);
    try {
        field.set(targetObject, fieldValue);
        return true;
    } catch (IllegalAccessException e) {
        return false;
    }
}

Answer 4

你可以试试这个：

//Your class instance
Publication publication = new Publication();

//Get class with full path(with package name)
Class<?> c = Class.forName("com.example.publication.models.Publication");

//Get method
Method  method = c.getDeclaredMethod ("setTitle", String.class);

//set value
method.invoke (publication,  "Value to want to set here...");

Answer 5

您可以尝试以下方法：

static class Student {
    private int age;
    private int number;

    public Student(int age, int number) {
        this.age = age;
        this.number = number;
    }

    public Student() {
    }
}
public static void main(String[] args) throws IllegalAccessException, NoSuchFieldException {
    Student student1=new Student();
   // Class g=student1.getClass();
    Field[]fields=student1.getClass().getDeclaredFields();
    Field age=student1.getClass().getDeclaredField("age");
    age.setAccessible(true);
    age.setInt(student1,13);
    Field number=student1.getClass().getDeclaredField("number");
    number.setAccessible(true);
    number.setInt(student1,936);

    for (Field f:fields
         ) {
        f.setAccessible(true);

        System.out.println(f.getName()+" "+f.getInt(student1));

    }
}

}

用反射设置字段值

5 个答案: