反射地实例化Java局部类

Question

在Java中，是否有一种方法或黑客来实例化一个本地类的反射?

这是我的测试类:

package com.example.localclass;
import org.junit.Test;
public class LocalClassInstantiationTest {
    @Test
    public void testInstantiateLocalClassReflectively() {
        System.out.println("About to define local class");
        class LocalClass {
            void print() {
                System.out.println("Hello world");
            }
        }
        System.out.println("--------------------Direct instantiation------------------------");
        LocalClass localClassInstance;
        localClassInstance = new LocalClass();
        localClassInstance.print();
        System.out.println("--------------------Reflective instantiation--------------------");
        Class<LocalClass> localClazz = LocalClass.class;
        System.out.println("Local class name: " + localClazz.getName() );
        System.out.println("Local class canonical name: " + localClazz.getCanonicalName() );
        System.out.println("Local class constructors array length: " + localClazz.getConstructors().length);
        System.out.println("About to create local class instance reflectively");
        try {
            localClassInstance = localClazz.newInstance();
            localClassInstance.print();
        } catch (InstantiationException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

当我运行它时，我得到以下结果:

About to define local class
--------------------Direct instantiation------------------------
Hello world
--------------------Reflective instantiation--------------------
Local class name: com.example.localclass.LocalClassInstantiationTest$1LocalClass
Local class canonical name: null
Local class constructors array length: 0
About to create local class instance reflectively
java.lang.InstantiationException: com.example.localclass.LocalClassInstantiationTest$1LocalClass
    at java.lang.Class.newInstance0(Class.java:357)
    at java.lang.Class.newInstance(Class.java:325)
    at com.example.localclass.LocalClassInstantiationTest.testInstantiateLocalClassReflectively(LocalClassInstantiationTest.java:32)

Answer 1

在您的例子中，问题是您的类没有空构造函数。newInstance()假定您有一个，如果没有则失败。

如果你在一个静态方法中运行这个，比如main，你不会得到这个错误。

这是因为您有一个非静态匿名类，并且无论您是否使用它，都隐式地需要对外部类LocalClassInstantiationTest的引用。

你可以试试这个

System.out.println("Local class constructors array length: " + 
                   localClazz.getDeclaredConstructors().length);
System.out.println("About to create local class instance reflectively");
try {
    localClassInstance = localClazz
            .getDeclaredConstructor(LocalClassInstantiationTest.class)
            .newInstance(this);
    localClassInstance.print();
} catch (NoSuchMethodException | InvocationTargetException | InstantiationException | IllegalAccessException e) {
    e.printStackTrace();
}

和你的程序打印

About to define local class
--------------------Direct instantiation------------------------
Hello world
--------------------Reflective instantiation--------------------
Local class name: LocalClassInstantiationTest$1LocalClass
Local class canonical name: null
Local class constructors array length: 1
About to create local class instance reflectively
Hello world