问题描述
这是有线的,但你能做到吗?
我想使用无参数构造函数(因此开发依赖于我的框架不需要扩展构造函数),但我想在字段中使用 final。所以:
注解.java
public @interface Annotation {
String value();
}
父.java
public abstract class Parent {
public final String name;
// this will cause you must extends constructor in any sub class
public Parent(Annotation annotation){
this.name = annotation.value();
}
}
接口定义
public abstract class Define extends Parent {
// You can't do that:
// Cannot reference 'this' before supertype constructor has been called
public Define (){
this(this.getClass().getAnnotation(Annotation.class).value());
}
// You can do that,but not working
// Define is point at Define as is rather I hope the sub class MyModule
public Define (){
this(Define.class.getAnnotation(Annotation.class).value());
}
public Define (Annotation annotation){
super(annotation); // You must extend this
}
public abstract void foo();
}
我希望开发人员像这样使用我的框架:
public class MyModule extends Define {
public void foo(){
// foo bar
}
}
但是由于 Cannot reference 'this' before supertype constructor has been called
,你必须写:
@Annotation
public class MyModule extends Define {
// my framework using scan an IoC auto invoke
public MyModule(Annotation annotation){
super(annotation.value())
}
public void foo(){
// foo bar
}
}
悖论是name
写在注解中,而this
必须在newInstance之后。所以问题更像是:
子类如何getClass()
?
所以唯一的解决方案是放弃 final 字段并使用类似 init() 之类的东西?
解决方法
你能接受使用reflect来解决这个问题吗?
public abstract class Parent {
final String name;
public Parent() {
this.name = null;
}
void setAnnotation(Annotation annotation) throws NoSuchFieldException,IllegalAccessException {
Field field = Parent.class.getDeclaredField("name");
field.setAccessible(true);
field.set(this,annotation.value());
field.setAccessible(false);
}
}
public abstract class Define extends Parent {
public Define() {
super();
try {
this.setAnnotation();
} catch (Exception e) {
}
}
public abstract void foo();
public void setAnnotation() throws NoSuchFieldException,IllegalAccessException {
Annotation annotation = this.getClass().getAnnotation(Annotation.class);
super.setAnnotation(annotation);
}
}
@Annotation(value = "foo")
public class MyModule extends Define {
@Override
public void foo() {
}
}
并且不要忘记在注释类上添加 Target
和 Retention
。
不是要求每个子类读取注解并将其传递给超类,您可以将读取注解移动到基类:
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation {
String value() default "defaultValue";
}
abstract class Base {
final String value;
Base() {
MyAnnotation myAnnotation= getClass().getAnnotation(MyAnnotation.class);
if (myAnnotation == null) {
throw new IllegalStateException("Class " + getClass().getName() + " does not have require annotation MyAnnotation");
}
this.value = myAnnotation.value();
}
}
@MyAnnotation
class DefaultValue extends Base {
}
@MyAnnotation("someValue")
class ExplicitValue extends Base {
}
class NoAnnotation extends Base {
}
给定这些类这两行
System.out.println(new DefaultValue().value);
System.out.println(new ExplicitValue().value);
将分别打印 defaultValue
和 someValue
。但是,这一行会抛出一个 IllegalStateException
:
new NoAnnotation();
不幸的是,Java 类型系统不允许在编译时强制要求对每个具体类进行注释,因此此运行时异常是您可以获得的最佳结果(除了其他静态分析工具或架构测试软件,如 ArchUnit) .