Java 8 - 如何访问被绑定为lambda的对象和方法

Question

在Java中,您可以"捕获"对象的"方法调用"作为Runnable,如下例所示.

之后,有权访问Runnable的这个实例,是否可以实际访问"捕获"对象和被调用方法的方法参数(如果可能,这可能需要通过反射来完成).

例如:

class SomePrintingClass {
  public void print(String myText) {
    System.out.println(myText);

  }
}


public class HowToAccess {
  public static void main(String[] args) throws Exception {
    final String myText = "How to access this?";

    final SomePrintingClass printer = new SomePrintingClass();

    Runnable r = () -> printer.print(myText); // capture as Runnable

    inspect(r);
  }


  private static void inspect(Runnable runnable) {
    // I have reference only to runnable... can I access "printer" here

  }


}

是否可以在"inspect"方法中访问(可能通过反射)"printer"对象和作为参数传递的"myText"？

Answer 1

这是可能的,因为捕获的引用被转换为runnable的字段(与所有匿名类一样).但名称不一致.

我通过测试发现你需要创建myText非final,否则它将被视为编译时常量和内联(并且不能作为字段访问):

private static void inspect(Runnable runnable) throws Exception {       
    for(Field f : runnable.getClass().getDeclaredFields()) {
        f.setAccessible(true);
        System.out.println("name: " + f.getName());
        Object o = f.get(runnable);
        System.out.println("value: " + o);
        System.out.println("class: " + o.getClass());
        System.out.println();
    }
}

打印:

name: arg$1
value: test.SomePrintingClass@1fb3ebeb
class: class test.SomePrintingClass

name: arg$2
value: How to access this?
class: class java.lang.String