问题描述
在Java中，你可以捕获对象的方法调用作为Runnable，如下例所示。
In Java you can "capture" a "method call on object" as a Runnable, as in belows example.
稍后，可以访问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完成）。
Later, having access to this instance of Runnable, is it possible to actually access the "captured" object and the method parameters of a method which is called (if possible this probably needs to be done via reflection).
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
}
}
是可以在inspect方法中访问（可能是通过反射）printer对象和作为参数传递的myText吗？
Is it possible in the "inspect" method to access (probably via reflection) "printer" object and "myText" which was passed as a parameter?
推荐答案
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）。但是，这些名称将不一致。
It is possible, because the captured references are translated into fields of the runnable (as with all anonymous classes). The names will be not be consistent however.
我通过测试发现你需要制作 myText 非 final ，否则它将被视为和内联（并且不能作为字段访问）：
I found by testing that you need to make myText non-final, otherwise it will be seen as a compile time constant and in-lined (and will not be accessible as a field):
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
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