Passing final variables to anonymous classes

问题:

In final variable passed to anonymous class via constructor, Jon Skeet mentioned that variables are passed to the anonymous class instance via an auto-generated constructor. Why would I not be able to see the constructor using reflection in that case:

public static void main(String... args) throws InterruptedException {
final int x = 100;
new Thread() {
    public void run() {
        System.out.println(x);      
        for (Constructor<?> cons : this.getClass()
                .getDeclaredConstructors()) {
            StringBuilder str = new StringBuilder();
            str.append("constructor : ").append(cons.getName())
                    .append("(");
            for (Class<?> param : cons.getParameterTypes()) {
                str.append(param.getSimpleName()).append(", ");
            }
            if (str.charAt(str.length() - 1) == ' ') {
                str.replace(str.length() - 2, str.length(), ")");
            } else
                str.append(')');
            System.out.println(str);
        }
    }

}.start();
Thread.sleep(2000);

}

The output is:

100
constructor : A$1()

回答1:

Here is what your program prints out on my system:

100
constructor : A$1()

So the constructor is there. However, it is parameterless. From looking at the disassembly, what happens is that the compiler figures out that it doesn't need to pass x to run() since its value is known at compile time.

If I change the code like so:

public class A {

    public static void test(final int x) throws InterruptedException {
        new Thread() {
            public void run() {
                System.out.println(x);
                for (Constructor<?> cons : this.getClass()
                        .getDeclaredConstructors()) {
                    StringBuilder str = new StringBuilder();
                    str.append("constructor : ").append(cons.getName())
                            .append("(");
                    for (Class<?> param : cons.getParameterTypes()) {
                        str.append(param.getSimpleName()).append(", ");
                    }
                    if (str.charAt(str.length() - 1) == ' ') {
                        str.replace(str.length() - 2, str.length(), ")");
                    } else
                        str.append(')');
                    System.out.println(str);
                }
            }

        }.start();
        Thread.sleep(2000);
        }

    public static void main(String[] args) throws InterruptedException {
        test(100);
    }

}

The constructor that gets generated is now:

constructor : A$1(int)

The sole argument is the value of x.

回答2:

In this case, it's because 100 is a constant. That gets baked into your class.

If you change x to be:

final int x = args.length;

... then you'll see Test$1(int) in the output. (This is despite it not being explicitly declared. And yes, capturing more variables adds parameters to the constructor.)