java - java中使用反射动态创建对象

Question

下面代码中的Class[] null和Object[] null是什么意思？

public Object selfFactory() {
    Class cl = this.getClass();
    Constructor cons;
    cons = cl.getConstructor((Class[]) null);
    return cons.newInstance((Object[]) null);
}

Answer 1

cl.getConstructor((Class[]) null);将参数的值和类型作为类型参数 <T> ，因此如果我们提供 null 则需要进行转换Class[]而不是null Object用于 .getConstructor() 的方法调用.

同样，cons.newInstance((Object[]) null);创建 cl 的新实例通过调用构造函数 cons这需要 Object[] ，给出为 null在这种情况下。如果没有类型转换，则为 null Object将会通过。

这在方法或构造函数重载时很有用。调用null如果没有强制转换，则类型为 Object并且常常含糊不清:

{
    eat(null); // ???
}

void eat(int[] arr);
void eat(Class arr);

通过强制转换，可以调用正确的构造函数或方法。

{
    eat((Class) null);
}

也如 polygenelubricants 所记录在 How to work with varargs and reflection ,

[...] invoke(Object obj, Object... args) makes it tricky if you need to pass an array of reference type as an only argument, because it is cast-able to Object[], even though it should be wrapped inside a new Object[1] instead.

You can do:

m.invoke(this, new Object[] {a}); // Bohzo's solution

This bypasses the vararg mechanism. More succinctly you can also do:

m.invoke(this, (Object) a);

The cast to Object makes the vararg mechanism do the work of creating the array for you.

The trick is also needed when passing a null as an argument to varargs, and has nothing to do with reflection.

public void foo(String... ss) {
    System.out.println(ss[0]);
}

    foo(null); // causes NullPointerException
    foo((String) null); // prints "null"