使用java.lang.invoke.MethodHandle调用私有方法 [英] Invoke private method with java.lang.invoke.MethodHandle

问题描述

如何使用方法句柄调用私有方法?

How can I invoke private method using method handles ?

据我所知，只有两种可公开访问的Lookup实例:

As far as I can see there are only two kinds of publicly accessible Lookup instances:

• MethodHandles.lookup()
• MethodHandles.publicLookup()

• MethodHandles.lookup()
• MethodHandles.publicLookup()

，并且都不允许无限制的私人访问.

and neither allows unrestricted private access.

有一个非公开的Lookup.IMPL_LOOKUP可以满足我的需求.是否有一些公开的方式来获取它(假设SecurityManager允许)?

There is the non-public Lookup.IMPL_LOOKUP that does what I want. Is there some public way to obtain it (assuming that SecurityManager allows it) ?

推荐答案

结果证明可以使用Lookup#unreflect(Method)并暂时使方法可访问(除非在程序初始化期间完成，否则可能会引入小的安全性问题).

Turns out it's possible with Lookup#unreflect(Method) and temporarily making method accessible (potentially introducing small security issue unless done during program initialization).

这是Thorben的答案中修改的主要方法:

Here is modified main method from Thorben's answer:

public static void main(String[] args) {

    Lookup lookup = MethodHandles.lookup();
    NestedTestClass ntc = new Program().new NestedTestClass();

    try {
        // Grab method using normal reflection and make it accessible
        Method pm = NestedTestClass.class.getDeclaredMethod("gimmeTheAnswer");
        pm.setAccessible(true);

        // Now convert reflected method into method handle
        MethodHandle pmh = lookup.unreflect(pm);
        System.out.println("reflection:" + pm.invoke(ntc));

        // We can now revoke access to original method
        pm.setAccessible(false);

        // And yet the method handle still works!
        System.out.println("handle:" + pmh.invoke(ntc));

        // While reflection is now denied again (throws exception)
        System.out.println("reflection:" + pm.invoke(ntc));

    } catch (Throwable e) {
        e.printStackTrace();
    }

}

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