如何在插入后更改MethodHandle参数？

Question

假设你有MethodHandle，并指定了一些参数，如何在设置后更改这些参数？

import static java.lang.invoke.MethodType.*;
import static java.lang.invoke.MethodHandles.*;

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;

public class SomeTest {

    public static void main(String[] args) throws Throwable {

        MethodHandle methodHandle = MethodHandles.lookup().findVirtual(SomeTest.class,
                "someMethod", methodType(void.class, String.class));

        methodHandle = MethodHandles.insertArguments(methodHandle, 1, "Hi");

        // this invoke calls with "Hi", which is fine
        methodHandle.invoke(new SomeTest());

        // here, how to change the arguments to be e.g. "Hello" instead of "Hi"

        methodHandle.invoke(new SomeTest());

    }

    public void someMethod(String a) {
        System.out.println("Called with " + a);
    }
}

我尝试过使用MethodHandles。filterArguments()

....
    methodHandle = MethodHandles.filterArguments(methodHandle, 1,
            MethodHandles.lookup().findStatic(SomeTest.class, "returnSomething",
                    methodType(String.class)));

    methodHandle.invoke(new SomeTest());
}

public static String returnSomething() {
    return "Hello";
}

但我得到了一个例外：

Exception in thread "main" java.lang.IllegalArgumentException: too many filters
    at java.lang.invoke.MethodHandleStatics.newIllegalArgumentException(MethodHandleStatics.java:139)
    at java.lang.invoke.MethodHandles.filterArgumentsCheckArity(MethodHandles.java:2623)
    at java.lang.invoke.MethodHandles.filterArguments(MethodHandles.java:2595)
    at test.test.SomeTest.main(SomeTest.java:22)

Answer 1

2种方法：

• 重用您的原始方法句柄并将其绑定到另一个字符串：

methodHandle MethodHandle = MethodHandles.lookup().findVirtual(SomeTest.class，"someMethod"，methodType(void.class，String.class))；MethodHandle hi = methodHandle.insertArguments(1，"Hi")；MethodHandle hello = methodHandle.insertArguments(1，"Hello")；hi.invoke(新SomeTest())；// "Hi“hello.invoke(新SomeTest())；//”Hello“

• 将第二个参数绑定到您操作的类成员的getter方法。您必须使用"exactInvoker“来过滤参数，该参数将执行getter以实际获得字符串值。请参见：

公共静态类StringHolder{公共字符串toPrint；StringHolder(String toPrint){ this.toPrint = toPrint；}}公共静态空main(String[] args)抛出Throwable { MethodHandle toPrintGetter = lookup().findGetter(StringHolder.class，"toPrint"，String.class)；MethodHandle someMethod = lookup().findVirtual(SomeTest.class，"someMethod"，MethodType.methodType(，))；holder = new (“Hi”)；someMethod = MethodHandles.filterArguments(someMethod，1，MethodHandles.exactInvoker(MethodType.methodType(String.class)))；MethodHandle stringPrinter = MethodHandles.insertArguments(someMethod，1，toPrintGetter.bindTo(holder))；stringPrinter.invokeExact(新SomeTest())；//打印"Hi“holder.toPrint = "Hello"；stringPrinter.invokeExact(新SomeTest())；//打印"Hello”}