Question

假设你有MethodHandle并且已经指定了一些参数，如何在设置之后更改这些参数？

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(new SomeTest()); // "Hi"
hello.invoke(new SomeTest()); // "Hello"

将第二个参数绑定到您操作的类成员的getter。您必须使用“exactInvoker”过滤参数，该“exactInvoker”将执行getter以实际获取String值。参见：

public static class StringHolder{
    public String toPrint;
    StringHolder(String toPrint){
        this.toPrint = toPrint;
    }
}

public static void main(String[] args) throws Throwable {
    MethodHandle toPrintGetter = lookup().findGetter(StringHolder.class, "toPrint", String.class);
    MethodHandle someMethod = lookup().findVirtual(SomeTest.class, "someMethod", MethodType.methodType(void.class, String.class));

    StringHolder holder = new StringHolder("Hi");
    someMethod = MethodHandles.filterArguments(someMethod, 1, MethodHandles.exactInvoker(MethodType.methodType(String.class)));
    MethodHandle stringPrinter = MethodHandles.insertArguments(someMethod, 1, toPrintGetter.bindTo(holder));

    stringPrinter.invokeExact(new SomeTest()); // prints "Hi"
    holder.toPrint = "Hello";
    stringPrinter.invokeExact(new SomeTest()); // prints "Hello"
}

如何在插入后更改MethodHandle参数？

1 个答案: