Comment modifier les arguments MethodHandle après leur insertion?

Question

Supposons que vous ayez MethodHandle et que certains arguments aient été spécifiés, comment changer ces arguments après avoir été défini?

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); 
    } 
} 

J'ai essayé d'utiliser 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"; 
} 

mais je reçois une exception:

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 méthodes:

• réutiliser votre poignée de méthode originale et le lier à une autre chaîne:

  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" 
  

• Bi sd le second argument d'un getter d'un membre de la classe, que vous manipulez. Vous devez filtrer les arguments avec un "exactInvoker" qui exécutera le getter pour obtenir la valeur String. Voir:

  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" 
  }