如何在插入後更改MethodHandle參數？

Question

假設您有MethodHandle並且已經指定了一些參數，如何在設置後更改這些參數？

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); 
    } 
} 

我試過使用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"; 
} 

，但我得到一個異常：

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種方法：

• 重用原來的方法處理，並將其綁定到另一個字符串：

  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" 
  

• Bi找到你操作的類成員的getter的第二個參數。您必須使用「exactInvoker」過濾參數，該參數將執行getter以實際獲取String值。請參閱：

  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" 
  }