MethodHandle InvokeExact parameter
java,invocation,invokedynamic,methodhandle
This appears in the Javadoc of findVirtual When called, the handle will treat the first argument as a receiver and dispatch on the receiver's type to determine which method implementation to enter. It's exactly as you described it. A static method does not have a receiver and therefore all arguments...
Forward call on a functional interface using method handles
java,reflection,methodhandle,functional-interface
While in principle, you can use MethodHandles to construct combining handles delegating to multiple targets, there is no way to provide it in form of an efficient implementation of the functional interface. LambdaMetafactory creates efficient implementation but currently supports direct method handles only, thus you can’t use a combining handle...
How can I use method handles with JDK 6?
java,reflection,jdk1.6,methodhandle
No, method handles were only added in JDK 7 and are not accessible before that. Neither can you compile a class against the method handle API as the JDK 6 does not understand the @PolymorphicSignature annotation; method handles are compiled slightly different that other methods using a synthetic signature. Beyond...
BootstrapMethodError caused by LambdaConversionException caused by using MethodHandle::invokeExact as a method reference
java,lambda,java-8,invokedynamic,methodhandle
Your code calling metafactory manually indeed shows that the meta factory will do it’s job if the method handle to MethodHandle.invokeExact has the right signature. Debugging revealed that in the second case the method handle has a (MethodHandle,MethodHandle)Object signature where it should be (MethodHandle)Object. While both can be created without...
MethodType transformation for MethodHandle to accept Array Object paramters
java,invokedynamic,methodhandle
The problem is with the invokeExact call -- because invokeExact is signature polymorphic, you need to specify the return type explicitly using a cast: System.out.println("Compare Result: "+ (boolean)adapt.invokeExact(myArgs)); That's what the stack trace is trying to tell you with expected (String[])boolean but found (String[])Object -- because you didn't have the...
Java code to be compiled into MethodHandle in Constant Pool
java,bytecode,decompiling,nashorn,methodhandle
There is no Java language construct to produce an ldc instruction loading a MethodHandle. Still, you can create an equivalent handle with a more complicated construct: MethodHandles.lookup().findStatic( jdk.nashorn.internal.objects.NativeFunction.class, "function", MethodType.fromMethodDescriptorString( "(ZLjava/lang/Object;[Ljava/lang/Object;)Ljdk/nashorn/internal/runtime/ScriptFunction;", null)) Not only is this more complicated than a single ldc bytecode instruction, you are also forced to deal...
Why can't I .invokeExact() here, even though the MethodType is OK?
When the compiler emits the invokeExact call, it records Object as the expected return type. From the MethodHandle javadoc (emphasis mine): As is usual with virtual methods, source-level calls to invokeExact and invoke compile to an invokevirtual instruction. More unusually, the compiler must record the actual argument types, and may...
Why does the return type (force cast) play a critical rule in MethodHandler performance?
java,performance,reflection,invokedynamic,methodhandle
Your benchmark is flawed for many reasons. You need to write a harnessed benchmark instead to create a controlled environment where the JIT compiler does not optimize away the code you are measuring. I have once written such a benchmark targeting method handles compared to reflection: https://gist.github.com/raphw/881e1745996f9d314ab0 Invokeexact does not...
InvokeExact on the object, whose type is dynamically loaded by classloader
java,casting,classloader,methodhandle,invokevirtual
You can’t use invokeExact if the compile-time type of an argument doesn’t match the MethodHandle’s parameter type. It doesn’t help to play around with the Generic constructs like invoking cast on a Class<T>, the dynamic type is still unknown to the compiler. Or, in other words, due to type erasure,...