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Interface MethodHandleInfo

public interface MethodHandleInfo

A symbolic reference obtained by cracking a direct method handle into its consitutent symbolic parts. To crack a direct method handle, call Lookup.revealDirect.

Direct Method Handles

A direct method handle represents a method, constructor, or field without any intervening argument bindings or other transformations. The method, constructor, or field referred to by a direct method handle is called its underlying member. Direct method handles may be obtained in any of these ways:

Restrictions on Cracking

Given a suitable Lookup object, it is possible to crack any direct method handle to recover a symbolic reference for the underlying method, constructor, or field. Cracking must be done via a Lookup object equivalent to that which created the target method handle, or which has enough access permissions to recreate an equivalent method handle.

If the underlying method is caller sensitive, the direct method handle will have been "bound" to a particular caller class, the lookup class of the lookup object used to create it. Cracking this method handle with a different lookup class will fail even if the underlying method is public (like Class.forName).

The requirement of lookup object matching provides a "fast fail" behavior for programs which may otherwise trust erroneous revelation of a method handle with symbolic information (or caller binding) from an unexpected scope. Use MethodHandles.reflectAs(java.lang.Class<T>, java.lang.invoke.MethodHandle) to override this limitation.

Reference kinds

The Lookup Factory Methods correspond to all major use cases for methods, constructors, and fields. These use cases may be distinguished using small integers as follows:
reference kind descriptive name scope member behavior
1 REF_getField class FT f; (T) this.f;
2 REF_getStatic class or interface static
FT f; 		(T) C.f;
3 REF_putField class FT f; this.f = x;
4 REF_putStatic class static
FT f; 		C.f = arg;
5 REF_invokeVirtual class T m(A*); (T) this.m(arg*);
6 REF_invokeStatic class or interface static
T m(A*); 		(T) C.m(arg*);
7 REF_invokeSpecial class or interface T m(A*); (T) super.m(arg*);
8 REF_newInvokeSpecial class C(A*); new C(arg*);
9 REF_invokeInterface interface T m(A*); (T) this.m(arg*);
Since:
1.8

Fields

REF_getField

static final int REF_getField

A direct method handle reference kind, as defined in the table above.

REF_getStatic

static final int REF_getStatic

A direct method handle reference kind, as defined in the table above.

REF_putField

static final int REF_putField

A direct method handle reference kind, as defined in the table above.

REF_putStatic

static final int REF_putStatic

A direct method handle reference kind, as defined in the table above.

REF_invokeVirtual

static final int REF_invokeVirtual

A direct method handle reference kind, as defined in the table above.

REF_invokeStatic

static final int REF_invokeStatic

A direct method handle reference kind, as defined in the table above.

REF_invokeSpecial

static final int REF_invokeSpecial

A direct method handle reference kind, as defined in the table above.

REF_newInvokeSpecial

static final int REF_newInvokeSpecial

A direct method handle reference kind, as defined in the table above.

REF_invokeInterface

static final int REF_invokeInterface

A direct method handle reference kind, as defined in the table above.

Methods

getReferenceKind

int getReferenceKind()

Returns the reference kind of the cracked method handle, which in turn determines whether the method handle's underlying member was a constructor, method, or field. See the table above for definitions.

Returns:
the integer code for the kind of reference used to access the underlying member

getDeclaringClass

Class<?> getDeclaringClass()

Returns the class in which the cracked method handle's underlying member was defined.

Returns:
the declaring class of the underlying member

getName

String getName()

Returns the name of the cracked method handle's underlying member. This is "<init>" if the underlying member was a constructor, else it is a simple method name or field name.

Returns:
the simple name of the underlying member

getMethodType

MethodType getMethodType()

Returns the nominal type of the cracked symbolic reference, expressed as a method type. If the reference is to a constructor, the return type will be void. If it is to a non-static method, the method type will not mention the this parameter. If it is to a field and the requested access is to read the field, the method type will have no parameters and return the field type. If it is to a field and the requested access is to write the field, the method type will have one parameter of the field type and return void.

Note that original direct method handle may include a leading this parameter, or (in the case of a constructor) will replace the void return type with the constructed class. The nominal type does not include any this parameter, and (in the case of a constructor) will return void.

Returns:
the type of the underlying member, expressed as a method type

reflectAs

<T extends Member> T reflectAs(Class<T> expected,
                               MethodHandles.Lookup lookup)

Reflects the underlying member as a method, constructor, or field object. If the underlying member is public, it is reflected as if by getMethod, getConstructor, or getField. Otherwise, it is reflected as if by getDeclaredMethod, getDeclaredConstructor, or getDeclaredField. The underlying member must be accessible to the given lookup object.

Type Parameters:
T - the desired type of the result, either Member or a subtype
Parameters:
expected - a class object representing the desired result type T
lookup - the lookup object that created this MethodHandleInfo, or one with equivalent access privileges
Returns:
a reference to the method, constructor, or field object
Throws:
ClassCastException - if the member is not of the expected type
NullPointerException - if either argument is null
IllegalArgumentException - if the underlying member is not accessible to the given lookup object

getModifiers

int getModifiers()

Returns the access modifiers of the underlying member.

Returns:
the Java language modifiers for underlying member, or -1 if the member cannot be accessed
See Also:
Modifier, reflectAs(java.lang.Class<T>, java.lang.invoke.MethodHandles.Lookup)

isVarArgs

default boolean isVarArgs()

Determines if the underlying member was a variable arity method or constructor. Such members are represented by method handles that are varargs collectors.

Implementation Requirements:
This produces a result equivalent to: 
getReferenceKind() >= REF_invokeVirtual && Modifier.isTransient(getModifiers())
Returns:
true if and only if the underlying member was declared with variable arity.

referenceKindToString

static String referenceKindToString(int referenceKind)

Returns the descriptive name of the given reference kind, as defined in the table above. The conventional prefix "REF_" is omitted.

Parameters:
referenceKind - an integer code for a kind of reference used to access a class member
Returns:
a mixed-case string such as "getField"
Throws:
IllegalArgumentException - if the argument is not a valid reference kind number

toString

static String toString(int kind,
                       Class<?> defc,
                       String name,
                       MethodType type)

Returns a string representation for a MethodHandleInfo, given the four parts of its symbolic reference. This is defined to be of the form "RK C.N:MT", where RK is the reference kind string for kind, C is the name of defc N is the name, and MT is the type. These four values may be obtained from the reference kind, declaring class, member name, and method type of a MethodHandleInfo object.

Implementation Requirements:
This produces a result equivalent to: 
String.format("%s %s.%s:%s", referenceKindToString(kind), defc.getName(), name, type)
Parameters:
kind - the reference kind part of the symbolic reference
defc - the declaring class part of the symbolic reference
name - the member name part of the symbolic reference
type - the method type part of the symbolic reference
Returns:
a string of the form "RK C.N:MT"
Throws:
IllegalArgumentException - if the first argument is not a valid reference kind number
NullPointerException - if any reference argument is null

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