Module java.base

Package java.lang.invoke

Interface MethodHandleInfo

public interface MethodHandleInfo

A symbolic reference obtained by cracking a direct method handle into its constituent 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:

• By executing an ldc instruction on a CONSTANT_MethodHandle constant. (See the Java Virtual Machine Specification, sections 4.4.8 and 5.4.3.)
• By calling one of the Lookup Factory Methods, such as Lookup.findVirtual, to resolve a symbolic reference into a method handle. A symbolic reference consists of a class, name string, and type.
• By calling the factory method Lookup.unreflect or Lookup.unreflectSpecial to convert a Method into a method handle.
• By calling the factory method Lookup.unreflectConstructor to convert a Constructor into a method handle.
• By calling the factory method Lookup.unreflectGetter or Lookup.unreflectSetter to convert a Field into a method handle.

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 kinds

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

Field Summary

Fields

Modifier and Type	Field	Description
static final int	REF_getField	A direct method handle reference kind, as defined in the table above.
static final int	REF_getStatic	A direct method handle reference kind, as defined in the table above.
static final int	REF_invokeInterface	A direct method handle reference kind, as defined in the table above.
static final int	REF_invokeSpecial	A direct method handle reference kind, as defined in the table above.
static final int	REF_invokeStatic	A direct method handle reference kind, as defined in the table above.
static final int	REF_invokeVirtual	A direct method handle reference kind, as defined in the table above.
static final int	REF_newInvokeSpecial	A direct method handle reference kind, as defined in the table above.
static final int	REF_putField	A direct method handle reference kind, as defined in the table above.
static final int	REF_putStatic	A direct method handle reference kind, as defined in the table above.

Method Summary

Modifier and Type	Method	Description
Class<?>	getDeclaringClass()	Returns the class in which the cracked method handle's underlying member was defined.
MethodType	getMethodType()	Returns the nominal type of the cracked symbolic reference, expressed as a method type.
int	getModifiers()	Returns the access modifiers of the underlying member.
String	getName()	Returns the name of the cracked method handle's underlying member.
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.
default boolean	isVarArgs()	Determines if the underlying member was a variable arity method or constructor.
static String	referenceKindToString(int referenceKind)	Returns the descriptive name of the given reference kind, as defined in the table above.
<T extends Member> T	reflectAs(Class<T> expected, MethodHandles.Lookup lookup)	Reflects the underlying member as a method, constructor, or field object.
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.

Field Details

REF_getField

static final int REF_getField

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

See Also:

• Constant Field Values

REF_getStatic

static final int REF_getStatic

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

See Also:

• Constant Field Values

REF_putField

static final int REF_putField

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

See Also:

• Constant Field Values

REF_putStatic

static final int REF_putStatic

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

See Also:

• Constant Field Values

REF_invokeVirtual

static final int REF_invokeVirtual

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

See Also:

• Constant Field Values

REF_invokeStatic

static final int REF_invokeStatic

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

See Also:

• Constant Field Values

REF_invokeSpecial

static final int REF_invokeSpecial

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

See Also:

• Constant Field Values

REF_newInvokeSpecial

static final int REF_newInvokeSpecial

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

See Also:

• Constant Field Values

REF_invokeInterface

static final int REF_invokeInterface

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

See Also:

• Constant Field Values

Method Details

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