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 aCONSTANT_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
orLookup.unreflectSpecial
to convert aMethod
into a method handle. - By calling the factory method
Lookup.unreflectConstructor
to convert aConstructor
into a method handle. - By calling the factory method
Lookup.unreflectGetter
orLookup.unreflectSetter
to convert aField
into a method handle.
Restrictions on Cracking
Given a suitableLookup
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
-
Field Summary
Modifier and TypeFieldDescriptionstatic final int
A direct method handle reference kind, as defined in the table above.static final int
A direct method handle reference kind, as defined in the table above.static final int
A direct method handle reference kind, as defined in the table above.static final int
A direct method handle reference kind, as defined in the table above.static final int
A direct method handle reference kind, as defined in the table above.static final int
A direct method handle reference kind, as defined in the table above.static final int
A direct method handle reference kind, as defined in the table above.static final int
A direct method handle reference kind, as defined in the table above.static final int
A direct method handle reference kind, as defined in the table above. -
Method Summary
Modifier and TypeMethodDescriptionClass
<?> Returns the class in which the cracked method handle's underlying member was defined.Returns the nominal type of the cracked symbolic reference, expressed as a method type.int
Returns the access modifiers of the underlying member.getName()
Returns the name of the cracked method handle's underlying member.int
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
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>
TreflectAs
(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 aMethodHandleInfo
, given the four parts of its symbolic reference.
-
Field Details
-
REF_getField
static final int REF_getFieldA direct method handle reference kind, as defined in the table above.- See Also:
-
REF_getStatic
static final int REF_getStaticA direct method handle reference kind, as defined in the table above.- See Also:
-
REF_putField
static final int REF_putFieldA direct method handle reference kind, as defined in the table above.- See Also:
-
REF_putStatic
static final int REF_putStaticA direct method handle reference kind, as defined in the table above.- See Also:
-
REF_invokeVirtual
static final int REF_invokeVirtualA direct method handle reference kind, as defined in the table above.- See Also:
-
REF_invokeStatic
static final int REF_invokeStaticA direct method handle reference kind, as defined in the table above.- See Also:
-
REF_invokeSpecial
static final int REF_invokeSpecialA direct method handle reference kind, as defined in the table above.- See Also:
-
REF_newInvokeSpecial
static final int REF_newInvokeSpecialA direct method handle reference kind, as defined in the table above.- See Also:
-
REF_invokeInterface
static final int REF_invokeInterfaceA direct method handle reference kind, as defined in the table above.- See Also:
-
-
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
-
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 bevoid
. If it is to a non-static method, the method type will not mention thethis
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 returnvoid
.Note that original direct method handle may include a leading
this
parameter, or (in the case of a constructor) will replace thevoid
return type with the constructed class. The nominal type does not include anythis
parameter, and (in the case of a constructor) will returnvoid
.- Returns:
- the type of the underlying member, expressed as a method type
-
reflectAs
Reflects the underlying member as a method, constructor, or field object. If the underlying member is public, it is reflected as if bygetMethod
,getConstructor
, orgetField
. Otherwise, it is reflected as if bygetDeclaredMethod
,getDeclaredConstructor
, orgetDeclaredField
. The underlying member must be accessible to the given lookup object.- Type Parameters:
T
- the desired type of the result, eitherMember
or a subtype- Parameters:
expected
- a class object representing the desired result typeT
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 typeNullPointerException
- if either argument isnull
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:
-
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
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
Returns a string representation for aMethodHandleInfo
, given the four parts of its symbolic reference. This is defined to be of the form"RK C.N:MT"
, whereRK
is the reference kind string forkind
,C
is the name ofdefc
N
is thename
, andMT
is thetype
. These four values may be obtained from the reference kind, declaring class, member name, and method type of aMethodHandleInfo
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 referencedefc
- the declaring class part of the symbolic referencename
- the member name part of the symbolic referencetype
- 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 numberNullPointerException
- if any reference argument isnull
-