MethodHandles.Lookup

public static final class MethodHandles.Lookup
extends Object

java.lang.Object
   ↳ java.lang.invoke.MethodHandles.Lookup


A lookup object is a factory for creating method handles, when the creation requires access checking. Method handles do not perform access checks when they are called, but rather when they are created. Therefore, method handle access restrictions must be enforced when a method handle is created. The caller class against which those restrictions are enforced is known as the lookup class.

A lookup class which needs to create method handles will call MethodHandles.lookup to create a factory for itself. When the Lookup factory object is created, the identity of the lookup class is determined, and securely stored in the Lookup object. The lookup class (or its delegates) may then use factory methods on the Lookup object to create method handles for access-checked members. This includes all methods, constructors, and fields which are allowed to the lookup class, even private ones.

Lookup Factory Methods

The factory methods on a Lookup object correspond to all major use cases for methods, constructors, and fields. Each method handle created by a factory method is the functional equivalent of a particular bytecode behavior. (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.) Here is a summary of the correspondence between these factory methods and the behavior the resulting method handles:
lookup expression member bytecode behavior
lookup.findGetter(C.class,"f",FT.class) FT f;(T) this.f;
lookup.findStaticGetter(C.class,"f",FT.class) static
FT f;
(T) C.f;
lookup.findSetter(C.class,"f",FT.class) FT f;this.f = x;
lookup.findStaticSetter(C.class,"f",FT.class) static
FT f;
C.f = arg;
lookup.findVirtual(C.class,"m",MT) T m(A*);(T) this.m(arg*);
lookup.findStatic(C.class,"m",MT) static
T m(A*);
(T) C.m(arg*);
lookup.findSpecial(C.class,"m",MT,this.class) T m(A*);(T) super.m(arg*);
lookup.findConstructor(C.class,MT) C(A*);new C(arg*);
lookup.unreflectGetter(aField) (static)?
FT f;
(FT) aField.get(thisOrNull);
lookup.unreflectSetter(aField) (static)?
FT f;
aField.set(thisOrNull, arg);
lookup.unreflect(aMethod) (static)?
T m(A*);
(T) aMethod.invoke(thisOrNull, arg*);
lookup.unreflectConstructor(aConstructor) C(A*);(C) aConstructor.newInstance(arg*);
lookup.unreflect(aMethod) (static)?
T m(A*);
(T) aMethod.invoke(thisOrNull, arg*);
Here, the type C is the class or interface being searched for a member, documented as a parameter named refc in the lookup methods. The method type MT is composed from the return type T and the sequence of argument types A*. The constructor also has a sequence of argument types A* and is deemed to return the newly-created object of type C. Both MT and the field type FT are documented as a parameter named type. The formal parameter this stands for the self-reference of type C; if it is present, it is always the leading argument to the method handle invocation. (In the case of some protected members, this may be restricted in type to the lookup class; see below.) The name arg stands for all the other method handle arguments. In the code examples for the Core Reflection API, the name thisOrNull stands for a null reference if the accessed method or field is static, and this otherwise. The names aMethod, aField, and aConstructor stand for reflective objects corresponding to the given members.

In cases where the given member is of variable arity (i.e., a method or constructor) the returned method handle will also be of variable arity. In all other cases, the returned method handle will be of fixed arity.

Discussion: The equivalence between looked-up method handles and underlying class members and bytecode behaviors can break down in a few ways:

  • If C is not symbolically accessible from the lookup class's loader, the lookup can still succeed, even when there is no equivalent Java expression or bytecoded constant.
  • Likewise, if T or MT is not symbolically accessible from the lookup class's loader, the lookup can still succeed. For example, lookups for MethodHandle.invokeExact and MethodHandle.invoke will always succeed, regardless of requested type.
  • If there is a security manager installed, it can forbid the lookup on various grounds (see below). By contrast, the ldc instruction on a CONSTANT_MethodHandle constant is not subject to security manager checks.
  • If the looked-up method has a very large arity, the method handle creation may fail, due to the method handle type having too many parameters.

Access checking

Access checks are applied in the factory methods of Lookup, when a method handle is created. This is a key difference from the Core Reflection API, since java.lang.reflect.Method.invoke performs access checking against every caller, on every call.

All access checks start from a Lookup object, which compares its recorded lookup class against all requests to create method handles. A single Lookup object can be used to create any number of access-checked method handles, all checked against a single lookup class.

A Lookup object can be shared with other trusted code, such as a metaobject protocol. A shared Lookup object delegates the capability to create method handles on private members of the lookup class. Even if privileged code uses the Lookup object, the access checking is confined to the privileges of the original lookup class.

A lookup can fail, because the containing class is not accessible to the lookup class, or because the desired class member is missing, or because the desired class member is not accessible to the lookup class, or because the lookup object is not trusted enough to access the member. In any of these cases, a ReflectiveOperationException will be thrown from the attempted lookup. The exact class will be one of the following:

  • NoSuchMethodException — if a method is requested but does not exist
  • NoSuchFieldException — if a field is requested but does not exist
  • IllegalAccessException — if the member exists but an access check fails

In general, the conditions under which a method handle may be looked up for a method M are no more restrictive than the conditions under which the lookup class could have compiled, verified, and resolved a call to M. Where the JVM would raise exceptions like NoSuchMethodError, a method handle lookup will generally raise a corresponding checked exception, such as NoSuchMethodException. And the effect of invoking the method handle resulting from the lookup is exactly equivalent to executing the compiled, verified, and resolved call to M. The same point is true of fields and constructors.

Discussion: Access checks only apply to named and reflected methods, constructors, and fields. Other method handle creation methods, such as MethodHandle.asType, do not require any access checks, and are used independently of any Lookup object.

If the desired member is protected, the usual JVM rules apply, including the requirement that the lookup class must be either be in the same package as the desired member, or must inherit that member. (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.) In addition, if the desired member is a non-static field or method in a different package, the resulting method handle may only be applied to objects of the lookup class or one of its subclasses. This requirement is enforced by narrowing the type of the leading this parameter from C (which will necessarily be a superclass of the lookup class) to the lookup class itself.

The JVM imposes a similar requirement on invokespecial instruction, that the receiver argument must match both the resolved method and the current class. Again, this requirement is enforced by narrowing the type of the leading parameter to the resulting method handle. (See the Java Virtual Machine Specification, section 4.10.1.9.)

The JVM represents constructors and static initializer blocks as internal methods with special names ("<init>" and "<clinit>"). The internal syntax of invocation instructions allows them to refer to such internal methods as if they were normal methods, but the JVM bytecode verifier rejects them. A lookup of such an internal method will produce a NoSuchMethodException.

In some cases, access between nested classes is obtained by the Java compiler by creating an wrapper method to access a private method of another class in the same top-level declaration. For example, a nested class C.D can access private members within other related classes such as C, C.D.E, or C.B, but the Java compiler may need to generate wrapper methods in those related classes. In such cases, a Lookup object on C.E would be unable to those private members. A workaround for this limitation is the Lookup.in method, which can transform a lookup on C.E into one on any of those other classes, without special elevation of privilege.

The accesses permitted to a given lookup object may be limited, according to its set of lookupModes, to a subset of members normally accessible to the lookup class. For example, the publicLookup method produces a lookup object which is only allowed to access public members in public classes. The caller sensitive method lookup produces a lookup object with full capabilities relative to its caller class, to emulate all supported bytecode behaviors. Also, the Lookup.in method may produce a lookup object with fewer access modes than the original lookup object.

Discussion of private access: We say that a lookup has private access if its lookup modes include the possibility of accessing private members. As documented in the relevant methods elsewhere, only lookups with private access possess the following capabilities:

Each of these permissions is a consequence of the fact that a lookup object with private access can be securely traced back to an originating class, whose bytecode behaviors and Java language access permissions can be reliably determined and emulated by method handles.

Security manager interactions

Although bytecode instructions can only refer to classes in a related class loader, this API can search for methods in any class, as long as a reference to its Class object is available. Such cross-loader references are also possible with the Core Reflection API, and are impossible to bytecode instructions such as invokestatic or getfield. There is a security manager API to allow applications to check such cross-loader references. These checks apply to both the MethodHandles.Lookup API and the Core Reflection API (as found on Class).

If a security manager is present, member lookups are subject to additional checks. From one to three calls are made to the security manager. Any of these calls can refuse access by throwing a SecurityException. Define smgr as the security manager, lookc as the lookup class of the current lookup object, refc as the containing class in which the member is being sought, and defc as the class in which the member is actually defined. The value lookc is defined as not present if the current lookup object does not have private access. The calls are made according to the following rules:

  • Step 1: If lookc is not present, or if its class loader is not the same as or an ancestor of the class loader of refc, then smgr.checkPackageAccess(refcPkg) is called, where refcPkg is the package of refc.
  • Step 2: If the retrieved member is not public and lookc is not present, then smgr.checkPermission with RuntimePermission("accessDeclaredMembers") is called.
  • Step 3: If the retrieved member is not public, and if lookc is not present, and if defc and refc are different, then smgr.checkPackageAccess(defcPkg) is called, where defcPkg is the package of defc.
Security checks are performed after other access checks have passed. Therefore, the above rules presuppose a member that is public, or else that is being accessed from a lookup class that has rights to access the member.

Caller sensitive methods

A small number of Java methods have a special property called caller sensitivity. A caller-sensitive method can behave differently depending on the identity of its immediate caller.

If a method handle for a caller-sensitive method is requested, the general rules for bytecode behaviors apply, but they take account of the lookup class in a special way. The resulting method handle behaves as if it were called from an instruction contained in the lookup class, so that the caller-sensitive method detects the lookup class. (By contrast, the invoker of the method handle is disregarded.) Thus, in the case of caller-sensitive methods, different lookup classes may give rise to differently behaving method handles.

In cases where the lookup object is publicLookup(), or some other lookup object without private access, the lookup class is disregarded. In such cases, no caller-sensitive method handle can be created, access is forbidden, and the lookup fails with an IllegalAccessException.

Discussion: For example, the caller-sensitive method Class.forName(x) can return varying classes or throw varying exceptions, depending on the class loader of the class that calls it. A public lookup of Class.forName will fail, because there is no reasonable way to determine its bytecode behavior.

If an application caches method handles for broad sharing, it should use publicLookup() to create them. If there is a lookup of Class.forName, it will fail, and the application must take appropriate action in that case. It may be that a later lookup, perhaps during the invocation of a bootstrap method, can incorporate the specific identity of the caller, making the method accessible.

The function MethodHandles.lookup is caller sensitive so that there can be a secure foundation for lookups. Nearly all other methods in the JSR 292 API rely on lookup objects to check access requests.

Summary

Constants

int PACKAGE

A single-bit mask representing package access (default access), which may contribute to the result of lookupModes.

int PRIVATE

A single-bit mask representing private access, which may contribute to the result of lookupModes.

int PROTECTED

A single-bit mask representing protected access, which may contribute to the result of lookupModes.

int PUBLIC

A single-bit mask representing public access, which may contribute to the result of lookupModes.

Public methods

MethodHandle bind(Object receiver, String name, MethodType type)

Produces an early-bound method handle for a non-static method.

MethodHandle findConstructor(Class<?> refc, MethodType type)

Produces a method handle which creates an object and initializes it, using the constructor of the specified type.

MethodHandle findGetter(Class<?> refc, String name, Class<?> type)

Produces a method handle giving read access to a non-static field.

MethodHandle findSetter(Class<?> refc, String name, Class<?> type)

Produces a method handle giving write access to a non-static field.

MethodHandle findSpecial(Class<?> refc, String name, MethodType type, Class<?> specialCaller)

Produces an early-bound method handle for a virtual method.

MethodHandle findStatic(Class<?> refc, String name, MethodType type)

Produces a method handle for a static method.

MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type)

Produces a method handle giving read access to a static field.

MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type)

Produces a method handle giving write access to a static field.

MethodHandle findVirtual(Class<?> refc, String name, MethodType type)

Produces a method handle for a virtual method.

MethodHandles.Lookup in(Class<?> requestedLookupClass)

Creates a lookup on the specified new lookup class.

Class<?> lookupClass()

Tells which class is performing the lookup.

int lookupModes()

Tells which access-protection classes of members this lookup object can produce.

MethodHandleInfo revealDirect(MethodHandle target)

Cracks a direct method handle created by this lookup object or a similar one.

String toString()

Displays the name of the class from which lookups are to be made.

MethodHandle unreflect(Method m)

Makes a direct method handle to m, if the lookup class has permission.

MethodHandle unreflectConstructor(Constructor<?> c)

Produces a method handle for a reflected constructor.

MethodHandle unreflectGetter(Field f)

Produces a method handle giving read access to a reflected field.

MethodHandle unreflectSetter(Field f)

Produces a method handle giving write access to a reflected field.

MethodHandle unreflectSpecial(Method m, Class<?> specialCaller)

Produces a method handle for a reflected method.

Inherited methods

Constants

PACKAGE

added in API level 26
int PACKAGE

A single-bit mask representing package access (default access), which may contribute to the result of lookupModes. The value is 0x08, which does not correspond meaningfully to any particular modifier bit.

Constant Value: 8 (0x00000008)

PRIVATE

added in API level 26
int PRIVATE

A single-bit mask representing private access, which may contribute to the result of lookupModes. The value, 0x02, happens to be the same as the value of the private modifier bit.

Constant Value: 2 (0x00000002)

PROTECTED

added in API level 26
int PROTECTED

A single-bit mask representing protected access, which may contribute to the result of lookupModes. The value, 0x04, happens to be the same as the value of the protected modifier bit.

Constant Value: 4 (0x00000004)

PUBLIC

added in API level 26
int PUBLIC

A single-bit mask representing public access, which may contribute to the result of lookupModes. The value, 0x01, happens to be the same as the value of the public modifier bit.

Constant Value: 1 (0x00000001)

Public methods

bind

added in API level 26
MethodHandle bind (Object receiver, 
                String name, 
                MethodType type)

Produces an early-bound method handle for a non-static method. The receiver must have a supertype defc in which a method of the given name and type is accessible to the lookup class. The method and all its argument types must be accessible to the lookup object. The type of the method handle will be that of the method, without any insertion of an additional receiver parameter. The given receiver will be bound into the method handle, so that every call to the method handle will invoke the requested method on the given receiver.

The returned method handle will have variable arity if and only if the method's variable arity modifier bit (0x0080) is set and the trailing array argument is not the only argument. (If the trailing array argument is the only argument, the given receiver value will be bound to it.)

This is equivalent to the following code:

import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle mh0 = lookup().findVirtual(defc, name, type);
MethodHandle mh1 = mh0.bindTo(receiver);
MethodType mt1 = mh1.type();
if (mh0.isVarargsCollector())
  mh1 = mh1.asVarargsCollector(mt1.parameterType(mt1.parameterCount()-1));
return mh1;
 
where defc is either receiver.getClass() or a super type of that class, in which the requested method is accessible to the lookup class. (Note that bindTo does not preserve variable arity.)

Parameters
receiver Object: the object from which the method is accessed

name String: the name of the method

type MethodType: the type of the method, with the receiver argument omitted

Returns
MethodHandle the desired method handle

Throws
NoSuchMethodException if the method does not exist
IllegalAccessException if access checking fails or if the method's variable arity modifier bit is set and asVarargsCollector fails
SecurityException if a security manager is present and it refuses access
NullPointerException if any argument is null

findConstructor

added in API level 26
MethodHandle findConstructor (Class<?> refc, 
                MethodType type)

Produces a method handle which creates an object and initializes it, using the constructor of the specified type. The parameter types of the method handle will be those of the constructor, while the return type will be a reference to the constructor's class. The constructor and all its argument types must be accessible to the lookup object.

The requested type must have a return type of void. (This is consistent with the JVM's treatment of constructor type descriptors.)

The returned method handle will have variable arity if and only if the constructor's variable arity modifier bit (0x0080) is set.

If the returned method handle is invoked, the constructor's class will be initialized, if it has not already been initialized.

Example:

import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle MH_newArrayList = publicLookup().findConstructor(
  ArrayList.class, methodType(void.class, Collection.class));
Collection orig = Arrays.asList("x", "y");
Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
assert(orig != copy);
assertEquals(orig, copy);
// a variable-arity constructor:
MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
  ProcessBuilder.class, methodType(void.class, String[].class));
ProcessBuilder pb = (ProcessBuilder)
  MH_newProcessBuilder.invoke("x", "y", "z");
assertEquals("[x, y, z]", pb.command().toString());
 

Parameters
refc Class: the class or interface from which the method is accessed

type MethodType: the type of the method, with the receiver argument omitted, and a void return type

Returns
MethodHandle the desired method handle

Throws
NoSuchMethodException if the constructor does not exist
IllegalAccessException if access checking fails or if the method's variable arity modifier bit is set and asVarargsCollector fails
SecurityException if a security manager is present and it refuses access
NullPointerException if any argument is null

findGetter

added in API level 26
MethodHandle findGetter (Class<?> refc, 
                String name, 
                Class<?> type)

Produces a method handle giving read access to a non-static field. The type of the method handle will have a return type of the field's value type. The method handle's single argument will be the instance containing the field. Access checking is performed immediately on behalf of the lookup class.

Parameters
refc Class: the class or interface from which the method is accessed

name String: the field's name

type Class: the field's type

Returns
MethodHandle a method handle which can load values from the field

Throws
NoSuchFieldException if the field does not exist
IllegalAccessException if access checking fails, or if the field is static
SecurityException if a security manager is present and it refuses access
NullPointerException if any argument is null

findSetter

added in API level 26
MethodHandle findSetter (Class<?> refc, 
                String name, 
                Class<?> type)

Produces a method handle giving write access to a non-static field. The type of the method handle will have a void return type. The method handle will take two arguments, the instance containing the field, and the value to be stored. The second argument will be of the field's value type. Access checking is performed immediately on behalf of the lookup class.

Parameters
refc Class: the class or interface from which the method is accessed

name String: the field's name

type Class: the field's type

Returns
MethodHandle a method handle which can store values into the field

Throws
NoSuchFieldException if the field does not exist
IllegalAccessException if access checking fails, or if the field is static
SecurityException if a security manager is present and it refuses access
NullPointerException if any argument is null

findSpecial

added in API level 26
MethodHandle findSpecial (Class<?> refc, 
                String name, 
                MethodType type, 
                Class<?> specialCaller)

Produces an early-bound method handle for a virtual method. It will bypass checks for overriding methods on the receiver, as if called from an invokespecial instruction from within the explicitly specified specialCaller. The type of the method handle will be that of the method, with a suitably restricted receiver type prepended. (The receiver type will be specialCaller or a subtype.) The method and all its argument types must be accessible to the lookup object.

Before method resolution, if the explicitly specified caller class is not identical with the lookup class, or if this lookup object does not have private access privileges, the access fails.

The returned method handle will have variable arity if and only if the method's variable arity modifier bit (0x0080) is set.

(Note: JVM internal methods named "<init>" are not visible to this API, even though the invokespecial instruction can refer to them in special circumstances. Use findConstructor to access instance initialization methods in a safe manner.)

Example:

import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
static class Listie extends ArrayList {
  public String toString() { return "[wee Listie]"; }
  static Lookup lookup() { return MethodHandles.lookup(); }
}
...
// no access to constructor via invokeSpecial:
MethodHandle MH_newListie = Listie.lookup()
  .findConstructor(Listie.class, methodType(void.class));
Listie l = (Listie) MH_newListie.invokeExact();
try { assertEquals("impossible", Listie.lookup().findSpecial(
        Listie.class, "<init>", methodType(void.class), Listie.class));
 } catch (NoSuchMethodException ex) { } // OK
// access to super and self methods via invokeSpecial:
MethodHandle MH_super = Listie.lookup().findSpecial(
  ArrayList.class, "toString" , methodType(String.class), Listie.class);
MethodHandle MH_this = Listie.lookup().findSpecial(
  Listie.class, "toString" , methodType(String.class), Listie.class);
MethodHandle MH_duper = Listie.lookup().findSpecial(
  Object.class, "toString" , methodType(String.class), Listie.class);
assertEquals("[]", (String) MH_super.invokeExact(l));
assertEquals(""+l, (String) MH_this.invokeExact(l));
assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
try { assertEquals("inaccessible", Listie.lookup().findSpecial(
        String.class, "toString", methodType(String.class), Listie.class));
 } catch (IllegalAccessException ex) { } // OK
Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
 

Parameters
refc Class: the class or interface from which the method is accessed

name String: the name of the method (which must not be "<init>")

type MethodType: the type of the method, with the receiver argument omitted

specialCaller Class: the proposed calling class to perform the invokespecial

Returns
MethodHandle the desired method handle

Throws
NoSuchMethodException if the method does not exist
IllegalAccessException if access checking fails or if the method's variable arity modifier bit is set and asVarargsCollector fails
SecurityException if a security manager is present and it refuses access
NullPointerException if any argument is null

findStatic

added in API level 26
MethodHandle findStatic (Class<?> refc, 
                String name, 
                MethodType type)

Produces a method handle for a static method. The type of the method handle will be that of the method. (Since static methods do not take receivers, there is no additional receiver argument inserted into the method handle type, as there would be with findVirtual or findSpecial.) The method and all its argument types must be accessible to the lookup object.

The returned method handle will have variable arity if and only if the method's variable arity modifier bit (0x0080) is set.

If the returned method handle is invoked, the method's class will be initialized, if it has not already been initialized.

Example:

import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
  "asList", methodType(List.class, Object[].class));
assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
 

Parameters
refc Class: the class from which the method is accessed

name String: the name of the method

type MethodType: the type of the method

Returns
MethodHandle the desired method handle

Throws
NoSuchMethodException if the method does not exist
IllegalAccessException if access checking fails, or if the method is not static, or if the method's variable arity modifier bit is set and asVarargsCollector fails
SecurityException if a security manager is present and it refuses access
NullPointerException if any argument is null

findStaticGetter

added in API level 26
MethodHandle findStaticGetter (Class<?> refc, 
                String name, 
                Class<?> type)

Produces a method handle giving read access to a static field. The type of the method handle will have a return type of the field's value type. The method handle will take no arguments. Access checking is performed immediately on behalf of the lookup class.

If the returned method handle is invoked, the field's class will be initialized, if it has not already been initialized.

Parameters
refc Class: the class or interface from which the method is accessed

name String: the field's name

type Class: the field's type

Returns
MethodHandle a method handle which can load values from the field

Throws
NoSuchFieldException if the field does not exist
IllegalAccessException if access checking fails, or if the field is not static
SecurityException if a security manager is present and it refuses access
NullPointerException if any argument is null

findStaticSetter

added in API level 26
MethodHandle findStaticSetter (Class<?> refc, 
                String name, 
                Class<?> type)

Produces a method handle giving write access to a static field. The type of the method handle will have a void return type. The method handle will take a single argument, of the field's value type, the value to be stored. Access checking is performed immediately on behalf of the lookup class.

If the returned method handle is invoked, the field's class will be initialized, if it has not already been initialized.

Parameters
refc Class: the class or interface from which the method is accessed

name String: the field's name

type Class: the field's type

Returns
MethodHandle a method handle which can store values into the field

Throws
NoSuchFieldException if the field does not exist
IllegalAccessException if access checking fails, or if the field is not static
SecurityException if a security manager is present and it refuses access
NullPointerException if any argument is null

findVirtual

added in API level 26
MethodHandle findVirtual (Class<?> refc, 
                String name, 
                MethodType type)

Produces a method handle for a virtual method. The type of the method handle will be that of the method, with the receiver type (usually refc) prepended. The method and all its argument types must be accessible to the lookup object.

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. (The dispatching action is identical with that performed by an invokevirtual or invokeinterface instruction.)

The first argument will be of type refc if the lookup class has full privileges to access the member. Otherwise the member must be protected and the first argument will be restricted in type to the lookup class.

The returned method handle will have variable arity if and only if the method's variable arity modifier bit (0x0080) is set.

Because of the general equivalence between invokevirtual instructions and method handles produced by findVirtual, if the class is MethodHandle and the name string is invokeExact or invoke, the resulting method handle is equivalent to one produced by MethodHandles.exactInvoker or MethodHandles.invoker with the same type argument. Example:

import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle MH_concat = publicLookup().findVirtual(String.class,
  "concat", methodType(String.class, String.class));
MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
  "hashCode", methodType(int.class));
MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
  "hashCode", methodType(int.class));
assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
// interface method:
MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
  "subSequence", methodType(CharSequence.class, int.class, int.class));
assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
// constructor "internal method" must be accessed differently:
MethodType MT_newString = methodType(void.class); //()V for new String()
try { assertEquals("impossible", lookup()
        .findVirtual(String.class, "<init>", MT_newString));
 } catch (NoSuchMethodException ex) { } // OK
MethodHandle MH_newString = publicLookup()
  .findConstructor(String.class, MT_newString);
assertEquals("", (String) MH_newString.invokeExact());
 

Parameters
refc Class: the class or interface from which the method is accessed

name String: the name of the method

type MethodType: the type of the method, with the receiver argument omitted

Returns
MethodHandle the desired method handle

Throws
NoSuchMethodException if the method does not exist
IllegalAccessException if access checking fails, or if the method is static or if the method's variable arity modifier bit is set and asVarargsCollector fails
SecurityException if a security manager is present and it refuses access
NullPointerException if any argument is null

in

added in API level 26
MethodHandles.Lookup in (Class<?> requestedLookupClass)

Creates a lookup on the specified new lookup class. The resulting object will report the specified class as its own lookupClass.

However, the resulting Lookup object is guaranteed to have no more access capabilities than the original. In particular, access capabilities can be lost as follows:

  • If the new lookup class differs from the old one, protected members will not be accessible by virtue of inheritance. (Protected members may continue to be accessible because of package sharing.)
  • If the new lookup class is in a different package than the old one, protected and default (package) members will not be accessible.
  • If the new lookup class is not within the same package member as the old one, private members will not be accessible.
  • If the new lookup class is not accessible to the old lookup class, then no members, not even public members, will be accessible. (In all other cases, public members will continue to be accessible.)

Parameters
requestedLookupClass Class: the desired lookup class for the new lookup object

Returns
MethodHandles.Lookup a lookup object which reports the desired lookup class

Throws
NullPointerException if the argument is null

lookupClass

added in API level 26
Class<?> lookupClass ()

Tells which class is performing the lookup. It is this class against which checks are performed for visibility and access permissions.

The class implies a maximum level of access permission, but the permissions may be additionally limited by the bitmask lookupModes, which controls whether non-public members can be accessed.

Returns
Class<?> the lookup class, on behalf of which this lookup object finds members

lookupModes

added in API level 26
int lookupModes ()

Tells which access-protection classes of members this lookup object can produce. The result is a bit-mask of the bits PUBLIC (0x01), PRIVATE (0x02), PROTECTED (0x04), and PACKAGE (0x08).

A freshly-created lookup object on the caller's class has all possible bits set, since the caller class can access all its own members. A lookup object on a new lookup class created from a previous lookup object may have some mode bits set to zero. The purpose of this is to restrict access via the new lookup object, so that it can access only names which can be reached by the original lookup object, and also by the new lookup class.

Returns
int the lookup modes, which limit the kinds of access performed by this lookup object

revealDirect

added in API level 26
MethodHandleInfo revealDirect (MethodHandle target)

Cracks a direct method handle created by this lookup object or a similar one. Security and access checks are performed to ensure that this lookup object is capable of reproducing the target method handle. This means that the cracking may fail if target is a direct method handle but was created by an unrelated lookup object. This can happen if the method handle is caller sensitive and was created by a lookup object for a different class.

Parameters
target MethodHandle: a direct method handle to crack into symbolic reference components

Returns
MethodHandleInfo a symbolic reference which can be used to reconstruct this method handle from this lookup object

Throws
SecurityException if a security manager is present and it refuses access
IllegalArgumentException if the target is not a direct method handle or if access checking fails
NullPointerException if the target is null

See also:

toString

added in API level 26
String toString ()

Displays the name of the class from which lookups are to be made. (The name is the one reported by Class.getName.) If there are restrictions on the access permitted to this lookup, this is indicated by adding a suffix to the class name, consisting of a slash and a keyword. The keyword represents the strongest allowed access, and is chosen as follows:

  • If no access is allowed, the suffix is "/noaccess".
  • If only public access is allowed, the suffix is "/public".
  • If only public and package access are allowed, the suffix is "/package".
  • If only public, package, and private access are allowed, the suffix is "/private".
If none of the above cases apply, it is the case that full access (public, package, private, and protected) is allowed. In this case, no suffix is added. This is true only of an object obtained originally from MethodHandles.lookup. Objects created by Lookup.in always have restricted access, and will display a suffix.

(It may seem strange that protected access should be stronger than private access. Viewed independently from package access, protected access is the first to be lost, because it requires a direct subclass relationship between caller and callee.)

Returns
String a string representation of the object.

See also:

unreflect

added in API level 26
MethodHandle unreflect (Method m)

Makes a direct method handle to m, if the lookup class has permission. If m is non-static, the receiver argument is treated as an initial argument. If m is virtual, overriding is respected on every call. Unlike the Core Reflection API, exceptions are not wrapped. The type of the method handle will be that of the method, with the receiver type prepended (but only if it is non-static). If the method's accessible flag is not set, access checking is performed immediately on behalf of the lookup class. If m is not public, do not share the resulting handle with untrusted parties.

The returned method handle will have variable arity if and only if the method's variable arity modifier bit (0x0080) is set.

If m is static, and if the returned method handle is invoked, the method's class will be initialized, if it has not already been initialized.

Parameters
m Method: the reflected method

Returns
MethodHandle a method handle which can invoke the reflected method

Throws
IllegalAccessException if access checking fails or if the method's variable arity modifier bit is set and asVarargsCollector fails
NullPointerException if the argument is null

unreflectConstructor

added in API level 26
MethodHandle unreflectConstructor (Constructor<?> c)

Produces a method handle for a reflected constructor. The type of the method handle will be that of the constructor, with the return type changed to the declaring class. The method handle will perform a newInstance operation, creating a new instance of the constructor's class on the arguments passed to the method handle.

If the constructor's accessible flag is not set, access checking is performed immediately on behalf of the lookup class.

The returned method handle will have variable arity if and only if the constructor's variable arity modifier bit (0x0080) is set.

If the returned method handle is invoked, the constructor's class will be initialized, if it has not already been initialized.

Parameters
c Constructor: the reflected constructor

Returns
MethodHandle a method handle which can invoke the reflected constructor

Throws
IllegalAccessException if access checking fails or if the method's variable arity modifier bit is set and asVarargsCollector fails
NullPointerException if the argument is null

unreflectGetter

added in API level 26
MethodHandle unreflectGetter (Field f)

Produces a method handle giving read access to a reflected field. The type of the method handle will have a return type of the field's value type. If the field is static, the method handle will take no arguments. Otherwise, its single argument will be the instance containing the field. If the field's accessible flag is not set, access checking is performed immediately on behalf of the lookup class.

If the field is static, and if the returned method handle is invoked, the field's class will be initialized, if it has not already been initialized.

Parameters
f Field: the reflected field

Returns
MethodHandle a method handle which can load values from the reflected field

Throws
IllegalAccessException if access checking fails
NullPointerException if the argument is null

unreflectSetter

added in API level 26
MethodHandle unreflectSetter (Field f)

Produces a method handle giving write access to a reflected field. The type of the method handle will have a void return type. If the field is static, the method handle will take a single argument, of the field's value type, the value to be stored. Otherwise, the two arguments will be the instance containing the field, and the value to be stored. If the field's accessible flag is not set, access checking is performed immediately on behalf of the lookup class.

If the field is static, and if the returned method handle is invoked, the field's class will be initialized, if it has not already been initialized.

Parameters
f Field: the reflected field

Returns
MethodHandle a method handle which can store values into the reflected field

Throws
IllegalAccessException if access checking fails
NullPointerException if the argument is null

unreflectSpecial

added in API level 26
MethodHandle unreflectSpecial (Method m, 
                Class<?> specialCaller)

Produces a method handle for a reflected method. It will bypass checks for overriding methods on the receiver, as if called from an invokespecial instruction from within the explicitly specified specialCaller. The type of the method handle will be that of the method, with a suitably restricted receiver type prepended. (The receiver type will be specialCaller or a subtype.) If the method's accessible flag is not set, access checking is performed immediately on behalf of the lookup class, as if invokespecial instruction were being linked.

Before method resolution, if the explicitly specified caller class is not identical with the lookup class, or if this lookup object does not have private access privileges, the access fails.

The returned method handle will have variable arity if and only if the method's variable arity modifier bit (0x0080) is set.

Parameters
m Method: the reflected method

specialCaller Class: the class nominally calling the method

Returns
MethodHandle a method handle which can invoke the reflected method

Throws
IllegalAccessException if access checking fails or if the method's variable arity modifier bit is set and asVarargsCollector fails
NullPointerException if any argument is null