Class Guards

public final class Guards
extends Object

Utility methods for creating typical guards for MethodHandles.guardWithTest(MethodHandle, MethodHandle, MethodHandle) and for adjusting their method types.

Methods

Modifier and Type Method Description
static MethodHandle asType​(MethodHandle test, MethodType type)

Takes a method handle intended to be used as a guard, and adapts it to the requested type, but returning a boolean.
static MethodHandle asType​(LinkerServices linkerServices, MethodHandle test, MethodType type)

Takes a method handle intended to be used as a guard, and adapts it to the requested type, but returning a boolean.
static MethodHandle getClassGuard​(Class<?> clazz)

Creates a guard method that tests its only argument for being of an exact particular class.
static MethodHandle getIdentityGuard​(Object obj)

Creates a guard method that tests its only argument for being referentially identical to another object
static MethodHandle getInstanceOfGuard​(Class<?> clazz)

Creates a guard method that tests its only argument for being an instance of a particular class.
static MethodHandle isArray​(int pos, MethodType type)

Creates a method handle that returns true if the argument in the specified position is a Java array.
static MethodHandle isInstance​(Class<?> clazz, int pos, MethodType type)

Creates a method handle with arguments of a specified type, but with boolean return value.
static MethodHandle isInstance​(Class<?> clazz, MethodType type)

Creates a method handle with arguments of a specified type, but with boolean return value.
static MethodHandle isNotNull()

Returns a guard that tests whether the first argument is not null.
static MethodHandle isNull()

Returns a guard that tests whether the first argument is null.
static MethodHandle isOfClass​(Class<?> clazz, MethodType type)

Creates a guard method handle with arguments of a specified type, but with boolean return value.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods

isOfClass

public static MethodHandle isOfClass(Class<?> clazz,
                                     MethodType type)

Creates a guard method handle with arguments of a specified type, but with boolean return value. When invoked, it returns true if the first argument is of the specified class (exactly of it, not a subclass). The rest of the arguments will be ignored.

Parameters:
clazz - the class of the first argument to test for
type - the method type
Returns:
a method handle testing whether its first argument is of the specified class.

isInstance

public static MethodHandle isInstance(Class<?> clazz,
                                      MethodType type)

Creates a method handle with arguments of a specified type, but with boolean return value. When invoked, it returns true if the first argument is instance of the specified class or its subclass). The rest of the arguments will be ignored.

Parameters:
clazz - the class of the first argument to test for
type - the method type
Returns:
a method handle testing whether its first argument is of the specified class or subclass.

isInstance

public static MethodHandle isInstance(Class<?> clazz,
                                      int pos,
                                      MethodType type)

Creates a method handle with arguments of a specified type, but with boolean return value. When invoked, it returns true if the n'th argument is instance of the specified class or its subclass). The rest of the arguments will be ignored.

Parameters:
clazz - the class of the first argument to test for
pos - the position on the argument list to test
type - the method type
Returns:
a method handle testing whether its first argument is of the specified class or subclass.

isArray

public static MethodHandle isArray(int pos,
                                   MethodType type)

Creates a method handle that returns true if the argument in the specified position is a Java array.

Parameters:
pos - the position in the argument lit
type - the method type of the handle
Returns:
a method handle that returns true if the argument in the specified position is a Java array; the rest of the arguments are ignored.

asType

public static MethodHandle asType(MethodHandle test,
                                  MethodType type)

Takes a method handle intended to be used as a guard, and adapts it to the requested type, but returning a boolean. Applies MethodHandle.asType(MethodType) to convert types and uses MethodHandles.dropArguments(MethodHandle, int, Class...) to match the requested type arity.

Parameters:
test - the test method handle
type - the type to adapt the method handle to
Returns:
the adapted method handle

asType

public static MethodHandle asType(LinkerServices linkerServices,
                                  MethodHandle test,
                                  MethodType type)

Takes a method handle intended to be used as a guard, and adapts it to the requested type, but returning a boolean. Applies LinkerServices.asType(MethodHandle, MethodType) to convert types and uses MethodHandles.dropArguments(MethodHandle, int, Class...) to match the requested type arity.

Parameters:
linkerServices - the linker services to use for type conversions
test - the test method handle
type - the type to adapt the method handle to
Returns:
the adapted method handle

getClassGuard

public static MethodHandle getClassGuard(Class<?> clazz)

Creates a guard method that tests its only argument for being of an exact particular class.

Parameters:
clazz - the class to test for.
Returns:
the desired guard method.

getInstanceOfGuard

public static MethodHandle getInstanceOfGuard(Class<?> clazz)

Creates a guard method that tests its only argument for being an instance of a particular class.

Parameters:
clazz - the class to test for.
Returns:
the desired guard method.

getIdentityGuard

public static MethodHandle getIdentityGuard(Object obj)

Creates a guard method that tests its only argument for being referentially identical to another object

Parameters:
obj - the object used as referential identity test
Returns:
the desired guard method.

isNull

public static MethodHandle isNull()

Returns a guard that tests whether the first argument is null.

Returns:
a guard that tests whether the first argument is null.

isNotNull

public static MethodHandle isNotNull()

Returns a guard that tests whether the first argument is not null.

Returns:
a guard that tests whether the first argument is not null.

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/11/docs/api/jdk.dynalink/jdk/dynalink/linker/support/Guards.html