Class ConstantBootstraps

java.lang.Object

java.lang.invoke.ConstantBootstraps

public final class ConstantBootstraps
extends Object

Bootstrap methods for dynamically-computed constants.

The bootstrap methods in this class will throw a NullPointerException for any reference argument that is null, unless the argument is specified to be unused or specified to accept a null value.

Since:

11

Method Summary

Modifier and Type	Method	Description
static VarHandle	arrayVarHandle(MethodHandles.Lookup lookup, String name, Class<VarHandle> type, Class<?> arrayClass)	Finds a VarHandle for an array type.
static <E extends Enum<E>> E	enumConstant(MethodHandles.Lookup lookup, String name, Class<E> type)	Returns an enum constant of the type specified by type with the name specified by name.
static Object	explicitCast(MethodHandles.Lookup lookup, String name, Class<?> dstType, Object value)	Applies a conversion from a source type to a destination type.
static VarHandle	fieldVarHandle(MethodHandles.Lookup lookup, String name, Class<VarHandle> type, Class<?> declaringClass, Class<?> fieldType)	Finds a VarHandle for an instance field.
static Object	getStaticFinal(MethodHandles.Lookup lookup, String name, Class<?> type)	Returns the value of a static final field declared in the class which is the same as the field's type (or, for primitive-valued fields, declared in the wrapper class.)
static Object	getStaticFinal(MethodHandles.Lookup lookup, String name, Class<?> type, Class<?> declaringClass)	Returns the value of a static final field.
static Object	invoke(MethodHandles.Lookup lookup, String name, Class<?> type, MethodHandle handle, Object... args)	Returns the result of invoking a method handle with the provided arguments.
static Object	nullConstant(MethodHandles.Lookup lookup, String name, Class<?> type)	Returns a null object reference for the reference type specified by type.
static Class<?>	primitiveClass(MethodHandles.Lookup lookup, String name, Class<?> type)	Returns a Class mirror for the primitive type whose type descriptor is specified by name.
static VarHandle	staticFieldVarHandle(MethodHandles.Lookup lookup, String name, Class<VarHandle> type, Class<?> declaringClass, Class<?> fieldType)	Finds a VarHandle for a static field.

Methods declared in class Object

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

Modifier and Type	Method	Description
protected Object	clone()	Answers a new instance of the same class as the receiver, whose slots have been filled in with the values in the slots of the receiver.
boolean	equals(Object o)	Compares the argument to the receiver, and answers true if they represent the same object using a class specific comparison.
protected void	finalize()	Deprecated, for removal: This API element is subject to removal in a future version. May cause performance issues, deadlocks and hangs.
final Class<? extends Object>	getClass()	Answers the unique instance of java.lang.Class which represents the class of the receiver.
int	hashCode()	Answers an integer hash code for the receiver.
final void	notify()	Causes one thread which is waiting on the receiver to be made ready to run.
final void	notifyAll()	Causes all threads which are waiting on the receiver to be made ready to run.
String	toString()	Answers a string containing a concise, human-readable description of the receiver.
final void	wait()	Causes the thread which sent this message to be made not ready to run pending some change in the receiver (as indicated by notify or notifyAll).
final void	wait(long time)	Causes the thread which sent this message to be made not ready to run either pending some change in the receiver (as indicated by notify or notifyAll) or the expiration of the timeout.
final void	wait(long time, int frac)	Causes the thread which sent this message to be made not ready to run either pending some change in the receiver (as indicated by notify or notifyAll) or the expiration of the timeout.

Method Details

nullConstant

public static Object nullConstant(MethodHandles.Lookup lookup,
 String name,
 Class<?> type)

Returns a null object reference for the reference type specified by type.

Parameters:

lookup - unused

name - unused

type - a reference type

Returns:

a null value

Throws:

IllegalArgumentException - if type is not a reference type

primitiveClass

public static Class<?> primitiveClass(MethodHandles.Lookup lookup,
 String name,
 Class<?> type)

Returns a Class mirror for the primitive type whose type descriptor is specified by name.

Parameters:

lookup - unused

name - the descriptor (JVMS §4.3) of the desired primitive type

type - the required result type (must be Class.class)

Returns:

the Class mirror

Throws:

IllegalArgumentException - if the name is not a descriptor for a primitive type or the type is not Class.class

enumConstant

public static <E extends Enum<E>> E enumConstant(MethodHandles.Lookup lookup,
 String name,
 Class<E> type)

Returns an enum constant of the type specified by type with the name specified by name.

Type Parameters:

E - The enum type for which a constant value is to be returned

Parameters:

lookup - the lookup context describing the class performing the operation (normally stacked by the JVM)

name - the name of the constant to return, which must exactly match an enum constant in the specified type.

type - the Class object describing the enum type for which a constant is to be returned

Returns:

the enum constant of the specified enum type with the specified name

Throws:

IllegalAccessError - if the declaring class or the field is not accessible to the class performing the operation

IllegalArgumentException - if the specified enum type has no constant with the specified name, or the specified class object does not represent an enum type

See Also:

• Enum.valueOf(Class, String)

getStaticFinal

public static Object getStaticFinal(MethodHandles.Lookup lookup,
 String name,
 Class<?> type,
 Class<?> declaringClass)

Returns the value of a static final field.

Parameters:

lookup - the lookup context describing the class performing the operation (normally stacked by the JVM)

name - the name of the field

type - the type of the field

declaringClass - the class in which the field is declared

Returns:

the value of the field

Throws:

IllegalAccessError - if the declaring class or the field is not accessible to the class performing the operation

NoSuchFieldError - if the specified field does not exist

IncompatibleClassChangeError - if the specified field is not final

getStaticFinal

public static Object getStaticFinal(MethodHandles.Lookup lookup,
 String name,
 Class<?> type)

Returns the value of a static final field declared in the class which is the same as the field's type (or, for primitive-valued fields, declared in the wrapper class.) This is a simplified form of getStaticFinal(MethodHandles.Lookup, String, Class, Class) for the case where a class declares distinguished constant instances of itself.

Parameters:

lookup - the lookup context describing the class performing the operation (normally stacked by the JVM)

name - the name of the field

type - the type of the field

Returns:

the value of the field

Throws:

IllegalAccessError - if the declaring class or the field is not accessible to the class performing the operation

NoSuchFieldError - if the specified field does not exist

IncompatibleClassChangeError - if the specified field is not final

See Also:

• getStaticFinal(MethodHandles.Lookup, String, Class, Class)

invoke

public static Object invoke(MethodHandles.Lookup lookup,
 String name,
 Class<?> type,
 MethodHandle handle,
 Object... args)
                     throws Throwable

Returns the result of invoking a method handle with the provided arguments.

This method behaves as if the method handle to be invoked is the result of adapting the given method handle, via MethodHandle.asType(MethodType), to adjust the return type to the desired type.

Parameters:

lookup - unused

name - unused

type - the desired type of the value to be returned, which must be compatible with the return type of the method handle

handle - the method handle to be invoked

args - the arguments to pass to the method handle, as if with MethodHandle.invokeWithArguments(Object...). Each argument may be null.

Returns:

the result of invoking the method handle

Throws:

WrongMethodTypeException - if the handle's method type cannot be adjusted to take the given number of arguments, or if the handle's return type cannot be adjusted to the desired type

ClassCastException - if an argument or the result produced by invoking the handle cannot be converted by reference casting

Throwable - anything thrown by the method handle invocation

fieldVarHandle

public static VarHandle fieldVarHandle(MethodHandles.Lookup lookup,
 String name,
 Class<VarHandle> type,
 Class<?> declaringClass,
 Class<?> fieldType)

Finds a VarHandle for an instance field.

Parameters:

lookup - the lookup context describing the class performing the operation (normally stacked by the JVM)

name - the name of the field

type - the required result type (must be Class<VarHandle>)

declaringClass - the class in which the field is declared

fieldType - the type of the field

Returns:

the VarHandle

Throws:

IllegalAccessError - if the declaring class or the field is not accessible to the class performing the operation

NoSuchFieldError - if the specified field does not exist

IllegalArgumentException - if the type is not VarHandle

staticFieldVarHandle

public static VarHandle staticFieldVarHandle(MethodHandles.Lookup lookup,
 String name,
 Class<VarHandle> type,
 Class<?> declaringClass,
 Class<?> fieldType)

Finds a VarHandle for a static field.

Parameters:

lookup - the lookup context describing the class performing the operation (normally stacked by the JVM)

name - the name of the field

type - the required result type (must be Class<VarHandle>)

declaringClass - the class in which the field is declared

fieldType - the type of the field

Returns:

the VarHandle

Throws:

IllegalAccessError - if the declaring class or the field is not accessible to the class performing the operation

NoSuchFieldError - if the specified field does not exist

IllegalArgumentException - if the type is not VarHandle

arrayVarHandle

public static VarHandle arrayVarHandle(MethodHandles.Lookup lookup,
 String name,
 Class<VarHandle> type,
 Class<?> arrayClass)

Finds a VarHandle for an array type.

Parameters:

lookup - the lookup context describing the class performing the operation (normally stacked by the JVM)

name - unused

type - the required result type (must be Class<VarHandle>)

arrayClass - the type of the array

Returns:

the VarHandle

Throws:

IllegalAccessError - if the component type of the array is not accessible to the class performing the operation

IllegalArgumentException - if the type is not VarHandle

explicitCast

public static Object explicitCast(MethodHandles.Lookup lookup,
 String name,
 Class<?> dstType,
 Object value)
                           throws ClassCastException

Applies a conversion from a source type to a destination type.

Given a destination type dstType and an input value value, one of the following will happen:

• If dstType is void.class, a ClassCastException is thrown.
• If dstType is Object.class, value is returned as is.

Otherwise one of the following conversions is applied to value:

1. If dstType is a reference type, a reference cast is applied to value as if by calling dstType.cast(value).
2. Otherwise, dstType is a primitive type:
   1. If value is null, the default value (JVMS §2.3) of dstType is returned.
   2. If the runtime type of value is a primitive wrapper type (such as Integer), a Java unboxing conversion is applied (JLS §5.1.8).
      • If the runtime type is Boolean, the unboxing result is then converted to int, where true becomes 1 and false becomes 0.
      Followed by a Java casting conversion (JLS §5.5):
      • If dstType is not boolean, the cast converts directly to dstType.
      • If dstType is boolean, the cast converts to int, and the resulting boolean is produced by testing whether the least significant bit of the cast int is 1.
   3. Otherwise, a ClassCastException is thrown.

The result is the same as when using the following code:

MethodHandle id = MethodHandles.identity(dstType);
MethodType mt = MethodType.methodType(dstType, Object.class);
MethodHandle conv = MethodHandles.explicitCastArguments(id, mt);
return conv.invoke(value);

Parameters:

lookup - unused

name - unused

dstType - the destination type of the conversion

value - the value to be converted, may be null

Returns:

the converted value

Throws:

ClassCastException - when dstType is void; when dstType is a reference type, and the reference cast fails; or when dstType is primitive, and value is an instance of a reference type that is not a wrapper class

Since:

15