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/OpenJDK 21

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 java.lang.Object

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

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:

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:

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(java.lang.invoke.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(java.lang.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. If dstType is a primitive type, then, if the runtime type of value is a primitive wrapper type (such as Integer), a Java unboxing conversion is applied 5.1.8 followed by a Java casting conversion 5.5 converting either directly to dstType, or, if dstType is boolean, to int, which is then converted to either true or false depending on whether the least-significant-bit is 1 or 0 respectively. If the runtime type of value is not a primitive wrapper type 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
Returns:
the converted value
Throws:
ClassCastException - when dstType is void, when a cast per (1) fails, or when dstType is a primitive type and the runtime type of value is not a primitive wrapper type (such as Integer)
Since:
15

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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.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/invoke/ConstantBootstraps.html