OpenJDK 27 · java.lang.invoke.MethodType · APIdia, the JavaDoc alternative

A method type represents the arguments and return type accepted and returned by a method handle, or the arguments and return type passed and expected by a method handle caller. Method types must be properly matched between a method handle and all its callers, and the JVM's operations enforce this matching at, specifically during calls to MethodHandle.invokeExact and MethodHandle.invoke, and during execution of invokedynamic instructions.

The structure is a return type accompanied by any number of parameter types. The types (primitive, void, and reference) are represented by Class objects. (For ease of exposition, we treat void as if it were a type. In fact, it denotes the absence of a return type.)

All instances of MethodType are immutable. Two instances are completely interchangeable if they compare equal. Equality depends on pairwise correspondence of the return and parameter types and on nothing else.

This type can be created only by factory methods. All factory methods may cache values, though caching is not guaranteed. Some factory methods are static, while others are virtual methods which modify precursor method types, e.g., by changing a selected parameter.

Factory methods which operate on groups of parameter types are systematically presented in two versions, so that both Java arrays and Java lists can be used to work with groups of parameter types. The query methods parameterArray and parameterList also provide a choice between arrays and lists.

MethodType objects are sometimes derived from bytecode instructions such as invokedynamic, specifically from the type descriptor strings associated with the instructions in a class file's constant pool.

Like classes and strings, method types can also be represented directly in a class file's constant pool as constants. A method type may be loaded by an ldc instruction which refers to a suitable CONSTANT_MethodType constant pool entry. The entry refers to a CONSTANT_Utf8 spelling for the descriptor string. (For full details on method type constants, see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)

When the JVM materializes a MethodType from a descriptor string, all classes named in the descriptor must be accessible, and will be loaded. (But the classes need not be initialized, as is the case with a CONSTANT_Class.) This loading may occur at any time before the MethodType object is first derived.

Nominal Descriptors

A MethodType can be described in nominal form if and only if all of the parameter types and return type can be described with a nominal descriptor represented by ClassDesc. If a method type can be described nominally, then:

The method type has a nominal descriptor returned by MethodType::describeConstable.
The descriptor string returned by MethodType::descriptorString or MethodType::toMethodDescriptorString for the method type is a method descriptor (JVMS 4.3.3).

If any of the parameter types or return type cannot be described nominally, i.e. Class::describeConstable returns an empty optional for that type, then the method type cannot be described nominally:

The method type has no nominal descriptor and MethodType::describeConstable returns an empty optional.
The descriptor string returned by MethodType::descriptorString or MethodType::toMethodDescriptorString for the method type is not a type descriptor.

Modifier and Type	Class and Description
private static class	MethodType.OffsetHolder

Modifier and Type	Field and Description
private MethodTypeForm	form
pack-priv static final ReferencedKeySet<MethodType>	internTable
private Invokers	invokers
pack-priv static final int	MAX_JVM_ARITY This number, mandated by the JVM spec as 255, is the maximum number of slots that any Java method can receive in its argument list.
pack-priv static final int	MAX_MH_ARITY This number is the maximum arity of a method handle, 254.
pack-priv static final int	MAX_MH_INVOKER_ARITY This number is the maximum arity of a method handle invoker, 253.
private static final Class<?>[]	METHOD_HANDLE_ARRAY
private String	methodDescriptor
pack-priv static final Class<?>[]	NO_PTYPES
private static final MethodType[]	objectOnlyTypes
private final Class<?>[]	ptypes
private final Class<?>	rtype
private static final ObjectStreamField[]	serialPersistentFields There are no serializable fields for `MethodType`.
private static final long	serialVersionUID
private Object	wrapAlt

Access	Constructor and Description
private	MethodType(Class<?> rtype, Class<?>[] ptypes) Constructor that performs no copying or validation.

Method Summary

Modifier and Type	Method and Description
public MethodType	Returns: the same type, except with the selected parameter(s) appended appendParameterTypes(Class<?>... zero or more new parameter types to insert after the end of the parameter list ptypesToInsert) Finds or creates a method type with additional parameter types.
public MethodType	Returns: the same type, except with the selected parameter(s) appended appendParameterTypes(List<Class<?>> zero or more new parameter types to insert after the end of the parameter list ptypesToInsert) Finds or creates a method type with additional parameter types.
pack-priv MethodType	Returns: the resulting type asCollectorType(Class<?> any array type arrayType, int position at which to insert parameters pos, int the number of parameter types to insert arrayLength) Delete the last parameter type and replace it with arrayLength copies of the component type of arrayType.
private static void	assemblySetup()
pack-priv MethodType	Returns: the resulting type asSpreaderType(Class<?> any array type arrayType, int position at which to spread pos, int the number of parameter types to change arrayLength) Replace the last arrayLength parameter types with the component type of arrayType.
pack-priv MethodType	Returns: a version of the original type with all reference and subword types replaced basicType() Erases all reference types to `Object`, and all subword types to `int`.
pack-priv static boolean	canConvert(Class<?> src, Class<?> dst)
private boolean	canConvertParameters(Class<?>[] srcTypes, Class<?>[] dstTypes)
public MethodType	Returns: the same type, except with the selected parameter changed changeParameterType(int the index (zero-based) of the parameter type to change num, Class<?> a new parameter type to replace the old one with nptype) Implements java.lang.invoke.TypeDescriptor.OfMethod.changeParameterType. Finds or creates a method type with a single different parameter type.
public MethodType	Returns: the same type, except with the return type change changeReturnType(Class<?> a return parameter type to replace the old one with nrtype) Implements java.lang.invoke.TypeDescriptor.OfMethod.changeReturnType. Finds or creates a method type with a different return type.
private static int	checkPtypes(Class<?>[] ptypes) Return number of extra slots (count of long/double args).
pack-priv static void	checkSlotCount(int count)
public Optional<MethodTypeDesc>	Returns: An `Optional` containing the resulting nominal descriptor, or an empty `Optional` if one cannot be constructed. describeConstable() Implements java.lang.constant.Constable.describeConstable. Returns a nominal descriptor for this instance, if one can be constructed, or an empty `Optional` if one cannot be.
public String	Returns: the descriptor string for this method type descriptorString() Implements java.lang.invoke.TypeDescriptor.descriptorString. Returns the descriptor string for this method type.
public MethodType	Returns: the same type, except with the selected parameter(s) removed dropParameterTypes(int the index (zero-based) of the first parameter type to remove start, int the index (greater than `start`) of the first parameter type after not to remove end) Implements java.lang.invoke.TypeDescriptor.OfMethod.dropParameterTypes. Finds or creates a method type with some parameter types omitted.
pack-priv boolean	effectivelyIdenticalParameters(int skipPos, List<Class<?>> fullList) True if my parameter list is effectively identical to the given full list, after skipping the given number of my own initial parameters.
public boolean	equals(Object object to compare x) Overrides java.lang.Object.equals. Compares the specified object with this type for equality.
private boolean	equals(MethodType that)
public MethodType	Returns: a version of the original type with all reference types replaced erase() Erases all reference types to `Object`.
pack-priv boolean	explicitCastEquivalentToAsType(MethodType newType) Returns true if MHs.explicitCastArguments produces the same result as MH.asType.
private static boolean	explicitCastEquivalentToAsType(Class<?> src, Class<?> dst) Reports true if the src can be converted to the dst, by both asType and MHs.eCE, and with the same effect.
pack-priv MethodTypeForm	form()
pack-priv static MethodType	fromDescriptor(String descriptor, ClassLoader loader) Same as `fromMethodDescriptorString(String, ClassLoader)`, but `null` ClassLoader means the bootstrap loader is used here.
public static MethodType	Returns: a method type of the given method descriptor fromMethodDescriptorString(String a method descriptor string descriptor, ClassLoader the class loader in which to look up the types loader) Finds or creates an instance of a method type of the given method descriptor (JVMS 4.3.3).
public MethodType	Returns: a version of the original type with all types replaced generic() Converts all types, both reference and primitive, to `Object`.
public static MethodType	Returns: a generally applicable method type, for all calls of the given fixed argument count and a collected array of further arguments genericMethodType(int number of parameters (excluding the final array parameter if any) objectArgCount, boolean whether there will be a trailing array parameter, of type `Object[]` finalArray) Finds or creates a method type whose components are `Object` with an optional trailing `Object[]` array.
public static MethodType	Returns: a generally applicable method type, for all calls of the given argument count genericMethodType(int number of parameters objectArgCount) Finds or creates a method type whose components are all `Object`.
public int	Returns: the hash code value for this method type hashCode() Overrides java.lang.Object.hashCode. Returns the hash code value for this method type.
public boolean	Returns: true if any of the types are primitives hasPrimitives() Reports if this type contains a primitive argument or return value.
public boolean	Returns: true if any of the types are wrappers hasWrappers() Reports if this type contains a wrapper argument or return value.
public MethodType	Returns: the same type, except with the selected parameter(s) inserted insertParameterTypes(int the position (zero-based) of the inserted parameter type(s) num, Class<?>... zero or more new parameter types to insert into the parameter list ptypesToInsert) Implements java.lang.invoke.TypeDescriptor.OfMethod.insertParameterTypes. Finds or creates a method type with additional parameter types.
public MethodType	Returns: the same type, except with the selected parameter(s) inserted insertParameterTypes(int the position (zero-based) of the inserted parameter type(s) num, List<Class<?>> zero or more new parameter types to insert into the parameter list ptypesToInsert) Finds or creates a method type with additional parameter types.
pack-priv Invokers	invokers()
pack-priv MethodType	invokerType() Returns: a version of the original type with MethodHandle prepended as the first argument
private static boolean	isAllObject(Class<?>[] ptypes, int to)
pack-priv boolean	isConvertibleTo(MethodType newType)
pack-priv boolean	isGeneric()
pack-priv boolean	isViewableAs(MethodType newType, boolean keepInterfaces) True if the old return type can always be viewed (w/o casting) under new return type, and the new parameters can be viewed (w/o casting) under the old parameter types.
public Class<?>	Returns: the last parameter type if any, else `void.class` lastParameterType() Returns the last parameter type of this method type.
pack-priv Class<?>	Returns: the leading parameter type, after error checks leadingReferenceParameter() Return the leading parameter type, which must exist and be a reference.
private static Class<?>[]	listToArray(List<Class<?>> ptypes)
private static MethodType	Returns: the unique method type of the desired structure makeImpl(Class<?> desired return type rtype, Class<?>[] desired parameter types ptypes, boolean whether the ptypes can be used without cloning trusted) Sole factory method to find or create an interned method type.
public static MethodType	Returns: a method type with the given components methodType(Class<?> the return type rtype, Class<?>[] the parameter types ptypes) Finds or creates an instance of the given method type.
public static MethodType	Returns: a method type with the given components methodType(Class<?> the return type rtype, List<Class<?>> the parameter types ptypes) Finds or creates a method type with the given components.
public static MethodType	Returns: a method type with the given components methodType(Class<?> the return type rtype, Class<?> the first parameter type ptype0, Class<?>... the remaining parameter types ptypes) Finds or creates a method type with the given components.
public static MethodType	Returns: a method type with the given return value methodType(Class<?> the return type rtype) Finds or creates a method type with the given components.
public static MethodType	Returns: a method type with the given return value and parameter type methodType(Class<?> the return type rtype, Class<?> the parameter type ptype0) Finds or creates a method type with the given components.
public static MethodType	Returns: a method type with the given components methodType(Class<?> the return type rtype, MethodType the method type which supplies the parameter types ptypes) Finds or creates a method type with the given components.
pack-priv static MethodType	methodType(Class<?> rtype, Class<?>[] ptypes, boolean trusted)
private static IndexOutOfBoundsException	newIndexOutOfBoundsException(Object num)
public Class<?>[]	Returns: the parameter types (as a fresh copy if necessary) parameterArray() Implements java.lang.invoke.TypeDescriptor.OfMethod.parameterArray. Presents the parameter types as an array (a convenience method).
public int	Returns: the number of parameter types parameterCount() Implements java.lang.invoke.TypeDescriptor.OfMethod.parameterCount. Returns the number of parameter types in this method type.
public List<Class<?>>	Returns: the parameter types (as an immutable list) parameterList() Implements java.lang.invoke.TypeDescriptor.OfMethod.parameterList. Presents the parameter types as a list (a convenience method).
pack-priv int	Returns: the number of JVM stack slots for this type's parameters parameterSlotCount() Reports the number of JVM stack slots required to invoke a method of this type.
public Class<?>	Returns: the selected parameter type parameterType(int the index (zero-based) of the desired parameter type num) Implements java.lang.invoke.TypeDescriptor.OfMethod.parameterType. Returns the parameter type at the specified index, within this method type.
pack-priv Class<?>[]	ptypes()
private void	readObject(ObjectInputStream the stream to read the object from s) Reconstitute the `MethodType` instance from a stream (that is, deserialize it).
private Object	Returns: the fully initialized `MethodType` object readResolve() Resolves and initializes a `MethodType` object after serialization.
pack-priv MethodType	Returns: the same type, except with the selected parameter(s) replaced replaceParameterTypes(int the position (zero-based) of the first replaced parameter type(s) start, int the position (zero-based) after the last replaced parameter type(s) end, Class<?>... zero or more new parameter types to insert into the parameter list ptypesToInsert) Finds or creates a method type with modified parameter types.
public Class<?>	Returns: the return type returnType() Implements java.lang.invoke.TypeDescriptor.OfMethod.returnType. Returns the return type of this method type.
pack-priv Class<?>	rtype()
pack-priv void	setForm(MethodTypeForm f)
pack-priv static String	toFieldDescriptorString(Class<?> cls)
public String	Returns: the descriptor string for this method type toMethodDescriptorString() Returns the descriptor string for this method type.
public String	toString() Overrides java.lang.Object.toString. Returns a string representation of the method type, of the form `"(PT0,PT1...)RT"`.
public MethodType	Returns: a version of the original type with all wrapper types replaced unwrap() Converts all wrapper types to their corresponding primitive types.
private static MethodType	unwrapWithNoPrims(MethodType wt)
public MethodType	Returns: a version of the original type with all primitive types replaced wrap() Converts all primitive types to their corresponding wrapper types.
private static MethodType	wrapWithPrims(MethodType pt)
private void	writeObject(ObjectOutputStream the stream to write the object to s) Save the `MethodType` instance to a stream.

Inherited from java.lang.Object:: clone finalize getClass notify notifyAll wait wait wait

public final Class MethodType

Nested and Inner Type Summary

Field Summary

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