LambdaMetafactory (Java SE 26 & JDK 26)
Description: declaration: module: java.base, package: java.lang.invoke, class: LambdaMetafactory
Keywords: java.lang.invoke.LambdaMetafactory class

# Class LambdaMetafactory

java.lang.Object
java.lang.invoke.LambdaMetafactory

public final class LambdaMetafactory extends Object

Methods to facilitate the creation of simple "function objects" that implement one or more interfaces by delegation to a provided `MethodHandle`, possibly after type adaptation and partial evaluation of arguments. These methods are typically used as bootstrap methods for `invokedynamic` call sites, to support the lambda expression and method reference expression features of the Java Programming Language.

Indirect access to the behavior specified by the provided `MethodHandle` proceeds in order through three phases:

- Linkage occurs when the methods in this class are invoked. They take as arguments an interface to be implemented (typically a functional interface, one with a single abstract method), a name and signature of a method from that interface to be implemented, a direct method handle describing the desired implementation behavior for that method, and possibly other additional metadata, and produce a `CallSite` whose target can be used to create suitable function objects.

  Linkage may involve dynamically loading a new class that implements the target interface, or re-using a suitable existing class.

  The `CallSite` can be considered a "factory" for function objects and so these linkage methods are referred to as "metafactories".

- Capture occurs when the `CallSite`'s target is invoked, typically through an `invokedynamic` call site, producing a function object. This may occur many times for a single factory `CallSite`.

  If the behavior `MethodHandle` has additional parameters beyond those of the specified interface method, these are referred to as captured parameters, which must be provided as arguments to the `CallSite` target. The expected number and types of captured parameters are determined during linkage.

  Capture may involve allocation of a new function object, or may return a suitable existing function object. The identity of a function object produced by capture is unpredictable, and therefore identity-sensitive operations (such as reference equality, object locking, and `System.identityHashCode()`) may produce different results in different implementations, or even upon different invocations in the same implementation.

- Invocation occurs when an implemented interface method is invoked on a function object. This may occur many times for a single function object. The method referenced by the implementation `MethodHandle` is invoked, passing to it the captured arguments and the invocation arguments. The result of the method is returned.

It is sometimes useful to restrict the set of inputs or results permitted at invocation. For example, when the generic interface `Predicate<T>` is parameterized as `Predicate<String>`, the input must be a `String`, even though the method to implement allows any `Object`. At linkage time, an additional `MethodType` parameter describes the "dynamic" method type; on invocation, the arguments and eventual result are checked against this `MethodType`.

This class provides two forms of linkage methods: a standard version (`metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)`) using an optimized protocol, and an alternate version `altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)`). The alternate version is a generalization of the standard version, providing additional control over the behavior of the generated function objects via flags and additional arguments. The alternate version adds the ability to manage the following attributes of function objects:

- Multiple methods. It is sometimes useful to implement multiple variations of the method signature, involving argument or return type adaptation. This occurs when multiple distinct VM signatures for a method are logically considered to be the same method by the language. The flag `FLAG_BRIDGES` indicates that a list of additional `MethodType`s will be provided, each of which will be implemented by the resulting function object. These methods will share the same name and instantiated type.
- Multiple interfaces. If needed, more than one interface can be implemented by the function object. (These additional interfaces are typically marker interfaces with no methods.) The flag `FLAG_MARKERS` indicates that a list of additional interfaces will be provided, each of which should be implemented by the resulting function object.
- Serializability. The generated function objects do not generally support serialization. If desired, `FLAG_SERIALIZABLE` can be used to indicate that the function objects should be serializable. Serializable function objects will use, as their serialized form, instances of the class `SerializedLambda`, which requires additional assistance from the capturing class (the class described by the `MethodHandles.Lookup` parameter `caller`); see `SerializedLambda` for details.

Assume the linkage arguments are as follows:

- `factoryType` (describing the `CallSite` signature) has K parameters of types (D1..Dk) and return type Rd;
- `interfaceMethodType` (describing the implemented method type) has N parameters, of types (U1..Un) and return type Ru;
- `implementation` (the `MethodHandle` providing the implementation) has M parameters, of types (A1..Am) and return type Ra (if the method describes an instance method, the method type of this method handle already includes an extra first argument corresponding to the receiver);
- `dynamicMethodType` (allowing restrictions on invocation) has N parameters, of types (T1..Tn) and return type Rt.

Then the following linkage invariants must hold:

- `interfaceMethodType` and `dynamicMethodType` have the same arity N, and for i=1..N, Ti and Ui are the same type, or Ti and Ui are both reference types and Ti is a subtype of Ui
- Either Rt and Ru are the same type, or both are reference types and Rt is a subtype of Ru
- K + N = M
- For i=1..K, Di = Ai
- For i=1..N, Ti is adaptable to Aj, where j=i+k
- The return type Rt is void, or the return type Ra is not void and is adaptable to Rt

Further, at capture time, if `implementation` corresponds to an instance method, and there are any capture arguments (`K > 0`), then the first capture argument (corresponding to the receiver) must be non-null.

A type Q is considered adaptable to S as follows:

adaptable types
Q | S | Link-time checks | Invocation-time checks
--- | --- | --- | ---
Primitive | Primitive | Q can be converted to S via a primitive widening conversion | None
Primitive | Reference | S is a supertype of the Wrapper(Q) | Cast from Wrapper(Q) to S
Reference | Primitive | for parameter types: Q is a primitive wrapper and Primitive(Q) can be widened to S for return types: If Q is a primitive wrapper, check that Primitive(Q) can be widened to S | If Q is not a primitive wrapper, cast Q to the base Wrapper(S); for example Number for numeric types
Reference | Reference | for parameter types: S is a supertype of Q for return types: none | Cast from Q to S

API Note:
  These linkage methods are designed to support the evaluation of lambda expressions and method references in the Java Language. For every lambda expressions or method reference in the source code, there is a target type which is a functional interface. Evaluating a lambda expression produces an object of its target type. The recommended mechanism for evaluating lambda expressions is to desugar the lambda body to a method, invoke an invokedynamic call site whose static argument list describes the sole method of the functional interface and the desugared implementation method, and returns an object (the lambda object) that implements the target type. (For method references, the implementation method is simply the referenced method; no desugaring is needed.)

  The argument list of the implementation method and the argument list of the interface method(s) may differ in several ways. The implementation methods may have additional arguments to accommodate arguments captured by the lambda expression; there may also be differences resulting from permitted adaptations of arguments, such as casting, boxing, unboxing, and primitive widening. (Varargs adaptations are not handled by the metafactories; these are expected to be handled by the caller.)

  Invokedynamic call sites have two argument lists: a static argument list and a dynamic argument list. The static argument list is stored in the constant pool; the dynamic argument is pushed on the operand stack at capture time. The bootstrap method has access to the entire static argument list (which in this case, includes information describing the implementation method, the target interface, and the target interface method(s)), as well as a method signature describing the number and static types (but not the values) of the dynamic arguments and the static return type of the invokedynamic site.

  The implementation method is described with a direct method handle referencing a method or constructor. In theory, any method handle could be used, but this is not compatible with some implementation techniques and would complicate the work implementations must do.

  Uses besides evaluation of lambda expressions and method references are unintended. These linkage methods may change their unspecified behaviors at any time to better suit the Java language features they were designed to support, and such changes may impact unintended uses. Unintended uses of these linkage methods may lead to resource leaks, or other unspecified negative effects.

Implementation Note:
  In the reference implementation, the classes implementing the created function objects are strongly reachable from the defining class loader of the caller, like classes and interfaces in Java source code. This technique reduces heap memory use, but as a consequence, the implementation classes can be unloaded only if the caller class can be unloaded. In particular, if the caller is a weak hidden class, the implementation class, a strong hidden class, may not be unloaded even if the caller may be unloaded.
Since:
  1.8

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `static final int`
  `FLAG_BRIDGES`
  Flag for alternate metafactories indicating the lambda object requires additional methods that invoke the `implementation`
  `static final int`
  `FLAG_MARKERS`
  Flag for `altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)` indicating the lambda object implements other interfaces besides `Serializable`
  `static final int`
  `FLAG_SERIALIZABLE`
  Flag for `altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)` indicating the lambda object must be serializable

- ## Method Summary

  Modifier and Type
  Method
  Description
  `static CallSite`
  `altMetafactory(MethodHandles.Lookup caller, String interfaceMethodName, MethodType factoryType, Object... args)`
  Facilitates the creation of simple "function objects" that implement one or more interfaces by delegation to a provided `MethodHandle`, after appropriate type adaptation and partial evaluation of arguments.
  `static CallSite`
  `metafactory(MethodHandles.Lookup caller, String interfaceMethodName, MethodType factoryType, MethodType interfaceMethodType, MethodHandle implementation, MethodType dynamicMethodType)`
  Facilitates the creation of simple "function objects" that implement one or more interfaces by delegation to a provided `MethodHandle`, after appropriate type adaptation and partial evaluation of arguments.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`
  Modifier and Type
  Method
  Description
  `protected Object`
  `clone()`
  Creates and returns a copy of this object.
  `boolean`
  `equals(Object obj)`
  Indicates whether some other object is "equal to" this one.
  `protected void`
  `finalize()`
  Deprecated, for removal: This API element is subject to removal in a future version.
  Finalization is deprecated and subject to removal in a future release.
  `final Class<?>`
  `getClass()`
  Returns the runtime class of this `Object`.
  `int`
  `hashCode()`
  Returns a hash code value for this object.
  `final void`
  `notify()`
  Wakes up a single thread that is waiting on this object's monitor.
  `final void`
  `notifyAll()`
  Wakes up all threads that are waiting on this object's monitor.
  `String`
  `toString()`
  Returns a string representation of the object.
  `final void`
  `wait()`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
  `final void`
  `wait(long timeoutMillis)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
  `final void`
  `wait(long timeoutMillis, int nanos)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

- ## Field Details

  - ### FLAG_SERIALIZABLE

    public static final int FLAG_SERIALIZABLE
    Flag for `altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)` indicating the lambda object must be serializable
    See Also:
      - Constant Field Values

  - ### FLAG_MARKERS

    public static final int FLAG_MARKERS
    Flag for `altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)` indicating the lambda object implements other interfaces besides `Serializable`
    See Also:
      - Constant Field Values

  - ### FLAG_BRIDGES

    public static final int FLAG_BRIDGES
    Flag for alternate metafactories indicating the lambda object requires additional methods that invoke the `implementation`
    See Also:
      - Constant Field Values

- ## Method Details

  - ### metafactory

    public static CallSite metafactory(MethodHandles.Lookup caller, String interfaceMethodName, MethodType factoryType, MethodType interfaceMethodType, MethodHandle implementation, MethodType dynamicMethodType) throws LambdaConversionException
    Facilitates the creation of simple "function objects" that implement one or more interfaces by delegation to a provided `MethodHandle`, after appropriate type adaptation and partial evaluation of arguments. Typically used as a bootstrap method for `invokedynamic` call sites, to support the lambda expression and method reference expression features of the Java Programming Language.

    This is the standard, streamlined metafactory; additional flexibility is provided by `altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)`. A general description of the behavior of this method is provided `above`.

    When the target of the `CallSite` returned from this method is invoked, the resulting function objects are instances of a class which implements the interface named by the return type of `factoryType`, declares a method with the name given by `interfaceMethodName` and the signature given by `interfaceMethodType`. It may also override additional methods from `Object`.

    Parameters:
      `caller` - Represents a lookup context with the accessibility privileges of the caller. Specifically, the lookup context must have full privilege access. When used with `invokedynamic`, this is stacked automatically by the VM.
      `interfaceMethodName` - The name of the method to implement. When used with `invokedynamic`, this is provided by the `NameAndType` of the `InvokeDynamic` structure and is stacked automatically by the VM.
      `factoryType` - The expected signature of the `CallSite`. The parameter types represent the types of capture variables; the return type is the interface to implement. When used with `invokedynamic`, this is provided by the `NameAndType` of the `InvokeDynamic` structure and is stacked automatically by the VM.
      `interfaceMethodType` - Signature and return type of method to be implemented by the function object.
      `implementation` - A direct method handle describing the implementation method which should be called (with suitable adaptation of argument types and return types, and with captured arguments prepended to the invocation arguments) at invocation time.
      `dynamicMethodType` - The signature and return type that should be enforced dynamically at invocation time. In simple use cases this is the same as `interfaceMethodType`.
    Returns:
      a CallSite whose target can be used to perform capture, generating instances of the interface named by `factoryType`
    Throws:
      `LambdaConversionException` - If `caller` does not have full privilege access, or if `interfaceMethodName` is not a valid JVM method name, or if the return type of `factoryType` is not an interface, or if `implementation` is not a direct method handle referencing a method or constructor, or if the linkage invariants are violated, as defined `above`.
      `NullPointerException` - If any argument is `null`.

  - ### altMetafactory

    public static CallSite altMetafactory(MethodHandles.Lookup caller, String interfaceMethodName, MethodType factoryType, Object... args) throws LambdaConversionException
    Facilitates the creation of simple "function objects" that implement one or more interfaces by delegation to a provided `MethodHandle`, after appropriate type adaptation and partial evaluation of arguments. Typically used as a bootstrap method for `invokedynamic` call sites, to support the lambda expression and method reference expression features of the Java Programming Language.

    This is the general, more flexible metafactory; a streamlined version is provided by `metafactory(java.lang.invoke.MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)`. A general description of the behavior of this method is provided `above`.

    The argument list for this method includes three fixed parameters, corresponding to the parameters automatically stacked by the VM for the bootstrap method in an `invokedynamic` invocation, and an `Object[]` parameter that contains additional parameters. The declared argument list for this method is:

         CallSite altMetafactory(MethodHandles.Lookup caller,
                                 String interfaceMethodName,
                                 MethodType factoryType,
                                 Object... args)

    but it behaves as if the argument list is as follows:

         CallSite altMetafactory(MethodHandles.Lookup caller,
                                 String interfaceMethodName,
                                 MethodType factoryType,
                                 MethodType interfaceMethodType,
                                 MethodHandle implementation,
                                 MethodType dynamicMethodType,
                                 int flags,
                                 int altInterfaceCount,        // IF flags has MARKERS set
                                 Class... altInterfaces,       // IF flags has MARKERS set
                                 int altMethodCount,           // IF flags has BRIDGES set
                                 MethodType... altMethods      // IF flags has BRIDGES set
                                 )

    Arguments that appear in the argument list for `metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)` have the same specification as in that method. The additional arguments are interpreted as follows:

    - `flags` indicates additional options; this is a bitwise OR of desired flags. Defined flags are `FLAG_BRIDGES`, `FLAG_MARKERS`, and `FLAG_SERIALIZABLE`.
    - `altInterfaceCount` is the number of additional interfaces the function object should implement, and is present if and only if the `FLAG_MARKERS` flag is set.
    - `altInterfaces` is a variable-length list of additional interfaces to implement, whose length equals `altInterfaceCount`, and is present if and only if the `FLAG_MARKERS` flag is set.
    - `altMethodCount` is the number of additional method signatures the function object should implement, and is present if and only if the `FLAG_BRIDGES` flag is set.
    - `altMethods` is a variable-length list of additional methods signatures to implement, whose length equals `altMethodCount`, and is present if and only if the `FLAG_BRIDGES` flag is set.

    Each class named by `altInterfaces` is subject to the same restrictions as `Rd`, the return type of `factoryType`, as described `above`. Each `MethodType` named by `altMethods` is subject to the same restrictions as `interfaceMethodType`, as described `above`.

    When FLAG_SERIALIZABLE is set in `flags`, the function objects will implement `Serializable`, and will have a `writeReplace` method that returns an appropriate `SerializedLambda`. The `caller` class must have an appropriate `$deserializeLambda$` method, as described in `SerializedLambda`.

    When the target of the `CallSite` returned from this method is invoked, the resulting function objects are instances of a class with the following properties:

    - The class implements the interface named by the return type of `factoryType` and any interfaces named by `altInterfaces`
    - The class declares methods with the name given by `interfaceMethodName`, and the signature given by `interfaceMethodType` and additional signatures given by `altMethods`
    - The class may override methods from `Object`, and may implement methods related to serialization.
    Parameters:
      `caller` - Represents a lookup context with the accessibility privileges of the caller. Specifically, the lookup context must have full privilege access. When used with `invokedynamic`, this is stacked automatically by the VM.
      `interfaceMethodName` - The name of the method to implement. When used with `invokedynamic`, this is provided by the `NameAndType` of the `InvokeDynamic` structure and is stacked automatically by the VM.
      `factoryType` - The expected signature of the `CallSite`. The parameter types represent the types of capture variables; the return type is the interface to implement. When used with `invokedynamic`, this is provided by the `NameAndType` of the `InvokeDynamic` structure and is stacked automatically by the VM.
      `args` - An array of `Object` containing the required arguments `interfaceMethodType`, `implementation`, `dynamicMethodType`, `flags`, and any optional arguments, as described above
    Returns:
      a CallSite whose target can be used to perform capture, generating instances of the interface named by `factoryType`
    Throws:
      `LambdaConversionException` - If `caller` does not have full privilege access, or if `interfaceMethodName` is not a valid JVM method name, or if the return type of `factoryType` is not an interface, or if any of `altInterfaces` is not an interface, or if `implementation` is not a direct method handle referencing a method or constructor, or if the linkage invariants are violated, as defined `above`.
      `NullPointerException` - If any argument, or any component of `args`, is `null`.
      `IllegalArgumentException` - If the number or types of the components of `args` do not follow the above rules, or if `altInterfaceCount` or `altMethodCount` are negative integers.