java.lang.invoke (Java SE 10 & JDK 10 )
Keywords: java.lang.invoke package

Module java.base

# Package java.lang.invoke

The `java.lang.invoke` package contains dynamic language support provided directly by the Java core class libraries and virtual machine.

As described in the Java Virtual Machine Specification, certain types in this package have special relations to dynamic language support in the virtual machine:

- The classes `MethodHandle` `VarHandle` contain signature polymorphic methods which can be linked regardless of their type descriptor. Normally, method linkage requires exact matching of type descriptors.
- The JVM bytecode format supports immediate constants of the classes `MethodHandle` and `MethodType`.

# Summary of relevant Java Virtual Machine changes

The following low-level information summarizes relevant parts of the Java Virtual Machine specification. For full details, please see the current version of that specification. Each occurrence of an `invokedynamic` instruction is called a dynamic call site.

## `invokedynamic` instructions

A dynamic call site is originally in an unlinked state. In this state, there is no target method for the call site to invoke.

Before the JVM can execute a dynamic call site (an `invokedynamic` instruction), the call site must first be linked. Linking is accomplished by calling a bootstrap method which is given the static information content of the call site, and which must produce a `method handle` that gives the behavior of the call site.

Each `invokedynamic` instruction statically specifies its own bootstrap method as a constant pool reference. The constant pool reference also specifies the call site's name and type descriptor, just like `invokevirtual` and the other invoke instructions.

Linking starts with resolving the constant pool entry for the bootstrap method, and resolving a `MethodType` object for the type descriptor of the dynamic call site. This resolution process may trigger class loading. It may therefore throw an error if a class fails to load. This error becomes the abnormal termination of the dynamic call site execution. Linkage does not trigger class initialization.

The bootstrap method is invoked on at least three values:

- a `MethodHandles.Lookup`, a lookup object on the caller class in which dynamic call site occurs
- a `String`, the method name mentioned in the call site
- a `MethodType`, the resolved type descriptor of the call
- optionally, any number of additional static arguments taken from the constant pool

In all cases, bootstrap method invocation is as if by `MethodHandle.invokeWithArguments`, (This is also equivalent to generic invocation if the number of arguments is small enough.)

For an `invokedynamic` instruction, the returned result must be convertible to a non-null reference to a `CallSite`. If the returned result cannot be converted to the expected type, `BootstrapMethodError` is thrown. The type of the call site's target must be exactly equal to the type derived from the dynamic call site's type descriptor and passed to the bootstrap method, otherwise a `BootstrapMethodError` is thrown. On success the call site then becomes permanently linked to the dynamic call site.

If an exception, `E` say, occurs when linking the call site then the linkage fails and terminates abnormally. `E` is rethrown if the type of `E` is `Error` or a subclass, otherwise a `BootstrapMethodError` that wraps `E` is thrown. If this happens, the same `Error` or subclass will the thrown for all subsequent attempts to execute the dynamic call site.

## timing of linkage

A dynamic call site is linked just before its first execution. The bootstrap method call implementing the linkage occurs within a thread that is attempting a first execution.

If there are several such threads, the bootstrap method may be invoked in several threads concurrently. Therefore, bootstrap methods which access global application data must take the usual precautions against race conditions. In any case, every `invokedynamic` instruction is either unlinked or linked to a unique `CallSite` object.

In an application which requires dynamic call sites with individually mutable behaviors, their bootstrap methods should produce distinct `CallSite` objects, one for each linkage request. Alternatively, an application can link a single `CallSite` object to several `invokedynamic` instructions, in which case a change to the target method will become visible at each of the instructions.

If several threads simultaneously execute a bootstrap method for a single dynamic call site, the JVM must choose one `CallSite` object and install it visibly to all threads. Any other bootstrap method calls are allowed to complete, but their results are ignored, and their dynamic call site invocations proceed with the originally chosen target object.

Discussion: These rules do not enable the JVM to duplicate dynamic call sites, or to issue “causeless” bootstrap method calls. Every dynamic call site transitions at most once from unlinked to linked, just before its first invocation. There is no way to undo the effect of a completed bootstrap method call.

## types of bootstrap methods

As long as each bootstrap method can be correctly invoked by `MethodHandle.invoke`, its detailed type is arbitrary. For example, the first argument could be `Object` instead of `MethodHandles.Lookup`, and the return type could also be `Object` instead of `CallSite`. (Note that the types and number of the stacked arguments limit the legal kinds of bootstrap methods to appropriately typed static methods and constructors of `CallSite` subclasses.)

If a given `invokedynamic` instruction specifies no static arguments, the instruction's bootstrap method will be invoked on three arguments, conveying the instruction's caller class, name, and method type. If the `invokedynamic` instruction specifies one or more static arguments, those values will be passed as additional arguments to the method handle. (Note that because there is a limit of 255 arguments to any method, at most 251 extra arguments can be supplied to a non-varargs bootstrap method, since the bootstrap method handle itself and its first three arguments must also be stacked.) The bootstrap method will be invoked as if by `MethodHandle.invokeWithArguments`. A variable-arity bootstrap method can accept thousands of static arguments, subject only by limits imposed by the class-file format.

The normal argument conversion rules for `MethodHandle.invoke` apply to all stacked arguments. For example, if a pushed value is a primitive type, it may be converted to a reference by boxing conversion. If the bootstrap method is a variable arity method (its modifier bit `0x0080` is set), then some or all of the arguments specified here may be collected into a trailing array parameter. (This is not a special rule, but rather a useful consequence of the interaction between `CONSTANT_MethodHandle` constants, the modifier bit for variable arity methods, and the `asVarargsCollector` transformation.)

Given these rules, here are examples of legal bootstrap method declarations, given various numbers `N` of extra arguments. The first row (marked `*`) will work for any number of extra arguments.

Static argument types
N | Sample bootstrap method
--- | ---
* | `CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)` `CallSite bootstrap(Object... args)` `CallSite bootstrap(Object caller, Object... nameAndTypeWithArgs)`
0 | `CallSite bootstrap(Lookup caller, String name, MethodType type)` `CallSite bootstrap(Lookup caller, Object... nameAndType)`
1 | `CallSite bootstrap(Lookup caller, String name, MethodType type, Object arg)`
2 | `CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)` `CallSite bootstrap(Lookup caller, String name, MethodType type, String... args)` `CallSite bootstrap(Lookup caller, String name, MethodType type, String x, int y)`

The last example assumes that the extra arguments are of type `String` and `Integer` (or `int`), respectively. The second-to-last example assumes that all extra arguments are of type `String`. The other examples work with all types of extra arguments.

As noted above, the actual method type of the bootstrap method can vary. For example, the fourth argument could be `MethodHandle`, if that is the type of the corresponding constant in the `CONSTANT_InvokeDynamic` entry. In that case, the `MethodHandle.invoke` call will pass the extra method handle constant as an `Object`, but the type matching machinery of `MethodHandle.invoke` will cast the reference back to `MethodHandle` before invoking the bootstrap method. (If a string constant were passed instead, by badly generated code, that cast would then fail, resulting in a `BootstrapMethodError`.)

Note that, as a consequence of the above rules, the bootstrap method may accept a primitive argument, if it can be represented by a constant pool entry. However, arguments of type `boolean`, `byte`, `short`, or `char` cannot be created for bootstrap methods, since such constants cannot be directly represented in the constant pool, and the invocation of the bootstrap method will not perform the necessary narrowing primitive conversions.

Extra bootstrap method arguments are intended to allow language implementors to safely and compactly encode metadata. In principle, the name and extra arguments are redundant, since each call site could be given its own unique bootstrap method. Such a practice would be likely to produce large class files and constant pools.

Since:
  1.7

- Interface Summary
  Interface | Description
  --- | ---
  MethodHandleInfo | A symbolic reference obtained by cracking a direct method handle into its consitutent symbolic parts.

- Class Summary
  Class | Description
  --- | ---
  CallSite | A `CallSite` is a holder for a variable `MethodHandle`, which is called its `target`.
  ConstantCallSite | A `ConstantCallSite` is a `CallSite` whose target is permanent, and can never be changed.
  LambdaMetafactory | 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.
  MethodHandle | A method handle is a typed, directly executable reference to an underlying method, constructor, field, or similar low-level operation, with optional transformations of arguments or return values.
  MethodHandleProxies | This class consists exclusively of static methods that help adapt method handles to other JVM types, such as interfaces.
  MethodHandles | This class consists exclusively of static methods that operate on or return method handles.
  MethodHandles.Lookup | A lookup object is a factory for creating method handles, when the creation requires access checking.
  MethodType | 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.
  MutableCallSite | A `MutableCallSite` is a `CallSite` whose target variable behaves like an ordinary field.
  SerializedLambda | Serialized form of a lambda expression.
  StringConcatFactory | Methods to facilitate the creation of String concatenation methods, that can be used to efficiently concatenate a known number of arguments of known types, possibly after type adaptation and partial evaluation of arguments.
  SwitchPoint | A `SwitchPoint` is an object which can publish state transitions to other threads.
  VarHandle | A VarHandle is a dynamically strongly typed reference to a variable, or to a parametrically-defined family of variables, including static fields, non-static fields, array elements, or components of an off-heap data structure.
  VolatileCallSite | A `VolatileCallSite` is a `CallSite` whose target acts like a volatile variable.

- Enum Summary
  Enum | Description
  --- | ---
  VarHandle.AccessMode | The set of access modes that specify how a variable, referenced by a VarHandle, is accessed.

- Exception Summary
  Exception | Description
  --- | ---
  LambdaConversionException | LambdaConversionException
  StringConcatException | StringConcatException is thrown by `StringConcatFactory` when linkage invariants are violated.
  WrongMethodTypeException | Thrown to indicate that code has attempted to call a method handle via the wrong method type.