Methods to facilitate the creation of simple "function objects" that + * implement one or more interfaces by delegation to a provided {@link MethodHandle}, + * possibly after type adaptation and partial evaluation of arguments. These + * methods are typically used as bootstrap methods for {@code 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 {@code MethodHandle} + * proceeds in order through three phases: + *
It is sometimes useful to restrict the set of inputs or results permitted
+ * at invocation. For example, when the generic interface {@code Predicate This class provides two forms of linkage methods: a standard version
+ * ({@link #metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)})
+ * using an optimized protocol, and an alternate version
+ * {@link #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:
+ *
+ * Assume the linkage arguments are as follows:
+ * Then the following linkage invariants must hold:
+ * Further, at capture time, if {@code implMethod} corresponds to an instance
+ * method, and there are any capture arguments ({@code 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:
+ * 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.
+ *
+ * @implNote The implementation method is described with a method handle. In
+ * theory, any method handle could be used. Currently supported are direct method
+ * handles representing invocation of virtual, interface, constructor and static
+ * methods.
+ */
+public class LambdaMetafactory {
+
+ /** Flag for alternate metafactories indicating the lambda object
+ * must be serializable */
+ public static final int FLAG_SERIALIZABLE = 1 << 0;
+
+ /**
+ * Flag for alternate metafactories indicating the lambda object implements
+ * other marker interfaces
+ * besides Serializable
+ */
+ public static final int FLAG_MARKERS = 1 << 1;
+
+ /**
+ * Flag for alternate metafactories indicating the lambda object requires
+ * additional bridge methods
+ */
+ public static final int FLAG_BRIDGES = 1 << 2;
+
+ private static final Class[] EMPTY_CLASS_ARRAY = new Class[0];
+ private static final MethodType[] EMPTY_MT_ARRAY = new MethodType[0];
+
+ /**
+ * Facilitates the creation of simple "function objects" that implement one
+ * or more interfaces by delegation to a provided {@link MethodHandle},
+ * after appropriate type adaptation and partial evaluation of arguments.
+ * Typically used as a bootstrap method for {@code 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 {@link #altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)}.
+ * A general description of the behavior of this method is provided
+ * {@link LambdaMetafactory above}.
+ *
+ * When the target of the {@code 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 {@code invokedType},
+ * declares a method with the name given by {@code invokedName} and the
+ * signature given by {@code samMethodType}. It may also override additional
+ * methods from {@code Object}.
+ *
+ * @param caller Represents a lookup context with the accessibility
+ * privileges of the caller. When used with {@code invokedynamic},
+ * this is stacked automatically by the VM.
+ * @param invokedName The name of the method to implement. When used with
+ * {@code invokedynamic}, this is provided by the
+ * {@code NameAndType} of the {@code InvokeDynamic}
+ * structure and is stacked automatically by the VM.
+ * @param invokedType The expected signature of the {@code CallSite}. The
+ * parameter types represent the types of capture variables;
+ * the return type is the interface to implement. When
+ * used with {@code invokedynamic}, this is provided by
+ * the {@code NameAndType} of the {@code InvokeDynamic}
+ * structure and is stacked automatically by the VM.
+ * In the event that the implementation method is an
+ * instance method and this signature has any parameters,
+ * the first parameter in the invocation signature must
+ * correspond to the receiver.
+ * @param samMethodType Signature and return type of method to be implemented
+ * by the function object.
+ * @param implMethod A direct method handle describing the implementation
+ * method which should be called (with suitable adaptation
+ * of argument types, return types, and with captured
+ * arguments prepended to the invocation arguments) at
+ * invocation time.
+ * @param instantiatedMethodType The signature and return type that should
+ * be enforced dynamically at invocation time.
+ * This may be the same as {@code samMethodType},
+ * or may be a specialization of it.
+ * @return a CallSite whose target can be used to perform capture, generating
+ * instances of the interface named by {@code invokedType}
+ * @throws LambdaConversionException If any of the linkage invariants
+ * described {@link LambdaMetafactory above}
+ * are violated
+ */
+ public static CallSite metafactory(MethodHandles.Lookup caller,
+ String invokedName,
+ MethodType invokedType,
+ MethodType samMethodType,
+ MethodHandle implMethod,
+ MethodType instantiatedMethodType)
+ throws LambdaConversionException {
+ AbstractValidatingLambdaMetafactory mf;
+ mf = new InnerClassLambdaMetafactory(caller, invokedType,
+ invokedName, samMethodType,
+ implMethod, instantiatedMethodType,
+ false, EMPTY_CLASS_ARRAY, EMPTY_MT_ARRAY);
+ mf.validateMetafactoryArgs();
+ return mf.buildCallSite();
+ }
+
+ /**
+ * Facilitates the creation of simple "function objects" that implement one
+ * or more interfaces by delegation to a provided {@link MethodHandle},
+ * after appropriate type adaptation and partial evaluation of arguments.
+ * Typically used as a bootstrap method for {@code 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 {@link #altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)}.
+ * A general description of the behavior of this method is provided
+ * {@link LambdaMetafactory 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 {@code invokedynamic} invocation, and an {@code Object[]}
+ * parameter that contains additional parameters. The declared argument
+ * list for this method is:
+ *
+ * but it behaves as if the argument list is as follows:
+ *
+ * Arguments that appear in the argument list for
+ * {@link #metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)}
+ * have the same specification as in that method. The additional arguments
+ * are interpreted as follows:
+ * Each class named by {@code markerInterfaces} is subject to the same
+ * restrictions as {@code Rd}, the return type of {@code invokedType},
+ * as described {@link LambdaMetafactory above}. Each {@code MethodType}
+ * named by {@code bridges} is subject to the same restrictions as
+ * {@code samMethodType}, as described {@link LambdaMetafactory above}.
+ *
+ * When FLAG_SERIALIZABLE is set in {@code flags}, the function objects
+ * will implement {@code Serializable}, and will have a {@code writeReplace}
+ * method that returns an appropriate {@link SerializedLambda}. The
+ * {@code caller} class must have an appropriate {@code $deserializeLambda$}
+ * method, as described in {@link SerializedLambda}.
+ *
+ * When the target of the {@code CallSite} returned from this method is
+ * invoked, the resulting function objects are instances of a class with
+ * the following properties:
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ * @apiNote 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.)
+ *
+ * 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 SIf 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: noneCast from Q to S
+ * {@code
+ * CallSite altMetafactory(MethodHandles.Lookup caller,
+ * String invokedName,
+ * MethodType invokedType,
+ * Object... args)
+ * }
+ *
+ * {@code
+ * CallSite altMetafactory(MethodHandles.Lookup caller,
+ * String invokedName,
+ * MethodType invokedType,
+ * MethodType samMethodType,
+ * MethodHandle implMethod,
+ * MethodType instantiatedMethodType,
+ * int flags,
+ * int markerInterfaceCount, // IF flags has MARKERS set
+ * Class... markerInterfaces, // IF flags has MARKERS set
+ * int bridgeCount, // IF flags has BRIDGES set
+ * MethodType... bridges // IF flags has BRIDGES set
+ * )
+ * }
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ * @param caller Represents a lookup context with the accessibility
+ * privileges of the caller. When used with {@code invokedynamic},
+ * this is stacked automatically by the VM.
+ * @param invokedName The name of the method to implement. When used with
+ * {@code invokedynamic}, this is provided by the
+ * {@code NameAndType} of the {@code InvokeDynamic}
+ * structure and is stacked automatically by the VM.
+ * @param invokedType The expected signature of the {@code CallSite}. The
+ * parameter types represent the types of capture variables;
+ * the return type is the interface to implement. When
+ * used with {@code invokedynamic}, this is provided by
+ * the {@code NameAndType} of the {@code InvokeDynamic}
+ * structure and is stacked automatically by the VM.
+ * In the event that the implementation method is an
+ * instance method and this signature has any parameters,
+ * the first parameter in the invocation signature must
+ * correspond to the receiver.
+ * @param args An {@code Object[]} array containing the required
+ * arguments {@code samMethodType}, {@code implMethod},
+ * {@code instantiatedMethodType}, {@code flags}, and any
+ * optional arguments, as described
+ * {@link #altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)} above}
+ * @return a CallSite whose target can be used to perform capture, generating
+ * instances of the interface named by {@code invokedType}
+ * @throws LambdaConversionException If any of the linkage invariants
+ * described {@link LambdaMetafactory above}
+ * are violated
+ */
+ public static CallSite altMetafactory(MethodHandles.Lookup caller,
+ String invokedName,
+ MethodType invokedType,
+ Object... args)
+ throws LambdaConversionException {
+ MethodType samMethodType = (MethodType)args[0];
+ MethodHandle implMethod = (MethodHandle)args[1];
+ MethodType instantiatedMethodType = (MethodType)args[2];
+ int flags = (Integer) args[3];
+ Class[] markerInterfaces;
+ MethodType[] bridges;
+ int argIndex = 4;
+ if ((flags & FLAG_MARKERS) != 0) {
+ int markerCount = (Integer) args[argIndex++];
+ markerInterfaces = new Class[markerCount];
+ System.arraycopy(args, argIndex, markerInterfaces, 0, markerCount);
+ argIndex += markerCount;
+ }
+ else
+ markerInterfaces = EMPTY_CLASS_ARRAY;
+ if ((flags & FLAG_BRIDGES) != 0) {
+ int bridgeCount = (Integer) args[argIndex++];
+ bridges = new MethodType[bridgeCount];
+ System.arraycopy(args, argIndex, bridges, 0, bridgeCount);
+ argIndex += bridgeCount;
+ }
+ else
+ bridges = EMPTY_MT_ARRAY;
+
+ boolean isSerializable = ((flags & FLAG_SERIALIZABLE) != 0);
+ if (isSerializable) {
+ boolean foundSerializableSupertype = Serializable.class.isAssignableFrom(invokedType.returnType());
+ for (Class c : markerInterfaces)
+ foundSerializableSupertype |= Serializable.class.isAssignableFrom(c);
+ if (!foundSerializableSupertype) {
+ markerInterfaces = Arrays.copyOf(markerInterfaces, markerInterfaces.length + 1);
+ markerInterfaces[markerInterfaces.length-1] = Serializable.class;
+ }
+ }
+
+ AbstractValidatingLambdaMetafactory mf
+ = new InnerClassLambdaMetafactory(caller, invokedType,
+ invokedName, samMethodType,
+ implMethod,
+ instantiatedMethodType,
+ isSerializable,
+ markerInterfaces, bridges);
+ mf.validateMetafactoryArgs();
+ return mf.buildCallSite();
+ }
+}