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package java.lang.invoke;

/**

 * <p>Bootstrap methods for converting lambda expressions and method references to functional interface objects.</p>

 *

 * <p>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 mechanism for

 * evaluating lambda expressions is to invoke an invokedynamic call site, which takes arguments describing the sole

 * method of the functional interface and the implementation method, and returns an object (the lambda object) that

 * implements the target type. Methods of the lambda object invoke the implementation method. For method

 * references, the implementation method is simply the referenced method; for lambda expressions, the

 * implementation method is produced by the compiler based on the body of the lambda expression. The methods in

 * this file are the bootstrap methods for those invokedynamic call sites, called lambda factories, and the

 * bootstrap methods responsible for linking the lambda factories are called lambda meta-factories.

 *

 * <p>The bootstrap methods in this class take the information about the functional interface, the implementation

 * method, and the static types of the captured lambda arguments, and link a call site which, when invoked,

 * produces the lambda object.

 *

 * <p>When parameterized types are used, the instantiated type of the functional interface method may be different

 * from that in the functional interface. For example, consider

 * <I>instantiated method type</I>), which has signature

 *

 * <p>The argument list of the implementation method and the argument list of the functional 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. They may also differ because of var-args,

 * but this is expected to be handled by the compiler.

 *

 * <p>Invokedynamic call sites have two argument lists: a static argument list and a dynamic argument list.  The

 * static argument list lives in the constant pool; the dynamic argument list lives on the operand stack at

 * invocation time.  The bootstrap method has access to the entire static argument list (which in this case,

 * contains method handles describing the implementation method and the canonical functional interface method),

 * 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.

 *

 * <p>The implementation method is described with a method handle. In theory, any method handle could be used.

 * Currently supported are method handles representing invocation of virtual, interface, constructor and static

 * methods.

 *

 * <p>Assume:

 * <ul>

 *      <li>the functional interface method has N arguments, of types (U1, U2, ... Un) and return type Ru</li>

 *      <li>then the instantiated method type also has N arguments, of types (T1, T2, ... Tn) and return type Rt</li>

 *      <li>the implementation method has M arguments, of types (A1..Am) and return type Ra,</li>

 *      <li>the dynamic argument list has K arguments of types (D1..Dk), and the invokedynamic return site has

 *          type Rd</li>

 *      <li>the functional interface type is F</li>

 * </ul>

 *

 * <p>The following signature invariants must hold:

 * <ul>

 *     <li>Rd is a subtype of F</li>

 *     <li>For i=1..N, Ti is a subtype of Ui</li>

 *     <li>Either Rt and Ru are primitive and are the same type, or both are reference types and

 *         Rt is a subtype of Ru</li>

 *     <li>If the implementation method is a static method:

 *     <ul>

 *         <li>K + N = M</li>

 *         <li>For i=1..K, Di = Ai</li>

 *         <li>For i=1..N, Ti is adaptable to Aj, where j=i+k</li>

 *     </ul></li>

 *     <li>If the implementation method is an instance method:

 *     <ul>

 *         <li>K + N = M + 1</li>

 *         <li>D1 must be a subtype of the enclosing class for the implementation method</li>

 *         <li>For i=2..K, Di = Aj, where j=i-1</li>

 *         <li>For i=1..N, Ti is adaptable to Aj, where j=i+k-1</li>

 *     </ul></li>

 *     <li>The return type Rt is void, or the return type Ra is not void and is adaptable to Rt</li>

 * </ul>

 *

 * <p>Note that the potentially parameterized implementation return type provides the value for the SAM. Whereas

 * the completely known instantiated return type is adapted to the implementation arguments. Because the

 * instantiated type of the implementation method is not available, the adaptability of return types cannot be

 * checked as precisely at link-time as the arguments can be checked. Thus a loose version of link-time checking is

 * done on return type, while a strict version is applied to arguments.

 *

 * <p>A type Q is considered adaptable to S as follows:

 * <table>

 *     <tr><th>Q</th><th>S</th><th>Link-time checks</th><th>Capture-time checks</th></tr>

 *     <tr>

 *         <td>Primitive</td><td>Primitive</td>

 *         <td>Q can be converted to S via a primitive widening conversion</td>

 *         <td>None</td>

 *     </tr>

 *     <tr>

 *         <td>Primitive</td><td>Reference</td>

 *         <td>S is a supertype of the Wrapper(Q)</td>

 *         <td>Cast from Wrapper(Q) to S</td>

 *     </tr>

 *     <tr>

 *         <td>Reference</td><td>Primitive</td>

 *         <td>strict: Q is a primitive wrapper and Primitive(Q) can be widened to S

 *         <br>loose: If Q is a primitive wrapper, check that Primitive(Q) can be widened to S</td>

 *         <td>If Q is not a primitive wrapper, cast Q to the base Wrapper(S); for example Number for numeric types</td>

 *     </tr>

 *     <tr>

 *         <td>Reference</td><td>Reference</td>

 *         <td>strict: S is a supertype of Q

 *         <br>loose: none</td>

 *         <td>Cast from Q to S</td>

 *     </tr>

 * </table>

 *

 * The default bootstrap ({@link #metaFactory}) represents the common cases and uses an optimized protocol.

 * Alternate bootstraps (e.g., {@link #altMetaFactory}) exist to support uncommon cases such as serialization

 * or additional marker superinterfaces.

 *

 */

public class LambdaMetafactory {

    public static final int FLAG_SERIALIZABLE = 1 << 0;

    /**

     * besides Serializable

     */

    public static final int FLAG_MARKERS = 1 << 1;

    private static final Class<?>[] EMPTY_CLASS_ARRAY = new Class<?>[0];

     *

     *                    Currently unused.

     * @throws ReflectiveOperationException

     */

    public static CallSite metaFactory(MethodHandles.Lookup caller,

                                       String invokedName,

                                       MethodType invokedType,

                                       MethodHandle samMethod,

                                       MethodHandle implMethod,

                                       MethodType instantiatedMethodType)

                   throws ReflectiveOperationException, LambdaConversionException {

        AbstractValidatingLambdaMetafactory mf;

        mf.validateMetafactoryArgs();

        return mf.buildCallSite();

    }

    /**

     *

     * The declared argument list for this method is:

     *

     *  CallSite altMetaFactory(MethodHandles.Lookup caller,

     *                          String invokedName,

     *                          MethodType invokedType,

     *                          Object... args)

     *

     * but it behaves as if the argument list is:

     *

     *  CallSite altMetaFactory(MethodHandles.Lookup caller,

     *                          String invokedName,

     *                          MethodType invokedType,

     *                          MethodHandle samMethod

     *                          MethodHandle implMethod,

     *                          MethodType instantiatedMethodType,

     *                          int flags,

     *                          int markerInterfaceCount, // IF flags has MARKERS set

     *                          Class... markerInterfaces // IF flags has MARKERS set

     *                          )

     *

     *

     * @throws ReflectiveOperationException

     */

    public static CallSite altMetaFactory(MethodHandles.Lookup caller,

                                          String invokedName,

                                          MethodType invokedType,

                                          Object... args)

            throws ReflectiveOperationException, LambdaConversionException {

        MethodHandle samMethod = (MethodHandle)args[0];

        MethodHandle implMethod = (MethodHandle)args[1];

        MethodType instantiatedMethodType = (MethodType)args[2];

        int flags = (Integer) args[3];

        Class<?>[] markerInterfaces;

        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;

        mf.validateMetafactoryArgs();

        return mf.buildCallSite();

    }

}