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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
 * <code>interface I&lt;T&gt; { int m(T x); }</code> if this functional interface type is used in a lambda
 * <code>I&lt;Byte&gt; v = ...</code>, we need both the actual functional interface method which has the signature
 * <code>(Object)int</code> and the erased instantiated type of the functional interface method (or simply
 * <I>instantiated method type</I>), which has signature
 * <code>(Byte)int</code>.
 *
 * <p>While functional interfaces only have a single abstract method from the language perspective (concrete
 * methods in Object are and default methods may be present), at the bytecode level they may actually have multiple
 * methods because of the need for bridge methods. Invoking any of these methods on the lambda object will result
 * in invoking the implementation method.
 *
 * <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 summary="adaptable types">
 *     <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 {

    /** Flag for alternate metafactories indicating the lambda object is must to 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;

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

    /**
     * Standard meta-factory for conversion of lambda expressions or method references to functional interfaces.
     *
     * @param caller Stacked automatically by VM; represents a lookup context with the accessibility privileges
     *               of the caller.
     * @param invokedName Stacked automatically by VM; the name of the invoked method as it appears at the call site.
     *                    Currently unused.
     * @param invokedType Stacked automatically by VM; the signature of the invoked method, which includes the
     *                    expected static type of the returned lambda object, and the static types of the captured
     *                    arguments for the lambda.  In the event that the implementation method is an instance method,
     *                    the first argument in the invocation signature will correspond to the receiver.
     * @param samMethod The primary method in the functional interface to which the lambda or method reference is
     *                  being converted, represented as a method handle.
     * @param implMethod The implementation method which should be called (with suitable adaptation of argument
     *                   types, return types, and adjustment for captured arguments) when methods of the resulting
     *                   functional interface instance are invoked.
     * @param instantiatedMethodType The signature of the primary functional interface method after type variables
     *                               are substituted with their instantiation from the capture site
     * @return a CallSite, which, when invoked, will return an instance of the functional interface
     * @throws ReflectiveOperationException if the caller is not able to reconstruct one of the method handles
     * @throws LambdaConversionException If any of the meta-factory protocol invariants are violated
     */
    public static CallSite metaFactory(MethodHandles.Lookup caller,
                                       String invokedName,
                                       MethodType invokedType,
                                       MethodHandle samMethod,
                                       MethodHandle implMethod,
                                       MethodType instantiatedMethodType)
                   throws ReflectiveOperationException, LambdaConversionException {
        AbstractValidatingLambdaMetafactory mf;
        mf = new InnerClassLambdaMetafactory(caller, invokedType, samMethod, implMethod, instantiatedMethodType,
                0, EMPTY_CLASS_ARRAY);
        mf.validateMetafactoryArgs();
        return mf.buildCallSite();
    }

    /**
     * Alternate meta-factory for conversion of lambda expressions or method references to functional interfaces,
     * which supports serialization and other uncommon options.
     *
     * 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
     *                          )
     *
     *
     * @param caller Stacked automatically by VM; represents a lookup context with the accessibility privileges
     *               of the caller.
     * @param invokedName Stacked automatically by VM; the name of the invoked method as it appears at the call site.
     *                    Currently unused.
     * @param invokedType Stacked automatically by VM; the signature of the invoked method, which includes thefu
     *                    expected static type of the returned lambda object, and the static types of the captured
     *                    arguments for the lambda.  In the event that the implementation method is an instance method,
     *                    the first argument in the invocation signature will correspond to the receiver.
     * @param  args       argument to pass, flags, marker interface count, and marker interfaces as described above
     * @return a CallSite, which, when invoked, will return an instance of the functional interface
     * @throws ReflectiveOperationException if the caller is not able to reconstruct one of the method handles
     * @throws LambdaConversionException If any of the meta-factory protocol invariants are violated
     */
    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;
        AbstractValidatingLambdaMetafactory mf;
        mf = new InnerClassLambdaMetafactory(caller, invokedType, samMethod, implMethod, instantiatedMethodType,
                                             flags, markerInterfaces);
        mf.validateMetafactoryArgs();
        return mf.buildCallSite();
    }
}