/*

 * Copyright (c) 2008, 2013, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package java.lang.invoke;

import java.lang.ref.WeakReference;

import java.lang.ref.Reference;

import java.lang.ref.ReferenceQueue;

import java.util.Arrays;

import java.util.Collections;

import java.util.List;

import java.util.Objects;

import java.util.concurrent.ConcurrentMap;

import java.util.concurrent.ConcurrentHashMap;

/**

 * 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.  Method types must be properly

 * matched between a method handle and all its callers,

 * and the JVM's operations enforce this matching at, specifically

 * during calls to {@link MethodHandle#invokeExact MethodHandle.invokeExact}

 * and {@link MethodHandle#invoke MethodHandle.invoke}, and during execution

 * of {@code invokedynamic} instructions.

 * <p>

 * The structure is a return type accompanied by any number of parameter types.

 * The types (primitive, {@code void}, and reference) are represented by {@link Class} objects.

 * (For ease of exposition, we treat {@code void} as if it were a type.

 * In fact, it denotes the absence of a return type.)

 * <p>

 * All instances of {@code MethodType} are immutable.

 * Two instances are completely interchangeable if they compare equal.

 * Equality depends on pairwise correspondence of the return and parameter types and on nothing else.

 * <p>

 * This type can be created only by factory methods.

 * All factory methods may cache values, though caching is not guaranteed.

 * Some factory methods are static, while others are virtual methods which

 * modify precursor method types, e.g., by changing a selected parameter.

 * <p>

 * Factory methods which operate on groups of parameter types

 * are systematically presented in two versions, so that both Java arrays and

 * Java lists can be used to work with groups of parameter types.

 * The query methods {@code parameterArray} and {@code parameterList}

 * also provide a choice between arrays and lists.

 * <p>

 * {@code MethodType} objects are sometimes derived from bytecode instructions

 * such as {@code invokedynamic}, specifically from the type descriptor strings associated

 * with the instructions in a class file's constant pool.

 * <p>

 * Like classes and strings, method types can also be represented directly

 * in a class file's constant pool as constants.

 * A method type may be loaded by an {@code ldc} instruction which refers

 * to a suitable {@code CONSTANT_MethodType} constant pool entry.

 * The entry refers to a {@code CONSTANT_Utf8} spelling for the descriptor string.

 * (For full details on method type constants,

 * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)

 * <p>

 * When the JVM materializes a {@code MethodType} from a descriptor string,

 * all classes named in the descriptor must be accessible, and will be loaded.

 * (But the classes need not be initialized, as is the case with a {@code CONSTANT_Class}.)

 * This loading may occur at any time before the {@code MethodType} object is first derived.

 * @author John Rose, JSR 292 EG

 */

public final

class MethodType implements java.io.Serializable {

    private static final long serialVersionUID = 292L;  // {rtype, {ptype...}}

    // The rtype and ptypes fields define the structural identity of the method type:

    private final Class<?>   rtype;

    private final Class<?>[] ptypes;

    /**

     * Check the given parameters for validity and store them into the final fields.

     */

    private MethodType(Class<?> rtype, Class<?>[] ptypes, boolean trusted) {

        this.rtype = rtype;

        // defensively copy the array passed in by the user

        this.ptypes = trusted ? ptypes : Arrays.copyOf(ptypes, ptypes.length);

    }

    /**

     * Construct a temporary unchecked instance of MethodType for use only as a key to the intern table.

     * Does not check the given parameters for validity, and must be discarded after it is used as a searching key.

     * The parameters are reversed for this constructor, so that is is not accidentally used.

     */

    private MethodType(Class<?>[] ptypes, Class<?> rtype) {

        this.rtype = rtype;

        this.ptypes = ptypes;

    }

    /** This number, mandated by the JVM spec as 255,

     *  is the maximum number of <em>slots</em>

     *  that any Java method can receive in its argument list.

     *  It limits both JVM signatures and method type objects.

     *  The longest possible invocation will look like

     *  {@code staticMethod(arg1, arg2, ..., arg255)} or

     *  {@code x.virtualMethod(arg1, arg2, ..., arg254)}.

     */

    /*non-public*/ static final int MAX_JVM_ARITY = 255;  // this is mandated by the JVM spec.

    /** This number is the maximum arity of a method handle, 254.

     *  It is derived from the absolute JVM-imposed arity by subtracting one,

     *  which is the slot occupied by the method handle itself at the

     *  beginning of the argument list used to invoke the method handle.

     *  The longest possible invocation will look like

     *  {@code mh.invoke(arg1, arg2, ..., arg254)}.

     */

    // Issue:  Should we allow MH.invokeWithArguments to go to the full 255?

    /*non-public*/ static final int MAX_MH_ARITY = MAX_JVM_ARITY-1;  // deduct one for mh receiver

    /** This number is the maximum arity of a method handle invoker, 253.

     *  It is derived from the absolute JVM-imposed arity by subtracting two,

     *  which are the slots occupied by invoke method handle, and the

     *  target method handle, which are both at the beginning of the argument

     *  list used to invoke the target method handle.

     *  The longest possible invocation will look like

     *  {@code invokermh.invoke(targetmh, arg1, arg2, ..., arg253)}.

     */

    /*non-public*/ static final int MAX_MH_INVOKER_ARITY = MAX_MH_ARITY-1;  // deduct one more for invoker

    private static void checkRtype(Class<?> rtype) {

        Objects.requireNonNull(rtype);

    }

    static final ConcurrentWeakInternSet<MethodType> internTable = new ConcurrentWeakInternSet<>();

    static final Class<?>[] NO_PTYPES = {};

    /**

     * Finds or creates an instance of the given method type.

     * @param rtype  the return type

     * @param ptypes the parameter types

     * @return a method type with the given components

     * @throws NullPointerException if {@code rtype} or {@code ptypes} or any element of {@code ptypes} is null

     * @throws IllegalArgumentException if any element of {@code ptypes} is {@code void.class}

     */

    public static

    MethodType methodType(Class<?> rtype, Class<?>[] ptypes) {

        return makeImpl(rtype, ptypes, false);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param rtype  the return type

     * @param ptypes the parameter types

     * @return a method type with the given components

     * @throws NullPointerException if {@code rtype} or {@code ptypes} or any element of {@code ptypes} is null

     * @throws IllegalArgumentException if any element of {@code ptypes} is {@code void.class}

     */

    public static

    MethodType methodType(Class<?> rtype, List<Class<?>> ptypes) {

        boolean notrust = false;  // random List impl. could return evil ptypes array

        return makeImpl(rtype, listToArray(ptypes), notrust);

    }

    private static Class<?>[] listToArray(List<Class<?>> ptypes) {

        return ptypes.toArray(NO_PTYPES);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * The leading parameter type is prepended to the remaining array.

     * @param rtype  the return type

     * @param ptype0 the first parameter type

     * @param ptypes the remaining parameter types

     * @return a method type with the given components

     * @throws NullPointerException if {@code rtype} or {@code ptype0} or {@code ptypes} or any element of {@code ptypes} is null

     * @throws IllegalArgumentException if {@code ptype0} or {@code ptypes} or any element of {@code ptypes} is {@code void.class}

     */

    public static

    MethodType methodType(Class<?> rtype, Class<?> ptype0, Class<?>... ptypes) {

        Class<?>[] ptypes1 = new Class<?>[1+ptypes.length];

        ptypes1[0] = ptype0;

        System.arraycopy(ptypes, 0, ptypes1, 1, ptypes.length);

        return makeImpl(rtype, ptypes1, true);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * The resulting method has no parameter types.

     * @param rtype  the return type

     * @return a method type with the given return value

     * @throws NullPointerException if {@code rtype} is null

     */

    public static

    MethodType methodType(Class<?> rtype) {

        return makeImpl(rtype, NO_PTYPES, true);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * The resulting method has the single given parameter type.

     * @param rtype  the return type

     * @param ptype0 the parameter type

     * @return a method type with the given return value and parameter type

     * @throws NullPointerException if {@code rtype} or {@code ptype0} is null

     * @throws IllegalArgumentException if {@code ptype0} is {@code void.class}

     */

    public static

    MethodType methodType(Class<?> rtype, Class<?> ptype0) {

        return makeImpl(rtype, new Class<?>[]{ ptype0 }, true);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * The resulting method has the same parameter types as {@code ptypes},

     * and the specified return type.

     * @param rtype  the return type

     * @param ptypes the method type which supplies the parameter types

     * @return a method type with the given components

     * @throws NullPointerException if {@code rtype} or {@code ptypes} is null

     */

    public static

    MethodType methodType(Class<?> rtype, MethodType ptypes) {

        return makeImpl(rtype, ptypes.ptypes, true);

    }

    /**

     * Sole factory method to find or create an interned method type.

     * @param rtype desired return type

     * @param ptypes desired parameter types

     * @param trusted whether the ptypes can be used without cloning

     * @return the unique method type of the desired structure

     */

    /*trusted*/ static

    MethodType makeImpl(Class<?> rtype, Class<?>[] ptypes, boolean trusted) {

        throw new IllegalStateException();

    }

    private static final MethodType[] objectOnlyTypes = new MethodType[20];

    /**

     * Finds or creates a method type whose components are {@code Object} with an optional trailing {@code Object[]} array.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All parameters and the return type will be {@code Object},

     * except the final array parameter if any, which will be {@code Object[]}.

     * @param objectArgCount number of parameters (excluding the final array parameter if any)

     * @param finalArray whether there will be a trailing array parameter, of type {@code Object[]}

     * @return a generally applicable method type, for all calls of the given fixed argument count and a collected array of further arguments

     * @throws IllegalArgumentException if {@code objectArgCount} is negative or greater than 255 (or 254, if {@code finalArray} is true)

     * @see #genericMethodType(int)

     */

    public static

    MethodType genericMethodType(int objectArgCount, boolean finalArray) {

        throw new IllegalStateException();

    }

    /**

     * Finds or creates a method type whose components are all {@code Object}.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All parameters and the return type will be Object.

     * @param objectArgCount number of parameters

     * @return a generally applicable method type, for all calls of the given argument count

     * @throws IllegalArgumentException if {@code objectArgCount} is negative or greater than 255

     * @see #genericMethodType(int, boolean)

     */

    public static

    MethodType genericMethodType(int objectArgCount) {

        return genericMethodType(objectArgCount, false);

    }

    /**

     * Finds or creates a method type with a single different parameter type.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param num    the index (zero-based) of the parameter type to change

     * @param nptype a new parameter type to replace the old one with

     * @return the same type, except with the selected parameter changed

     * @throws IndexOutOfBoundsException if {@code num} is not a valid index into {@code parameterArray()}

     * @throws IllegalArgumentException if {@code nptype} is {@code void.class}

     * @throws NullPointerException if {@code nptype} is null

     */

    public MethodType changeParameterType(int num, Class<?> nptype) {

        throw new IllegalStateException();

    }

    /**

     * Finds or creates a method type with additional parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param num    the position (zero-based) of the inserted parameter type(s)

     * @param ptypesToInsert zero or more new parameter types to insert into the parameter list

     * @return the same type, except with the selected parameter(s) inserted

     * @throws IndexOutOfBoundsException if {@code num} is negative or greater than {@code parameterCount()}

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    public MethodType insertParameterTypes(int num, Class<?>... ptypesToInsert) {

        throw new IllegalStateException();

    }

    /**

     * Finds or creates a method type with additional parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param ptypesToInsert zero or more new parameter types to insert after the end of the parameter list

     * @return the same type, except with the selected parameter(s) appended

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    public MethodType appendParameterTypes(Class<?>... ptypesToInsert) {

        return insertParameterTypes(parameterCount(), ptypesToInsert);

    }

    /**

     * Finds or creates a method type with additional parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param num    the position (zero-based) of the inserted parameter type(s)

     * @param ptypesToInsert zero or more new parameter types to insert into the parameter list

     * @return the same type, except with the selected parameter(s) inserted

     * @throws IndexOutOfBoundsException if {@code num} is negative or greater than {@code parameterCount()}

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    public MethodType insertParameterTypes(int num, List<Class<?>> ptypesToInsert) {

        return insertParameterTypes(num, listToArray(ptypesToInsert));

    }

    /**

     * Finds or creates a method type with additional parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param ptypesToInsert zero or more new parameter types to insert after the end of the parameter list

     * @return the same type, except with the selected parameter(s) appended

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    public MethodType appendParameterTypes(List<Class<?>> ptypesToInsert) {

        return insertParameterTypes(parameterCount(), ptypesToInsert);

    }

     /**

     * Finds or creates a method type with modified parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param start  the position (zero-based) of the first replaced parameter type(s)

     * @param end    the position (zero-based) after the last replaced parameter type(s)

     * @param ptypesToInsert zero or more new parameter types to insert into the parameter list

     * @return the same type, except with the selected parameter(s) replaced

     * @throws IndexOutOfBoundsException if {@code start} is negative or greater than {@code parameterCount()}

     *                                  or if {@code end} is negative or greater than {@code parameterCount()}

     *                                  or if {@code start} is greater than {@code end}

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    /*non-public*/ MethodType replaceParameterTypes(int start, int end, Class<?>... ptypesToInsert) {

        throw new IllegalStateException();

    }

    /**

     * Finds or creates a method type with some parameter types omitted.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param start  the index (zero-based) of the first parameter type to remove

     * @param end    the index (greater than {@code start}) of the first parameter type after not to remove

     * @return the same type, except with the selected parameter(s) removed

     * @throws IndexOutOfBoundsException if {@code start} is negative or greater than {@code parameterCount()}

     *                                  or if {@code end} is negative or greater than {@code parameterCount()}

     *                                  or if {@code start} is greater than {@code end}

     */

    public MethodType dropParameterTypes(int start, int end) {

        throw new IllegalStateException();

    }

    /**

     * Finds or creates a method type with a different return type.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param nrtype a return parameter type to replace the old one with

     * @return the same type, except with the return type change

     * @throws NullPointerException if {@code nrtype} is null

     */

    public MethodType changeReturnType(Class<?> nrtype) {

        throw new IllegalStateException();

    }

    /**

     * Reports if this type contains a primitive argument or return value.

     * The return type {@code void} counts as a primitive.

     * @return true if any of the types are primitives

     */

    public boolean hasPrimitives() {

        throw new IllegalStateException();

    }

    /**

     * Reports if this type contains a wrapper argument or return value.

     * Wrappers are types which box primitive values, such as {@link Integer}.

     * The reference type {@code java.lang.Void} counts as a wrapper,

     * if it occurs as a return type.

     * @return true if any of the types are wrappers

     */

    public boolean hasWrappers() {

        return unwrap() != this;

    }

    /**

     * Erases all reference types to {@code Object}.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All primitive types (including {@code void}) will remain unchanged.

     * @return a version of the original type with all reference types replaced

     */

    public MethodType erase() {

        throw new IllegalStateException();

    }

    /**

     * Erases all reference types to {@code Object}, and all subword types to {@code int}.

     * This is the reduced type polymorphism used by private methods

     * such as {@link MethodHandle#invokeBasic invokeBasic}.

     * @return a version of the original type with all reference and subword types replaced

     */

    /*non-public*/ MethodType basicType() {

        throw new IllegalStateException();

    }

    /**

     * @return a version of the original type with MethodHandle prepended as the first argument

     */

    /*non-public*/ MethodType invokerType() {

        throw new IllegalStateException();

    }

    /**

     * Converts all types, both reference and primitive, to {@code Object}.

     * Convenience method for {@link #genericMethodType(int) genericMethodType}.

     * The expression {@code type.wrap().erase()} produces the same value

     * as {@code type.generic()}.

     * @return a version of the original type with all types replaced

     */

    public MethodType generic() {

        return genericMethodType(parameterCount());

    }

    /**

     * Converts all primitive types to their corresponding wrapper types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All reference types (including wrapper types) will remain unchanged.

     * A {@code void} return type is changed to the type {@code java.lang.Void}.

     * The expression {@code type.wrap().erase()} produces the same value

     * as {@code type.generic()}.

     * @return a version of the original type with all primitive types replaced

     */

    public MethodType wrap() {

        return hasPrimitives() ? wrapWithPrims(this) : this;

    }

    /**

     * Converts all wrapper types to their corresponding primitive types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All primitive types (including {@code void}) will remain unchanged.

     * A return type of {@code java.lang.Void} is changed to {@code void}.

     * @return a version of the original type with all wrapper types replaced

     */

    public MethodType unwrap() {

        MethodType noprims = !hasPrimitives() ? this : wrapWithPrims(this);

        return unwrapWithNoPrims(noprims);

    }

    private static MethodType wrapWithPrims(MethodType pt) {

        throw new IllegalStateException();

    }

    private static MethodType unwrapWithNoPrims(MethodType wt) {

        throw new IllegalStateException();

    }

    /**

     * Returns the parameter type at the specified index, within this method type.

     * @param num the index (zero-based) of the desired parameter type

     * @return the selected parameter type

     * @throws IndexOutOfBoundsException if {@code num} is not a valid index into {@code parameterArray()}

     */

    public Class<?> parameterType(int num) {

        return ptypes[num];

    }

    /**

     * Returns the number of parameter types in this method type.

     * @return the number of parameter types

     */

    public int parameterCount() {

        return ptypes.length;

    }

    /**

     * Returns the return type of this method type.

     * @return the return type

     */

    public Class<?> returnType() {

        return rtype;

    }

    /**

     * Presents the parameter types as a list (a convenience method).

     * The list will be immutable.

     * @return the parameter types (as an immutable list)

     */

    public List<Class<?>> parameterList() {

        return Collections.unmodifiableList(Arrays.asList(ptypes));

    }

    /*non-public*/ Class<?> lastParameterType() {

        int len = ptypes.length;

        return len == 0 ? void.class : ptypes[len-1];

    }

    /**

     * Presents the parameter types as an array (a convenience method).

     * Changes to the array will not result in changes to the type.

     * @return the parameter types (as a fresh copy if necessary)

     */

    public Class<?>[] parameterArray() {

        return ptypes.clone();

    }

    /**

     * Compares the specified object with this type for equality.

     * That is, it returns <tt>true</tt> if and only if the specified object

     * is also a method type with exactly the same parameters and return type.

     * @param x object to compare

     * @see Object#equals(Object)

     */

    @Override

    public boolean equals(Object x) {

        return this == x || x instanceof MethodType && equals((MethodType)x);

    }

    private boolean equals(MethodType that) {

        return this.rtype == that.rtype

            && Arrays.equals(this.ptypes, that.ptypes);

    }

    /**

     * Returns the hash code value for this method type.

     * It is defined to be the same as the hashcode of a List

     * whose elements are the return type followed by the

     * parameter types.

     * @return the hash code value for this method type

     * @see Object#hashCode()

     * @see #equals(Object)

     * @see List#hashCode()

     */

    @Override

    public int hashCode() {

      int hashCode = 31 + rtype.hashCode();

      for (Class<?> ptype : ptypes)

          hashCode = 31*hashCode + ptype.hashCode();

      return hashCode;

    }

    /**

     * Returns a string representation of the method type,

     * of the form {@code "(PT0,PT1...)RT"}.

     * The string representation of a method type is a

     * parenthesis enclosed, comma separated list of type names,

     * followed immediately by the return type.

     * <p>

     * Each type is represented by its

     * {@link java.lang.Class#getSimpleName simple name}.

     */

    @Override

    public String toString() {

        StringBuilder sb = new StringBuilder();

        sb.append("(");

        for (int i = 0; i < ptypes.length; i++) {

            if (i > 0)  sb.append(",");

            sb.append(ptypes[i].getSimpleName());

        }

        sb.append(")");

        sb.append(rtype.getSimpleName());

        return sb.toString();

    }

    /**

     * Finds or creates an instance of a method type, given the spelling of its bytecode descriptor.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * Any class or interface name embedded in the descriptor string

     * will be resolved by calling {@link ClassLoader#loadClass(java.lang.String)}

     * on the given loader (or if it is null, on the system class loader).

     * <p>

     * Note that it is possible to encounter method types which cannot be

     * constructed by this method, because their component types are

     * not all reachable from a common class loader.

     * <p>

     * This method is included for the benefit of applications that must

     * generate bytecodes that process method handles and {@code invokedynamic}.

     * @param descriptor a bytecode-level type descriptor string "(T...)T"

     * @param loader the class loader in which to look up the types

     * @return a method type matching the bytecode-level type descriptor

     * @throws NullPointerException if the string is null

     * @throws IllegalArgumentException if the string is not well-formed

     * @throws TypeNotPresentException if a named type cannot be found

     */

    public static MethodType fromMethodDescriptorString(String descriptor, ClassLoader loader)

        throws IllegalArgumentException

    {

        throw new IllegalStateException();

    }

    /**

     * Produces a bytecode descriptor representation of the method type.

     * <p>

     * Note that this is not a strict inverse of {@link #fromMethodDescriptorString fromMethodDescriptorString}.

     * Two distinct classes which share a common name but have different class loaders

     * will appear identical when viewed within descriptor strings.

     * <p>

     * This method is included for the benefit of applications that must

     * generate bytecodes that process method handles and {@code invokedynamic}.

     * {@link #fromMethodDescriptorString(java.lang.String, java.lang.ClassLoader) fromMethodDescriptorString},

     * because the latter requires a suitable class loader argument.

     * @return the bytecode type descriptor representation

     */

    public String toMethodDescriptorString() {

        throw new IllegalStateException();

    }

    /// Serialization.

    /**

     * There are no serializable fields for {@code MethodType}.

     */

    private static final java.io.ObjectStreamField[] serialPersistentFields = { };

//    /**

//     * Save the {@code MethodType} instance to a stream.

//     *

//     * @serialData

//     * For portability, the serialized format does not refer to named fields.

//     * Instead, the return type and parameter type arrays are written directly

//     * from the {@code writeObject} method, using two calls to {@code s.writeObject}

//     * as follows:

//     * <blockquote><pre>{@code

//s.writeObject(this.returnType());

//s.writeObject(this.parameterArray());

//     * }</pre></blockquote>

//     * <p>

//     * The deserialized field values are checked as if they were

//     * provided to the factory method {@link #methodType(Class,Class[]) methodType}.

//     * For example, null values, or {@code void} parameter types,

//     * will lead to exceptions during deserialization.

//     * @param s the stream to write the object to

//     * @throws java.io.IOException if there is a problem writing the object

//     */

//    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {

//        s.defaultWriteObject();  // requires serialPersistentFields to be an empty array

//        s.writeObject(returnType());

//        s.writeObject(parameterArray());

//    }

//

//    /**

//     * Reconstitute the {@code MethodType} instance from a stream (that is,

//     * deserialize it).

//     * This instance is a scratch object with bogus final fields.

//     * It provides the parameters to the factory method called by

//     * {@link #readResolve readResolve}.

//     * After that call it is discarded.

//     * @param s the stream to read the object from

//     * @throws java.io.IOException if there is a problem reading the object

//     * @throws ClassNotFoundException if one of the component classes cannot be resolved

//     * @see #MethodType()

//     * @see #readResolve

//     * @see #writeObject

//     */

//    private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {

//        s.defaultReadObject();  // requires serialPersistentFields to be an empty array

//

//        Class<?>   returnType     = (Class<?>)   s.readObject();

//        Class<?>[] parameterArray = (Class<?>[]) s.readObject();

//

//        // Probably this object will never escape, but let's check

//        // the field values now, just to be sure.

//        checkRtype(returnType);

//        checkPtypes(parameterArray);

//

//        parameterArray = parameterArray.clone();  // make sure it is unshared

//        MethodType_init(returnType, parameterArray);

//    }

    /**

     * For serialization only.

     * Sets the final fields to null, pending {@code Unsafe.putObject}.

     */

    private MethodType() {

        this.rtype = null;

        this.ptypes = null;

    }

//    private void MethodType_init(Class<?> rtype, Class<?>[] ptypes) {

//        // In order to communicate these values to readResolve, we must

//        // store them into the implementation-specific final fields.

//        checkRtype(rtype);

//        checkPtypes(ptypes);

//        UNSAFE.putObject(this, rtypeOffset, rtype);

//        UNSAFE.putObject(this, ptypesOffset, ptypes);

//    }

    // Support for resetting final fields while deserializing

//    private static final long rtypeOffset, ptypesOffset;

//    static {

//        try {

//            rtypeOffset = UNSAFE.objectFieldOffset

//                (MethodType.class.getDeclaredField("rtype"));

//            ptypesOffset = UNSAFE.objectFieldOffset

//                (MethodType.class.getDeclaredField("ptypes"));

//        } catch (Exception ex) {

//            throw new Error(ex);

//        }

//    }

    /**

     * Resolves and initializes a {@code MethodType} object

     * after serialization.

     * @return the fully initialized {@code MethodType} object

     */

    private Object readResolve() {

        // Do not use a trusted path for deserialization:

        //return makeImpl(rtype, ptypes, true);

        // Verify all operands, and make sure ptypes is unshared:

        return methodType(rtype, ptypes);

    }

    /**

     * Simple implementation of weak concurrent intern set.

     *

     * @param <T> interned type

     */

    private static class ConcurrentWeakInternSet<T> {

        private final ConcurrentMap<WeakEntry<T>, WeakEntry<T>> map;

        private final ReferenceQueue<T> stale;

        public ConcurrentWeakInternSet() {

            this.map = new ConcurrentHashMap<>();

            this.stale = new ReferenceQueue<>();

        }

        /**

         * Get the existing interned element.

         * This method returns null if no element is interned.

         *

         * @param elem element to look up

         * @return the interned element

         */

        public T get(T elem) {

            if (elem == null) throw new NullPointerException();

            expungeStaleElements();

            WeakEntry<T> value = map.get(new WeakEntry<>(elem));

            if (value != null) {

                T res = value.get();

                if (res != null) {

                    return res;

                }

            }

            return null;

        }

        /**

         * Interns the element.

         * Always returns non-null element, matching the one in the intern set.

         * Under the race against another add(), it can return <i>different</i>

         * element, if another thread beats us to interning it.

         *

         * @param elem element to add

         * @return element that was actually added

         */

        public T add(T elem) {

            if (elem == null) throw new NullPointerException();

            // Playing double race here, and so spinloop is required.

            // First race is with two concurrent updaters.

            // Second race is with GC purging weak ref under our feet.

            // Hopefully, we almost always end up with a single pass.

            T interned;

            WeakEntry<T> e = new WeakEntry<>(elem, stale);

            do {

                expungeStaleElements();

                WeakEntry<T> exist = map.putIfAbsent(e, e);

                interned = (exist == null) ? elem : exist.get();

            } while (interned == null);

            return interned;

        }

        private void expungeStaleElements() {

            Reference<? extends T> reference;

            while ((reference = stale.poll()) != null) {

                map.remove(reference);

            }

        }

        private static class WeakEntry<T> extends WeakReference<T> {

            public final int hashcode;

            public WeakEntry(T key, ReferenceQueue<T> queue) {

                super(key, queue);

                hashcode = key.hashCode();

            }

            public WeakEntry(T key) {

                super(key);

                hashcode = key.hashCode();

            }

            @Override

            public boolean equals(Object obj) {

                if (obj instanceof WeakEntry) {

                    Object that = ((WeakEntry) obj).get();

                    Object mine = get();

                    return (that == null || mine == null) ? (this == obj) : mine.equals(that);

                }

                return false;

            }

            @Override

            public int hashCode() {

                return hashcode;

            }

        }

    }

}