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package java.lang.invoke;

import java.util.*;

import static java.lang.invoke.MethodHandleStatics.*;

/**

 * 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.

 * These transformations are quite general, and include such patterns as

 * {@linkplain #asType conversion},

 * {@linkplain #bindTo insertion},

 * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},

 * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.

 *

 * <h1>Method handle contents</h1>

 * Method handles are dynamically and strongly typed according to their parameter and return types.

 * They are not distinguished by the name or the defining class of their underlying methods.

 * A method handle must be invoked using a symbolic type descriptor which matches

 * the method handle's own {@linkplain #type type descriptor}.

 * <p>

 * Every method handle reports its type descriptor via the {@link #type type} accessor.

 * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,

 * whose structure is a series of classes, one of which is

 * the return type of the method (or {@code void.class} if none).

 * <p>

 * A method handle's type controls the types of invocations it accepts,

 * and the kinds of transformations that apply to it.

 * <p>

 * A method handle contains a pair of special invoker methods

 * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.

 * Both invoker methods provide direct access to the method handle's

 * underlying method, constructor, field, or other operation,

 * as modified by transformations of arguments and return values.

 * Both invokers accept calls which exactly match the method handle's own type.

 * The plain, inexact invoker also accepts a range of other call types.

 * <p>

 * Method handles are immutable and have no visible state.

 * Of course, they can be bound to underlying methods or data which exhibit state.

 * With respect to the Java Memory Model, any method handle will behave

 * as if all of its (internal) fields are final variables.  This means that any method

 * handle made visible to the application will always be fully formed.

 * This is true even if the method handle is published through a shared

 * variable in a data race.

 * <p>

 * Method handles cannot be subclassed by the user.

 * Implementations may (or may not) create internal subclasses of {@code MethodHandle}

 * which may be visible via the {@link java.lang.Object#getClass Object.getClass}

 * operation.  The programmer should not draw conclusions about a method handle

 * from its specific class, as the method handle class hierarchy (if any)

 * may change from time to time or across implementations from different vendors.

 *

 * <h1>Method handle compilation</h1>

 * A Java method call expression naming {@code invokeExact} or {@code invoke}

 * can invoke a method handle from Java source code.

 * From the viewpoint of source code, these methods can take any arguments

 * and their result can be cast to any return type.

 * Formally this is accomplished by giving the invoker methods

 * {@code Object} return types and variable arity {@code Object} arguments,

 * but they have an additional quality called <em>signature polymorphism</em>

 * which connects this freedom of invocation directly to the JVM execution stack.

 * <p>

 * As is usual with virtual methods, source-level calls to {@code invokeExact}

 * and {@code invoke} compile to an {@code invokevirtual} instruction.

 * More unusually, the compiler must record the actual argument types,

 * and may not perform method invocation conversions on the arguments.

 * Instead, it must push them on the stack according to their own unconverted types.

 * The method handle object itself is pushed on the stack before the arguments.

 * The compiler then calls the method handle with a symbolic type descriptor which

 * describes the argument and return types.

 * <p>

 * To issue a complete symbolic type descriptor, the compiler must also determine

 * the return type.  This is based on a cast on the method invocation expression,

 * if there is one, or else {@code Object} if the invocation is an expression

 * or else {@code void} if the invocation is a statement.

 * The cast may be to a primitive type (but not {@code void}).

 * <p>

 * As a corner case, an uncasted {@code null} argument is given

 * a symbolic type descriptor of {@code java.lang.Void}.

 * The ambiguity with the type {@code Void} is harmless, since there are no references of type

 * {@code Void} except the null reference.

 *

 * <h1>Method handle invocation</h1>

 * The first time a {@code invokevirtual} instruction is executed

 * it is linked, by symbolically resolving the names in the instruction

 * and verifying that the method call is statically legal.

 * This is true of calls to {@code invokeExact} and {@code invoke}.

 * In this case, the symbolic type descriptor emitted by the compiler is checked for

 * correct syntax and names it contains are resolved.

 * Thus, an {@code invokevirtual} instruction which invokes

 * a method handle will always link, as long

 * as the symbolic type descriptor is syntactically well-formed

 * and the types exist.

 * <p>

 * When the {@code invokevirtual} is executed after linking,

 * the receiving method handle's type is first checked by the JVM

 * to ensure that it matches the symbolic type descriptor.

 * If the type match fails, it means that the method which the

 * caller is invoking is not present on the individual

 * method handle being invoked.

 * <p>

 * In the case of {@code invokeExact}, the type descriptor of the invocation

 * (after resolving symbolic type names) must exactly match the method type

 * of the receiving method handle.

 * In the case of plain, inexact {@code invoke}, the resolved type descriptor

 * must be a valid argument to the receiver's {@link #asType asType} method.

 * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.

 * <p>

 * After type matching, a call to {@code invokeExact} directly

 * and immediately invoke the method handle's underlying method

 * (or other behavior, as the case may be).

 * <p>

 * A call to plain {@code invoke} works the same as a call to

 * {@code invokeExact}, if the symbolic type descriptor specified by the caller

 * exactly matches the method handle's own type.

 * If there is a type mismatch, {@code invoke} attempts

 * to adjust the type of the receiving method handle,

 * as if by a call to {@link #asType asType},

 * to obtain an exactly invokable method handle {@code M2}.

 * This allows a more powerful negotiation of method type

 * between caller and callee.

 * <p>

 * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,

 * and implementations are therefore not required to materialize it.)

 *

 * <h1>Invocation checking</h1>

 * In typical programs, method handle type matching will usually succeed.

 * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},

 * either directly (in the case of {@code invokeExact}) or indirectly as if

 * by a failed call to {@code asType} (in the case of {@code invoke}).

 * <p>

 * Thus, a method type mismatch which might show up as a linkage error

 * in a statically typed program can show up as

 * a dynamic {@code WrongMethodTypeException}

 * in a program which uses method handles.

 * <p>

 * Because method types contain "live" {@code Class} objects,

 * method type matching takes into account both types names and class loaders.

 * Thus, even if a method handle {@code M} is created in one

 * class loader {@code L1} and used in another {@code L2},

 * method handle calls are type-safe, because the caller's symbolic type

 * descriptor, as resolved in {@code L2},

 * is matched against the original callee method's symbolic type descriptor,

 * as resolved in {@code L1}.

 * The resolution in {@code L1} happens when {@code M} is created

 * and its type is assigned, while the resolution in {@code L2} happens

 * when the {@code invokevirtual} instruction is linked.

 * <p>

 * Apart from the checking of type descriptors,

 * a method handle's capability to call its underlying method is unrestricted.

 * If a method handle is formed on a non-public method by a class

 * that has access to that method, the resulting handle can be used

 * in any place by any caller who receives a reference to it.

 * <p>

 * Unlike with the Core Reflection API, where access is checked every time

 * a reflective method is invoked,

 * method handle access checking is performed

 * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.

 * In the case of {@code ldc} (see below), access checking is performed as part of linking

 * the constant pool entry underlying the constant method handle.

 * <p>

 * Thus, handles to non-public methods, or to methods in non-public classes,

 * should generally be kept secret.

 * They should not be passed to untrusted code unless their use from

 * the untrusted code would be harmless.

 *

 * <h1>Method handle creation</h1>

 * Java code can create a method handle that directly accesses

 * any method, constructor, or field that is accessible to that code.

 * This is done via a reflective, capability-based API called

 * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}

 * For example, a static method handle can be obtained

 * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.

 * There are also conversion methods from Core Reflection API objects,

 * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.

 * <p>

 * Like classes and strings, method handles that correspond to accessible

 * fields, methods, and constructors can also be represented directly

 * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.

 * A new type of constant pool entry, {@code CONSTANT_MethodHandle},

 * refers directly to an associated {@code CONSTANT_Methodref},

 * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}

 * constant pool entry.

 * (For full details on method handle constants,

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

 * <p>

 * Method handles produced by lookups or constant loads from methods or

 * constructors with the variable arity modifier bit ({@code 0x0080})

 * have a corresponding variable arity, as if they were defined with

 * the help of {@link #asVarargsCollector asVarargsCollector}.

 * <p>

 * A method reference may refer either to a static or non-static method.

 * In the non-static case, the method handle type includes an explicit

 * receiver argument, prepended before any other arguments.

 * In the method handle's type, the initial receiver argument is typed

 * according to the class under which the method was initially requested.

 * (E.g., if a non-static method handle is obtained via {@code ldc},

 * the type of the receiver is the class named in the constant pool entry.)

 * <p>

 * Method handle constants are subject to the same link-time access checks

 * their corresponding bytecode instructions, and the {@code ldc} instruction

 * will throw corresponding linkage errors if the bytecode behaviors would

 * throw such errors.

 * <p>

 * As a corollary of this, access to protected members is restricted

 * to receivers only of the accessing class, or one of its subclasses,

 * and the accessing class must in turn be a subclass (or package sibling)

 * of the protected member's defining class.

 * If a method reference refers to a protected non-static method or field

 * of a class outside the current package, the receiver argument will

 * be narrowed to the type of the accessing class.

 * <p>

 * When a method handle to a virtual method is invoked, the method is

 * always looked up in the receiver (that is, the first argument).

 * <p>

 * A non-virtual method handle to a specific virtual method implementation

 * can also be created.  These do not perform virtual lookup based on

 * receiver type.  Such a method handle simulates the effect of

 * an {@code invokespecial} instruction to the same method.

 *

 * <h1>Usage examples</h1>

 * Here are some examples of usage:

 * <blockquote><pre>{@code

Object x, y; String s; int i;

MethodType mt; MethodHandle mh;

MethodHandles.Lookup lookup = MethodHandles.lookup();

// mt is (char,char)String

mt = MethodType.methodType(String.class, char.class, char.class);

mh = lookup.findVirtual(String.class, "replace", mt);

s = (String) mh.invokeExact("daddy",'d','n');

// invokeExact(Ljava/lang/String;CC)Ljava/lang/String;

assertEquals(s, "nanny");

// weakly typed invocation (using MHs.invoke)

s = (String) mh.invokeWithArguments("sappy", 'p', 'v');

assertEquals(s, "savvy");

// mt is (Object[])List

mt = MethodType.methodType(java.util.List.class, Object[].class);

mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);

assert(mh.isVarargsCollector());

x = mh.invoke("one", "two");

// invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;

assertEquals(x, java.util.Arrays.asList("one","two"));

// mt is (Object,Object,Object)Object

mt = MethodType.genericMethodType(3);

mh = mh.asType(mt);

x = mh.invokeExact((Object)1, (Object)2, (Object)3);

// invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;

assertEquals(x, java.util.Arrays.asList(1,2,3));

// mt is ()int

mt = MethodType.methodType(int.class);

mh = lookup.findVirtual(java.util.List.class, "size", mt);

i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));

// invokeExact(Ljava/util/List;)I

assert(i == 3);

mt = MethodType.methodType(void.class, String.class);

mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);

mh.invokeExact(System.out, "Hello, world.");

// invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V

 * }</pre></blockquote>

 * Each of the above calls to {@code invokeExact} or plain {@code invoke}

 * generates a single invokevirtual instruction with

 * the symbolic type descriptor indicated in the following comment.

 * In these examples, the helper method {@code assertEquals} is assumed to

 * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}

 * on its arguments, and asserts that the result is true.

 *

 * <h1>Exceptions</h1>

 * The methods {@code invokeExact} and {@code invoke} are declared

 * to throw {@link java.lang.Throwable Throwable},

 * which is to say that there is no static restriction on what a method handle

 * can throw.  Since the JVM does not distinguish between checked

 * and unchecked exceptions (other than by their class, of course),

 * there is no particular effect on bytecode shape from ascribing

 * checked exceptions to method handle invocations.  But in Java source

 * code, methods which perform method handle calls must either explicitly

 * throw {@code Throwable}, or else must catch all

 * throwables locally, rethrowing only those which are legal in the context,

 * and wrapping ones which are illegal.

 *

 * <h1><a name="sigpoly"></a>Signature polymorphism</h1>

 * The unusual compilation and linkage behavior of

 * {@code invokeExact} and plain {@code invoke}

 * is referenced by the term <em>signature polymorphism</em>.

 * As defined in the Java Language Specification,

 * a signature polymorphic method is one which can operate with

 * any of a wide range of call signatures and return types.

 * <p>

 * In source code, a call to a signature polymorphic method will

 * compile, regardless of the requested symbolic type descriptor.

 * As usual, the Java compiler emits an {@code invokevirtual}

 * instruction with the given symbolic type descriptor against the named method.

 * The unusual part is that the symbolic type descriptor is derived from

 * the actual argument and return types, not from the method declaration.

 * <p>

 * When the JVM processes bytecode containing signature polymorphic calls,

 * it will successfully link any such call, regardless of its symbolic type descriptor.

 * (In order to retain type safety, the JVM will guard such calls with suitable

 * dynamic type checks, as described elsewhere.)

 * <p>

 * Bytecode generators, including the compiler back end, are required to emit

 * untransformed symbolic type descriptors for these methods.

 * Tools which determine symbolic linkage are required to accept such

 * untransformed descriptors, without reporting linkage errors.

 *

 * <h1>Interoperation between method handles and the Core Reflection API</h1>

 * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,

 * any class member represented by a Core Reflection API object

 * can be converted to a behaviorally equivalent method handle.

 * For example, a reflective {@link java.lang.reflect.Method Method} can

 * be converted to a method handle using

 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.

 * The resulting method handles generally provide more direct and efficient

 * access to the underlying class members.

 * <p>

 * As a special case,

 * when the Core Reflection API is used to view the signature polymorphic

 * methods {@code invokeExact} or plain {@code invoke} in this class,

 * they appear as ordinary non-polymorphic methods.

 * Their reflective appearance, as viewed by

 * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},

 * is unaffected by their special status in this API.

 * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}

 * will report exactly those modifier bits required for any similarly

 * declared method, including in this case {@code native} and {@code varargs} bits.

 * <p>

 * As with any reflected method, these methods (when reflected) may be

 * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.

 * However, such reflective calls do not result in method handle invocations.

 * Such a call, if passed the required argument

 * (a single one, of type {@code Object[]}), will ignore the argument and

 * will throw an {@code UnsupportedOperationException}.

 * <p>

 * Since {@code invokevirtual} instructions can natively

 * invoke method handles under any symbolic type descriptor, this reflective view conflicts

 * with the normal presentation of these methods via bytecodes.

 * Thus, these two native methods, when reflectively viewed by

 * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.

 * <p>

 * In order to obtain an invoker method for a particular type descriptor,

 * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},

 * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.

 * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}

 * API is also able to return a method handle

 * to call {@code invokeExact} or plain {@code invoke},

 * for any specified type descriptor .

 *

 * <h1>Interoperation between method handles and Java generics</h1>

 * A method handle can be obtained on a method, constructor, or field

 * which is declared with Java generic types.

 * As with the Core Reflection API, the type of the method handle

 * will constructed from the erasure of the source-level type.

 * When a method handle is invoked, the types of its arguments

 * or the return value cast type may be generic types or type instances.

 * If this occurs, the compiler will replace those

 * types by their erasures when it constructs the symbolic type descriptor

 * for the {@code invokevirtual} instruction.

 * <p>

 * Method handles do not represent

 * their function-like types in terms of Java parameterized (generic) types,

 * because there are three mismatches between function-like types and parameterized

 * Java types.

 * <ul>

 * <li>Method types range over all possible arities,

 * from no arguments to up to the  <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.

 * Generics are not variadic, and so cannot represent this.</li>

 * <li>Method types can specify arguments of primitive types,

 * which Java generic types cannot range over.</li>

 * <li>Higher order functions over method handles (combinators) are

 * often generic across a wide range of function types, including

 * those of multiple arities.  It is impossible to represent such

 * genericity with a Java type parameter.</li>

 * </ul>

 *

 * <h1><a name="maxarity"></a>Arity limits</h1>

 * The JVM imposes on all methods and constructors of any kind an absolute

 * limit of 255 stacked arguments.  This limit can appear more restrictive

 * in certain cases:

 * <ul>

 * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.

 * <li>A non-static method consumes an extra argument for the object on which the method is called.

 * <li>A constructor consumes an extra argument for the object which is being constructed.

 * <li>Since a method handle&rsquo;s {@code invoke} method (or other signature-polymorphic method) is non-virtual,

 *     it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.

 * </ul>

 * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.

 * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.

 * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.

 * In particular, a method handle’s type must not have an arity of the exact maximum 255.

 *

 * @see MethodType

 * @see MethodHandles

 * @author John Rose, JSR 292 EG

 */

public abstract class MethodHandle {

    static { MethodHandleImpl.initStatics(); }

    /**

     * Internal marker interface which distinguishes (to the Java compiler)

     * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.

     */

    @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})

    @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)

    @interface PolymorphicSignature { }

    private final MethodType type;

    /*private*/ final LambdaForm form;

    // form is not private so that invokers can easily fetch it

    /*private*/ MethodHandle asTypeCache;

    // asTypeCache is not private so that invokers can easily fetch it

    /*non-public*/ byte customizationCount;

    // customizationCount should be accessible from invokers

    /**

     * Reports the type of this method handle.

     * Every invocation of this method handle via {@code invokeExact} must exactly match this type.

     * @return the method handle type

     */

    public MethodType type() {

        return type;

    }

    /**

     * Package-private constructor for the method handle implementation hierarchy.

     * Method handle inheritance will be contained completely within

     * the {@code java.lang.invoke} package.

     */

    // @param type type (permanently assigned) of the new method handle

    /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {

        this.form.prepare();  // TO DO:  Try to delay this step until just before invocation.

    }

    /**

     * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.

     * The symbolic type descriptor at the call site of {@code invokeExact} must

     * exactly match this method handle's {@link #type type}.

     * No conversions are allowed on arguments or return values.

     * <p>

     * When this method is observed via the Core Reflection API,

     * it will appear as a single native method, taking an object array and returning an object.

     * If this native method is invoked directly via

     * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,

     * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},

     * it will throw an {@code UnsupportedOperationException}.

     * @param args the signature-polymorphic parameter list, statically represented using varargs

     * @return the signature-polymorphic result, statically represented using {@code Object}

     * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor

     * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call

     */

    public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;

    /**

     * Invokes the method handle, allowing any caller type descriptor,

     * and optionally performing conversions on arguments and return values.

     * <p>