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/**

 * The {@code java.lang.invoke} package contains dynamic language support provided directly by

 * the Java core class libraries and virtual machine.

 *

 * <p>

 * Certain types in this package have special relations to dynamic

 * language support in the virtual machine:

 * <ul>

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

 * which can be linked regardless of their type descriptor.

 * Normally, method linkage requires exact matching of type descriptors.

 * </li>

 *

 * <li>The JVM bytecode format supports immediate constants of

 * </li>

 * </ul>

 *

 * <h2><a name="jvm_mods"></a>Corresponding JVM bytecode format changes</h2>

 * <em>The following low-level information is presented here as a preview of

 * changes being made to the Java Virtual Machine specification for JSR 292.

 * This information will be incorporated in a future version of the JVM specification.</em>

 *

 * <h3><a name="indyinsn"></a>{@code invokedynamic} instruction format</h3>

 * In bytecode, an {@code invokedynamic} instruction is formatted as five bytes.

 * The first byte is the opcode 186 (hexadecimal {@code BA}).

 * The next two bytes are a constant pool index (in the same format as for the other {@code invoke} instructions).

 * The final two bytes are reserved for future use and required to be zero.

 * The constant pool reference of an {@code invokedynamic} instruction is to a entry

 * with tag {@code CONSTANT_InvokeDynamic} (decimal 18).  See below for its format.

 * The entry specifies the following information:

 * <ul>

 * <li>the dynamic invocation name (a UTF8 string)</li>

 * <li>the argument and return types of the call (encoded as a type descriptor in a UTF8 string)</li>

 * <li>optionally, a sequence of additional <em>static arguments</em> to the bootstrap method ({@code ldc}-type constants)</li>

 * </ul>

 * <p>

 * Each instance of an {@code invokedynamic} instruction is called a <em>dynamic call site</em>.

 * Multiple instances of an {@code invokedynamic} instruction can share a single

 * {@code CONSTANT_InvokeDynamic} entry.

 * In any case, distinct call sites always have distinct linkage state.

 * <p>

 * A dynamic call site is originally in an unlinked state.  In this state, there is

 * no target method for the call site to invoke.

 * A dynamic call site is linked by means of a bootstrap method,

 * as <a href="#bsm">described below</a>.

 *

 * <h3><a name="indycon"></a>constant pool entries for {@code invokedynamic} instructions</h3>

 * If a constant pool entry has the tag {@code CONSTANT_InvokeDynamic} (decimal 18),

 * it must contain exactly four more bytes after the tag.

 * These bytes are interpreted as two 16-bit indexes, in the usual {@code u2} format.

 * The first pair of bytes after the tag must be an index into a side table called the

 * <em>bootstrap method table</em>, which is stored in the {@code BootstrapMethods}

 * attribute as <a href="#bsmattr">described below</a>.

 * The second pair of bytes must be an index to a {@code CONSTANT_NameAndType}.

 * <p>

 * The first index specifies a bootstrap method used by the associated dynamic call sites.

 * The second index specifies the method name, argument types, and return type of the dynamic call site.

 * The structure of such an entry is therefore analogous to a {@code CONSTANT_Methodref},

 * except that the bootstrap method specifier reference replaces

 * the {@code CONSTANT_Class} reference of a {@code CONSTANT_Methodref} entry.

 *

 * If a constant pool entry has the tag {@code CONSTANT_MethodType} (decimal 16),

 * it must contain exactly two more bytes, which must be an index to a {@code CONSTANT_Utf8}

 * entry which represents a method type descriptor.

 * <p>

 * The JVM will ensure that on first

 * will be created which represents the type descriptor.

 * Any classes mentioned in the {@code MethodType} will be loaded if necessary,

 * but not initialized.

 * Access checking and error reporting is performed exactly as it is for

 * references by {@code ldc} instructions to {@code CONSTANT_Class} constants.

 *

 * If a constant pool entry has the tag {@code CONSTANT_MethodHandle} (decimal 15),

 * it must contain exactly three more bytes.  The first byte after the tag is a subtag

 * value which must be in the range 1 through 9, and the last two must be an index to a

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

 * {@code CONSTANT_InterfaceMethodref} entry which represents a field or method

 * for which a method handle is to be created.

 * Furthermore, the subtag value and the type of the constant index value

 * must agree according to the table below.

 * <p>

 * The JVM will ensure that on first execution of an {@code ldc} instruction

 * the field or method reference, according to the specific mode implied by the subtag.

 * <p>

 * As with {@code CONSTANT_Class} and {@code CONSTANT_MethodType} constants,

 * the {@code Class} or {@code MethodType} object which reifies the field or method's

 * type is created.  Any classes mentioned in this reification will be loaded if necessary,

 * but not initialized, and access checking and error reporting performed as usual.

 * <p>

 * Unlike the reflective {@code Lookup} API, there are no security manager calls made

 * when these constants are resolved.

 * <p>

 * The method handle itself will have a type and behavior determined by the subtag as follows:

 * <code>

 * <table border=1 cellpadding=5 summary="CONSTANT_MethodHandle subtypes">

 * <tr><th>N</th><th>subtag name</th><th>member</th><th>MH type</th><th>bytecode behavior</th><th>lookup expression</th></tr>

 * <tr><td>1</td><td>REF_getField</td><td>C.f:T</td><td>(C)T</td><td>getfield C.f:T</td>

 * <tr><td>2</td><td>REF_getStatic</td><td>C.f:T</td><td>(&nbsp;)T</td><td>getstatic C.f:T</td>

 * <tr><td>3</td><td>REF_putField</td><td>C.f:T</td><td>(C,T)void</td><td>putfield C.f:T</td>

 * <tr><td>4</td><td>REF_putStatic</td><td>C.f:T</td><td>(T)void</td><td>putstatic C.f:T</td>

 * <tr><td>5</td><td>REF_invokeVirtual</td><td>C.m(A*)T</td><td>(C,A*)T</td><td>invokevirtual C.m(A*)T</td>

 * <tr><td>6</td><td>REF_invokeStatic</td><td>C.m(A*)T</td><td>(C,A*)T</td><td>invokestatic C.m(A*)T</td>

 * <tr><td>7</td><td>REF_invokeSpecial</td><td>C.m(A*)T</td><td>(C,A*)T</td><td>invokespecial C.m(A*)T</td>

 * <tr><td>8</td><td>REF_newInvokeSpecial</td><td>C.&lt;init&gt;(A*)void</td><td>(A*)C</td><td>new C; dup; invokespecial C.&lt;init&gt;(A*)void</td>

 * <tr><td>9</td><td>REF_invokeInterface</td><td>C.m(A*)T</td><td>(C,A*)T</td><td>invokeinterface C.m(A*)T</td>

 * </table>

 * </code>

 * Here, the type {@code C} is taken from the {@code CONSTANT_Class} reference associated

 * with the {@code CONSTANT_NameAndType} descriptor.

 * The field name {@code f} or method name {@code m} is taken from the {@code CONSTANT_NameAndType}

 * as is the result type {@code T} and (in the case of a method or constructor) the argument type sequence

 * {@code A*}.

 * <p>

 * Each method handle constant has an equivalent instruction sequence called its <em>bytecode behavior</em>.

 * In general, creating a method handle constant can be done in exactly the same circumstances that

 * the JVM would successfully resolve the symbolic references in the bytecode behavior.

 * Also, the type of a method handle constant is such that a valid {@code invokeExact} call

 * on the method handle has exactly the same JVM stack effects as the <em>bytecode behavior</em>.

 * Finally, calling a method handle constant on a valid set of arguments has exactly the same effect

 * and returns the same result (if any) as the corresponding <em>bytecode behavior</em>.

 * <p>

 * Each method handle constant also has an equivalent reflective <em>lookup expression</em>,

 * In the example lookup method expression given in the table above, the name {@code MT}

 * stands for a {@code MethodType} built from {@code T} and the sequence of argument types {@code A*}.

 * (Note that the type {@code C} is not prepended to the query type {@code MT} even if the member is non-static.)

 * In the case of {@code findSpecial}, the name {@code this.class} refers to the class containing

 * the bytecodes.

 * <p>

 * The special name {@code <clinit>} is not allowed.

 * The special name {@code <init>} is not allowed except for subtag 8 as shown.

 * <p>

 * The JVM verifier and linker apply the same access checks and restrictions for these references as for the hypothetical

 * bytecode instructions specified in the last column of the table.

 * A method handle constant will successfully resolve to a method handle if the symbolic references

 * of the corresponding bytecode instruction(s) would also resolve successfully.

 * Otherwise, an attempt to resolve the constant will throw equivalent linkage errors.

 * In particular, method handles to

 * private and protected members can be created in exactly those classes for which the corresponding

 * normal accesses are legal.

 * <p>

 * A constant may refer to a method or constructor with the {@code varargs}

 * bit (hexadecimal {@code 0x0080}) set in its modifier bitmask.

 * The method handle constant produced for such a method behaves as if

 * In other words, the constant method handle will exhibit variable arity,

 * when invoked via {@code invokeGeneric}.

 * On the other hand, its behavior with respect to {@code invokeExact} will be the same

 * as if the {@code varargs} bit were not set.

 * <p>

 * Although the {@code CONSTANT_MethodHandle} and {@code CONSTANT_MethodType} constant types

 * resolve class names, they do not force class initialization.

 * Method handle constants for subtags {@code REF_getStatic}, {@code REF_putStatic}, and {@code REF_invokeStatic}

 * may force class initialization on their first invocation, just like the corresponding bytecodes.

 * <p>

 * The rules of section 5.4.3 of the

 * <a href="http://java.sun.com/docs/books/jvms/second_edition/html/ConstantPool.doc.html#73492">JVM Specification</a>

 * apply to the resolution of {@code CONSTANT_MethodType}, {@code CONSTANT_MethodHandle},

 * and {@code CONSTANT_InvokeDynamic} constants,

 * by the execution of {@code invokedynamic} and {@code ldc} instructions.

 * (Roughly speaking, this means that every use of a constant pool entry

 * must lead to the same outcome.

 * If the resolution succeeds, the same object reference is produced

 * by every subsequent execution of the same instruction.

 * If the resolution of the constant causes an error to occur,

 * the same error will be re-thrown on every subsequent attempt

 * to use this particular constant.)

 * <p>

 * Constants created by the resolution of these constant pool types are not necessarily

 * interned.  Except for {@code CONSTANT_Class} and {@code CONSTANT_String} entries,

 * two distinct constant pool entries might not resolve to the same reference

 * even if they contain the same symbolic reference.

 *

 * <h2><a name="bsm"></a>Bootstrap Methods</h2>

 * Before the JVM can execute a dynamic call site (an {@code invokedynamic} instruction),

 * the call site must first be <em>linked</em>.

 * Linking is accomplished by calling a <em>bootstrap method</em>

 * which is given the static information content of the call site,

 * that gives the behavior of the call site.

 * <p>

 * Each {@code invokedynamic} instruction statically specifies its own

 * bootstrap method as a constant pool reference.

 * The constant pool reference also specifies the call site's name and type descriptor,

 * just like {@code invokevirtual} and the other invoke instructions.

 * <p>

 * Linking starts with resolving the constant pool entry for the

 * the type descriptor of the dynamic call site.

 * This resolution process may trigger class loading.

 * It may therefore throw an error if a class fails to load.

 * This error becomes the abnormal termination of the dynamic

 * call site execution.

 * Linkage does not trigger class initialization.

 * <p>

 * Next, the bootstrap method call is started, with at least four values being stacked:

 * <ul>

 * <li>a {@code MethodHandle}, the resolved bootstrap method itself </li>

 * <li>a {@code MethodHandles.Lookup}, a lookup object on the <em>caller class</em> in which dynamic call site occurs </li>

 * <li>a {@code String}, the method name mentioned in the call site </li>

 * <li>a {@code MethodType}, the resolved type descriptor of the call </li>

 * <li>optionally, one or more <a href="#args">additional static arguments</a> </li>

 * </ul>

 * The method handle is then applied to the other values as if by

 * The type of the call site's target must be exactly equal to the type

 * derived from the dynamic call site's type descriptor and passed to

 * the bootstrap method.

 * The call site then becomes permanently linked to the dynamic call site.

 * <p>

 * As long as each bootstrap method can be correctly invoked

 * by <code>invokeGeneric</code>, its detailed type is arbitrary.

 * For example, the first argument could be {@code Object}

 * instead of {@code MethodHandles.Lookup}, and the return type

 * could also be {@code Object} instead of {@code CallSite}.

 * <p>

 * After resolution, the linkage process may fail in a variety of ways.

 * which is thrown as the abnormal termination of the dynamic call

 * site execution.

 * The following circumstances will cause this:

 * <ul>

 * <li>the index to the bootstrap method specifier is out of range </li>

 * <li>the bootstrap method cannot be resolved </li>

 * <li>the {@code MethodType} to pass to the bootstrap method cannot be resolved </li>

 * <li>a static argument to the bootstrap method cannot be resolved

 *     (i.e., a {@code CONSTANT_Class}, {@code CONSTANT_MethodType},

 *     or {@code CONSTANT_MethodHandle} argument cannot be linked) </li>

 * <li>the bootstrap method has the wrong arity,

 *     causing {@code invokeGeneric} to throw {@code WrongMethodTypeException} </li>

 * <li>the bootstrap method has a wrong argument or return type </li>

 * <li>the bootstrap method invocation completes abnormally </li>

 * <li>the result from the bootstrap invocation is not a reference to

 * <li>the target of the {@code CallSite} does not have a target of

 *     the expected {@code MethodType} </li>

 * </ul>

 *

 * <h3><a name="linktime"></a>timing of linkage</h3>

 * A dynamic call site is linked just before its first execution.

 * The bootstrap method call implementing the linkage occurs within

 * a thread that is attempting a first execution.

 * <p>

 * If there are several such threads, the bootstrap method may be

 * invoked in several threads concurrently.

 * Therefore, bootstrap methods which access global application

 * data must take the usual precautions against race conditions.

 * In any case, every {@code invokedynamic} instruction is either

 * unlinked or linked to a unique {@code CallSite} object.

 * <p>

 * In an application which requires dynamic call sites with individually

 * mutable behaviors, their bootstrap methods should produce distinct

 * Alternatively, an application can link a single {@code CallSite} object

 * to several {@code invokedynamic} instructions, in which case

 * a change to the target method will become visible at each of

 * the instructions.

 * <p>

 * If several threads simultaneously execute a bootstrap method for a single dynamic

 * call site, the JVM must choose one {@code CallSite} object and install it visibly to

 * all threads.  Any other bootstrap method calls are allowed to complete, but their

 * results are ignored, and their dynamic call site invocations proceed with the originally

 * chosen target object.

 *

 * <p style="font-size:smaller;">

 * or to issue “causeless” bootstrap method calls.

 * Every dynamic call site transitions at most once from unlinked to linked,

 * just before its first invocation.

 *

 * <h3><a name="bsmattr">the {@code BootstrapMethods} attribute </h3>

 * Each {@code CONSTANT_InvokeDynamic} entry contains an index which references

 * a bootstrap method specifier; all such specifiers are contained in a separate array.

 * This array is defined by a class attribute named {@code BootstrapMethods}.

 * The body of this attribute consists of a sequence of byte pairs, all interpreted as

 * as 16-bit counts or constant pool indexes, in the {@code u2} format.

 * The attribute body starts with a count of bootstrap method specifiers,

 * which is immediately followed by the sequence of specifiers.

 * <p>

 * Each bootstrap method specifier contains an index to a

 * {@code CONSTANT_MethodHandle} constant, which is the bootstrap

 * method itself.

 * This is followed by a count, and then a sequence (perhaps empty) of

 * indexes to <a href="#args">additional static arguments</a>

 * for the bootstrap method.

 * <p>

 * During class loading, the verifier must check the structure of the

 * {@code BootstrapMethods} attribute.  In particular, each constant

 * pool index must be of the correct type.  A bootstrap method index

 * must refer to a {@code CONSTANT_MethodHandle} (tag 15).

 * Every other index must refer to a valid operand of an

 * {@code ldc_w} or {@code ldc2_w} instruction (tag 3..8 or 15..16).

 *

 * <h3><a name="args">static arguments to the bootstrap method</h3>

 * An {@code invokedynamic} instruction specifies at least three arguments

 * to pass to its bootstrap method:

 * the name (extracted from the {@code CONSTANT_NameAndType} entry),

 * and the type (also extracted from the {@code CONSTANT_NameAndType} entry).

 * The {@code invokedynamic} instruction may specify additional metadata values

 * to pass to its bootstrap method.

 * Collectively, these values are called <em>static arguments</em> to the

 * {@code invokedynamic} instruction, because they are used once at link

 * time to determine the instruction's behavior on subsequent sets of

 * <em>dynamic arguments</em>.

 * <p>

 * Static arguments are used to communicate application-specific meta-data

 * to the bootstrap method.

 * Drawn from the constant pool, they may include references to classes, method handles,

 * strings, or numeric data that may be relevant to the task of linking that particular call site.

 * <p>

 * Static arguments are specified constant pool indexes stored in the {@code BootstrapMethods} attribute.

 * Before the bootstrap method is invoked, each index is used to compute an {@code Object}

 * reference to the indexed value in the constant pool.

 * The valid constant pool entries are listed in this table:

 * <code>

 * <table border=1 cellpadding=5 summary="Static argument types">

 * <tr><th>entry type</th><th>argument type</th><th>argument value</th></tr>

 * <tr><td>CONSTANT_String</td><td><code>java.lang.String</code></td><td>the indexed string literal</td></tr>

 * <tr><td>CONSTANT_Class</td><td><code>java.lang.Class</code></td><td>the indexed class, resolved</td></tr>

 * <tr><td>CONSTANT_Integer</td><td><code>java.lang.Integer</code></td><td>the indexed int value</td></tr>

 * <tr><td>CONSTANT_Long</td><td><code>java.lang.Long</code></td><td>the indexed long value</td></tr>

 * <tr><td>CONSTANT_Float</td><td><code>java.lang.Float</code></td><td>the indexed float value</td></tr>

 * <tr><td>CONSTANT_Double</td><td><code>java.lang.Double</code></td><td>the indexed double value</td></tr>

 * </table>

 * </code>

 * <p>

 * If a given {@code invokedynamic} instruction specifies no static arguments,

 * the instruction's bootstrap method will be invoked on three arguments,

 * conveying the instruction's caller class, name, and method type.

 * If the {@code invokedynamic} instruction specifies one or more static arguments,

 * those values will be passed as additional arguments to the method handle.

 * (Note that because there is a limit of 255 arguments to any method,

 * at most 252 extra arguments can be supplied.)

 * The bootstrap method will be invoked as if by either {@code invokeGeneric}

 * or {@code invokeWithArguments}.  (There is no way to tell the difference.)

 * <p>

 * The normal argument conversion rules for {@code invokeGeneric} apply to all stacked arguments.

 * For example, if a pushed value is a primitive type, it may be converted to a reference by boxing conversion.

 * If the bootstrap method is a variable arity method (its modifier bit {@code 0x0080} is set),

 * then some or all of the arguments specified here may be collected into a trailing array parameter.

 * (This is not a special rule, but rather a useful consequence of the interaction

 * between {@code CONSTANT_MethodHandle} constants, the modifier bit for variable arity methods,

 * <p>

 * Given these rules, here are examples of legal bootstrap method declarations,

 * given various numbers {@code N} of extra arguments.

 * The first rows (marked {@code *}) will work for any number of extra arguments.

 * <code>

 * <table border=1 cellpadding=5 summary="Static argument types">

 * <tr><th>N</th><th>sample bootstrap method</th></tr>

 * <tr><td>*</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)</code></td></tr>

 * <tr><td>*</td><td><code>CallSite bootstrap(Object... args)</code></td></tr>

 * <tr><td>*</td><td><code>CallSite bootstrap(Object caller, Object... nameAndTypeWithArgs)</code></td></tr>

 * <tr><td>0</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type)</code></td></tr>

 * <tr><td>0</td><td><code>CallSite bootstrap(Lookup caller, Object... nameAndType)</code></td></tr>

 * <tr><td>1</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object arg)</code></td></tr>

 * <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)</code></td></tr>

 * <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, String... args)</code></td></tr>

 * <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, String x, int y)</code></td></tr>

 * </table>

 * </code>

 * The last example assumes that the extra arguments are of type

 * {@code CONSTANT_String} and {@code CONSTANT_Integer}, respectively.

 * The second-to-last example assumes that all extra arguments are of type

 * {@code CONSTANT_String}.

 * The other examples work with all types of extra arguments.

 * <p>

 * As noted above, the actual method type of the bootstrap method can vary.

 * For example, the fourth argument could be {@code MethodHandle},

 * if that is the type of the corresponding constant in

 * the {@code CONSTANT_InvokeDynamic} entry.

 * In that case, the {@code invokeGeneric} call will pass the extra method handle

 * constant as an {@code Object}, but the type matching machinery of {@code invokeGeneric}

 * will cast the reference back to {@code MethodHandle} before invoking the bootstrap method.

 * (If a string constant were passed instead, by badly generated code, that cast would then fail,

 * <p>

 * Extra bootstrap method arguments are intended to allow language implementors

 * to safely and compactly encode metadata.

 * In principle, the name and extra arguments are redundant,

 * since each call site could be given its own unique bootstrap method.

 * Such a practice is likely to produce large class files and constant pools.

 *

 * <h2><a name="structs"></a>Structure Summary</h2>

 * <blockquote><pre>// summary of constant and attribute structures

struct CONSTANT_MethodHandle_info {

  u1 tag = 15;

  u1 reference_kind;       // 1..8 (one of REF_invokeVirtual, etc.)

  u2 reference_index;      // index to CONSTANT_Fieldref or *Methodref

}

struct CONSTANT_MethodType_info {

  u1 tag = 16;

  u2 descriptor_index;    // index to CONSTANT_Utf8, as in NameAndType

}

struct CONSTANT_InvokeDynamic_info {

  u1 tag = 18;

  u2 bootstrap_method_attr_index;  // index into BootstrapMethods_attr

  u2 name_and_type_index;          // index to CONSTANT_NameAndType, as in Methodref

}

struct BootstrapMethods_attr {

 u2 name;  // CONSTANT_Utf8 = "BootstrapMethods"

 u4 size;

 u2 bootstrap_method_count;

 struct bootstrap_method_specifier {

   u2 bootstrap_method_ref;  // index to CONSTANT_MethodHandle

   u2 bootstrap_argument_count;

   u2 bootstrap_arguments[bootstrap_argument_count];  // constant pool indexes

 } bootstrap_methods[bootstrap_method_count];

}

 * </pre></blockquote>

 *

 * @author John Rose, JSR 292 EG

 */