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25
26package java.lang.invoke;
27
28import java.lang.reflect.*;
29import java.util.*;
30import java.lang.invoke.MethodHandleNatives.Constants;
31import java.lang.invoke.MethodHandles.Lookup;
32import static java.lang.invoke.MethodHandleStatics.*;
33
34/**
35 * A symbolic reference obtained by cracking a direct method handle
36 * into its consitutent symbolic parts.
37 * To crack a direct method handle, call {@link Lookup#revealDirect Lookup.revealDirect}.
38 * <h1><a id="directmh"></a>Direct Method Handles</h1>
39 * A <em>direct method handle</em> represents a method, constructor, or field without
40 * any intervening argument bindings or other transformations.
41 * The method, constructor, or field referred to by a direct method handle is called
42 * its <em>underlying member</em>.
43 * Direct method handles may be obtained in any of these ways:
44 * <ul>
45 * <li>By executing an {@code ldc} instruction on a {@code CONSTANT_MethodHandle} constant.
46 *     (See the Java Virtual Machine Specification, sections 4.4.8 and 5.4.3.)
47 * <li>By calling one of the <a href="MethodHandles.Lookup.html#lookups">Lookup Factory Methods</a>,
48 *     such as {@link Lookup#findVirtual Lookup.findVirtual},
49 *     to resolve a symbolic reference into a method handle.
50 *     A symbolic reference consists of a class, name string, and type.
51 * <li>By calling the factory method {@link Lookup#unreflect Lookup.unreflect}
52 *     or {@link Lookup#unreflectSpecial Lookup.unreflectSpecial}
53 *     to convert a {@link Method} into a method handle.
54 * <li>By calling the factory method {@link Lookup#unreflectConstructor Lookup.unreflectConstructor}
55 *     to convert a {@link Constructor} into a method handle.
56 * <li>By calling the factory method {@link Lookup#unreflectGetter Lookup.unreflectGetter}
57 *     or {@link Lookup#unreflectSetter Lookup.unreflectSetter}
58 *     to convert a {@link Field} into a method handle.
59 * </ul>
60 *
61 * <h1>Restrictions on Cracking</h1>
62 * Given a suitable {@code Lookup} object, it is possible to crack any direct method handle
63 * to recover a symbolic reference for the underlying method, constructor, or field.
64 * Cracking must be done via a {@code Lookup} object equivalent to that which created
65 * the target method handle, or which has enough access permissions to recreate
66 * an equivalent method handle.
67 * <p>
68 * If the underlying method is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>,
69 * the direct method handle will have been "bound" to a particular caller class, the
70 * {@linkplain java.lang.invoke.MethodHandles.Lookup#lookupClass() lookup class}
71 * of the lookup object used to create it.
72 * Cracking this method handle with a different lookup class will fail
73 * even if the underlying method is public (like {@code Class.forName}).
74 * <p>
75 * The requirement of lookup object matching provides a "fast fail" behavior
76 * for programs which may otherwise trust erroneous revelation of a method
77 * handle with symbolic information (or caller binding) from an unexpected scope.
78 * Use {@link java.lang.invoke.MethodHandles#reflectAs} to override this limitation.
79 *
80 * <h1><a id="refkinds"></a>Reference kinds</h1>
81 * The <a href="MethodHandles.Lookup.html#lookups">Lookup Factory Methods</a>
82 * correspond to all major use cases for methods, constructors, and fields.
83 * These use cases may be distinguished using small integers as follows:
84 * <table class="striped">
85 * <caption style="display:none">reference kinds</caption>
86 * <thead>
87 * <tr><th>reference kind</th><th>descriptive name</th><th>scope</th><th>member</th><th>behavior</th></tr>
88 * </thead>
89 * <tbody>
90 * <tr>
91 *     <td>{@code 1}</td><td>{@code REF_getField}</td><td>{@code class}</td>
92 *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
93 * </tr>
94 * <tr>
95 *     <td>{@code 2}</td><td>{@code REF_getStatic}</td><td>{@code class} or {@code interface}</td>
96 *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
97 * </tr>
98 * <tr>
99 *     <td>{@code 3}</td><td>{@code REF_putField}</td><td>{@code class}</td>
100 *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
101 * </tr>
102 * <tr>
103 *     <td>{@code 4}</td><td>{@code REF_putStatic}</td><td>{@code class}</td>
104 *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
105 * </tr>
106 * <tr>
107 *     <td>{@code 5}</td><td>{@code REF_invokeVirtual}</td><td>{@code class}</td>
108 *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
109 * </tr>
110 * <tr>
111 *     <td>{@code 6}</td><td>{@code REF_invokeStatic}</td><td>{@code class} or {@code interface}</td>
112 *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
113 * </tr>
114 * <tr>
115 *     <td>{@code 7}</td><td>{@code REF_invokeSpecial}</td><td>{@code class} or {@code interface}</td>
116 *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
117 * </tr>
118 * <tr>
119 *     <td>{@code 8}</td><td>{@code REF_newInvokeSpecial}</td><td>{@code class}</td>
120 *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
121 * </tr>
122 * <tr>
123 *     <td>{@code 9}</td><td>{@code REF_invokeInterface}</td><td>{@code interface}</td>
124 *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
125 * </tr>
126 * </tbody>
127 * </table>
128 * @since 1.8
129 */
130public
131interface MethodHandleInfo {
132    /**
133     * A direct method handle reference kind,
134     * as defined in the <a href="MethodHandleInfo.html#refkinds">table above</a>.
135     */
136    public static final int
137        REF_getField                = Constants.REF_getField,
138        REF_getStatic               = Constants.REF_getStatic,
139        REF_putField                = Constants.REF_putField,
140        REF_putStatic               = Constants.REF_putStatic,
141        REF_invokeVirtual           = Constants.REF_invokeVirtual,
142        REF_invokeStatic            = Constants.REF_invokeStatic,
143        REF_invokeSpecial           = Constants.REF_invokeSpecial,
144        REF_newInvokeSpecial        = Constants.REF_newInvokeSpecial,
145        REF_invokeInterface         = Constants.REF_invokeInterface;
146
147    /**
148     * Returns the reference kind of the cracked method handle, which in turn
149     * determines whether the method handle's underlying member was a constructor, method, or field.
150     * See the <a href="MethodHandleInfo.html#refkinds">table above</a> for definitions.
151     * @return the integer code for the kind of reference used to access the underlying member
152     */
153    public int getReferenceKind();
154
155    /**
156     * Returns the class in which the cracked method handle's underlying member was defined.
157     * @return the declaring class of the underlying member
158     */
159    public Class<?> getDeclaringClass();
160
161    /**
162     * Returns the name of the cracked method handle's underlying member.
163     * This is {@code "<init>"} if the underlying member was a constructor,
164     * else it is a simple method name or field name.
165     * @return the simple name of the underlying member
166     */
167    public String getName();
168
169    /**
170     * Returns the nominal type of the cracked symbolic reference, expressed as a method type.
171     * If the reference is to a constructor, the return type will be {@code void}.
172     * If it is to a non-static method, the method type will not mention the {@code this} parameter.
173     * If it is to a field and the requested access is to read the field,
174     * the method type will have no parameters and return the field type.
175     * If it is to a field and the requested access is to write the field,
176     * the method type will have one parameter of the field type and return {@code void}.
177     * <p>
178     * Note that original direct method handle may include a leading {@code this} parameter,
179     * or (in the case of a constructor) will replace the {@code void} return type
180     * with the constructed class.
181     * The nominal type does not include any {@code this} parameter,
182     * and (in the case of a constructor) will return {@code void}.
183     * @return the type of the underlying member, expressed as a method type
184     */
185    public MethodType getMethodType();
186
187    // Utility methods.
188    // NOTE: class/name/type and reference kind constitute a symbolic reference
189    // member and modifiers are an add-on, derived from Core Reflection (or the equivalent)
190
191    /**
192     * Reflects the underlying member as a method, constructor, or field object.
193     * If the underlying member is public, it is reflected as if by
194     * {@code getMethod}, {@code getConstructor}, or {@code getField}.
195     * Otherwise, it is reflected as if by
196     * {@code getDeclaredMethod}, {@code getDeclaredConstructor}, or {@code getDeclaredField}.
197     * The underlying member must be accessible to the given lookup object.
198     * @param <T> the desired type of the result, either {@link Member} or a subtype
199     * @param expected a class object representing the desired result type {@code T}
200     * @param lookup the lookup object that created this MethodHandleInfo, or one with equivalent access privileges
201     * @return a reference to the method, constructor, or field object
202     * @exception ClassCastException if the member is not of the expected type
203     * @exception NullPointerException if either argument is {@code null}
204     * @exception IllegalArgumentException if the underlying member is not accessible to the given lookup object
205     */
206    public <T extends Member> T reflectAs(Class<T> expected, Lookup lookup);
207
208    /**
209     * Returns the access modifiers of the underlying member.
210     * @return the Java language modifiers for underlying member,
211     *         or -1 if the member cannot be accessed
212     * @see Modifier
213     * @see #reflectAs
214     */
215    public int getModifiers();
216
217    /**
218     * Determines if the underlying member was a variable arity method or constructor.
219     * Such members are represented by method handles that are varargs collectors.
220     * @implSpec
221     * This produces a result equivalent to:
222     * <pre>{@code
223     *     getReferenceKind() >= REF_invokeVirtual && Modifier.isTransient(getModifiers())
224     * }</pre>
225     *
226     *
227     * @return {@code true} if and only if the underlying member was declared with variable arity.
228     */
229    // spelling derived from java.lang.reflect.Executable, not MethodHandle.isVarargsCollector
230    public default boolean isVarArgs()  {
231        // fields are never varargs:
232        if (MethodHandleNatives.refKindIsField((byte) getReferenceKind()))
233            return false;
234        // not in the public API: Modifier.VARARGS
235        final int ACC_VARARGS = 0x00000080;  // from JVMS 4.6 (Table 4.20)
236        assert(ACC_VARARGS == Modifier.TRANSIENT);
237        return Modifier.isTransient(getModifiers());
238    }
239
240    /**
241     * Returns the descriptive name of the given reference kind,
242     * as defined in the <a href="MethodHandleInfo.html#refkinds">table above</a>.
243     * The conventional prefix "REF_" is omitted.
244     * @param referenceKind an integer code for a kind of reference used to access a class member
245     * @return a mixed-case string such as {@code "getField"}
246     * @exception IllegalArgumentException if the argument is not a valid
247     *            <a href="MethodHandleInfo.html#refkinds">reference kind number</a>
248     */
249    public static String referenceKindToString(int referenceKind) {
250        if (!MethodHandleNatives.refKindIsValid(referenceKind))
251            throw newIllegalArgumentException("invalid reference kind", referenceKind);
252        return MethodHandleNatives.refKindName((byte)referenceKind);
253    }
254
255    /**
256     * Returns a string representation for a {@code MethodHandleInfo},
257     * given the four parts of its symbolic reference.
258     * This is defined to be of the form {@code "RK C.N:MT"}, where {@code RK} is the
259     * {@linkplain #referenceKindToString reference kind string} for {@code kind},
260     * {@code C} is the {@linkplain java.lang.Class#getName name} of {@code defc}
261     * {@code N} is the {@code name}, and
262     * {@code MT} is the {@code type}.
263     * These four values may be obtained from the
264     * {@linkplain #getReferenceKind reference kind},
265     * {@linkplain #getDeclaringClass declaring class},
266     * {@linkplain #getName member name},
267     * and {@linkplain #getMethodType method type}
268     * of a {@code MethodHandleInfo} object.
269     *
270     * @implSpec
271     * This produces a result equivalent to:
272     * <pre>{@code
273     *     String.format("%s %s.%s:%s", referenceKindToString(kind), defc.getName(), name, type)
274     * }</pre>
275     *
276     * @param kind the {@linkplain #getReferenceKind reference kind} part of the symbolic reference
277     * @param defc the {@linkplain #getDeclaringClass declaring class} part of the symbolic reference
278     * @param name the {@linkplain #getName member name} part of the symbolic reference
279     * @param type the {@linkplain #getMethodType method type} part of the symbolic reference
280     * @return a string of the form {@code "RK C.N:MT"}
281     * @exception IllegalArgumentException if the first argument is not a valid
282     *            <a href="MethodHandleInfo.html#refkinds">reference kind number</a>
283     * @exception NullPointerException if any reference argument is {@code null}
284     */
285    public static String toString(int kind, Class<?> defc, String name, MethodType type) {
286        Objects.requireNonNull(name); Objects.requireNonNull(type);
287        return String.format("%s %s.%s:%s", referenceKindToString(kind), defc.getName(), name, type);
288    }
289}
290