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25
26 /**
27 * The {@code java.lang.invoke} package contains dynamic language support provided directly by
28 * the Java core class libraries and virtual machine.
29 *
30 * <p>
31 * As described in the Java Virtual Machine Specification,
32 * certain types in this package have special relations to dynamic
33 * language support in the virtual machine:
34 * <ul>
35 * <li>The classes {@link java.lang.invoke.MethodHandle MethodHandle}
36 * {@link java.lang.invoke.VarHandle VarHandle} contain
37 * <a href="MethodHandle.html#sigpoly">signature polymorphic methods</a>
38 * which can be linked regardless of their type descriptor.
39 * Normally, method linkage requires exact matching of type descriptors.
40 * </li>
41 *
42 * <li>The JVM bytecode format supports immediate constants of
43 * the classes {@link java.lang.invoke.MethodHandle MethodHandle} and {@link java.lang.invoke.MethodType MethodType}.
44 * </li>
45 * </ul>
46 *
47 * <h1><a name="jvm_mods"></a>Summary of relevant Java Virtual Machine changes</h1>
48 * The following low-level information summarizes relevant parts of the
49 * Java Virtual Machine specification. For full details, please see the
50 * current version of that specification.
51 *
52 * Each occurrence of an {@code invokedynamic} instruction is called a <em>dynamic call site</em>.
53 * <h2><a name="indyinsn"></a>{@code invokedynamic} instructions</h2>
54 * A dynamic call site is originally in an unlinked state. In this state, there is
55 * no target method for the call site to invoke.
56 * <p>
57 * Before the JVM can execute a dynamic call site (an {@code invokedynamic} instruction),
58 * the call site must first be <em>linked</em>.
59 * Linking is accomplished by calling a <em>bootstrap method</em>
60 * which is given the static information content of the call site,
61 * and which must produce a {@link java.lang.invoke.MethodHandle method handle}
62 * that gives the behavior of the call site.
63 * <p>
64 * Each {@code invokedynamic} instruction statically specifies its own
65 * bootstrap method as a constant pool reference.
66 * The constant pool reference also specifies the call site's name and type descriptor,
67 * just like {@code invokevirtual} and the other invoke instructions.
68 * <p>
69 * Linking starts with resolving the constant pool entry for the
70 * bootstrap method, and resolving a {@link java.lang.invoke.MethodType MethodType} object for
71 * the type descriptor of the dynamic call site.
72 * This resolution process may trigger class loading.
73 * It may therefore throw an error if a class fails to load.
74 * This error becomes the abnormal termination of the dynamic
75 * call site execution.
76 * Linkage does not trigger class initialization.
77 * <p>
78 * The bootstrap method is invoked on at least three values:
79 * <ul>
80 * <li>a {@code MethodHandles.Lookup}, a lookup object on the <em>caller class</em> in which dynamic call site occurs </li>
81 * <li>a {@code String}, the method name mentioned in the call site </li>
82 * <li>a {@code MethodType}, the resolved type descriptor of the call </li>
83 * <li>optionally, between 1 and 251 additional static arguments taken from the constant pool </li>
84 * </ul>
85 * Invocation is as if by
86 * {@link java.lang.invoke.MethodHandle#invoke MethodHandle.invoke}.
87 * The returned result must be a {@link java.lang.invoke.CallSite CallSite}
88 * (or a subclass), otherwise a
89 * {@link java.lang.BootstrapMethodError BootstrapMethodError} is thrown.
90 * The type of the call site's target must be exactly equal to the type
91 * derived from the dynamic call site's type descriptor and passed to
92 * the bootstrap method, otherwise a {@code BootstrapMethodError} is thrown.
93 * On success the call site then becomes permanently linked to the dynamic call
94 * site.
95 * <p>
96 * If an exception, {@code E} say, occurs when linking the call site then the
97 * linkage fails and terminates abnormally. {@code E} is rethrown if the type of
98 * {@code E} is {@code Error} or a subclass, otherwise a
99 * {@code BootstrapMethodError} that wraps {@code E} is thrown.
100 * If this happens, the same {@code Error} or subclass will the thrown for all
101 * subsequent attempts to execute the dynamic call site.
102 * <h2>timing of linkage</h2>
103 * A dynamic call site is linked just before its first execution.
104 * The bootstrap method call implementing the linkage occurs within
105 * a thread that is attempting a first execution.
106 * <p>
107 * If there are several such threads, the bootstrap method may be
108 * invoked in several threads concurrently.
109 * Therefore, bootstrap methods which access global application
110 * data must take the usual precautions against race conditions.
111 * In any case, every {@code invokedynamic} instruction is either
112 * unlinked or linked to a unique {@code CallSite} object.
113 * <p>
114 * In an application which requires dynamic call sites with individually
115 * mutable behaviors, their bootstrap methods should produce distinct
116 * {@link java.lang.invoke.CallSite CallSite} objects, one for each linkage request.
117 * Alternatively, an application can link a single {@code CallSite} object
118 * to several {@code invokedynamic} instructions, in which case
119 * a change to the target method will become visible at each of
120 * the instructions.
121 * <p>
122 * If several threads simultaneously execute a bootstrap method for a single dynamic
123 * call site, the JVM must choose one {@code CallSite} object and install it visibly to
124 * all threads. Any other bootstrap method calls are allowed to complete, but their
125 * results are ignored, and their dynamic call site invocations proceed with the originally
126 * chosen target object.
127
128 * <p style="font-size:smaller;">
129 * <em>Discussion:</em>
130 * These rules do not enable the JVM to duplicate dynamic call sites,
131 * or to issue “causeless” bootstrap method calls.
132 * Every dynamic call site transitions at most once from unlinked to linked,
133 * just before its first invocation.
134 * There is no way to undo the effect of a completed bootstrap method call.
135 *
136 * <h2>types of bootstrap methods</h2>
137 * As long as each bootstrap method can be correctly invoked
138 * by {@code MethodHandle.invoke}, its detailed type is arbitrary.
139 * For example, the first argument could be {@code Object}
140 * instead of {@code MethodHandles.Lookup}, and the return type
141 * could also be {@code Object} instead of {@code CallSite}.
142 * (Note that the types and number of the stacked arguments limit
143 * the legal kinds of bootstrap methods to appropriately typed
144 * static methods and constructors of {@code CallSite} subclasses.)
145 * <p>
146 * If a given {@code invokedynamic} instruction specifies no static arguments,
147 * the instruction's bootstrap method will be invoked on three arguments,
148 * conveying the instruction's caller class, name, and method type.
149 * If the {@code invokedynamic} instruction specifies one or more static arguments,
150 * those values will be passed as additional arguments to the method handle.
151 * (Note that because there is a limit of 255 arguments to any method,
152 * at most 251 extra arguments can be supplied, since the bootstrap method
153 * handle itself and its first three arguments must also be stacked.)
154 * The bootstrap method will be invoked as if by either {@code MethodHandle.invoke}
155 * or {@code invokeWithArguments}. (There is no way to tell the difference.)
156 * <p>
157 * The normal argument conversion rules for {@code MethodHandle.invoke} apply to all stacked arguments.
158 * For example, if a pushed value is a primitive type, it may be converted to a reference by boxing conversion.
159 * If the bootstrap method is a variable arity method (its modifier bit {@code 0x0080} is set),
160 * then some or all of the arguments specified here may be collected into a trailing array parameter.
161 * (This is not a special rule, but rather a useful consequence of the interaction
162 * between {@code CONSTANT_MethodHandle} constants, the modifier bit for variable arity methods,
163 * and the {@link java.lang.invoke.MethodHandle#asVarargsCollector asVarargsCollector} transformation.)
164 * <p>
165 * Given these rules, here are examples of legal bootstrap method declarations,
166 * given various numbers {@code N} of extra arguments.
167 * The first rows (marked {@code *}) will work for any number of extra arguments.
168 * <table border=1 cellpadding=5 summary="Static argument types">
169 * <tr><th>N</th><th>sample bootstrap method</th></tr>
170 * <tr><td>*</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)</code></td></tr>
171 * <tr><td>*</td><td><code>CallSite bootstrap(Object... args)</code></td></tr>
172 * <tr><td>*</td><td><code>CallSite bootstrap(Object caller, Object... nameAndTypeWithArgs)</code></td></tr>
173 * <tr><td>0</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type)</code></td></tr>
174 * <tr><td>0</td><td><code>CallSite bootstrap(Lookup caller, Object... nameAndType)</code></td></tr>
175 * <tr><td>1</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object arg)</code></td></tr>
176 * <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, Object... args)</code></td></tr>
177 * <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, String... args)</code></td></tr>
178 * <tr><td>2</td><td><code>CallSite bootstrap(Lookup caller, String name, MethodType type, String x, int y)</code></td></tr>
179 * </table>
180 * The last example assumes that the extra arguments are of type
181 * {@code CONSTANT_String} and {@code CONSTANT_Integer}, respectively.
182 * The second-to-last example assumes that all extra arguments are of type
183 * {@code CONSTANT_String}.
184 * The other examples work with all types of extra arguments.
185 * <p>
186 * As noted above, the actual method type of the bootstrap method can vary.
187 * For example, the fourth argument could be {@code MethodHandle},
188 * if that is the type of the corresponding constant in
189 * the {@code CONSTANT_InvokeDynamic} entry.
190 * In that case, the {@code MethodHandle.invoke} call will pass the extra method handle
191 * constant as an {@code Object}, but the type matching machinery of {@code MethodHandle.invoke}
192 * will cast the reference back to {@code MethodHandle} before invoking the bootstrap method.
193 * (If a string constant were passed instead, by badly generated code, that cast would then fail,
194 * resulting in a {@code BootstrapMethodError}.)
195 * <p>
196 * Note that, as a consequence of the above rules, the bootstrap method may accept a primitive
197 * argument, if it can be represented by a constant pool entry.
198 * However, arguments of type {@code boolean}, {@code byte}, {@code short}, or {@code char}
199 * cannot be created for bootstrap methods, since such constants cannot be directly
200 * represented in the constant pool, and the invocation of the bootstrap method will
201 * not perform the necessary narrowing primitive conversions.
202 * <p>
203 * Extra bootstrap method arguments are intended to allow language implementors
204 * to safely and compactly encode metadata.
205 * In principle, the name and extra arguments are redundant,
206 * since each call site could be given its own unique bootstrap method.
207 * Such a practice is likely to produce large class files and constant pools.
208 *
209 * @author John Rose, JSR 292 EG
210 * @since 1.7
211 */
212
213 package java.lang.invoke;