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25 
26 package java.lang.invoke;
27 
28 /**
29  * A {@code ConstantCallSite} is a {@link CallSite} whose target is permanent, and can never be changed.
30  * An {@code invokedynamic} instruction linked to a {@code ConstantCallSite} is permanently
31  * bound to the call site's target.
32  * @author John Rose, JSR 292 EG
33  */
34 public class ConstantCallSite extends CallSite {
35     private final boolean isFrozen;
36 
37     /**
38      * Creates a call site with a permanent target.
39      * @param target the target to be permanently associated with this call site
40      * @throws NullPointerException if the proposed target is null
41      */
ConstantCallSite(MethodHandle target)42     public ConstantCallSite(MethodHandle target) {
43         super(target);
44         isFrozen = true;
45     }
46 
47     /**
48      * Creates a call site with a permanent target, possibly bound to the call site itself.
49      * <p>
50      * During construction of the call site, the {@code createTargetHook} is invoked to
51      * produce the actual target, as if by a call of the form
52      * {@code (MethodHandle) createTargetHook.invoke(this)}.
53      * <p>
54      * Note that user code cannot perform such an action directly in a subclass constructor,
55      * since the target must be fixed before the {@code ConstantCallSite} constructor returns.
56      * <p>
57      * The hook is said to bind the call site to a target method handle,
58      * and a typical action would be {@code someTarget.bindTo(this)}.
59      * However, the hook is free to take any action whatever,
60      * including ignoring the call site and returning a constant target.
61      * <p>
62      * The result returned by the hook must be a method handle of exactly
63      * the same type as the call site.
64      * <p>
65      * While the hook is being called, the new {@code ConstantCallSite}
66      * object is in a partially constructed state.
67      * In this state,
68      * a call to {@code getTarget}, or any other attempt to use the target,
69      * will result in an {@code IllegalStateException}.
70      * It is legal at all times to obtain the call site's type using the {@code type} method.
71      *
72      * @param targetType the type of the method handle to be permanently associated with this call site
73      * @param createTargetHook a method handle to invoke (on the call site) to produce the call site's target
74      * @throws WrongMethodTypeException if the hook cannot be invoked on the required arguments,
75      *         or if the target returned by the hook is not of the given {@code targetType}
76      * @throws NullPointerException if the hook returns a null value
77      * @throws ClassCastException if the hook returns something other than a {@code MethodHandle}
78      * @throws Throwable anything else thrown by the hook function
79      */
ConstantCallSite(MethodType targetType, MethodHandle createTargetHook)80     protected ConstantCallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {
81         super(targetType, createTargetHook);
82         isFrozen = true;
83     }
84 
85     /**
86      * Returns the target method of the call site, which behaves
87      * like a {@code final} field of the {@code ConstantCallSite}.
88      * That is, the target is always the original value passed
89      * to the constructor call which created this instance.
90      *
91      * @return the immutable linkage state of this call site, a constant method handle
92      * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed
93      */
getTarget()94     @Override public final MethodHandle getTarget() {
95         if (!isFrozen)  throw new IllegalStateException();
96         return target;
97     }
98 
99     /**
100      * Always throws an {@link UnsupportedOperationException}.
101      * This kind of call site cannot change its target.
102      * @param ignore a new target proposed for the call site, which is ignored
103      * @throws UnsupportedOperationException because this kind of call site cannot change its target
104      */
setTarget(MethodHandle ignore)105     @Override public final void setTarget(MethodHandle ignore) {
106         throw new UnsupportedOperationException();
107     }
108 
109     /**
110      * Returns this call site's permanent target.
111      * Since that target will never change, this is a correct implementation
112      * of {@link CallSite#dynamicInvoker CallSite.dynamicInvoker}.
113      * @return the immutable linkage state of this call site, a constant method handle
114      * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed
115      */
116     @Override
dynamicInvoker()117     public final MethodHandle dynamicInvoker() {
118         return getTarget();
119     }
120 }
121