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28 import java.lang.Class;
29 import org.apidesign.bck2brwsr.core.JavaScriptBody;
32 * Lazily associate a computed value with (potentially) every type.
33 * For example, if a dynamic language needs to construct a message dispatch
34 * table for each class encountered at a message send call site,
35 * it can use a {@code ClassValue} to cache information needed to
36 * perform the message send quickly, for each class encountered.
37 * @author John Rose, JSR 292 EG
40 public abstract class ClassValue<T> {
42 * Sole constructor. (For invocation by subclass constructors, typically
45 protected ClassValue() {
49 * Computes the given class's derived value for this {@code ClassValue}.
51 * This method will be invoked within the first thread that accesses
52 * the value with the {@link #get get} method.
54 * Normally, this method is invoked at most once per class,
55 * but it may be invoked again if there has been a call to
56 * {@link #remove remove}.
58 * If this method throws an exception, the corresponding call to {@code get}
59 * will terminate abnormally with that exception, and no class value will be recorded.
61 * @param type the type whose class value must be computed
62 * @return the newly computed value associated with this {@code ClassValue}, for the given class or interface
66 protected abstract T computeValue(Class<?> type);
69 * Returns the value for the given class.
70 * If no value has yet been computed, it is obtained by
71 * an invocation of the {@link #computeValue computeValue} method.
73 * The actual installation of the value on the class
74 * is performed atomically.
75 * At that point, if several racing threads have
76 * computed values, one is chosen, and returned to
77 * all the racing threads.
79 * The {@code type} parameter is typically a class, but it may be any type,
80 * such as an interface, a primitive type (like {@code int.class}), or {@code void.class}.
82 * In the absence of {@code remove} calls, a class value has a simple
83 * state diagram: uninitialized and initialized.
84 * When {@code remove} calls are made,
85 * the rules for value observation are more complex.
86 * See the documentation for {@link #remove remove} for more information.
88 * @param type the type whose class value must be computed or retrieved
89 * @return the current value associated with this {@code ClassValue}, for the given class or interface
90 * @throws NullPointerException if the argument is null
94 public T get(Class<?> type) {
95 T value = access(type, id, false, null);
96 if (value == undefined()) {
97 value = access(type, id, true, computeValue(type));
103 * Removes the associated value for the given class.
104 * If this value is subsequently {@linkplain #get read} for the same class,
105 * its value will be reinitialized by invoking its {@link #computeValue computeValue} method.
106 * This may result in an additional invocation of the
107 * {@code computeValue} method for the given class.
109 * In order to explain the interaction between {@code get} and {@code remove} calls,
110 * we must model the state transitions of a class value to take into account
111 * the alternation between uninitialized and initialized states.
112 * To do this, number these states sequentially from zero, and note that
113 * uninitialized (or removed) states are numbered with even numbers,
114 * while initialized (or re-initialized) states have odd numbers.
116 * When a thread {@code T} removes a class value in state {@code 2N},
117 * nothing happens, since the class value is already uninitialized.
118 * Otherwise, the state is advanced atomically to {@code 2N+1}.
120 * When a thread {@code T} queries a class value in state {@code 2N},
121 * the thread first attempts to initialize the class value to state {@code 2N+1}
122 * by invoking {@code computeValue} and installing the resulting value.
124 * When {@code T} attempts to install the newly computed value,
125 * if the state is still at {@code 2N}, the class value will be initialized
126 * with the computed value, advancing it to state {@code 2N+1}.
128 * Otherwise, whether the new state is even or odd,
129 * {@code T} will discard the newly computed value
130 * and retry the {@code get} operation.
132 * Discarding and retrying is an important proviso,
133 * since otherwise {@code T} could potentially install
134 * a disastrously stale value. For example:
136 * <li>{@code T} calls {@code CV.get(C)} and sees state {@code 2N}
137 * <li>{@code T} quickly computes a time-dependent value {@code V0} and gets ready to install it
138 * <li>{@code T} is hit by an unlucky paging or scheduling event, and goes to sleep for a long time
139 * <li>...meanwhile, {@code T2} also calls {@code CV.get(C)} and sees state {@code 2N}
140 * <li>{@code T2} quickly computes a similar time-dependent value {@code V1} and installs it on {@code CV.get(C)}
141 * <li>{@code T2} (or a third thread) then calls {@code CV.remove(C)}, undoing {@code T2}'s work
142 * <li> the previous actions of {@code T2} are repeated several times
143 * <li> also, the relevant computed values change over time: {@code V1}, {@code V2}, ...
144 * <li>...meanwhile, {@code T} wakes up and attempts to install {@code V0}; <em>this must fail</em>
146 * We can assume in the above scenario that {@code CV.computeValue} uses locks to properly
147 * observe the time-dependent states as it computes {@code V1}, etc.
148 * This does not remove the threat of a stale value, since there is a window of time
149 * between the return of {@code computeValue} in {@code T} and the installation
150 * of the the new value. No user synchronization is possible during this time.
152 * @param type the type whose class value must be removed
153 * @throws NullPointerException if the argument is null
155 public void remove(Class<?> type) {
156 access(type, id, true, undefined());
159 /// Implementation...
161 private static int COUNTER = 0;
162 private final int id = COUNTER++;
164 @JavaScriptBody(args = {}, body = "return undefined;")
165 private static native Object undefined();
167 @JavaScriptBody(args = { "where", "index", "set", "newValue" }, body =
168 "var data = where['values'];\n" +
170 " data = where['values'] = [];\n" +
173 " data[index] = newValue;\n" +
175 "return data[index];\n" +
178 private static native <T> T access(Class<?> where, int index, boolean set, T newValue);