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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

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 * accompanied this code).

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package java.lang.invoke;

/**

 * A {@code CallSite} is a holder for a variable {@link MethodHandle},

 * which is called its {@code target}.

 * An {@code invokedynamic} instruction linked to a {@code CallSite} delegates

 * all calls to the site's current target.

 * A {@code CallSite} may be associated with several {@code invokedynamic}

 * instructions, or it may be "free floating", associated with none.

 * In any case, it may be invoked through an associated method handle

 * called its {@linkplain #dynamicInvoker dynamic invoker}.

 * <p>

 * {@code CallSite} is an abstract class which does not allow

 * direct subclassing by users.  It has three immediate,

 * concrete subclasses that may be either instantiated or subclassed.

 * <ul>

 * <li>If a mutable target is not required, an {@code invokedynamic} instruction

 * may be permanently bound by means of a {@linkplain ConstantCallSite constant call site}.

 * <li>If a mutable target is required which has volatile variable semantics,

 * because updates to the target must be immediately and reliably witnessed by other threads,

 * a {@linkplain VolatileCallSite volatile call site} may be used.

 * <li>Otherwise, if a mutable target is required,

 * a {@linkplain MutableCallSite mutable call site} may be used.

 * </ul>

 * <p>

 * A non-constant call site may be <em>relinked</em> by changing its target.

 * The new target must have the same {@linkplain MethodHandle#type() type}

 * as the previous target.

 * Thus, though a call site can be relinked to a series of

 * successive targets, it cannot change its type.

 * <p>

 * Here is a sample use of call sites and bootstrap methods which links every

 * dynamic call site to print its arguments:

<blockquote><pre>{@code

static void test() throws Throwable {

    // THE FOLLOWING LINE IS PSEUDOCODE FOR A JVM INSTRUCTION

    InvokeDynamic[#bootstrapDynamic].baz("baz arg", 2, 3.14);

}

private static void printArgs(Object... args) {

  System.out.println(java.util.Arrays.deepToString(args));

}

private static final MethodHandle printArgs;

static {

  MethodHandles.Lookup lookup = MethodHandles.lookup();

  Class thisClass = lookup.lookupClass();  // (who am I?)

  printArgs = lookup.findStatic(thisClass,

      "printArgs", MethodType.methodType(void.class, Object[].class));

}

private static CallSite bootstrapDynamic(MethodHandles.Lookup caller, String name, MethodType type) {

  // ignore caller and name, but match the type:

  return new ConstantCallSite(printArgs.asType(type));

}

}</pre></blockquote>

 * @author John Rose, JSR 292 EG

 */

abstract

public class CallSite {

    // The actual payload of this call site:

    /*package-private*/

    MethodHandle target;    // Note: This field is known to the JVM.  Do not change.

    /**

     * Make a blank call site object with the given method type.

     * An initial target method is supplied which will throw

     * an {@link IllegalStateException} if called.

     * <p>

     * Before this {@code CallSite} object is returned from a bootstrap method,

     * it is usually provided with a more useful target method,

     * via a call to {@link CallSite#setTarget(MethodHandle) setTarget}.

     * @throws NullPointerException if the proposed type is null

     */

    /*package-private*/

    CallSite(MethodType type) {

        throw new IllegalStateException();

    }

    /**

     * Make a call site object equipped with an initial target method handle.

     * @param target the method handle which will be the initial target of the call site

     * @throws NullPointerException if the proposed target is null

     */

    /*package-private*/

    CallSite(MethodHandle target) {

        target.type();  // null check

        this.target = target;

    }

    /**

     * Make a call site object equipped with an initial target method handle.

     * @param targetType the desired type of the call site

     * @param createTargetHook a hook which will bind the call site to the target method handle

     * @throws WrongMethodTypeException if the hook cannot be invoked on the required arguments,

     *         or if the target returned by the hook is not of the given {@code targetType}

     * @throws NullPointerException if the hook returns a null value

     * @throws ClassCastException if the hook returns something other than a {@code MethodHandle}

     * @throws Throwable anything else thrown by the hook function

     */

    /*package-private*/

    CallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {

        throw new IllegalStateException();

    }

    /**

     * Returns the type of this call site's target.

     * Although targets may change, any call site's type is permanent, and can never change to an unequal type.

     * The {@code setTarget} method enforces this invariant by refusing any new target that does

     * not have the previous target's type.

     * @return the type of the current target, which is also the type of any future target

     */

    public MethodType type() {

        // warning:  do not call getTarget here, because CCS.getTarget can throw IllegalStateException

        return target.type();

    }

    /**

     * Returns the target method of the call site, according to the

     * behavior defined by this call site's specific class.

     * The immediate subclasses of {@code CallSite} document the

     * class-specific behaviors of this method.

     *

     * @return the current linkage state of the call site, its target method handle

     * @see ConstantCallSite

     * @see VolatileCallSite

     * @see #setTarget

     * @see ConstantCallSite#getTarget

     * @see MutableCallSite#getTarget

     * @see VolatileCallSite#getTarget

     */

    public abstract MethodHandle getTarget();

    /**

     * Updates the target method of this call site, according to the

     * behavior defined by this call site's specific class.

     * The immediate subclasses of {@code CallSite} document the

     * class-specific behaviors of this method.

     * <p>

     * The type of the new target must be {@linkplain MethodType#equals equal to}

     * the type of the old target.

     *

     * @param newTarget the new target

     * @throws NullPointerException if the proposed new target is null

     * @throws WrongMethodTypeException if the proposed new target

     *         has a method type that differs from the previous target

     * @see CallSite#getTarget

     * @see ConstantCallSite#setTarget

     * @see MutableCallSite#setTarget

     * @see VolatileCallSite#setTarget

     */

    public abstract void setTarget(MethodHandle newTarget);

    /**

     * Produces a method handle equivalent to an invokedynamic instruction

     * which has been linked to this call site.

     * <p>

     * This method is equivalent to the following code:

     * <blockquote><pre>{@code

     * MethodHandle getTarget, invoker, result;

     * getTarget = MethodHandles.publicLookup().bind(this, "getTarget", MethodType.methodType(MethodHandle.class));

     * invoker = MethodHandles.exactInvoker(this.type());

     * result = MethodHandles.foldArguments(invoker, getTarget)

     * }</pre></blockquote>

     *

     * @return a method handle which always invokes this call site's current target

     */

    public abstract MethodHandle dynamicInvoker();

}