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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

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

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

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package java.lang.invoke;

import static java.lang.invoke.MethodHandleStatics.*;

import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP;

import jdk.internal.vm.annotation.Stable;

/**

 * 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

 * @since 1.7

 */

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) {

        target = makeUninitializedCallSite(type);

    }

    /**

     * 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 {

        this(targetType);

        ConstantCallSite selfCCS = (ConstantCallSite) this;

        MethodHandle boundTarget = (MethodHandle) createTargetHook.invokeWithArguments(selfCCS);

        checkTargetChange(this.target, boundTarget);

        this.target = boundTarget;

    }

    /**

     * {@code CallSite} dependency context.

     * JVM uses CallSite.context to store nmethod dependencies on the call site target.

     */

    private final MethodHandleNatives.CallSiteContext context = MethodHandleNatives.CallSiteContext.make(this);

    /**

     * 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);

    void checkTargetChange(MethodHandle oldTarget, MethodHandle newTarget) {

        MethodType oldType = oldTarget.type();

        MethodType newType = newTarget.type();  // null check!

        if (!newType.equals(oldType))

            throw wrongTargetType(newTarget, oldType);

    }

    private static WrongMethodTypeException wrongTargetType(MethodHandle target, MethodType type) {

        return new WrongMethodTypeException(String.valueOf(target)+" should be of type "+type);

    }

    /**

     * 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();

        MethodHandle getTarget = getTargetHandle().bindArgumentL(0, this);

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

        return MethodHandles.foldArguments(invoker, getTarget);

    }

    private static @Stable MethodHandle GET_TARGET;

    private static MethodHandle getTargetHandle() {

        MethodHandle handle = GET_TARGET;

        if (handle != null) {

            return handle;

        }

        try {

            return GET_TARGET = IMPL_LOOKUP.

                    findVirtual(CallSite.class, "getTarget",

                                MethodType.methodType(MethodHandle.class));

        } catch (ReflectiveOperationException e) {

            throw newInternalError(e);

        }

    }

    private static @Stable MethodHandle THROW_UCS;

    private static MethodHandle uninitializedCallSiteHandle() {

        MethodHandle handle = THROW_UCS;

        if (handle != null) {

            return handle;

        }

        try {

            return THROW_UCS = IMPL_LOOKUP.

                findStatic(CallSite.class, "uninitializedCallSite",

                           MethodType.methodType(Object.class, Object[].class));

        } catch (ReflectiveOperationException e) {

            throw newInternalError(e);

        }

    }

    /** This guy is rolled into the default target if a MethodType is supplied to the constructor. */

    private static Object uninitializedCallSite(Object... ignore) {

        throw new IllegalStateException("uninitialized call site");

    }

    private MethodHandle makeUninitializedCallSite(MethodType targetType) {

        MethodType basicType = targetType.basicType();

        MethodHandle invoker = basicType.form().cachedMethodHandle(MethodTypeForm.MH_UNINIT_CS);

        if (invoker == null) {

            invoker = uninitializedCallSiteHandle().asType(basicType);

            invoker = basicType.form().setCachedMethodHandle(MethodTypeForm.MH_UNINIT_CS, invoker);

        }

        // unchecked view is OK since no values will be received or returned

        return invoker.viewAsType(targetType, false);

    }

    // unsafe stuff:

    private static @Stable long TARGET_OFFSET;

    private static long getTargetOffset() {

        long offset = TARGET_OFFSET;

        if (offset > 0) {

            return offset;

        }

        offset = TARGET_OFFSET = UNSAFE.objectFieldOffset(CallSite.class, "target");

        assert(offset > 0);

        return offset;

    }

    /*package-private*/

    void setTargetNormal(MethodHandle newTarget) {

        MethodHandleNatives.setCallSiteTargetNormal(this, newTarget);

    }

    /*package-private*/

    MethodHandle getTargetVolatile() {

        return (MethodHandle) UNSAFE.getReferenceVolatile(this, getTargetOffset());

    }

    /*package-private*/

    void setTargetVolatile(MethodHandle newTarget) {

        MethodHandleNatives.setCallSiteTargetVolatile(this, newTarget);

    }

    // this implements the upcall from the JVM, MethodHandleNatives.linkCallSite:

    static CallSite makeSite(MethodHandle bootstrapMethod,

                             // Callee information:

                             String name, MethodType type,

                             // Extra arguments for BSM, if any:

                             Object info,

                             // Caller information:

                             Class<?> callerClass) {

        CallSite site;

        try {

            Object binding = BootstrapMethodInvoker.invoke(

                    CallSite.class, bootstrapMethod, name, type, info, callerClass);

            if (binding instanceof CallSite) {

                site = (CallSite) binding;

            } else {

                // See the "Linking Exceptions" section for the invokedynamic

                // instruction in JVMS 6.5.

                // Throws a runtime exception defining the cause that is then

                // in the "catch (Throwable ex)" a few lines below wrapped in

                // BootstrapMethodError

                throw new ClassCastException("CallSite bootstrap method failed to produce an instance of CallSite");

            }

            if (!site.getTarget().type().equals(type)) {

                // See the "Linking Exceptions" section for the invokedynamic

                // instruction in JVMS 6.5.

                // Throws a runtime exception defining the cause that is then

                // in the "catch (Throwable ex)" a few lines below wrapped in

                // BootstrapMethodError

                throw wrongTargetType(site.getTarget(), type);

            }

        } catch (Error e) {

            // Pass through an Error, including BootstrapMethodError, any other

            // form of linkage error, such as IllegalAccessError if the bootstrap

            // method is inaccessible, or say ThreadDeath/OutOfMemoryError

            // See the "Linking Exceptions" section for the invokedynamic

            // instruction in JVMS 6.5.

            throw e;

        } catch (Throwable ex) {

            // Wrap anything else in BootstrapMethodError

            throw new BootstrapMethodError("CallSite bootstrap method initialization exception", ex);

        }

        return site;

    }

}