# Class ChainedCallSite

java.lang.Object
java.lang.invoke.CallSite
java.lang.invoke.MutableCallSite
jdk.dynalink.support.AbstractRelinkableCallSite
jdk.dynalink.support.ChainedCallSite

All Implemented Interfaces:
  `RelinkableCallSite`

public class ChainedCallSite extends AbstractRelinkableCallSite
A relinkable call site that implements a polymorphic inline caching strategy. It remembers up to 8 `GuardedInvocation`s it was linked with, and on each relink request builds a cascading chain of method handles of one invocation falling back to the next one. The number of remembered invocations can be customized by overriding `getMaxChainLength()` in a subclass. When this call site is relinked with a new invocation and the length of the chain is already at the maximum, it will throw away the oldest invocation. Invocations with invalidated switch points and ones for which their invalidating exception triggered are removed eagerly from the chain. The invocations are never reordered; the most recently linked method handle is always at the start of the chain and the least recently linked at its end. The call site can be safely relinked on more than one thread concurrently. Race conditions in linking are resolved by throwing away the `GuardedInvocation` produced on the losing thread without incorporating it into the chain, so it can lead to repeated linking for the same arguments.

Since:
  9

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `ChainedCallSite(CallSiteDescriptor descriptor)`
  Creates a new chained call site.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected int`
  `getMaxChainLength()`
  The maximum number of method handles in the chain.
  `void`
  `relink(GuardedInvocation guardedInvocation, MethodHandle relinkAndInvoke)`
  This method will be called by the dynamic linker every time the call site is relinked (but see `RelinkableCallSite.resetAndRelink(GuardedInvocation, MethodHandle)` for an exception).
  `void`
  `resetAndRelink(GuardedInvocation guardedInvocation, MethodHandle relinkAndInvoke)`
  This method will be called by the dynamic linker every time the call site is relinked and the linker wishes the call site to throw away any prior linkage state (that is how it differs from `RelinkableCallSite.relink(GuardedInvocation, MethodHandle)`).

  ### Methods declared in class AbstractRelinkableCallSite

  `getDescriptor, initialize`
  Modifier and Type
  Method
  Description
  `CallSiteDescriptor`
  `getDescriptor()`
  Returns the descriptor for this call site.
  `void`
  `initialize(MethodHandle relinkAndInvoke)`
  Invoked by dynamic linker to initialize the relinkable call site by setting a relink-and-invoke method handle.

  ### Methods declared in class MutableCallSite

  `dynamicInvoker, getTarget, setTarget, syncAll`
  Modifier and Type
  Method
  Description
  `final MethodHandle`
  `dynamicInvoker()`
  Produces a method handle equivalent to an invokedynamic instruction which has been linked to this call site.
  `final MethodHandle`
  `getTarget()`
  Returns the target method of the call site, which behaves like a normal field of the `MutableCallSite`.
  `void`
  `setTarget(MethodHandle newTarget)`
  Updates the target method of this call site, as a normal variable.
  `static void`
  `syncAll(MutableCallSite[] sites)`
  Performs a synchronization operation on each call site in the given array, forcing all other threads to throw away any cached values previously loaded from the target of any of the call sites.

  ### Methods declared in class CallSite

  `type`
  Modifier and Type
  Method
  Description
  `MethodType`
  `type()`
  Returns the type of this call site's target.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`
  Modifier and Type
  Method
  Description
  `protected Object`
  `clone()`
  Creates and returns a copy of this object.
  `boolean`
  `equals(Object obj)`
  Indicates whether some other object is "equal to" this one.
  `protected void`
  `finalize()`
  Deprecated, for removal: This API element is subject to removal in a future version.
  Finalization is deprecated and subject to removal in a future release.
  `final Class<?>`
  `getClass()`
  Returns the runtime class of this `Object`.
  `int`
  `hashCode()`
  Returns a hash code value for this object.
  `final void`
  `notify()`
  Wakes up a single thread that is waiting on this object's monitor.
  `final void`
  `notifyAll()`
  Wakes up all threads that are waiting on this object's monitor.
  `String`
  `toString()`
  Returns a string representation of the object.
  `final void`
  `wait()`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
  `final void`
  `wait(long timeoutMillis)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
  `final void`
  `wait(long timeoutMillis, int nanos)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

- ## Constructor Details

  - ### ChainedCallSite

    public ChainedCallSite(CallSiteDescriptor descriptor)
    Creates a new chained call site.
    Parameters:
      `descriptor` - the descriptor for the call site.

- ## Method Details

  - ### getMaxChainLength

    protected int getMaxChainLength()
    The maximum number of method handles in the chain. Defaults to 8. You can override it in a subclass if you need to change the value.
    Returns:
      the maximum number of method handles in the chain. The return value is checked, and if your override returns a value less than 1, a `RuntimeException` will be thrown.

  - ### relink

    public void relink(GuardedInvocation guardedInvocation, MethodHandle relinkAndInvoke)
    Description copied from interface: `RelinkableCallSite`
    This method will be called by the dynamic linker every time the call site is relinked (but see `RelinkableCallSite.resetAndRelink(GuardedInvocation, MethodHandle)` for an exception). It will be passed a `GuardedInvocation` that the call site should incorporate into its target method handle. When this method is called, the call site is allowed to keep other non-invalidated invocations around for implementation of polymorphic inline caches and compose them with this invocation to form its final target.
    Parameters:
      `guardedInvocation` - the guarded invocation that the call site should incorporate into its target method handle.
      `relinkAndInvoke` - a relink-and-invoke method handle. This is a method handle matching the method type of the call site that is supplied by the `DynamicLinker` as a callback. It should be used by this call site as the ultimate fallback when it can't invoke its target with the passed arguments. The fallback method is such that when it's invoked, it'll try to obtain an adequate target `GuardedInvocation` for the invocation, and subsequently invoke `RelinkableCallSite.relink(GuardedInvocation, MethodHandle)` or `RelinkableCallSite.resetAndRelink(GuardedInvocation, MethodHandle)`, and finally invoke the target.

  - ### resetAndRelink

    public void resetAndRelink(GuardedInvocation guardedInvocation, MethodHandle relinkAndInvoke)
    Description copied from interface: `RelinkableCallSite`
    This method will be called by the dynamic linker every time the call site is relinked and the linker wishes the call site to throw away any prior linkage state (that is how it differs from `RelinkableCallSite.relink(GuardedInvocation, MethodHandle)`). It will be passed a `GuardedInvocation` that the call site should use to build its new target method handle. When this method is called, the call site is discouraged from keeping any previous state, and is supposed to only link the current invocation.
    Parameters:
      `guardedInvocation` - the guarded invocation that the call site should use to build its target method handle.
      `relinkAndInvoke` - a relink-and-invoke method handle. This is a method handle matching the method type of the call site that is supplied by the `DynamicLinker` as a callback. It should be used by this call site as the ultimate fallback when it can't invoke its target with the passed arguments. The fallback method is such that when it's invoked, it'll try to obtain an adequate target `GuardedInvocation` for the invocation, and subsequently invoke `RelinkableCallSite.relink(GuardedInvocation, MethodHandle)` or `RelinkableCallSite.resetAndRelink(GuardedInvocation, MethodHandle)`, and finally invoke the target.