# Interface ExecutionControl

All Superinterfaces:
  `AutoCloseable`

All Known Implementing Classes:
  `DirectExecutionControl, JdiDefaultExecutionControl, JdiExecutionControl, LocalExecutionControl, RemoteExecutionControl, StreamingExecutionControl`

public interface ExecutionControl extends AutoCloseable
This interface specifies the functionality that must provided to implement a pluggable JShell execution engine.

The audience for this Service Provider Interface is engineers wishing to implement their own version of the execution engine in support of the JShell API.

A Snippet is compiled into code wrapped in a 'wrapper class'. The execution engine is used by the core JShell implementation to load and, for executable Snippets, execute the Snippet.

Methods defined in this interface should only be called by the core JShell implementation.

Since:
  9

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Interface
  Description
  `static final class`
  `ExecutionControl.ClassBytecodes`
  Bundles class name with class bytecodes.
  `static class`
  `ExecutionControl.ClassInstallException`
  A class install (load or redefine) encountered a problem.
  `static class`
  `ExecutionControl.EngineTerminationException`
  Unbidden execution engine termination has occurred.
  `static class`
  `ExecutionControl.ExecutionControlException`
  The abstract base of all `ExecutionControl` exceptions.
  `static class`
  `ExecutionControl.InternalException`
  An internal problem has occurred.
  `static class`
  `ExecutionControl.NotImplementedException`
  The command is not implemented.
  `static class`
  `ExecutionControl.ResolutionException`
  An exception indicating that a `DeclarationSnippet` with unresolved references has been encountered.
  `static class`
  `ExecutionControl.RunException`
  The abstract base of of exceptions specific to running user code.
  `static class`
  `ExecutionControl.StoppedException`
  An exception indicating that an `invoke(java.lang.String, java.lang.String)` (or theoretically a `varValue(java.lang.String, java.lang.String)`) has been interrupted by a `stop()`.
  `static class`
  `ExecutionControl.UserException`
  A 'normal' user exception occurred.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `addToClasspath(String path)`
  Adds the path to the execution class path.
  `void`
  `close()`
  Shuts down this execution engine.
  `Object`
  `extensionCommand(String command, Object arg)`
  Run a non-standard command (or a standard command from a newer version).
  `static ExecutionControl`
  `generate(ExecutionEnv env, String spec)`
  Search for a provider, then create and return the `ExecutionControl` instance.
  `static ExecutionControl`
  `generate(ExecutionEnv env, String name, Map<String,String> parameters)`
  Search for a provider, then create and return the `ExecutionControl` instance.
  `String`
  `invoke(String className, String methodName)`
  Invokes an executable Snippet by calling a method on the specified wrapper class.
  `void`
  `load(ExecutionControl.ClassBytecodes[] cbcs)`
  Attempts to load new classes.
  `void`
  `redefine(ExecutionControl.ClassBytecodes[] cbcs)`
  Attempts to redefine previously loaded classes.
  `void`
  `stop()`
  Interrupts a running invoke.
  `String`
  `varValue(String className, String varName)`
  Returns the value of a variable.

- ## Method Details

  - ### load

    void load(ExecutionControl.ClassBytecodes[] cbcs) throws ExecutionControl.ClassInstallException, ExecutionControl.NotImplementedException, ExecutionControl.EngineTerminationException
    Attempts to load new classes.
    Parameters:
      `cbcs` - the class name and bytecodes to load
    Throws:
      `ExecutionControl.ClassInstallException` - exception occurred loading the classes, some or all were not loaded
      `ExecutionControl.NotImplementedException` - if not implemented
      `ExecutionControl.EngineTerminationException` - the execution engine has terminated

  - ### redefine

    void redefine(ExecutionControl.ClassBytecodes[] cbcs) throws ExecutionControl.ClassInstallException, ExecutionControl.NotImplementedException, ExecutionControl.EngineTerminationException
    Attempts to redefine previously loaded classes.
    Parameters:
      `cbcs` - the class name and bytecodes to redefine
    Throws:
      `ExecutionControl.ClassInstallException` - exception occurred redefining the classes, some or all were not redefined
      `ExecutionControl.NotImplementedException` - if not implemented
      `ExecutionControl.EngineTerminationException` - the execution engine has terminated

  - ### invoke

    String invoke(String className, String methodName) throws ExecutionControl.RunException, ExecutionControl.EngineTerminationException, ExecutionControl.InternalException
    Invokes an executable Snippet by calling a method on the specified wrapper class. The method must have no arguments and return String.
    Parameters:
      `className` - the class whose method should be invoked
      `methodName` - the name of method to invoke
    Returns:
      the result of the execution or null if no result
    Throws:
      `ExecutionControl.UserException` - the invoke raised a user exception
      `ExecutionControl.ResolutionException` - the invoke attempted to directly or indirectly invoke an unresolved snippet
      `ExecutionControl.StoppedException` - if the `invoke()` was canceled by `stop()`
      `ExecutionControl.EngineTerminationException` - the execution engine has terminated
      `ExecutionControl.InternalException` - an internal problem occurred
      `ExecutionControl.RunException`

  - ### varValue

    String varValue(String className, String varName) throws ExecutionControl.RunException, ExecutionControl.EngineTerminationException, ExecutionControl.InternalException
    Returns the value of a variable.
    Parameters:
      `className` - the name of the wrapper class of the variable
      `varName` - the name of the variable
    Returns:
      the value of the variable
    Throws:
      `ExecutionControl.UserException` - formatting the value raised a user exception
      `ExecutionControl.ResolutionException` - formatting the value attempted to directly or indirectly invoke an unresolved snippet
      `ExecutionControl.StoppedException` - if the formatting the value was canceled by `stop()`
      `ExecutionControl.EngineTerminationException` - the execution engine has terminated
      `ExecutionControl.InternalException` - an internal problem occurred
      `ExecutionControl.RunException`

  - ### addToClasspath

    void addToClasspath(String path) throws ExecutionControl.EngineTerminationException, ExecutionControl.InternalException
    Adds the path to the execution class path.
    Parameters:
      `path` - the path to add
    Throws:
      `ExecutionControl.EngineTerminationException` - the execution engine has terminated
      `ExecutionControl.InternalException` - an internal problem occurred

  - ### stop

    void stop() throws ExecutionControl.EngineTerminationException, ExecutionControl.InternalException
    Interrupts a running invoke.
    Throws:
      `ExecutionControl.EngineTerminationException` - the execution engine has terminated
      `ExecutionControl.InternalException` - an internal problem occurred

  - ### extensionCommand

    Object extensionCommand(String command, Object arg) throws ExecutionControl.RunException, ExecutionControl.EngineTerminationException, ExecutionControl.InternalException
    Run a non-standard command (or a standard command from a newer version).
    Parameters:
      `command` - the non-standard command
      `arg` - the commands argument
    Returns:
      the commands return value
    Throws:
      `ExecutionControl.UserException` - the command raised a user exception
      `ExecutionControl.ResolutionException` - the command attempted to directly or indirectly invoke an unresolved snippet
      `ExecutionControl.StoppedException` - if the command was canceled by `stop()`
      `ExecutionControl.EngineTerminationException` - the execution engine has terminated
      `ExecutionControl.NotImplementedException` - if not implemented
      `ExecutionControl.InternalException` - an internal problem occurred
      `ExecutionControl.RunException`

  - ### close

    void close()
    Shuts down this execution engine. Implementation should free all resources held by this execution engine.

    No calls to methods on this interface should be made after close.

    Specified by:
      `close` in interface `AutoCloseable`

  - ### generate

    static ExecutionControl generate(ExecutionEnv env, String name, Map<String,String> parameters) throws Throwable
    Search for a provider, then create and return the `ExecutionControl` instance.
    Parameters:
      `env` - the execution environment (provided by JShell)
      `name` - the name of provider
      `parameters` - the parameter map.
    Returns:
      the execution engine
    Throws:
      `Throwable` - an exception that occurred attempting to find or create the execution engine.
      `IllegalArgumentException` - if no ExecutionControlProvider has the specified `name` and `parameters`.

  - ### generate

    static ExecutionControl generate(ExecutionEnv env, String spec) throws Throwable
    Search for a provider, then create and return the `ExecutionControl` instance.
    Parameters:
      `env` - the execution environment (provided by JShell)
      `spec` - the `ExecutionControl` spec, which is described in the documentation of this package documentation.
    Returns:
      the execution engine
    Throws:
      `Throwable` - an exception that occurred attempting to find or create the execution engine.
      `IllegalArgumentException` - if no ExecutionControlProvider has the specified `name` and `parameters`.
      `IllegalArgumentException` - if `spec` is malformed