# Class JShell

java.lang.Object
jdk.jshell.JShell

All Implemented Interfaces:
  `AutoCloseable`

public class JShell extends Object implements AutoCloseable
The JShell evaluation state engine. This is the central class in the JShell API. A `JShell` instance holds the evolving compilation and execution state. The state is changed with the instance methods `eval(String)`, `drop(Snippet)` and `addToClasspath(String)`. The majority of methods query the state. A `JShell` instance also allows registering for events with `onSnippetEvent(Consumer)` and `onShutdown(Consumer)`, which are unregistered with `unsubscribe(Subscription)`. Access to the source analysis utilities is via `sourceCodeAnalysis()`. When complete the instance should be closed to free resources -- `close()`.

An instance of `JShell` is created with `JShell.create()`.

This class is not thread safe, except as noted, all access should be through a single thread.

Since:
  9

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `static class`
  `JShell.Builder`
  Builder for `JShell` instances.
  `class`
  `JShell.Subscription`
  Subscription is a token for referring to subscriptions so they can be unsubscribed.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `addToClasspath(String path)`
  The specified path is added to the end of the classpath used in eval().
  `static JShell.Builder`
  `builder()`
  Factory method for `JShell.Builder` which, in-turn, is used for creating instances of `JShell`.
  `void`
  `close()`
  Close this state engine.
  `static JShell`
  `create()`
  Create a new JShell state engine.
  `Stream<Diag>`
  `diagnostics(Snippet snippet)`
  Return the diagnostics of the most recent evaluation of the snippet.
  `List<SnippetEvent>`
  `drop(Snippet snippet)`
  Remove a declaration from the state.
  `List<SnippetEvent>`
  `eval(String input)`
  Evaluate the input String, including definition and/or execution, if applicable.
  `Stream<ImportSnippet>`
  `imports()`
  Returns the active import snippets.
  `Stream<MethodSnippet>`
  `methods()`
  Returns the active method snippets.
  `JShell.Subscription`
  `onShutdown(Consumer<JShell> listener)`
  Register a callback to be called when this JShell instance terminates.
  `JShell.Subscription`
  `onSnippetEvent(Consumer<SnippetEvent> listener)`
  Register a callback to be called when the Status of a snippet changes.
  `Stream<Snippet>`
  `snippets()`
  Return all snippets.
  `SourceCodeAnalysis`
  `sourceCodeAnalysis()`
  Access to source code analysis functionality.
  `Snippet.Status`
  `status(Snippet snippet)`
  Return the status of the snippet.
  `void`
  `stop()`
  Attempt to stop currently running evaluation.
  `Stream<TypeDeclSnippet>`
  `types()`
  Returns the active type declaration (class, interface, annotation type, and enum) snippets.
  `Stream<String>`
  `unresolvedDependencies(DeclarationSnippet snippet)`
  For `RECOVERABLE_DEFINED` or `RECOVERABLE_NOT_DEFINED` declarations, the names of current unresolved dependencies for the snippet.
  `void`
  `unsubscribe(JShell.Subscription token)`
  Cancel a callback subscription.
  `Stream<VarSnippet>`
  `variables()`
  Returns the active variable snippets.
  `String`
  `varValue(VarSnippet snippet)`
  Get the current value of a variable.

  ### Methods declared in class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Method Details

  - ### create

    public static JShell create() throws IllegalStateException
    Create a new JShell state engine. That is, create an instance of `JShell`.

    Equivalent to `JShell.builder()``.build()`.

    Returns:
      an instance of `JShell`.
    Throws:
      `IllegalStateException` - if the `JShell` instance could not be created.

  - ### builder

    public static JShell.Builder builder()
    Factory method for `JShell.Builder` which, in-turn, is used for creating instances of `JShell`. Create a default instance of `JShell` with `JShell.builder().build()`. For more construction options see `JShell.Builder`.
    Returns:
      an instance of `Builder`.
    See Also:
      - `JShell.Builder`

  - ### sourceCodeAnalysis

    public SourceCodeAnalysis sourceCodeAnalysis()
    Access to source code analysis functionality. An instance of `JShell` will always return the same `SourceCodeAnalysis` instance from `sourceCodeAnalysis()`.
    Returns:
      an instance of `SourceCodeAnalysis` which can be used for source analysis such as completion detection and completion suggestions.

  - ### eval

    public List<SnippetEvent> eval(String input) throws IllegalStateException
    Evaluate the input String, including definition and/or execution, if applicable. The input is checked for errors, unless the errors can be deferred (as is the case with some unresolvedDependencies references), errors will abort evaluation.

    The input should be exactly one complete snippet of source code, that is, one expression, statement, variable declaration, method declaration, class declaration, or import. To break arbitrary input into individual complete snippets, use `SourceCodeAnalysis.analyzeCompletion(String)`.

    For imports, the import is added. Classes, interfaces. methods, and variables are defined. The initializer of variables, statements, and expressions are executed. The modifiers public, protected, private, static, and final are not allowed on op-level declarations and are ignored with a warning. Synchronized, native, abstract, and default top-level methods are not allowed and are errors. If a previous definition of a declaration is overwritten then there will be an event showing its status changed to OVERWRITTEN, this will not occur for dropped, rejected, or already overwritten declarations.

    If execution environment is out of process, as is the default case, then if the evaluated code causes the execution environment to terminate, this `JShell` instance will be closed but the calling process and VM remain valid.

    Parameters:
      `input` - The input String to evaluate
    Returns:
      the list of events directly or indirectly caused by this evaluation.
    Throws:
      `IllegalStateException` - if this `JShell` instance is closed.
    See Also:
      - `SourceCodeAnalysis.analyzeCompletion(String)`
      - `onShutdown(java.util.function.Consumer)`

  - ### drop

    public List<SnippetEvent> drop(Snippet snippet) throws IllegalStateException
    Remove a declaration from the state. That is, if the snippet is an active persistent snippet, remove the snippet and update the JShell evaluation state accordingly. For all active snippets, change the status to `DROPPED`.
    Parameters:
      `snippet` - The snippet to remove
    Returns:
      The list of events from updating declarations dependent on the dropped snippet.
    Throws:
      `IllegalStateException` - if this `JShell` instance is closed.
      `IllegalArgumentException` - if the snippet is not associated with this `JShell` instance.

  - ### addToClasspath

    public void addToClasspath(String path)
    The specified path is added to the end of the classpath used in eval(). Note that the unnamed package is not accessible from the package in which `eval(String)` code is placed.
    Parameters:
      `path` - the path to add to the classpath.
    Throws:
      `IllegalStateException` - if this `JShell` instance is closed.

  - ### stop

    public void stop()
    Attempt to stop currently running evaluation. When called while the `eval(java.lang.String)` method is running and the user's code being executed, an attempt will be made to stop user's code. Note that typically this method needs to be called from a different thread than the one running the `eval` method.

    If the `eval(java.lang.String)` method is not running, does nothing.

    The attempt to stop the user's code may fail in some case, which may include when the execution is blocked on an I/O operation, or when the user's code is catching the `ThreadDeath` exception.

  - ### close

    public void close()
    Close this state engine. Frees resources. Should be called when this state engine is no longer needed.
    Specified by:
      `close` in interface `AutoCloseable`

  - ### snippets

    public Stream<Snippet> snippets()
    Return all snippets.
    Returns:
      the snippets for all current snippets in id order.

  - ### variables

    public Stream<VarSnippet> variables()
    Returns the active variable snippets. This convenience method is equivalent to `snippets()` filtered for `status(snippet).isActive()` `&& snippet.kind() == Kind.VARIABLE` and cast to `VarSnippet`.
    Returns:
      the active declared variables.

  - ### methods

    public Stream<MethodSnippet> methods()
    Returns the active method snippets. This convenience method is equivalent to `snippets()` filtered for `status(snippet).isActive()` `&& snippet.kind() == Kind.METHOD` and cast to MethodSnippet.
    Returns:
      the active declared methods.

  - ### types

    public Stream<TypeDeclSnippet> types()
    Returns the active type declaration (class, interface, annotation type, and enum) snippets. This convenience method is equivalent to `snippets()` filtered for `status(snippet).isActive()` `&& snippet.kind() == Kind.TYPE_DECL` and cast to TypeDeclSnippet.
    Returns:
      the active declared type declarations.

  - ### imports

    public Stream<ImportSnippet> imports()
    Returns the active import snippets. This convenience method is equivalent to `snippets()` filtered for `status(snippet).isActive()` `&& snippet.kind() == Kind.IMPORT` and cast to ImportSnippet.
    Returns:
      the active declared import declarations.

  - ### status

    public Snippet.Status status(Snippet snippet)
    Return the status of the snippet. This is updated either because of an explicit `eval()` call or an automatic update triggered by a dependency.
    Parameters:
      `snippet` - the `Snippet` to look up
    Returns:
      the status corresponding to this snippet
    Throws:
      `IllegalStateException` - if this `JShell` instance is closed.
      `IllegalArgumentException` - if the snippet is not associated with this `JShell` instance.

  - ### diagnostics

    public Stream<Diag> diagnostics(Snippet snippet)
    Return the diagnostics of the most recent evaluation of the snippet. The evaluation can either because of an explicit `eval()` call or an automatic update triggered by a dependency.

    This method will return best-effort diagnostics for snippets returned from `SourceCodeAnalysis.sourceToSnippets(java.lang.String)`. The diagnostics returned for such snippets may differ from diagnostics provided after the snippet is `eval(java.lang.String)`-ed.

    Parameters:
      `snippet` - the `Snippet` to look up
    Returns:
      the diagnostics corresponding to this snippet. This does not include unresolvedDependencies references reported in `unresolvedDependencies()`.
    Throws:
      `IllegalStateException` - if this `JShell` instance is closed.
      `IllegalArgumentException` - if the snippet is not associated with this `JShell` instance.

  - ### unresolvedDependencies

    public Stream<String> unresolvedDependencies(DeclarationSnippet snippet)
    For `RECOVERABLE_DEFINED` or `RECOVERABLE_NOT_DEFINED` declarations, the names of current unresolved dependencies for the snippet. The returned value of this method, for a given method may change when an `eval()` or `drop()` of another snippet causes an update of a dependency.
    Parameters:
      `snippet` - the declaration `Snippet` to look up
    Returns:
      a stream of symbol names that are currently unresolvedDependencies.
    Throws:
      `IllegalStateException` - if this `JShell` instance is closed.
      `IllegalArgumentException` - if the snippet is not associated with this `JShell` instance.

  - ### varValue

    public String varValue(VarSnippet snippet) throws IllegalStateException
    Get the current value of a variable.
    Parameters:
      `snippet` - the variable Snippet whose value is queried.
    Returns:
      the current value of the variable referenced by snippet.
    Throws:
      `IllegalStateException` - if this `JShell` instance is closed.
      `IllegalArgumentException` - if the snippet is not associated with this `JShell` instance.
      `IllegalArgumentException` - if the variable's status is anything but `Snippet.Status.VALID`.

  - ### onSnippetEvent

    public JShell.Subscription onSnippetEvent(Consumer<SnippetEvent> listener) throws IllegalStateException
    Register a callback to be called when the Status of a snippet changes. Each call adds a new subscription.
    Parameters:
      `listener` - Action to perform when the Status changes.
    Returns:
      A token which can be used to unsubscribe this subscription.
    Throws:
      `IllegalStateException` - if this `JShell` instance is closed.

  - ### onShutdown

    public JShell.Subscription onShutdown(Consumer<JShell> listener) throws IllegalStateException
    Register a callback to be called when this JShell instance terminates. This occurs either because the client process has ended (e.g. called System.exit(0)) or the connection has been shutdown, as by close(). Each call adds a new subscription.
    Parameters:
      `listener` - Action to perform when the state terminates.
    Returns:
      A token which can be used to unsubscribe this subscription.
    Throws:
      `IllegalStateException` - if this JShell instance is closed

  - ### unsubscribe

    public void unsubscribe(JShell.Subscription token)
    Cancel a callback subscription.
    Parameters:
      `token` - The token corresponding to the subscription to be unsubscribed.