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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

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:

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:

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.
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.

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
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https://docs.oracle.com/en/java/javase/17/docs/api/jdk.jshell/jdk/jshell/JShell.html