package jdk.jshell
Provides interfaces for creating tools, such as a Read-Eval-Print Loop (REPL), which interactively evaluate "snippets" of Java programming language code. Where a "snippet" is a single expression, statement, or declaration. This functionality can be used to enhance tools such as IDEs or can be stand-alone.
JShell is the central class. An instance of JShell holds the evaluation state, which is both the current set of source snippets and the execution state they have produced.
Each source snippet is represented by an instance of a subclass of Snippet. For example, a statement is represented by an instance of StatementSnippet, and a method declaration is represented by an instance of MethodSnippet. Snippets are created when JShell.eval(String) is invoked with an input which includes one or more snippets of code.
Any change to the compilation status of a snippet is reported with a SnippetEvent. There are three major kinds of changes to the status of a snippet: it can be created with eval, it can be dropped from the active source state with JShell.drop(jdk.jshell.Snippet), and it can have its status updated as a result of a status change in another snippet. For example: given js, an instance of JShell, executing js.eval("int x = 5;") will add the variable x to the source state and will generate an event describing the creation of a VarSnippet for x. Then executing js.eval("int timesx(int val) { return val * x; }") will add a method to the source state and will generate an event describing the creation of a MethodSnippet for timesx. Assume that varx holds the snippet created by the first call to eval, executing js.drop(varx) will generate two events: one for changing the status of the variable snippet to DROPPED and one for updating the method snippet (which now has an unresolved reference to x).
Of course, for any general application of the API, the input would not be fixed strings, but would come from the user. Below is a very simplified example of how the API might be used to implement a REPL.
import java.io.ByteArrayInputStream;
import java.io.Console;
import java.util.List;
import jdk.jshell.*;
import jdk.jshell.Snippet.Status;
class ExampleJShell {
public static void main(String[] args) {
Console console = System.console();
try (JShell js = JShell.create()) {
do {
System.out.print("Enter some Java code: ");
String input = console.readLine();
if (input == null) {
break;
}
List<SnippetEvent> events = js.eval(input);
for (SnippetEvent e : events) {
StringBuilder sb = new StringBuilder();
if (e.causeSnippet() == null) {
// We have a snippet creation event
switch (e.status()) {
case VALID:
sb.append("Successful ");
break;
case RECOVERABLE_DEFINED:
sb.append("With unresolved references ");
break;
case RECOVERABLE_NOT_DEFINED:
sb.append("Possibly reparable, failed ");
break;
case REJECTED:
sb.append("Failed ");
break;
}
if (e.previousStatus() == Status.NONEXISTENT) {
sb.append("addition");
} else {
sb.append("modification");
}
sb.append(" of ");
sb.append(e.snippet().source());
System.out.println(sb);
if (e.value() != null) {
System.out.printf("Value is: %s\n", e.value());
}
System.out.flush();
}
}
} while (true);
}
System.out.println("\nGoodbye");
}
}
To register for status change events use JShell.onSnippetEvent(java.util.function.Consumer). These events are only generated by eval and drop, the return values of these methods are the list of events generated by that call. So, as in the example above, events can be used without registering to receive events.
If you experiment with this example, you will see that failing to terminate a statement or variable declaration with a semi-colon will simply fail. An unfinished entry (for example a desired multi-line method) will also just fail after one line. The utilities in SourceCodeAnalysis provide source boundary and completeness analysis to address cases like those. SourceCodeAnalysis also provides suggested completions of input, as might be used in tab-completion.
Diagnostic information for a Snippet.
A snippet of code that is not valid Java programming language code.
Wraps an throwable thrown in the executing client.
Snippet for an assignment or variable-value expression.
Snippet for an import declaration.
The JShell evaluation state engine.
Builder for JShell instances.
The superclass of JShell generated exceptions
Snippet for a method definition.
Grouping for Snippets which persist and influence future code.
Describes the general kind of snippet.
Describes the current state of a Snippet.
The detailed variety of a snippet.
A description of a change to a Snippet.
Provides analysis utilities for source code input.
Describes the completeness of the given input.
The result of analyzeCompletion(String input).
A documentation for a candidate for continuation of the given user's input.
List of possible qualified names.
The wrapping of a snippet of Java source into valid top-level Java source.
A candidate for continuation of the given user's input.
Snippet for a type definition (a class, interface, enum, or annotation interface definition).
Snippet for a variable definition.
Report a bug or suggest an enhancement
For further API reference and developer documentation see the Java SE Documentation, which contains more detailed, developer-targeted descriptions with conceptual overviews, definitions of terms, workarounds, and working code examples. Other versions.
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All rights reserved. Use is subject to license terms and the documentation redistribution policy.
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