xref: /openjdk10/langtools/src/jdk.jshell/share/classes/jdk/jshell/package-info.java

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 * accompanied this code).
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/**
 * 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.
 * <p>
 * {@link jdk.jshell.JShell} is the central class.  An instance of
 * <code>JShell</code> holds the evaluation state, which is both the current
 * set of source snippets and the execution state they have produced.
 * <p>
 * Each source snippet is represented by an instance of a subclass of
 * {@link jdk.jshell.Snippet}. For example, a statement is represented by an
 * instance of {@link jdk.jshell.StatementSnippet}, and a method declaration is
 * represented by an instance of {@link jdk.jshell.MethodSnippet}.
 * Snippets are created when {@link jdk.jshell.JShell#eval(java.lang.String)}
 * is invoked with an input which includes one or more snippets of code.
 * <p>
 * Any change to the compilation status of a snippet is reported with a
 * {@link jdk.jshell.SnippetEvent}.  There are three major kinds of
 * changes to the status of a snippet: it can created with <code>eval</code>,
 * it can be dropped from the active source state with
 * {@link jdk.jshell.JShell#drop(jdk.jshell.PersistentSnippet)}, and it can have
 * its status updated as a result of a status change in another snippet.
 * For
 * example: given <code>js</code>, an instance of <code>JShell</code>, executing
 * <code>js.eval("int x = 5;")</code> will add the variable <code>x</code> to
 * the source state and will generate an event describing the creation of a
 * {@link jdk.jshell.VarSnippet} for <code>x</code>. Then executing
 * <code>js.eval("int timesx(int val) { return val * x; }")</code> will add
 * a method to the source state and will generate an event
 * describing the creation of a {@link jdk.jshell.MethodSnippet} for
 * <code>timesx</code>.
 * Assume that <code>varx</code> holds the snippet created by the first
 * call to <code>eval</code>, executing <code>js.drop(varx)</code> will
 * generate two events: one for changing the status of the
 * variable snippet to <code>DROPPED</code> and one for
 * updating the method snippet (which now has an unresolved reference to
 * <code>x</code>).
 * <p>
 * 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.
 * <pre>
* {@code
 *     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&lt;SnippetEvent&gt; 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");
 *         }
 *     }
 * }
 * </pre>
 * <p>
 * To register for status change events use
 * {@link jdk.jshell.JShell#onSnippetEvent(java.util.function.Consumer)}.
 * These events are only generated by <code>eval</code> and <code>drop</code>,
 * 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.
 * <p>
 * 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 {@link jdk.jshell.SourceCodeAnalysis}
 * provide source boundary and completeness analysis to address cases like
 * those.  <code>SourceCodeAnalysis</code> also provides suggested completions
 * of input, as might be used in tab-completion.
 */


package jdk.jshell;