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Java Platform, Standard Edition Java Scripting Programmer's Guide
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# 2 The Java Scripting API

This chapter describes how the Java Scripting API (defined by JSR 223) is used to embed scripts in your Java applications, and provides a number of examples with Java classes, which demostrate the features of the Java Scripting API.

The Java Scripting API consists of classes and interfaces from the `javax.script` package. It is a relatively small and simple package with the `ScriptEngineManager` class as the starting point. A `ScriptEngineManager` object can discover script engines through the JAR file service discovery mechanism, and instantiate `ScriptEngine` objects that interpret scripts written in a specific scripting language. For more information about the `javax.script` package, see the Java SE specification at `http://docs.oracle.com/javase/8/docs/api/javax/script/package-summary.html`

The Nashorn engine is the default ECMAScript (JavaScript) engine bundled with the Java SE Development Kit (JDK). The Nashorn engine was developed fully in Java by Oracle as part of an OpenJDK project. You can find more information about the Nashorn project at `http://openjdk.java.net/projects/nashorn/`

Although Nashorn is the default ECMAScript engine used by the Java Scripting API, you can use any script engine compliant with JSR 223, or you can implement your own. This document does not cover the implementation of script engines compliant with JSR 223, but at the most basic level, you must implement the `javax.script.ScriptEngine` and `javax.script.ScriptEngineFactory` interfaces. The abstract class `javax.script.AbstractScriptEngine` provides useful defaults for a few methods in the `ScriptEngine` interface.

To use the Java Scripting API:

1. Create a `ScriptEngineManager` object.

2. Get a `ScriptEngine` object from the manager.

3. Evaluate the script using the script engine's `eval()` method.

The following examples shows you how to use the Java Scripting API in Java. To keep the examples simple, exceptions are not handled. However, there are checked and runtime exceptions thrown by the Java Scripting API, and they should be properly handled. In every example, an instance of the `ScriptEngineManager` class is used to request the Nashorn engine (an object of the `ScriptEngine` class) using the `getEngineByName()` method. If the engine with the specified name is not present, `null` is returned. For more information about using the Nashorn engine, see the Nashorn User's Guide.

Note: Each `ScriptEngine` object has its own variable scope. For information about using multiple variable scopes, see Example 8.

Example 1 - Evaluating a Statement

  In this example, the `eval()` method is called on the script engine instance to execute JavaScript code from a `String` object.

      import javax.script.*;

      public class EvalScript {
          public static void main(String[] args) throws Exception {
              ScriptEngineManager manager = new ScriptEngineManager();
              ScriptEngine engine = manager.getEngineByName("nashorn");

              // evaluate JavaScript code
              engine.eval("print('Hello, World')");
          }
      }

Example 2 - Evaluating a Script File

  In this example, the `eval()` method takes in a `FileReader` object that reads JavaScript code from a file named `script.js`. By wrapping various input stream objects as readers, it is possible to execute scripts from files, URLs, and other resources.

      import javax.script.*;

      public class EvalFile {
          public static void main(String[] args) throws Exception {
              ScriptEngineManager manager = new ScriptEngineManager();
              ScriptEngine engine = manager.getEngineByName("nashorn");

              // evaluate JavaScript code
              engine.eval(new java.io.FileReader("script.js"));
          }
      }

Example 3 - Exposing a Java Object as a Global Variable

  In this example, a `File` object is created and exposed to the engine as a global variable named `file` using the `put()` method. Then the `eval()` method is called with JavaScript code that accesses the variable and calls the `getAbsolutePath()` method.

  Note: The syntax to access fields and call methods of Java objects exposed as variables depends on the scripting language. This example uses JavaScript syntax, which is similar to Java.

      import javax.script.*;
      import java.io.*;

      public class ScriptVars {
          public static void main(String[] args) throws Exception {
              ScriptEngineManager manager = new ScriptEngineManager();
              ScriptEngine engine = manager.getEngineByName("nashorn");

              // create File object
              File f = new File("test.txt");

              // expose File object as a global variable to the engine
              engine.put("file", f);

              // evaluate JavaScript code and access the variable
              engine.eval("print(file.getAbsolutePath())");
          }
      }

Example 4 - Invoking a Script Function

  In this example, the `eval()` method is called with JavaScript code that defines a function with one parameter. Then, an `Invocable` object is created and its `invokeFunction()` method is used to invoke the function.

  Note: Not all script engines implement the `Invocable` interface. This example uses the Nashorn engine, which can invoke functions in scripts that have previously been evaluated by this engine.

      import javax.script.*;

      public class InvokeScriptFunction {
          public static void main(String[] args) throws Exception {
              ScriptEngineManager manager = new ScriptEngineManager();
              ScriptEngine engine = manager.getEngineByName("nashorn");

              // evaluate JavaScript code that defines a function with one parameter
              engine.eval("function hello(name) { print('Hello, ' + name) }");

              // create an Invocable object by casting the script engine object
              Invocable inv = (Invocable) engine;

              // invoke the function named "hello" with "Scripting!" as the argument
              inv.invokeFunction("hello", "Scripting!");
          }
      }

Example 5 - Invoking a Script Object's Method

  In this example, the `eval()` method is called with JavaScript code that defines an object with a method. This object is then exposed from the script to the Java application using the script engine's `get()` method. Then, an `Invocable` object is created, and its `invokeMethod()` method is used to invoke the method defined for the script object.

  Note: Not all script engines implement the `Invocable` interface. This example uses the Nashorn engine, which can invoke methods in scripts that have previously been evaluated by this engine.

      import javax.script.*;

      public class InvokeScriptMethod {
          public static void main(String[] args) throws Exception {
              ScriptEngineManager manager = new ScriptEngineManager();
              ScriptEngine engine = manager.getEngineByName("nashorn");

              // evaluate JavaScript code that defines an object with one method
              engine.eval("var obj = new Object()");
              engine.eval("obj.hello = function(name) { print('Hello, ' + name) }");

              // expose object defined in the script to the Java application
              Object obj = engine.get("obj");

              // create an Invocable object by casting the script engine object
              Invocable inv = (Invocable) engine;

              // invoke the method named "hello" on the object defined in the script
              // with "Script Method!" as the argument
              inv.invokeMethod(obj, "hello", "Script Method!");
          }
      }

Example 6 - Implementing a Java Interface with Script Functions

  In this example, the `eval()` method is called with JavaScript code that defines a function. Then, an `Invocable` object is created, and its `getInterface()` method is used to create a `Runnable` interface object. The methods of the interface are implemented by script functions with matching names (in this case, the `run()` function is used to implement the `run()` method in the interface object). Finally, a new thread is started that runs the script function.

      import javax.script.*;

      public class ImplementRunnable {
          public static void main(String[] args) throws Exception {
              ScriptEngineManager manager = new ScriptEngineManager();
              ScriptEngine engine = manager.getEngineByName("nashorn");

              // evaluate JavaScript code that defines a function with one parameter
              engine.eval("function run() { print('run() function called') }");

              // create an Invocable object by casting the script engine object
              Invocable inv = (Invocable) engine;

              // get Runnable interface object
              Runnable r = inv.getInterface(Runnable.class);

              // start a new thread that runs the script
              Thread th = new Thread(r);
              th.start();
              th.join();
          }
      }

Example 7 - Implementing a Java Interface with the Script Object's Methods

  In this example, the `eval()` method is called with JavaScript code that defines an object with a method. This object is then exposed from the script to the Java application using the script engine's `get()` method. Then, an `Invocable` object is created, and its `getInterface()` method is used to create a `Runnable` interface object. The methods of the interface are implemented by the script object's methods with matching names (in this case, the `run` method of the `obj` object is used to implement the `run()` method in the interface object). Finally, a new thread is started that runs the script object's method.

      import javax.script.*;

      public class ImplementRunnableObject {
          public static void main(String[] args) throws Exception {
              ScriptEngineManager manager = new ScriptEngineManager();
              ScriptEngine engine = manager.getEngineByName("nashorn");

              // evaluate JavaScript code that defines a function with one parameter
              engine.eval("var obj = new Object()")
              engine.eval("obj.run = function() { print('obj.run() method called') }");

              // expose object defined in the script to the Java application
              Object obj = engine.get("obj");

              // create an Invocable object by casting the script engine object
              Invocable inv = (Invocable) engine;

              // get Runnable interface object
              Runnable r = inv.getInterface(obj, Runnable.class);

              // start a new thread that runs the script
              Thread th = new Thread(r);
              th.start();
              th.join();
          }
      }

Example 8 - Using Multiple Scopes

  In this example, the script engine's `put()` method is used to set the variable `x` to a `String` object `hello`. Then, the `eval()` method is used to print the variable in the default scope. Then, a different script context is defined, and its scope is used to set the same variable to a different value (a `String` object `world`). Finally, the variable is printed in the new script context that displays a different value.

  A single scope is an instance of the `javax.script.Bindings` interface. This interface is derived from the `java.util.Map<String, Object>` interface. A scope is a set of name and value pairs where the name is a non-empty, non-null `String` object. The `javax.script.ScriptContext` interface supports multiple scopes with associated `Bindings` for each scope. By default, every script engine has a default script context. The default script context has at least one scope represented by the static field `ENGINE_SCOPE`. Various scopes supported by a script context are available through the `getScopes()` method.

      import javax.script.*;

      public class MultipleScopes {
          public static void main(String[] args) throws Exception {
              ScriptEngineManager manager = new ScriptEngineManager();
              ScriptEngine engine = manager.getEngineByName("nashorn");

              // set global variable
              engine.put("x","hello");

              // evaluate JavaScript code that prints the variable (x = "hello")
              engine.eval("print(x)");

              // define a different script context
              ScriptContext newContext = new SimpleScriptContext();
              newContext.setBindings(engine.createBindings(), ScriptContext.ENGINE_SCOPE);
              Bindings engineScope = newContext.getBindings(ScriptContext.ENGINE_SCOPE);

              // set the variable to a different value in another scope
              engineScope.put("x", "world");

              // evaluate the same code but in a different script context (x = "world")
              engine.eval("print(x)", newContext);

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