Java Scripting API 使用示例

虽然 Nashorn 是 Java Scripting API 默认使用的 ECMAScript 引擎，但你也可以使用其他兼容 JSR 223 的脚本引擎，或者你可以实现自己的引擎。该文档不涉及脚本引擎的实现方法，但最最基础的级别，你必须实现 javax.script.ScriptEngine 和 javax.script.ScriptEngineFactory 接口。抽象类 javax.script.AbstractScriptEngine 提供了很多默认 ScriptEngine 接口的方法实现。

1. 创建一个  ScriptEngineManager  对象.
2. 从管理器对象中获取  ScriptEngine  对象
3. 使用脚本引擎的 eval() 方法来执行脚本

下面我们提供了几个例子来展示如何使用 Java Scripting API。为了让例子尽可能的简单，我们没有对执行过程中的异常进行处理。Java Scripting API 的异常有受检查异常和运行时异常两种，这些异常必须进行正确处理。在每个例子中，ScriptEngineManager 类的实例是通过 Nashorn 引擎的 getEngineByName() 方法来获取的。如果指定名称的引擎不存在，该方法会返回 null 。更多关于 Nashorn 引擎的信息请看 Nashorn User’s Guide .

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')");
    }
}

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"));
    }
}

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())");
    }
}

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!");
    }
}

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!");
    }
}

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();
    }
}

示例 7 —— 在脚本对象方法中实现 Java 接口

本例中在 JavaScript 代码中调用 eval() 方法来定义一个包含单个方法的对象。该对象在脚本中通过 get() 方法暴露给 Java 应用。然后创建一个 Invocable 对象并通过其  getInterface() 方法来创建一个 Runnable 接口对象。该接口的方法实现是在脚本对象中相同匹配名称的函数中（本例中对象的 run 方法对应接口的 run 方法的实现）。最后启动一个新线程来运行脚本对象的方法。

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();
    }
}

单一的作用域是 javax.script.Bindings 接口的实例。这个接口继承自 java.util.Map<String,Object> 接口。一个作用域相当于是成对的名称和值的列表，其中名称不能为空。javax.script.ScriptContext 接口通过为每个作用域关联一个 Bindings 对象来实现对多个作用域的支持。默认情况下，每个脚本应用有自己独立的上下文。默认的脚本上下文至少有一个作用域，这是通过静态属性 ENGINE_SCOPE 来定义的。脚本上下文可以通过 getScopes() 方法来实现对不同作用域的支持。

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);