Java_脚本引擎_01_用法入门

一、前言

最近有个需求，需要在js中调用java，这样能避免更新java，从而实现代码的热更新。

于是想到用 Nashorn JavaScript 引擎。

二、概述

通过 JDK 8 的 Nashorn JavaScript 引擎 ，可以很方便的实现在java中调用js，以及在js中调用java。

三、简单示例

1.java中调用js

    @Test
    public void testDemo1() throws ScriptException {
        //1.创建引擎
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("JavaScript");

        //2.要执行的脚本
        String script = "var sum = n1 + n2;print(msg + ' n1= '+ n1 + '; n2 ='+ n2 + '; sum=' + sum);";
        manager.put("n1", 1);

        //3.执行脚本
        //3.1 直接传参，并执行脚本
        engine.put("n2", 2);             //使用put(key,value)直接传参到到脚本中，在脚本中通过key来取值
        engine.put("msg", "a string");
        engine.eval(script);

        //3.2 通过绑定传参，并执行脚本
        Bindings bindings = engine.createBindings();
        bindings.put("n2", 3);              // AbstractScriptEngine.put(key,value)的内部实现就是使用的Bindings.put(key,value)
        bindings.put("msg", "another string");
        engine.eval(script, bindings);

        //3.3 通过上下文来传参，并执行脚本
        ScriptContext ctx = new SimpleScriptContext();
        Bindings ctxGlobalBindings = engine.createBindings();
        ctx.setBindings(ctxGlobalBindings, GLOBAL_SCOPE);
        ctx.setAttribute("n1", 4, GLOBAL_SCOPE);
        ctx.setAttribute("n2", 5, ENGINE_SCOPE);
        ctx.setAttribute("msg", "ScriptContext:", ENGINE_SCOPE);
        engine.eval(script, ctx);  //使用指定上下文来执行脚本

        engine.eval(script);   //使用默认上下文执行脚本
    }

2.js中调用java

HelloWorld.java

public class HelloWorld {

    public void sayHello(){
        System.out.println("hello");
    }
}

测试用例：

    @Test
    public void test2() throws ScriptException {
        //1.创建引擎
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("JavaScript");

        //2.设置要执行的脚本
        String script="$hello.sayHello();";

        //3.传参，并执行脚本
        engine.put("$hello", new HelloWorld());
        engine.eval(script);  //使用指定上下文来执行脚本
        
    }

3.通过js实现java接口

参见：Java脚本教程 - Java脚本实现接口

四、脚本引擎类结构图

脚本引擎相关源码的类和接口如下：

先关注这几个基本实现类

1. ScriptEngineManager

源码如下：

/*
 * Copyright (c) 2005, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package javax.script;
import java.util.*;
import java.security.*;
import java.util.ServiceLoader;
import java.util.ServiceConfigurationError;

/**
 * The <code>ScriptEngineManager</code> implements a discovery and instantiation
 * mechanism for <code>ScriptEngine</code> classes and also maintains a
 * collection of key/value pairs storing state shared by all engines created
 * by the Manager. This class uses the <a href="../../../technotes/guides/jar/jar.html#Service%20Provider">service provider</a> mechanism to enumerate all the
 * implementations of <code>ScriptEngineFactory</code>. <br><br>
 * The <code>ScriptEngineManager</code> provides a method to return a list of all these factories
 * as well as utility methods which look up factories on the basis of language name, file extension
 * and mime type.
 * <p>
 * The <code>Bindings</code> of key/value pairs, referred to as the "Global Scope"  maintained
 * by the manager is available to all instances of <code>ScriptEngine</code> created
 * by the <code>ScriptEngineManager</code>.  The values in the <code>Bindings</code> are
 * generally exposed in all scripts.
 *
 * @author Mike Grogan
 * @author A. Sundararajan
 * @since 1.6
 */
public class ScriptEngineManager  {
    private static final boolean DEBUG = false;
    /**
     * The effect of calling this constructor is the same as calling
     * <code>ScriptEngineManager(Thread.currentThread().getContextClassLoader())</code>.
     *
     * @see java.lang.Thread#getContextClassLoader
     */
    public ScriptEngineManager() {
        ClassLoader ctxtLoader = Thread.currentThread().getContextClassLoader();
        init(ctxtLoader);
    }

    /**
     * This constructor loads the implementations of
     * <code>ScriptEngineFactory</code> visible to the given
     * <code>ClassLoader</code> using the <a href="../../../technotes/guides/jar/jar.html#Service%20Provider">service provider</a> mechanism.<br><br>
     * If loader is <code>null</code>, the script engine factories that are
     * bundled with the platform and that are in the usual extension
     * directories (installed extensions) are loaded. <br><br>
     *
     * @param loader ClassLoader used to discover script engine factories.
     */
    public ScriptEngineManager(ClassLoader loader) {
        init(loader);
    }

    private void init(final ClassLoader loader) {
        globalScope = new SimpleBindings();
        engineSpis = new HashSet<ScriptEngineFactory>();
        nameAssociations = new HashMap<String, ScriptEngineFactory>();
        extensionAssociations = new HashMap<String, ScriptEngineFactory>();
        mimeTypeAssociations = new HashMap<String, ScriptEngineFactory>();
        initEngines(loader);
    }

    private ServiceLoader<ScriptEngineFactory> getServiceLoader(final ClassLoader loader) {
        if (loader != null) {
            return ServiceLoader.load(ScriptEngineFactory.class, loader);
        } else {
            return ServiceLoader.loadInstalled(ScriptEngineFactory.class);
        }
    }

    private void initEngines(final ClassLoader loader) {
        Iterator<ScriptEngineFactory> itr = null;
        try {
            ServiceLoader<ScriptEngineFactory> sl = AccessController.doPrivileged(
                new PrivilegedAction<ServiceLoader<ScriptEngineFactory>>() {
                    @Override
                    public ServiceLoader<ScriptEngineFactory> run() {
                        return getServiceLoader(loader);
                    }
                });

            itr = sl.iterator();
        } catch (ServiceConfigurationError err) {
            System.err.println("Can't find ScriptEngineFactory providers: " +
                          err.getMessage());
            if (DEBUG) {
                err.printStackTrace();
            }
            // do not throw any exception here. user may want to
            // manage his/her own factories using this manager
            // by explicit registratation (by registerXXX) methods.
            return;
        }

        try {
            while (itr.hasNext()) {
                try {
                    ScriptEngineFactory fact = itr.next();
                    engineSpis.add(fact);
                } catch (ServiceConfigurationError err) {
                    System.err.println("ScriptEngineManager providers.next(): "
                                 + err.getMessage());
                    if (DEBUG) {
                        err.printStackTrace();
                    }
                    // one factory failed, but check other factories...
                    continue;
                }
            }
        } catch (ServiceConfigurationError err) {
            System.err.println("ScriptEngineManager providers.hasNext(): "
                            + err.getMessage());
            if (DEBUG) {
                err.printStackTrace();
            }
            // do not throw any exception here. user may want to
            // manage his/her own factories using this manager
            // by explicit registratation (by registerXXX) methods.
            return;
        }
    }

    /**
     * <code>setBindings</code> stores the specified <code>Bindings</code>
     * in the <code>globalScope</code> field. ScriptEngineManager sets this
     * <code>Bindings</code> as global bindings for <code>ScriptEngine</code>
     * objects created by it.
     *
     * @param bindings The specified <code>Bindings</code>
     * @throws IllegalArgumentException if bindings is null.
     */
    public void setBindings(Bindings bindings) {
        if (bindings == null) {
            throw new IllegalArgumentException("Global scope cannot be null.");
        }

        globalScope = bindings;
    }

    /**
     * <code>getBindings</code> returns the value of the <code>globalScope</code> field.
     * ScriptEngineManager sets this <code>Bindings</code> as global bindings for
     * <code>ScriptEngine</code> objects created by it.
     *
     * @return The globalScope field.
     */
    public Bindings getBindings() {
        return globalScope;
    }

    /**
     * Sets the specified key/value pair in the Global Scope.
     * @param key Key to set
     * @param value Value to set.
     * @throws NullPointerException if key is null.
     * @throws IllegalArgumentException if key is empty string.
     */
    public void put(String key, Object value) {
        globalScope.put(key, value);
    }

    /**
     * Gets the value for the specified key in the Global Scope
     * @param key The key whose value is to be returned.
     * @return The value for the specified key.
     */
    public Object get(String key) {
        return globalScope.get(key);
    }

    /**
     * Looks up and creates a <code>ScriptEngine</code> for a given  name.
     * The algorithm first searches for a <code>ScriptEngineFactory</code> that has been
     * registered as a handler for the specified name using the <code>registerEngineName</code>
     * method.
     * <br><br> If one is not found, it searches the set of <code>ScriptEngineFactory</code> instances
     * stored by the constructor for one with the specified name.  If a <code>ScriptEngineFactory</code>
     * is found by either method, it is used to create instance of <code>ScriptEngine</code>.
     * @param shortName The short name of the <code>ScriptEngine</code> implementation.
     * returned by the <code>getNames</code> method of its <code>ScriptEngineFactory</code>.
     * @return A <code>ScriptEngine</code> created by the factory located in the search.  Returns null
     * if no such factory was found.  The <code>ScriptEngineManager</code> sets its own <code>globalScope</code>
     * <code>Bindings</code> as the <code>GLOBAL_SCOPE</code> <code>Bindings</code> of the newly
     * created <code>ScriptEngine</code>.
     * @throws NullPointerException if shortName is null.
     */
    public ScriptEngine getEngineByName(String shortName) {
        if (shortName == null) throw new NullPointerException();
        //look for registered name first
        Object obj;
        if (null != (obj = nameAssociations.get(shortName))) {
            ScriptEngineFactory spi = (ScriptEngineFactory)obj;
            try {
                ScriptEngine engine = spi.getScriptEngine();
                engine.setBindings(getBindings(), ScriptContext.GLOBAL_SCOPE);
                return engine;
            } catch (Exception exp) {
                if (DEBUG) exp.printStackTrace();
            }
        }

        for (ScriptEngineFactory spi : engineSpis) {
            List<String> names = null;
            try {
                names = spi.getNames();
            } catch (Exception exp) {
                if (DEBUG) exp.printStackTrace();
            }

            if (names != null) {
                for (String name : names) {
                    if (shortName.equals(name)) {
                        try {
                            ScriptEngine engine = spi.getScriptEngine();
                            engine.setBindings(getBindings(), ScriptContext.GLOBAL_SCOPE);
                            return engine;
                        } catch (Exception exp) {
                            if (DEBUG) exp.printStackTrace();
                        }
                    }
                }
            }
        }

        return null;
    }

    /**
     * Look up and create a <code>ScriptEngine</code> for a given extension.  The algorithm
     * used by <code>getEngineByName</code> is used except that the search starts
     * by looking for a <code>ScriptEngineFactory</code> registered to handle the
     * given extension using <code>registerEngineExtension</code>.
     * @param extension The given extension
     * @return The engine to handle scripts with this extension.  Returns <code>null</code>
     * if not found.
     * @throws NullPointerException if extension is null.
     */
    public ScriptEngine getEngineByExtension(String extension) {
        if (extension == null) throw new NullPointerException();
        //look for registered extension first
        Object obj;
        if (null != (obj = extensionAssociations.get(extension))) {
            ScriptEngineFactory spi = (ScriptEngineFactory)obj;
            try {
                ScriptEngine engine = spi.getScriptEngine();
                engine.setBindings(getBindings(), ScriptContext.GLOBAL_SCOPE);
                return engine;
            } catch (Exception exp) {
                if (DEBUG) exp.printStackTrace();
            }
        }

        for (ScriptEngineFactory spi : engineSpis) {
            List<String> exts = null;
            try {
                exts = spi.getExtensions();
            } catch (Exception exp) {
                if (DEBUG) exp.printStackTrace();
            }
            if (exts == null) continue;
            for (String ext : exts) {
                if (extension.equals(ext)) {
                    try {
                        ScriptEngine engine = spi.getScriptEngine();
                        engine.setBindings(getBindings(), ScriptContext.GLOBAL_SCOPE);
                        return engine;
                    } catch (Exception exp) {
                        if (DEBUG) exp.printStackTrace();
                    }
                }
            }
        }
        return null;
    }

    /**
     * Look up and create a <code>ScriptEngine</code> for a given mime type.  The algorithm
     * used by <code>getEngineByName</code> is used except that the search starts
     * by looking for a <code>ScriptEngineFactory</code> registered to handle the
     * given mime type using <code>registerEngineMimeType</code>.
     * @param mimeType The given mime type
     * @return The engine to handle scripts with this mime type.  Returns <code>null</code>
     * if not found.
     * @throws NullPointerException if mimeType is null.
     */
    public ScriptEngine getEngineByMimeType(String mimeType) {
        if (mimeType == null) throw new NullPointerException();
        //look for registered types first
        Object obj;
        if (null != (obj = mimeTypeAssociations.get(mimeType))) {
            ScriptEngineFactory spi = (ScriptEngineFactory)obj;
            try {
                ScriptEngine engine = spi.getScriptEngine();
                engine.setBindings(getBindings(), ScriptContext.GLOBAL_SCOPE);
                return engine;
            } catch (Exception exp) {
                if (DEBUG) exp.printStackTrace();
            }
        }

        for (ScriptEngineFactory spi : engineSpis) {
            List<String> types = null;
            try {
                types = spi.getMimeTypes();
            } catch (Exception exp) {
                if (DEBUG) exp.printStackTrace();
            }
            if (types == null) continue;
            for (String type : types) {
                if (mimeType.equals(type)) {
                    try {
                        ScriptEngine engine = spi.getScriptEngine();
                        engine.setBindings(getBindings(), ScriptContext.GLOBAL_SCOPE);
                        return engine;
                    } catch (Exception exp) {
                        if (DEBUG) exp.printStackTrace();
                    }
                }
            }
        }
        return null;
    }

    /**
     * Returns a list whose elements are instances of all the <code>ScriptEngineFactory</code> classes
     * found by the discovery mechanism.
     * @return List of all discovered <code>ScriptEngineFactory</code>s.
     */
    public List<ScriptEngineFactory> getEngineFactories() {
        List<ScriptEngineFactory> res = new ArrayList<ScriptEngineFactory>(engineSpis.size());
        for (ScriptEngineFactory spi : engineSpis) {
            res.add(spi);
        }
        return Collections.unmodifiableList(res);
    }

    /**
     * Registers a <code>ScriptEngineFactory</code> to handle a language
     * name.  Overrides any such association found using the Discovery mechanism.
     * @param name The name to be associated with the <code>ScriptEngineFactory</code>.
     * @param factory The class to associate with the given name.
     * @throws NullPointerException if any of the parameters is null.
     */
    public void registerEngineName(String name, ScriptEngineFactory factory) {
        if (name == null || factory == null) throw new NullPointerException();
        nameAssociations.put(name, factory);
    }

    /**
     * Registers a <code>ScriptEngineFactory</code> to handle a mime type.
     * Overrides any such association found using the Discovery mechanism.
     *
     * @param type The mime type  to be associated with the
     * <code>ScriptEngineFactory</code>.
     *
     * @param factory The class to associate with the given mime type.
     * @throws NullPointerException if any of the parameters is null.
     */
    public void registerEngineMimeType(String type, ScriptEngineFactory factory) {
        if (type == null || factory == null) throw new NullPointerException();
        mimeTypeAssociations.put(type, factory);
    }

    /**
     * Registers a <code>ScriptEngineFactory</code> to handle an extension.
     * Overrides any such association found using the Discovery mechanism.
     *
     * @param extension The extension type  to be associated with the
     * <code>ScriptEngineFactory</code>.
     * @param factory The class to associate with the given extension.
     * @throws NullPointerException if any of the parameters is null.
     */
    public void registerEngineExtension(String extension, ScriptEngineFactory factory) {
        if (extension == null || factory == null) throw new NullPointerException();
        extensionAssociations.put(extension, factory);
    }

    /** Set of script engine factories discovered. */
    private HashSet<ScriptEngineFactory> engineSpis;

    /** Map of engine name to script engine factory. */
    private HashMap<String, ScriptEngineFactory> nameAssociations;

    /** Map of script file extension to script engine factory. */
    private HashMap<String, ScriptEngineFactory> extensionAssociations;

    /** Map of script script MIME type to script engine factory. */
    private HashMap<String, ScriptEngineFactory> mimeTypeAssociations;

    /** Global bindings associated with script engines created by this manager. */
    private Bindings globalScope;
}

此类主要有三个作用：

（1）设置全局变量

（2）获取脚本引擎

（3）注册脚本引擎工厂

（1）设置全局变量

    /** Global bindings associated with script engines created by this manager. */
    private Bindings globalScope;

    public void put(String key, Object value) {
        globalScope.put(key, value);
    }

    public Object get(String key) {
        return globalScope.get(key);
    }

可以看到，通过 ScriptEngineManager 设置的变量都绑定到了 globalScope 这个绑定中。

这里并不能说明这个一个全局绑定。

再往下看：

 此处设置了绑定的作用域为全局。因此 可通过 ScriptEngineManager  设置全局变量

 （2）获取脚本引擎

可通过以下方法获取脚本引擎

 

 2. SimpleBindings

绑定是用来存放变量的。

simpleBingdings是 绑定的基本实现类，源码如下

/*
 * Copyright (c) 2005, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package javax.script;

import java.util.Map;
import java.util.HashMap;
import java.util.Collection;
import java.util.Set;

/**
 * A simple implementation of Bindings backed by
 * a <code>HashMap</code> or some other specified <code>Map</code>.
 *
 * @author Mike Grogan
 * @since 1.6
 */
public class SimpleBindings implements Bindings {

    /**
     * The <code>Map</code> field stores the attributes.
     */
    private Map<String,Object> map;

    /**
     * Constructor uses an existing <code>Map</code> to store the values.
     * @param m The <code>Map</code> backing this <code>SimpleBindings</code>.
     * @throws NullPointerException if m is null
     */
    public SimpleBindings(Map<String,Object> m) {
        if (m == null) {
            throw new NullPointerException();
        }
        this.map = m;
    }

    /**
     * Default constructor uses a <code>HashMap</code>.
     */
    public SimpleBindings() {
        this(new HashMap<String,Object>());
    }

    /**
     * Sets the specified key/value in the underlying <code>map</code> field.
     *
     * @param name Name of value
     * @param value Value to set.
     *
     * @return Previous value for the specified key.  Returns null if key was previously
     * unset.
     *
     * @throws NullPointerException if the name is null.
     * @throws IllegalArgumentException if the name is empty.
     */
    public Object put(String name, Object value) {
        checkKey(name);
        return map.put(name,value);
    }

    /**
     * <code>putAll</code> is implemented using <code>Map.putAll</code>.
     *
     * @param toMerge The <code>Map</code> of values to add.
     *
     * @throws NullPointerException
     *         if toMerge map is null or if some key in the map is null.
     * @throws IllegalArgumentException
     *         if some key in the map is an empty String.
     */
    public void putAll(Map<? extends String, ? extends Object> toMerge) {
        if (toMerge == null) {
            throw new NullPointerException("toMerge map is null");
        }
        for (Map.Entry<? extends String, ? extends Object> entry : toMerge.entrySet()) {
            String key = entry.getKey();
            checkKey(key);
            put(key, entry.getValue());
        }
    }

    /** {@inheritDoc} */
    public void clear() {
        map.clear();
    }

    /**
     * Returns <tt>true</tt> if this map contains a mapping for the specified
     * key.  More formally, returns <tt>true</tt> if and only if
     * this map contains a mapping for a key <tt>k</tt> such that
     * <tt>(key==null ? k==null : key.equals(k))</tt>.  (There can be
     * at most one such mapping.)
     *
     * @param key key whose presence in this map is to be tested.
     * @return <tt>true</tt> if this map contains a mapping for the specified
     *         key.
     *
     * @throws NullPointerException if key is null
     * @throws ClassCastException if key is not String
     * @throws IllegalArgumentException if key is empty String
     */
    public boolean containsKey(Object key) {
        checkKey(key);
        return map.containsKey(key);
    }

    /** {@inheritDoc} */
    public boolean containsValue(Object value) {
        return map.containsValue(value);
    }

    /** {@inheritDoc} */
    public Set<Map.Entry<String, Object>> entrySet() {
        return map.entrySet();
    }

    /**
     * Returns the value to which this map maps the specified key.  Returns
     * <tt>null</tt> if the map contains no mapping for this key.  A return
     * value of <tt>null</tt> does not <i>necessarily</i> indicate that the
     * map contains no mapping for the key; it's also possible that the map
     * explicitly maps the key to <tt>null</tt>.  The <tt>containsKey</tt>
     * operation may be used to distinguish these two cases.
     *
     * <p>More formally, if this map contains a mapping from a key
     * <tt>k</tt> to a value <tt>v</tt> such that <tt>(key==null ? k==null :
     * key.equals(k))</tt>, then this method returns <tt>v</tt>; otherwise
     * it returns <tt>null</tt>.  (There can be at most one such mapping.)
     *
     * @param key key whose associated value is to be returned.
     * @return the value to which this map maps the specified key, or
     *         <tt>null</tt> if the map contains no mapping for this key.
     *
     * @throws NullPointerException if key is null
     * @throws ClassCastException if key is not String
     * @throws IllegalArgumentException if key is empty String
     */
    public Object get(Object key) {
        checkKey(key);
        return map.get(key);
    }

    /** {@inheritDoc} */
    public boolean isEmpty() {
        return map.isEmpty();
    }

    /** {@inheritDoc} */
    public Set<String> keySet() {
        return map.keySet();
    }

    /**
     * Removes the mapping for this key from this map if it is present
     * (optional operation).   More formally, if this map contains a mapping
     * from key <tt>k</tt> to value <tt>v</tt> such that
     * <code>(key==null ?  k==null : key.equals(k))</code>, that mapping
     * is removed.  (The map can contain at most one such mapping.)
     *
     * <p>Returns the value to which the map previously associated the key, or
     * <tt>null</tt> if the map contained no mapping for this key.  (A
     * <tt>null</tt> return can also indicate that the map previously
     * associated <tt>null</tt> with the specified key if the implementation
     * supports <tt>null</tt> values.)  The map will not contain a mapping for
     * the specified  key once the call returns.
     *
     * @param key key whose mapping is to be removed from the map.
     * @return previous value associated with specified key, or <tt>null</tt>
     *         if there was no mapping for key.
     *
     * @throws NullPointerException if key is null
     * @throws ClassCastException if key is not String
     * @throws IllegalArgumentException if key is empty String
     */
    public Object remove(Object key) {
        checkKey(key);
        return map.remove(key);
    }

    /** {@inheritDoc} */
    public int size() {
        return map.size();
    }

    /** {@inheritDoc} */
    public Collection<Object> values() {
        return map.values();
    }

    private void checkKey(Object key) {
        if (key == null) {
            throw new NullPointerException("key can not be null");
        }
        if (!(key instanceof String)) {
            throw new ClassCastException("key should be a String");
        }
        if (key.equals("")) {
            throw new IllegalArgumentException("key can not be empty");
        }
    }
}

可以发现绑定的内部实现时通过HashMap来实现的。

Bingding通过HashMap来存放变量。

3.SimpleScriptContext

脚本引擎是在脚本上下文中执行的。

脚本上下文由四个组件组成：

• 与不同范围相关联的绑定。上下文中的绑定将参数传递给脚本。
• 读取器读取输入
• 写入输出
• 写入错误输出的错误写入器

绑定是是有作用域的，有两个作用域：

• ScriptContext.ENGINE_SCOPE       （引擎范围，值为100）
• ScriptContext.GLOBAL_SCOPE      （全局范围，值为200）

(1) 设置变量：

    public void setAttribute(String var1, Object var2, int var3) {
        this.checkName(var1);
        switch(var3) {
        case 100:
            this.engineScope.put(var1, var2);
            return;
        case 200:
            if (this.globalScope != null) {
                this.globalScope.put(var1, var2);
            }

            return;
        default:
            throw new IllegalArgumentException("Illegal scope value.");
        }
    }

    protected Bindings engineScope = new SimpleBindings();
    protected Bindings globalScope = null;

可以看到全局绑定默认为空。回到 三、简单示例 中，如下：

        //3.3 通过上下文来传参，并执行脚本
        ScriptContext ctx = new SimpleScriptContext();
        Bindings ctxGlobalBindings = engine.createBindings();
        ctx.setBindings(ctxGlobalBindings, GLOBAL_SCOPE);
        ctx.setAttribute("n1", 4, GLOBAL_SCOPE);
        ctx.setAttribute("n2", 5, ENGINE_SCOPE);
        ctx.setAttribute("msg", "ScriptContext:", ENGINE_SCOPE);
        engine.eval(script, ctx);  //使用指定上下文来执行脚本

设置全局变量的步骤：我们先设置了一个全局绑定，这样全局绑定就不为空，然后再设置全局变量。

 4.ScriptEngine

 ScriptEngine提供了如下接口，

抽象类AbstractScriptEngine实现了ScriptEngine的部分基本方法。

脚本引擎实际的执行者 NashornScriptEngine 继承自 AbstractScriptEngine 实现了 ScriptEngine的全部方法，功能强大。

五、参考资料

1.Java 脚本引擎—w3cschool

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