Java 腳本上下文

Java腳本教程 - Java腳本上下文

腳本引擎在上下文或環(huán)境中執(zhí)行腳本。

ScriptContext 接口的實(shí)例表示腳本的上下文。

SimpleScriptContext 類是 ScriptContext 接口的實(shí)現(xiàn)。

腳本上下文由四個(gè)組件組成：

• 與不同范圍相關(guān)聯(lián)的綁定。上下文中的綁定將參數(shù)傳遞給腳本。
• 讀取器讀取輸入
• 寫入輸出
• 寫入錯(cuò)誤輸出的錯(cuò)誤寫入器

例子

以下代碼使用SimpleScriptContext類創(chuàng)建ScriptContext。

SimpleScriptContext類的一個(gè)實(shí)例維護(hù)Bindings的兩個(gè)實(shí)例：一個(gè)用于引擎范圍，一個(gè)用于全局范圍。

默認(rèn)情況下，SimpleScriptContext類分別將上下文的輸入閱讀器，輸出寫入程序和錯(cuò)誤寫入程序初始化為 System.in ， System.out 和 System.err 。

ScriptContext接口的getReader()，getWriter()和getErrorWriter()方法分別返回來(lái)自ScriptContext的讀者，寫者和錯(cuò)誤寫入者的引用。

Setter方法可以設(shè)置讀寫器。

以下代碼顯示如何獲取讀寫器。它還顯示如何設(shè)置寫入器到FileWriter以將腳本輸出寫入文件：

import java.io.FileWriter;
import java.io.Reader;
import java.io.Writer;

import javax.script.ScriptContext;
import javax.script.SimpleScriptContext;

public class Main {
  public static void main(String[] args) throws Exception{
    ScriptContext ctx = new SimpleScriptContext();

    // Get the reader and writers from the script context
    Reader inputReader = ctx.getReader();
    Writer outputWriter = ctx.getWriter();
    Writer errWriter = ctx.getErrorWriter();

    // Write all script outputs to an out.txt file
    Writer fileWriter = new FileWriter("out.txt");
    ctx.setWriter(fileWriter);
  }
}

添加上下文

創(chuàng)建 SimpleScriptContext 后，您可以開始在引擎范圍綁定中存儲(chǔ)鍵值對(duì)。

setAttribute()方法用于向Bindings添加鍵值對(duì)。

以下代碼顯示如何添加三個(gè)鍵值對(duì)。

import javax.script.ScriptContext;
import javax.script.SimpleScriptContext;

public class Main {
  public static void main(String[] args) throws Exception{
    ScriptContext ctx = new SimpleScriptContext();
        ctx.setAttribute("year", 2015, ScriptContext.ENGINE_SCOPE);
        ctx.setAttribute("month", 9, ScriptContext.ENGINE_SCOPE);
        ctx.setAttribute("day", 21, ScriptContext.ENGINE_SCOPE);

  }
}

將綁定添加到GLOBAL_SCOPE

要將鍵值對(duì)添加到全局范圍中的綁定，請(qǐng)首先創(chuàng)建和設(shè)置綁定。

import javax.script.Bindings;
import javax.script.ScriptContext;
import javax.script.SimpleBindings;
import javax.script.SimpleScriptContext;

public class Main {
  public static void main(String[] args) throws Exception {
    ScriptContext ctx = new SimpleScriptContext();
    Bindings globalBindings = new SimpleBindings();
    ctx.setBindings(globalBindings, ScriptContext.GLOBAL_SCOPE);
    ctx.setAttribute("year", 2015, ScriptContext.GLOBAL_SCOPE);
    ctx.setAttribute("name", "Java", ScriptContext.GLOBAL_SCOPE);
  }
}

解決不同范圍的變量

import static javax.script.ScriptContext.ENGINE_SCOPE;
import static javax.script.ScriptContext.GLOBAL_SCOPE;

import java.util.List;

import javax.script.Bindings;
import javax.script.ScriptContext;
import javax.script.SimpleBindings;
import javax.script.SimpleScriptContext;

public class Main {
  public static void main(String[] args) {
    ScriptContext ctx = new SimpleScriptContext();
    List<Integer> scopes = ctx.getScopes();
    System.out.println("Supported Scopes: " + scopes);

    ctx.setAttribute("year", 2015, ENGINE_SCOPE);

    Bindings globalBindings = new SimpleBindings();
    ctx.setBindings(globalBindings, GLOBAL_SCOPE);

    ctx.setAttribute("year", 2014, GLOBAL_SCOPE);
    ctx.setAttribute("name", "Jack", GLOBAL_SCOPE);
    
    String nameValue = (String) ctx.getAttribute("name");
    System.out.println("nameValue = " + nameValue);

    int engineScopeYear = (Integer) ctx.getAttribute("year", ENGINE_SCOPE);
    int globalScopeYear = (Integer) ctx.getAttribute("year", GLOBAL_SCOPE);

    System.out.println("engineScopeYear = " + engineScopeYear);
    System.out.println("globalScopeYear = " + globalScopeYear);
  }
}

上面的代碼生成以下結(jié)果。

例3

使用由同一ScriptEngineManager創(chuàng)建的引擎的全局和引擎范圍綁定。

import static javax.script.ScriptContext.ENGINE_SCOPE;
import static javax.script.ScriptContext.GLOBAL_SCOPE;

import javax.script.Bindings;
import javax.script.ScriptContext;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import javax.script.SimpleScriptContext;

public class Main {
  public static void main(String[] args) throws ScriptException {
    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("JavaScript");

    manager.put("n1", 1);
    String script = "var sum = n1 + n2;print(msg + "
        + "" n1=" + n1 + ", n2=" + n2 + " + "", sum=" + sum);";

    engine.put("n2", 2);
    engine.put("msg", "a string");
    engine.eval(script);

    Bindings bindings = engine.createBindings();
    bindings.put("n2", 3);
    bindings.put("msg", "another string");
    engine.eval(script, bindings);

    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);

    engine.eval(script);
  }
}

上面的代碼生成以下結(jié)果。

更改默認(rèn)ScriptContext

分別使用getContext()和setContext()方法獲取和設(shè)置ScriptEngine的默認(rèn)上下文，如下所示：

import javax.script.ScriptContext;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import javax.script.SimpleScriptContext;

public class Main {
  public static void main(String[] args) throws ScriptException {
    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("JavaScript");

        ScriptContext defaultCtx = engine.getContext();
        // Work with defaultCtx here

        // Create a new context
        ScriptContext ctx = new SimpleScriptContext();
        // Configure ctx here

        engine.setContext(ctx);
  }
}

要在新的默認(rèn)上下文中使用ScriptEngineManager的綁定，請(qǐng)如下所示明確設(shè)置：

import javax.script.ScriptContext;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import javax.script.SimpleScriptContext;

public class Main {
  public static void main(String[] args) throws ScriptException {
    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("JavaScript");

        ScriptContext defaultCtx = engine.getContext();
        // Work with defaultCtx here

        // Create a new context
        ScriptContext ctx = new SimpleScriptContext();

        // Configure ctx here

        // Set ctx as the new default context for the engine
        engine.setContext(ctx);
        

        ctx.setBindings(manager.getBindings(), ScriptContext.GLOBAL_SCOPE);

        engine.setContext(ctx);
  }
}