jsr223 +编写脚本解释器

Question

好.ScriptEngine.eval(String string)完整地计算字符串,并ScriptEngine.eval(Reader reader)从整体评估输入Reader.

因此,如果我有一个文件,我可以打开一个FileInputStream,在它周围包装一个Reader,然后调用scriptEngine.eval(reader).

如果我有一个完整的语句作为字符串,我可以打电话scriptEngine.eval(string).

如果我需要实现交互式解释器,该怎么办？我有一个用户在多行语句中交互式输入,例如

 function f() {
     return 3;
 }

如果我逐行读取输入,并使用字符串形式eval(),我将最终传递它不完整的语句,例如,function f() {并得到一个错误.

如果我传入一个Reader,ScriptEngine它将永远等待输入完成,并且它不是交互式的.

救命!

---

只是为了澄清:这里的问题是我只能传递ScriptEngine.eval()完整的语句,而作为ScriptEngine的客户,我不知道什么时候输入行是完整的,没有ScriptEngine本身的帮助.

---

Rhino的交互式shell使用Rhino Context.stringIsCompilableUnit()(参见LXR的使用和实现).

Answer 1

我实现了一些可以与 Java SE 6 Rhino (Javascript) 和 Jython 1.5.2 (Python) 配合使用的东西，使用了一种相当简单的方法，类似于 Rhino 的交互式 shell（请参阅我在问题末尾的评论）：

• 保留尚未评估的输入行的待处理列表。
• 尝试编译（但不评估）待处理的输入行。
  • 如果编译正常，我们也许能够执行待处理的输入行。
  • 如果编译抛出异常，并且有错误位置（行+列号）的指示，并且这与待处理输入的末尾匹配，那么这就是我们期待更多输入的线索，因此吞下异常并等待下一行。
  • 否则，我们要么不知道错误在哪里，要么发生在待处理输入结束之前，因此重新抛出异常。
• 如果我们不需要更多的输入行，并且只有一行待处理的输入，则对其进行评估并重新启动。
• 如果我们不期望有更多的输入行，并且最后一行是空白行（根据 @karakuricoder 的答案），并且我们有不止一行待处理的输入，则对其进行评估并重新启动。Python 的交互式 shell 似乎就是这样做的。
• 否则，继续读取输入行。

我不想发生的是：

• 用户在单行输入后必须输入额外的空行感到恼火
• 用户输入一个很长的多行语句，直到第二行出现语法错误后才发现。

这是我编写的一个实现我的方法的辅助类：

import java.lang.reflect.Method;
import javax.script.Bindings;
import javax.script.Compilable;
import javax.script.CompiledScript;
import javax.script.ScriptEngine;
import javax.script.ScriptException;

public class ScriptEngineInterpreter
{
    private static final boolean DEBUG = false;
    final private ScriptEngine engine;
    final private Bindings bindings;
    final private StringBuilder sb;
    private int lineNumber;
    private int pendingLineCount;
    private boolean expectingMoreInput;

    /**
     * @param engine ScriptingEngine to use in this interpreter
     * @param bindings Bindings to use in this interpreter
     */
    public ScriptEngineInterpreter(ScriptEngine engine, Bindings bindings) 
    { 
        this.engine = engine; 
        this.bindings = bindings;
        this.sb = new StringBuilder();
        this.lineNumber = 0;
        reset();
    }       
    /** @return ScriptEngine used by this interpreter */
    public ScriptEngine getEngine() { return this.engine; }
    protected void reset() { 
        this.sb.setLength(0);
        this.pendingLineCount = 0;
        setExpectingMoreInput(false);
    }
    /** @return whether the interpreter is ready for a brand new statement. */
    public boolean isReady() { return this.sb.length() == 0; }
    /**
     * @return whether the interpreter expects more input
     * 
     * A true value means there is definitely more input needed.
     * A false value means no more input is needed, but it may not yet
     * be appropriate to evaluate all the pending lines.
     * (there's some ambiguity depending on the language)
     */
    public boolean isExpectingMoreInput() { return this.expectingMoreInput; }
    protected void setExpectingMoreInput(boolean b) { this.expectingMoreInput = b; }
    /**
     * @return number of lines pending execution
     */
    protected int getPendingLineCount() { return this.pendingLineCount; }
    /**
     * @param lineIsEmpty whether the last line is empty
     * @return whether we should evaluate the pending input
     * The default behavior is to evaluate if we only have one line of input,
     * or if the user enters a blank line.
     * This behavior should be overridden where appropriate.
     */
    protected boolean shouldEvaluatePendingInput(boolean lineIsEmpty) 
    {
        if (isExpectingMoreInput())
            return false;
        else
            return (getPendingLineCount() == 1) || lineIsEmpty; 
    } 
    /**
     * @param line line to interpret
     * @return value of the line (or null if there is still pending input)
     * @throws ScriptException in case of an exception
     */
    public Object interpret(String line) throws ScriptException
    {
        ++this.lineNumber;
        if (line.isEmpty())
        {
            if (!shouldEvaluatePendingInput(true))
                return null;
        }

        ++this.pendingLineCount;        
        this.sb.append(line);
        this.sb.append("\n");
        CompiledScript cs = tryCompiling(this.sb.toString(), getPendingLineCount(), line.length());

        if (cs == null)
        {
            return null;
        }
        else if (shouldEvaluatePendingInput(line.isEmpty()))
        {
            try
            {
                Object result = cs.eval(this.bindings);
                return result;
            }
            finally
            {
                reset();
            }
        }
        else
        {
            return null;
        }
    }
    private CompiledScript tryCompiling(String string, int lineCount, int lastLineLength)
        throws ScriptException 
    {
        CompiledScript result = null;
        try
        {
            Compilable c = (Compilable)this.engine;
            result = c.compile(string);
        }
        catch (ScriptException se) {
            boolean rethrow = true;
            if (se.getCause() != null)
            {
                Integer col = columnNumber(se);
                Integer line = lineNumber(se);
                /* swallow the exception if it occurs at the last character
                 * of the input (we may need to wait for more lines)
                 */
                if (col != null
                 && line != null 
                 && line.intValue() == lineCount 
                 && col.intValue() == lastLineLength)
                {
                    rethrow = false;
                }
                else if (DEBUG)
                {
                    String msg = se.getCause().getMessage();
                    System.err.println("L"+line+" C"+col+"("+lineCount+","+lastLineLength+"): "+msg);
                    System.err.println("in '"+string+"'");
                }
            }

            if (rethrow)
            {
                reset();
                throw se;
            }
        }

        setExpectingMoreInput(result == null);
        return result;
    }
    private Integer columnNumber(ScriptException se)
    {       
        if (se.getColumnNumber() >= 0)
            return se.getColumnNumber();
        return callMethod(se.getCause(), "columnNumber", Integer.class);
    }
    private Integer lineNumber(ScriptException se)
    {       
        if (se.getLineNumber() >= 0)
            return se.getLineNumber();
        return callMethod(se.getCause(), "lineNumber", Integer.class);
    }
    static private Method getMethod(Object object, String methodName)
    {
        try {
            return object.getClass().getMethod(methodName);
        }
        catch (NoSuchMethodException e) {
            return null;
            /* gulp */ 
        }
    }
    static private <T> T callMethod(Object object, String methodName, Class<T> cl) {
        try {
            Method m = getMethod(object, methodName);
            if (m != null)
            {
                Object result = m.invoke(object); 
                return cl.cast(result);
            }
        }
        catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

}