设计模式-解释器模式

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解释器模式

定义

解释器模式（Interpreter Pattern）提供了评估语言的语法或表达式的方式。这种模式实现了一个表达式接口，该接口解释一个特定的上下文。这种模式被用在 SQL 解析、符号处理引擎等。

类图

解释器模式类图

AbstractExpression（抽象表达式）

定义了语法树节点的共同接口。

TerminalExpression（终结符表达式）

对应BNF中的终结符表达式

NonterminalExpression（非终结符表达式）

对应BNF中的非终结符表达式

Context（文脉，上下文）

为解释器语法提供了必要的信息

Client（请求者）

为了推导语法数，Client会调用TerminalExpression和NonterminalExpression。

示例

类图

解释器模式示例类图

CommandListNode

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// <command list> ::= <command>* end
public class CommandListNode extends Node {
    private ArrayList<Node> list = new ArrayList<>();
    public void parse(Context context) throws ParseException {
        while (true) {
            if (context.currentToken() == null) {
                throw new ParseException("Missing 'end'");
            } else if (context.currentToken().equals("end")) {
                context.skipToken("end");
                break;
            } else {
                Node commandNode = new CommandNode();
                commandNode.parse(context);
                list.add(commandNode);
            }
        }
    }
    public String toString() {
        return list.toString();
    }
}

CommandNode

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// <command> ::= <repeat command> | <primitive command>
public class CommandNode extends Node {
    private Node node;
    public void parse(Context context) throws ParseException {
        if (context.currentToken().equals("repeat")) {
            node = new RepeatCommandNode();
            node.parse(context);
        } else {
            node = new PrimitiveCommandNode();
            node.parse(context);
        }
    }
    public String toString() {
        return node.toString();
    }
}

Context

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public class Context {
    private StringTokenizer tokenizer;
    private String currentToken;
    public Context(String text) {
        tokenizer = new StringTokenizer(text);
        nextToken();
    }
    public String nextToken() {
        if (tokenizer.hasMoreTokens()) {
            currentToken = tokenizer.nextToken();
        } else {
            currentToken = null;
        }
        return currentToken;
    }
    public String currentToken() {
        return currentToken;
    }
    public void skipToken(String token) throws ParseException {
        if (!token.equals(currentToken)) {
            throw new ParseException("Warning: " + token + " is expected, but " + currentToken + " is found.");
        }
        nextToken();
    }
    public int currentNumber() throws ParseException {
        int number = 0;
        try {
            number = Integer.parseInt(currentToken);
        } catch (NumberFormatException e) {
            throw new ParseException("Warning: " + e);
        }
        return number;
    }
}

Node

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public abstract class Node {
    public abstract void parse(Context context) throws ParseException;
}

ParseException

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public class ParseException extends Exception {
    public ParseException(String msg) {
        super(msg);
    }
}

PrimitiveCommandNode

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// <primitive command> ::= go | right | left
public class PrimitiveCommandNode extends Node {
    private String name;
    public void parse(Context context) throws ParseException {
        name = context.currentToken();
        context.skipToken(name);
        if (!name.equals("go") && !name.equals("right") && !name.equals("left")) {
            throw new ParseException(name + " is undefined");
        }
    }
    public String toString() {
        return name;
    }
}

ProgramNode

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// <program> ::= program <command list>
public class ProgramNode extends Node {
    private Node commandListNode;
    public void parse(Context context) throws ParseException {
        context.skipToken("program");
        commandListNode = new CommandListNode();
        commandListNode.parse(context);
    }
    public String toString() {
        return "[program " + commandListNode + "]";
    }
}

RepeatCommandNode

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// <repeat command> ::= repeat <number> <command list>
public class RepeatCommandNode extends Node {
    private int number;
    private Node commandListNode;
    public void parse(Context context) throws ParseException {
        context.skipToken("repeat");
        number = context.currentNumber();
        context.nextToken();
        commandListNode = new CommandListNode();
        commandListNode.parse(context);
    }
    public String toString() {
        return "[repeat " + number + " " + commandListNode + "]";
    }
}

Main

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public class Main {
    public static void main(String[] args) {
        try {
            BufferedReader reader = new BufferedReader(new FileReader("program.txt"));
            String text;
            while ((text = reader.readLine()) != null) {
                System.out.println("text = \"" + text + "\"");
                Node node = new ProgramNode();
                node.parse(new Context(text));
                System.out.println("node = " + node);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

program.txt

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program end
program go end
program go right go right go right go right end
program repeat 4 go right end end
program repeat 4 repeat 3 go right go left end right end end

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