.NET NLog 详解(五) - Condition Expression

Sample

<!-- during normal execution only log Info messages -->

    <defaultFilter>level >= LogLevel.Info</defaultFilter>

    <!-- if there is at least one error, log everything from trace level -->

    <when exists="level >= LogLevel.Error" filter="level >= LogLevel.Trace" />

实现将字符串转换成表达式进行逻辑判断，然后做出相应的动作。这个过程的实现使用Condition Expression。

从简单的例子开始：

public void BooleanOperatorTest()

    {

        AssertEvaluationResult(false, "false or false");

        AssertEvaluationResult(true, "false or true");

        AssertEvaluationResult(true, "true or false");

        AssertEvaluationResult(true, "true or true");

        AssertEvaluationResult(false, "false and false");

        AssertEvaluationResult(false, "false and true");

        AssertEvaluationResult(false, "true and false");

        AssertEvaluationResult(true, "true and true");

        AssertEvaluationResult(false, "not true");

        AssertEvaluationResult(true, "not false");

        AssertEvaluationResult(false, "not not false");

        AssertEvaluationResult(true, "not not true");

    }

输入的是字符串false or false，我们希望的结果是得到false，大概的过程是这样的:

CondtionParser

 /// <summary>

/// Condition parser. Turns a string representation of condition expression

/// into an expression tree.

/// </summary>

public class ConditionParser

第一步就是初始化这个Parser

var parser = new ConditionParser(new SimpleStringReader(expressionText), configurationItemFactories);

顺带初始化了tokenizer

    private ConditionParser(SimpleStringReader stringReader, ConfigurationItemFactory configurationItemFactory)

    {

        this.configurationItemFactory = configurationItemFactory;

        this.tokenizer = new ConditionTokenizer(stringReader);

    }

构造函数里直接取到了第一个token false

    public ConditionTokenizer(SimpleStringReader stringReader)

    {

        this.stringReader = stringReader;

        this.TokenType = ConditionTokenType.BeginningOfInput;

        this.GetNextToken();

    }

获取的过程也比较的简单，一个接一个的读入char，遇到非指定的字符即停止

private void ParseKeyword(char ch)

    {

        int i;

        this.TokenType = ConditionTokenType.Keyword;

        StringBuilder sb = new StringBuilder();

        sb.Append((char)ch);

        this.ReadChar();

        while ((i = this.PeekChar()) != -1)

        {

            if ((char)i == '_' || (char)i == '-' || char.IsLetterOrDigit((char)i))

            {

                sb.Append((char)this.ReadChar());

            }

            else

            {

                break;

            }

        }

        this.TokenValue = sb.ToString();

    }

这里有一系列比较复杂的表达式树生成的过程：

private ConditionExpression ParseBooleanOr()

    {

        ConditionExpression expression = this.ParseBooleanAnd();

        while (this.tokenizer.IsKeyword("or") || this.tokenizer.IsToken(ConditionTokenType.Or))

        {

            this.tokenizer.GetNextToken();

            expression = new ConditionOrExpression(expression, this.ParseBooleanAnd());

        }

        return expression;

    }

最后拿到的表达式树是这样的:

左右的表达式为ConditionLiteralExpression。

     private ConditionExpression ParseLiteralExpression()

    {

        //......

        if (this.tokenizer.TokenType == ConditionTokenType.Keyword)

        {

            string keyword = this.tokenizer.EatKeyword();

            if (0 == string.Compare(keyword, "level", StringComparison.OrdinalIgnoreCase))

            {

                return new ConditionLevelExpression();

            }

            if (0 == string.Compare(keyword, "logger", StringComparison.OrdinalIgnoreCase))

            {

                return new ConditionLoggerNameExpression();

            }

            if (0 == string.Compare(keyword, "message", StringComparison.OrdinalIgnoreCase))

            {

                return new ConditionMessageExpression();

            }

            if (0 == string.Compare(keyword, "loglevel", StringComparison.OrdinalIgnoreCase))

            {

                this.tokenizer.Expect(ConditionTokenType.Dot);

                return new ConditionLiteralExpression(LogLevel.FromString(this.tokenizer.EatKeyword()));

            }

            if (0 == string.Compare(keyword, "true", StringComparison.OrdinalIgnoreCase))

            {

                return new ConditionLiteralExpression(true);

            }

再来个例子:

public void ConditionMethodsTest()

    {

        AssertEvaluationResult(true, "starts-with('foobar','foo')");

        AssertEvaluationResult(false, "starts-with('foobar','bar')");

        AssertEvaluationResult(true, "ends-with('foobar','bar')");

        AssertEvaluationResult(false, "ends-with('foobar','foo')");

        AssertEvaluationResult(0, "length('')");

        AssertEvaluationResult(4, "length('${level}')");

        AssertEvaluationResult(false, "equals(1, 2)");

        AssertEvaluationResult(true, "equals(3.14, 3.14)");

        AssertEvaluationResult(true, "contains('foobar','ooba')");

        AssertEvaluationResult(false, "contains('foobar','oobe')");

        AssertEvaluationResult(false, "contains('','foo')");

        AssertEvaluationResult(true, "contains('foo','')");

    }

这里出现了一些条件方法starts-with,ends-with,equals,contains,可以通过这些操作，当log的message包含特定的字符串的时候才写日志

[ConditionMethod("starts-with")]是以属性反射的方式在初始化的时候加载的。

在parse的过程中，多一步判断是否是ConditionMethods并创建实例

private ConditionMethodExpression ParsePredicate(string functionName)

    {

        try

        {

            var methodInfo = this.configurationItemFactory.ConditionMethods.CreateInstance(functionName);

            return new ConditionMethodExpression(functionName, methodInfo, par);

        }

        catch (Exception exception)

        {

            if (exception.MustBeRethrown())

            {

                throw;

            }

            throw new ConditionParseException("Cannot resolve function '" + functionName + "'", exception);

        }

    }

那么怎么知道该字符串是否是ConditionMethods呢？这里还是简单从左到右一个个的吃字符。

public string EatKeyword()

    {

        if (this.TokenType != ConditionTokenType.Keyword)

        {

            throw new ConditionParseException("Identifier expected");

        }

        string s = (string)this.TokenValue;

        this.GetNextToken();

        return s;

    }

当出现特殊的char时候做出判断。

     private static ConditionTokenType[] BuildCharIndexToTokenType()

    {

        CharToTokenType[] charToTokenType =

        {

            new CharToTokenType('(', ConditionTokenType.LeftParen),

            new CharToTokenType(')', ConditionTokenType.RightParen),

            new CharToTokenType('.', ConditionTokenType.Dot),

            new CharToTokenType(',', ConditionTokenType.Comma),

            new CharToTokenType('!', ConditionTokenType.Not),

            new CharToTokenType('-', ConditionTokenType.Minus),

        };

巴特西

.NET NLog 详解(五) - Condition Expression

CondtionParser

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