SpelEvaluationException: EL1001E:类型转换问题

Question

我使用SpEL ()计算模板字符串，代码如下所示：

Expression exp = parser.parseExpression(templatedString, templateParserContext);
return exp.getValue(readOnlyEvalContext, contextRoot);

当templatedString == "flow.config：${flow.config}“和contextRoot中的"flow.config”是LinkedHashMap (由杰克逊在JSON反序列化期间创建)时，我得到以下异常：

org.springframework.expression.spel.SpelEvaluationException: EL1001E: Type conversion problem, cannot convert from java.util.LinkedHashMap<?, ?> to java.lang.String

为了解决这个问题，我尝试创建一个转换器来使用ObjectMapper序列化LinkedHashMap，以便在我的计算表达式中显示映射的内联JSON。

@Configuration
public class ConverterConfig implements WebMvcConfigurer {

    @Override
    public void addFormatters(FormatterRegistry registry) {
        registry.addConverter(new ConverterLinkedHashMapToString());
    }
}

@Component
public class ConverterLinkedHashMapToString implements Converter<LinkedHashMap<?, ?>, String> {

    @Override
    public String convert(LinkedHashMap<?, ?> source) {
        return objMapper.writeValueAsString(source);
    }

    @Override
    public <U> Converter<LinkedHashMap<?, ?>, U> andThen(Converter<? super String, ? extends U> after) {
        return Converter.super.andThen(after);
    }
}

不管我做什么，我似乎都做不到。我只是一直得到同样的例外。我可能做错了什么，但尽管我花了大约一天的时间寻找和挣扎，但我一直未能解决这个问题。任何帮助都将不胜感激！谢谢!

Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1001E: Type conversion problem, cannot convert from java.util.LinkedHashMap<?, ?> to java.lang.String
    at org.springframework.expression.spel.support.StandardTypeConverter.convertValue(StandardTypeConverter.java:87) ~[spring-expression-5.3.22.jar:5.3.22]
    at org.springframework.expression.common.ExpressionUtils.convertTypedValue(ExpressionUtils.java:57) ~[spring-expression-5.3.22.jar:5.3.22]
    at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:378) ~[spring-expression-5.3.22.jar:5.3.22]
    at org.springframework.expression.common.CompositeStringExpression.getValue(CompositeStringExpression.java:129) ~[spring-expression-5.3.22.jar:5.3.22]
    at org.springframework.expression.common.CompositeStringExpression.getValue(CompositeStringExpression.java:43) ~[spring-expression-5.3.22.jar:5.3.22]

[...]

    ... 3 common frames omitted
Caused by: org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.util.LinkedHashMap<?, ?>] to type [java.lang.String]
    at org.springframework.core.convert.support.GenericConversionService.handleConverterNotFound(GenericConversionService.java:322) ~[spring-core-5.3.22.jar:5.3.22]
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:195) ~[spring-core-5.3.22.jar:5.3.22]
    at org.springframework.expression.spel.support.StandardTypeConverter.convertValue(StandardTypeConverter.java:82) ~[spring-expression-5.3.22.jar:5.3.22]
    ... 38 common frames omitted

Answer 1

通过将Converter添加到EvaluationContext中，通过TypeConverter，我能够让它与您的转换器一起工作。将Converter添加到EvaluationContext的代码如下所示：

GenericConversionService conversionService = new DefaultConversionService();
conversionService.addConverter(new ConverterLinkedHashMapToString());
TypeConverter typeConverter = new StandardTypeConverter(conversionService);
StandardEvaluationContext evalContext = new StandardEvaluationContext();
evalContext.setTypeConverter(typeConverter);

Expression exp = parser.parseExpression(templatedString, templateParserContext);
String message = (String) exp.getValue(evalContext, contextRoot);
System.out.println(message);

不过，我没有在spring/spring引导应用程序中测试这个。我使用独立的groovy脚本进行了测试。下面是供参考的完整脚本：

@Grab(group='org.springframework', module='spring-expression', version='5.3.22')
@Grab(group='com.fasterxml.jackson.core', module='jackson-databind', version='2.13.3')

import com.fasterxml.jackson.databind.ObjectMapper;

import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;
import org.springframework.expression.spel.support.StandardTypeConverter;
import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.ParserContext;
import org.springframework.expression.common.TemplateParserContext;
import org.springframework.expression.TypeConverter;
import org.springframework.core.convert.support.GenericConversionService;
import org.springframework.core.convert.converter.Converter;
import org.springframework.core.convert.support.DefaultConversionService;

ExpressionParser parser = new SpelExpressionParser();
ParserContext templateParserContext = new TemplateParserContext('${', '}');
String templatedString = "flow.config: \${flow['config']}";
Dummy contextRoot = new Dummy();

GenericConversionService conversionService = new DefaultConversionService();
conversionService.addConverter(new ConverterLinkedHashMapToString());
TypeConverter typeConverter = new StandardTypeConverter(conversionService);
StandardEvaluationContext evalContext = new StandardEvaluationContext();
evalContext.setTypeConverter(typeConverter);

Expression exp = parser.parseExpression(templatedString, templateParserContext);
String message = (String) exp.getValue(evalContext, contextRoot);
System.out.println(message);

class Dummy {
    Map<String, ?> flow = ['config': ['hola': 'hello']];
}

class ConverterLinkedHashMapToString implements Converter<LinkedHashMap<?, ?>, String> {
    private ObjectMapper objMapper = new ObjectMapper();

    @Override
    public String convert(LinkedHashMap<?, ?> source) {
        return objMapper.writeValueAsString(source);
    }
}

在将Converter添加到EvaluationContext之前，我得到了相同的SpelEvaluationException。添加转换器后，代码运行良好，并将包含映射的JSON值的解析字符串打印为flow.config: {"hola":"hello"}