Spring Document supported characters for identifiers in SpEL expressions

Original title: Does SpEL support Chinese variables?

I try to use SpEL to execute some code which includes some Chinese variables.

void test() {
        SimpleEvaluationContext evaluationContext = SimpleEvaluationContext
                .forReadWriteDataBinding()
                .withMethodResolvers(DataBindingMethodResolver.forInstanceMethodInvocation())
                .build();

        evaluationContext.setVariable("中文", 1);

        ExpressionParser parser = new SpelExpressionParser();
        Expression expression = parser.parseExpression("#中文 == 1");

        Boolean value = expression.getValue(evaluationContext, Boolean.class);

        System.out.println(value);
 }

Exception stack trace:

java.lang.IllegalStateException: Cannot handle (20013) '中'
    at org.springframework.expression.spel.standard.Tokenizer.process(Tokenizer.java:268)
    at org.springframework.expression.spel.standard.InternalSpelExpressionParser.doParseExpression(InternalSpelExpressionParser.java:127)
    at org.springframework.expression.spel.standard.SpelExpressionParser.doParseExpression(SpelExpressionParser.java:61)
    at org.springframework.expression.spel.standard.SpelExpressionParser.doParseExpression(SpelExpressionParser.java:33)
    at org.springframework.expression.common.TemplateAwareExpressionParser.parseExpression(TemplateAwareExpressionParser.java:52)
    at org.springframework.expression.common.TemplateAwareExpressionParser.parseExpression(TemplateAwareExpressionParser.java:43)

Comment From: sbrannen

For identifiers such as variables, SpEL does not support characters whose integer value is greater than 255.

https://github.com/spring-projects/spring-framework/blob/6c2cb8ecf5d1d755f09aff80489aa8b6e49d70b1/spring-expression/src/main/java/org/springframework/expression/spel/standard/Tokenizer.java#L569-L574

However, the documentation in the reference manual does not explicitly state this fact.

Thus, we can improve the documentation here to be explicit.